Abstract
We describe the Gems of the Galaxy Zoos (Zoo Gems) project, a gap-filler project using short windows in the Hubble Space Telescope's schedule. As with previous snapshot programs, targets are taken from a pool based on position; we combine objects selected by volunteers in both the Galaxy Zoo and Radio Galaxy Zoo citizen-science projects. Zoo Gems uses exposures with the Advanced Camera for Surveys to address a broad range of topics in galaxy morphology, interstellar-medium content, host galaxies of active galactic nuclei, and galaxy evolution. Science cases include studying galaxy interactions, backlit dust in galaxies, post-starburst systems, rings and peculiar spiral patterns, outliers from the usual color–morphology relation, Green Pea compact starburst systems, double radio sources with spiral host galaxies, and extended emission-line regions around active galactic nuclei. For many of these science categories, final selection of targets from a larger list used public input via a voting process. Highlights to date include the prevalence of tightly wound spiral structure in blue, apparently early-type galaxies, a nearly complete Einstein ring from a group lens, redder components at lower surface brightness surrounding compact Green Pea starbursts, and high-probability examples of spiral galaxies hosting large double radio sources.
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1. Introduction
Astronomy enjoys a rich history of knowledge gained from objects at the extremes of sample properties, and outliers to common correlations. Our experience with spinoff studies from the Galaxy Zoo projects has certainly borne this out, leading to further observation of rare and unusual galaxies that in turn yielded insights regarding a range of questions in galaxy evolution. This paper describes one such project, delivering Hubble Space Telescope (HST) images of galaxies randomly selected from a list chosen for science value in a number of contexts.
Galaxy Zoo has encompassed several iterations of classification based on volunteer examination of galaxies in digital sky surveys. Initially, "classic" Galaxy Zoo (Lintott et al. 2008) provided broad morphological information (spiral/elliptical/merging, and direction of spiral arms) for over 900,000 galaxies from data release 7 (DR7, Abazajian et al. 2009) of the Sloan Digital Sky Survey (SDSS; York et al. 2000). Galaxy Zoo 2 (Willett et al. 2013) built on the demonstrated ability of volunteers to consistently provide finer-grained morphological information, now working with about 250,000 of the brightest SDSS galaxies. The approach was extended, broadening the decision tree to encompass clumpy galaxies, to deep optical HST fields in Galaxy Zoo Hubble (Willett et al. 2017) and the near-IR CANDELS data (Simmons et al. 2017), and most recently images from the Legacy Survey (Dey et al. 2019). The results of these studies led to recognition of the importance of blue early-type galaxies (blue ellipticals for short; Schawinski et al. 2009) and red spiral galaxies (Bamford et al. 2009; Masters et al. 2010); Masters & Galaxy Zoo Team (2020) summarize the first twelve years of Galaxy Zoo results. It quickly became clear that the project discussion forum 15 , where volunteers could ask questions and exchange comments about galaxy images, was drawing attention to very rare phenomena, leading to the identification of Green Pea compact starburst systems (Cardamone et al. 2009), nearly 2000 pairs of galaxies with overlapping images for dust analysis (Keel et al. 2013), and giant extended emission-line regions (EELRs) around active galactic nuclei (AGN), many of which are so luminous as to suggest that the central AGN must have faded within the relevant light-travel time (Lintott et al. 2009; Keel et al. 2012). Beyond these, numerous other galaxy images of special interest have been brought up for discussion on the Forum and its successor in the project's Talk interface, 16 providing ready sets of objects for follow-up observation. As this became clear, team members could call for specific kinds of objects, and were often answered with great energy by volunteers. Beyond the primary statistical goals of the various iterations of the Galaxy Zoo public-participation project, some of its hundreds of thousands of volunteer participants have identified rare and unusual galaxies for which further data would be particularly interesting.
The more recent launch of Radio Galaxy Zoo (Banfield et al. 2015), in which participants examine optical and near-infrared images in concert with radio data, likewise makes use of the Talk interface for exchange of more detailed information, particularly on the rare radio galaxies with possibly spiral host morphology and on active galactic nuclei (AGN) with extensive emission-line clouds. For many of the objects, their nature would become much clearer with higher-resolution optical images than the SDSS data used for the initial rounds of Galaxy Zoo classifications, and numerous specific science goals could be addressed with even a modest set of such follow-up images.
Galaxy Zoo team members had long joked about the appropriate follow-up observing proposal being "We have a bunch of weird galaxies, and need a closer look to understand them better." This was essentially what the 2017 STScI gap-filler opportunity offered. We describe in this paper the resulting program, "Gems of the Galaxy Zoos" (Zoo Gems for short, program 15445), which has provided HST images relevant to a wide range of science cases drawn from Galaxy Zoo and Radio Galaxy Zoo. In this paper, we describe these aims, detail how we incorporated public input in selecting the target lists for many of the science cases we could address, document the setup of the observations, and present some initial results. While many of the results of Zoo Gems will appear in further papers, we think it useful to provide here the common background and rationale of the observations.
2. Science Cases
This section sets out the science rationales for various object categories, organized into broad morphological themes. Some categories have had only a single example observed at this point.
2.1. Galaxy Disks
2.1.1. Galaxy Zoo: Unusual Spirals
This category furnishes a catch-all for spiral galaxies: three-armed grand-design systems, galaxies with very asymmetric patterns but no obvious interacting companion, or dominant resonance ring structures. The three-armed spirals took on particular interest with the finding from Galaxy Zoo 2 classifications that they are not more common in low-density environments, as had been expected from relative growth properties of various Fourier modes (Elmegreen et al. 1992; Durbala et al. 2009), and that bars are just as common in these as in the two-armed examples where the 180° symmetry of bar and arms matches (Hancock 2019).
2.1.2. Galaxy Zoo: Nuclear Disks and Bars in Spirals
High-resolution images have shown some spiral galaxies, especially those with bars and outer rings, to have analogous circumnuclear structures—known as nuclear disks. We included galaxies seen (or very likely) to have unusually large central disks within bars, or central bars misaligned with the outer structures.
2.1.3. Galaxy Zoo: Backlit Galaxies
Noninteracting galaxy pairs whose images overlap in projection offer a way to study dust attenuation independent of dust temperature or structure of the galaxy, and subject to completely different systematics than methods relying on IR emission or modeling of the galaxy's spectral-energy distribution (SED). This approach has been applied to very limited sets of galaxies, using ground-based data by White & Keel (1992), Berlind et al. (1997), and White et al. (2000), and with the improved angular resolution of HST imaging by Elmegreen et al. (2000), Keel & White (2001), Keel & White (2001), and Holwerda et al. (2009). Applicability of this technique remained limited by the very modest number of suitable backlit galaxies then known, a situation that was dramatically improved by the sensitivity and (especially) dynamic range of SDSS data. Galaxy Zoo participants provided an extensive finding list of candidate pairs, for a catalog of nearly 2000 such pairs after validation from their initial examination of DSSS DR7 images alone (Keel et al. 2013). This list has been supplemented by the second pass through DR7 images during Galaxy Zoo 2. Similar pairs were also noted during Hubble Zoo, but not considered here since Zoo Gems images would not improve their data quality. The ideal pair would have galaxies with redshifts so different as to rule out physical association, containing a smooth early-type galaxy behind a relatively symmetric spiral. In practice, these criteria can be relaxed—for example, to include pairs with particular geometries or evidence for very distant attenuation regions—as long as the image information is properly used to estimate uncertainties due to departures from symmetry.
2.2. Starbursts and Star-forming Regions
2.2.1. Galaxy Zoo: Green Peas
Green Pea systems, as described by Cardamone et al. (2009), were initially identified as a class by Galaxy Zoo volunteers. The name arises from their combination of small size (SDSS Petrosian radius petrorad_r < 2
0) and green appearance in SDSS gri composite images, arising from a strong emission line in the r band (refined to ugriz color criteria by Cardamone et al. (2009)). The dominant emission line for these "green" objects is redshifted [O iii] λ5007. Inspection of the SDSS spectra showed most of these to be star-forming systems; their small angular sizes and redshifts mean these are therefore among the most compact star-forming galaxies, and the large emission-line equivalent widths responsible for their color selection mark them firmly as starburst systems. Only a handful of Green Peas appeared serendipitously in previous HST imagery, so we incorporated Peas into the Zoo Gems list to enable a more systematic study of their structures. In particular, we selected filters that emphasized the stellar continuum, for a better view of the structure of the galaxies themselves, and included systems in three redshift slices. Improved measurements or limits on the sizes of the stellar structures would lead to better understanding of how intense the starbursts are, and to what extent these extreme star-forming regions reside in systems with previous histories of star formation, or show evidence of tidal disturbance that could trigger these extreme starbursts. These red ACS WFC passbands also provide a contrast with the UV bands previously observed for some Green Pea samples using HST, which represent the young starburst populations well—but not any older stellar components.
The Galaxy Zoo sample of Green Peas has generated extensive follow-up work as sources of Lyman-continuum leakage (Yang et al. 2017; Malkan & Malkan 2021), Lyman α emission sources (Orlitová et al. 2018), extreme starbursts driving global winds (Jaskot et al. 2017; Bosch et al. 2019; Hogarth et al. 2020), and testbeds for chemical evolution scenarios at high star formation rates (Hawley 2012; Amorín et al. 2012). These compact, intense starbursts are likely related to the less-massive "Little Blue Spheroids" (Moffett et al. 2019).
2.2.2. Galaxy Zoo: Post-starbursts
Spectroscopically selected post-starburst galaxies 17 (using the combination of Hδ in absorption with equivalent width >3 Å and Hα emission undetected at the 4σ level) show central concentrations (SDSS fracdev parameter) intermediate between disk and early-type galaxies (Wong et al. 2012). This could reflect genuine morphological transformation or preferential scales for the starburst, both issues that higher-resolution images could shed light on. This selection on specific absorption line properties is distinct from the color/luminosity selection often defining the "Green Valley," being influenced by star formation events that are more localized or involve smaller gas masses, but the possibility of post-starburst systems being seen during morphological transformation does parallel the inferences about galaxies in the Green Valley largely undergoing a one-time morphological change (Mendez et al. 2011; Schawinski et al. 2014; Smethurst et al. 2015; Kelvin et al. 2018). In addition, the fading starbursts are often concentrated in knots, which are often well-detected in snapshot observations, allowing broad comparison of the starburst properties. These properties in combination could suggest to what extent processes in local post-starburst galaxies are, or are not, useful analogs for the quenching of star formation in the galaxy population more generally.
2.2.3. Galaxy Zoo: Blue Ellipticals
One of the earliest Galaxy Zoo science results was the existence of galaxies robustly classified as early-type, with colors much bluer than the usual red sequence (Schawinski et al. 2009), indicating an unusual level of recent star formation or contamination by the light of an AGN. High-resolution images can trace the distribution of star-forming regions, indicating whether they form disks, rings, or the kinds of asymmetric patches that could be infalling star-forming regions. We tracked three separate subcategories among blue ellipticals (selected using a color cut based on the red sequence track, as in Schawinski et al. 2009), so the final list included the highest-ranked examples of blue early-type galaxies with emission-line ratios from SDSS spectra indicating that AGN or star formation is dominant (five and four targets, respectively), and star-forming examples with detected CO or H I emission (three targets; Schawinski et al. 2009; Wong et al. 2015).
2.2.4. Galaxy Zoo: Red Spirals
Analogous to blue ellipticals, Galaxy Zoo classifications led to identification of a population of red spiral galaxies (Masters et al. 2010), defined by color offset from typical spirals at a given luminosity. While present in all environments, they are most abundant in the high-density regions just outside cluster cores (Bamford et al. 2009). Galaxies for voting were selected to be nearly face-on, so the red color is not due solely to attenuation in the disk. The HST images could show whether the star-forming regions in these systems are of unusually low luminosity or unusually sparse compared to spirals of typical colors, and address the incidence of bars at small scales for comparison with the global bars that are common in these systems (Masters et al. 2011; Kruk et al. 2018). The properties of red spirals may give clues to the processes quenching star formation when seen independently of morphological transformations.
2.2.5. Galaxy Zoo: Luminous Star-forming Clumps in Galaxies
While rare in the local Universe, these may be helpful analogs to the luminous star-forming clumps that are ubiquitous in the high-redshift galaxy population (Cowie et al. 1995; Elmegreen et al. 2004). The relative handful of nearby analogous objects will be much better resolved, providing information on scales of star formation (and sometimes the properties of the most luminous clusters in these regions).
2.3. Interacting and Merging Galaxies
2.3.1. Galaxy Zoo: Mergers That Are Very Distorted or Have Very Long/Luminous Tails
Among the many interacting and merging systems flagged by Galaxy Zoo (Darg et al. 2010), some stand out even in such exceptional company as having tidal tails of unusual length or brightness, or main galaxy bodies that are unusually distorted. We include some of these in the target list, to sample the properties of galaxy interactions that produce such extreme stages. Outcomes might include populations of luminous star clusters, morphological information on scales beyond the SDSS resolution limit, and the role of dust attenuation (which can change our interpretation of a system's components and their spatial relationships).
2.3.2. Galaxy Zoo: Collisional and Polar Rings (Including Possible Lenses)
Numerous candidates for these rare interaction signatures were noted by Galaxy Zoo participants. They offer particular insight into not only the prevalence of these kinds of galaxy encounters, but such disparate issues as the shapes of dark-matter halos, properties of star formation with time mapped to location, and the degree of self-gravitation in polar rings (Reshetnikov & Combes 1994; Bizyaev et al. 2007; Egorov & Moiseev 2019). The top-voted polar-ring candidate was earlier cataloged as PRC A-1 in the catalog by Whitmore et al. (1990), supporting the identification of this system as hosting a classic polar-ring structure. This category also includes objects that may turn out to be gravitationally lensed arcs when examined at high angular resolution, as indeed happened in one spectacular and near-complete Einstein ring (Section 5.5).
2.3.3. Galaxy Zoo: Red/Blue Pairs
Particularly in the earliest examination of Legacy Survey images (Dey et al. 2019), participants have found a small category of close pairs of marginally resolved images with very different colors. At first, we suspected these might be star/galaxy superpositions, but one Zoo Gems image shows at least some to be interesting galaxy interactions, of the general kind long discussed as mixed-morphology pairs (Rampazzo & Sulentic 1992; Hernández Toledo et al. 1999) with contrasting morphologies, colors, and star-forming properties.
2.3.4. Galaxy Zoo: Regrowing Disks
The "merger hypothesis" for making elliptical galaxies by merging disk systems (Toomre 1977) was largely suggested by the tendency for disk mergers in simulations to yield elliptical-like, diskless remnants for mass ratios near unity (in practice, a cutoff near 3:1 has often been taken to give the right fraction of mergers, if not always the right outcomes in individual cases when such parameters as approach geometry and gas fraction are added to the mass ratio). However, as merger simulations achieve higher fidelity and are run for more combinations of the numerous initial conditions, some mergers of near-equal mass galaxies are shown to retain disks afterward (Barnes & Hernquist 1996; Governato et al. 2009; Hopkins et al. 2009). In parallel, we have noticed a category of interacting-galaxy pairs that show single disks and spiral patterns surrounding two distinct bulges, which could represent this process in action (and existence of a remnant disk before the nuclei merge). We included all these systems in the Zoo Gems object list, to give the possibility of empirical information on the survival or reformation of disks in major mergers. Such events may be important in producing the population of high-luminosity "super spiral galaxies" identified by Ogle et al. (2016) and Ogle et al. (2019). They note that a significant fraction of these gigantic disk systems show signs of at least minor meters; retaining or regrowing disks after some major mergers would make it easier to understand the existence of disk-dominated systems at the highest galaxy luminosities.
2.4. Active Galactic Nuclei and Their Host Galaxies
2.4.1. Galaxy Zoo: EELRs
Galaxy Zoo participants have proven to be adept at identifying candidates for extended emission-line regions (EELRs) associated with AGN, based on the distinctive colors in the SDSS gri composite images produced by strong [O iii] emission appearing in g or r at different redshifts (Lintott et al. 2009; Keel et al. 2012). The distance of these clouds from the AGN, and their luminosity compared to what we see from the AGN directly, constrain both the duty cycle of rapid accretion and characteristic duration of accretion episodes. Fine structure in these clouds strengthens constraints on the required ionizing luminosity of the associated AGN (Keel et al. 2012, 2017), motivating us to include the strongest EELR candidates in filters matching the SDSS detection bands, even in the absence of separate HST continuum data.
2.4.2. Radio Galaxy Zoo: EELRs
As in Galaxy Zoo, Radio Galaxy Zoo participants identified many galaxies, or galaxy components, with such extreme colors that there are almost certainly strong, resolved emission-line clouds (some tagged as "RGZ Green" objects, since many are at redshifts z ≈ 0.25 where [O iii] emission appears in the r band, mapped to green in the SDSS and analogous color-composite displays). For radio-loud AGN, in addition to photoionization, emerging jets add the possibility of shock ionization on large scales. Here again, we specified filters closely matching the detection band from Radio Galaxy Zoo. The input list for voting incorporated both SDSS colors of the host and spatially resolved structure in the r filter.
2.4.3. Radio Galaxy Zoo: SDRAGNs
For convenience, we follow Leahy (1993) in using DRAGNS (Double-lobed Radio sources Associated with Galactic Nuclei) to describe typical double-lobed radio galaxies. A long-known property of the population of galaxies hosting DRAGNs is that they are overwhelmingly elliptical galaxies or merger remnants. However, detailed study has revealed a handful of galaxies, associated at high probability with DRAGNs, with clear spiral structure (Ledlow et al. 2001; Keel et al. 2006; Hota et al. 2011; Mao et al. 2015; Mulcahy et al. 2016). We will call these rare DRAGNs with spiral host galaxies "SDRAGNs." We concentrate on these because they go against the dominant correlation of luminous double sources with elliptical or post-merger host galaxies, offering the possibility of a way to understand which host-galaxy properties drive (or allow) production of large-scale double sources. The initial list was selected based on location of the putative host galaxy with respect to the radio lobes, and evidence of a disk (a large exponential-disk fraction in the SDSS image analysis or from a full two-dimensional fit to the SDSS images using GALFIT (Peng et al. 2002)).
The angular resolution of HST imaging can make the morphology of these galaxies, many as distant as z ≈ 0.2, much clearer, resolving spiral arms, dust lanes, and star-forming regions, allowing a much more confident morphological assessment than the photometric bulge-disk decompositions that were our starting points. By the same token, the bulge properties will also be much better determined, giving an improved understanding of likely black hole masses and evolutionary paths of these galaxies. For those radio sources that actually arise in a more distant galaxy than the spiral observed, improved astrometry with the HST data can identify such false associations.
2.5. Galaxy Zoo: Unusual Bulges
This category includes bulges that are unusually shaped, unusually prominent for the galaxy's morphological type, or appear prolate with respect to the disk orientation. The latter may indicate that the disk is more like a polar ring, resulting from material starting in a second galaxy. An important subtype is formed by so-called X-bulges (extreme versions of "peanut bulges"), which have attracted attention as being edge-on views of bars, allowing us to visualize the distribution of stars along a direction that remains unseen in images of more face-on systems (Bureau & Freeman 1999; Kruk et al. 2019).
3. Final Target List
Our original proposal estimated 1100 targets available. The program was allocated 300 targets for entry into the whole gap-filler coordinate pool, 18 so much of the time available for detailed preparation went into winnowing the target list. Sparse categories (in practice, those with fewer than 10 examples) were carried along "as is."
Because of the public-participation nature of the Galaxy Zoo projects, and further encouraged by comments from STScI, we solicited public input to select the objects to be listed from categories with a large enough number of galaxies. After members of the science team inspected them for suitability, we set up a web-based voting system, and advertised this opportunity widely through each project's Web and social media presence, and coordinated with the STScI social media team for announcement through their accounts as well. We used the Zooniverse Project Builder interface 19 (Trouille et al. 2019), producing parallel selection interfaces for the Galaxy Zoo and Radio Galaxy Zoo subsets. The voting was open from 2018 February 2 to 18, to meet the February 28 deadline for submission of the Phase 2 proposal with target names, coordinates, and observation details. The Galaxy Zoo categories drew 6199 votes among 292 galaxies (mean 21.2 per object), while the Radio Galaxy Zoo objects attracted 9730 votes among 286 galaxies (mean 34.0 each). Numbers of votes per object varied, as users did different numbers per session. The Project Builder interface was set up so that volunteers would cycle through all members of each science category in sequence, with the option to see a montage of available objects in that category in order to allow a more informed comparative ranking. As illustrated in Figure 1, each was presented with a question of the form "What priority would you give this galaxy with an unusual spiral pattern for possible Hubble Space Telescope observations?" and possible answers lowest priority, low, medium priority, high, or highest priority.
Figure 1. Screen capture of the voting interface for Galaxy Zoo targets. Clicking on "Need some help with this task?" displayed a longer description and a montage of all galaxies in the same voting category. The red banner at upper left indicates that a user has already seen this object.
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Standard image High-resolution imageIn collating votes, we could distinguish between users who signed in with a Zooniverse account and those participating anonymously. For the former, registered users, we could recognize multiple votes for a single object, and counted only the last one. We examined the anonymous votes for any evidence of such misbehavior as robotic software packing votes, which could affect the outcome. There were very few anonymous votes (180 for Galaxy Zoo, 247 for Radio Galaxy Zoo), widely spread across galaxies, so we saw no evidence of problematic behavior and included these votes in our rankings. Within each science category being voted on, we ranked objects using a straightforward mean of votes weighted by priority. Users were given a five-point scale to select on seeing each galaxy image; we ranked on the mean with highest priority = 1, lowest priority = 5. The highest-ranked objects in each science category were carried into the final target list. (The number of slots allocated for each science category was arbitrarily set by the PI, attempting to reflect the number of input objects and scientific interest).
The preliminary target list was shared for discussion with the Galaxy Zoo community on the Talk interface. Volunteers identified some duplicates, because objects could enter in different science categories by different names. We also coordinated our target list with that of gap-filler program 15446 (Arp and Arp-Madore interacting galaxies, P.I. Julianne Dalcanton), to eliminate duplications while ensuring that some especially interesting systems were on one list or the other.
For Green Peas, whose unresolved SDSS images made visual selection superfluous, the input list consisted of SDSS DR12 objects with secure matches to the Portsmouth spectral fitting catalog (Thomas et al. 2013), and BPT type "Star-forming" from that catalog. Further selection was for the brightest objects in each of three redshift ranges where ACS filters (mostly) isolate continuum.
Our category for each targeted object was entered in the phase 2 proposal file 20 under the "Comments" field.
Some of the flavor of the voting outcomes can be seen in Figure 3, showing the top-ranked galaxies in four categories with numerous objects examined.
Table 1 lists the number of target objects in each category, and the number observed as of submission of this paper. Only primary categories are given, although some fit in multiple categories. For example, all five of the Galaxy Zoo EELR systems occur in interacting galaxies, and one Radio Galaxy Zoo lens candidate lies in the same ACS field as the Galaxy Zoo EELR candidate SDSS J160646.74+565139.2. For categories where targets were selected by voting, the table also lists the number of objects voted on in each category and the number of votes received. "Number input" is the number of targets from that category included in the final observing list, and "Observed" is the number actually observed to date.
Table 1. Target and Filter Selection by Object Categories
| Category | Number Voted | Votes | Number Input | Observed | Filter | Goal |
|---|---|---|---|---|---|---|
| Galaxy Zoo ring systems | 26 | 799 | 15 | 6 | F606W | Overall structure |
| Galaxy Zoo EELRs | ⋯ | ⋯ | 5 | 1 | F475W | Line emission |
| Galaxy Zoo regrowing disks | ⋯ | ⋯ | 7 | 6 | F606W | Overall structure |
| Galaxy Zoo bulges | ⋯ | ⋯ | 9 | 5 | F814W | Old populations |
| Galaxy Zoo interacting systems | 15 | 515 | 10 | 7 | 606W | Overall structure |
| Galaxy Zoo red/blue pairs | ⋯ | ⋯ | 5 | 2 | F606W | Overall structure |
| Galaxy Zoo star-forming clumps | 36 | 904 | 7 | 1 | F606W | Overall structure |
| Galaxy Zoo spiral patterns | 17 | 490 | 7 | 4 | F475W | Young populations |
| Galaxy Zoo post-starbursts | 56 | 1396 | 20 | 5 | F475W | Young clusters |
| Galaxy Zoo overlapping pairs | 63 | 1566 | 21 | 7 | F606W | Dust structure |
| Galaxy Zoo nuclear disks/bars | ⋯ | ⋯ | 8 | 4 | F814W | Old populations |
| Galaxy Zoo blue ellipticals | 61 | 1569 | 12 | 6 | F475W | Spiral structure |
| Galaxy Zoo red spirals | 19 | 517 | 5 | 1 | F606W | Overall structure |
| Galaxy Zoo Green Peas: all | ⋯ | ⋯ | 74 | 38 | Stellar continuum | |
| Green Peas: single filter | ⋯ | ⋯ | 47 | 23 | F775W | Stellar continuum |
| Green Peas: two filters | ⋯ | ⋯ | 27 | 15 | F555W+F850LP | Stellar continuum |
| Radio Galaxy Zoo SDRAGNs | 214 | 7110 | 65 | 32 | F475W | Spiral structure |
| Radio Galaxy Zoo EELRs | 72 | 2620 | 36 | 18 | F625W | Line emission |
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4. Observation Setup
The gap-filler proposal category originated with the realization within STScI that there were schedule gaps too short for typical snapshot programs, such that the observational output of HST could be further enhanced by programs with large target lists, ideally spread around the sky or at least around all right ascensions, which could make effective use of short observation windows. This rationale, and the review process for gap-filler programs, are described by MacKenty (2017). Use of the Wide-Field Camera (WFC) mode of the Advanced Camera for Surveys (ACS; Ford et al. 1998) was mandated for gap-filler observations, because of it having a larger field of view than the Wide Field Camera 3 (WFC3), and to minimize use of a moving mirror in WFC3 identified as a potential failure mode.
An internal STScI pilot project (program 14840) observed bright NGC galaxies, using pairs of 337 s exposures with short dither motions in between. All three gap-filler programs finally scheduled use these same exposure times, since the pilot program had demonstrated that there were a significant number of schedule gaps that could accommodate this sequence after guide-star acquisition.
Having settled on the exposure strategy for each object, the available choices were filter, dither strategy, target location in the ACS field, and (in a few cases among Green Peas) whether to do two dithered exposures or one exposure each in two filters. The filter selection followed the science goal—if we were interested mostly in spiral structure, the bluer F475W (roughly SDSS g) filter was chosen to enhance its contrast. If the goal was bulge structure, its signal-to-noise ratio would be best in the F814W filter. When the goal was emission-line structure already identified in the SDSS r filter, we used the closely matching F625W filter. The filter choices and rationale for each object category are listed in Table 1.
For Green Pea systems, our aim was to study the continuum structure, using filters dominated by the stellar population rather than ionized gas. We were guided by the results of numerically redshifting a typical Pea spectrum and folding through the system response for several filters (Figure 2). For 35 Green Peas in the redshift range z = 0.32–0.36, we used the F775W filter, with emission-line fraction <0.3%. For Green Peas near z = 0.15 (12 objects), we used F850LP where that is dominated by continuum, while for the 27 targets near z = 0.25, we took single exposures in F555W and F850LP that are each mostly continuum, tolerating the numerous cosmic-ray events because the targets are small. Except for this small number of two-filter Green Pea observations, we used a common two-point dither pattern designed to fill the chip gap (albeit with a central band of cosmic-ray features resulting from coverage with only one exposure in this area).
Figure 2. Emission-line contribution in the F555W, F775W, and F850LP passbands for a typical Green Pea spectrum, SDSS J131036.73+214817.0 at z = 0.2832, evaluated at various redshifts, showing the range near z = 0.24 where both the very broad F555W and F850LP filters have <10% contribution from emission lines, and the very deep minimum in the F775W filter from z = 0.32–0.36 where the contamination from line emission is well below 1%.
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Figure 3. Montage of SDSS gri images of galaxies highest-ranked in the voting among four large categories. Object names are shown for those that have been observed at the time of submission.
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Standard image High-resolution imageTarget locations on the ACS chips were set in view of the charge-transfer trailing occurring in these CCDs. While the effects can be substantially reduced with the pixel-based algorithm, essentially a deconvolution, by Anderson & Bedin (2010), the effect is reduced in the first place if the number of charge transfers can be minimized by placing the target close to the readout amplifier. Since orientation had to be unconstrained for snapshot observations, we identified a circular region of interest for each target, including the SDSS extent of the galaxy and any nearby obvious companions, and used the interactive ALADIN viewing feature of the Astronomers' Proposal Tool (APT) to define a POS TARG coordinate offset, so that circular region closely abutted the edge of CCD WFC1. We considered putting targets closer to the corner of the overall ACS WFC field where optical distortion gives the largest pixels on the sky, in order to improve surface-brightness sensitivity, but that region falls on CCD WFC2, whose slightly higher readout noise more than compensates for the pixel-area difference in sensitivity.
During this stage, we also caught some pasting errors in the sign of target declinations, possibly fostered by having target lists and formats from multiple sources.
5. Sample Results
The Zoo Gems observations to date are listed in Table 2, organized by science topic and observation date. As of 31 January 2022, 146 objects had been observed, 49% of the input list. The total exposure time in each case is 674 s, in two dithered exposures when a single filter was used, or two individual 337 s exposure for those Green Peas with two filters listed. For these two-filter objects, we list the final two characters of the second data set identifier after the complete identifier for the first exposure. In a few cases, one exposure was terminated onboard before the planned duration.
Table 2. Zoo Gems Observations
| Data Set | Target Name | α2000 | δ2000 | UT Start Time | Filter |
|---|---|---|---|---|---|
| Unusual Spiral Patterns: | |||||
| JDS452010 | SDSS-170414.33+620234.0 | 17 04 16.225 | +62 02 57.91 | 2018-07-06 08:19:20 | F475W |
| JDS450010 | UGC-4250 | 08 10 05.486 | +46 11 34.90 | 2021-01-11 05:23:05 | F475W |
| JDS451010 | NGC-2595 | 08 27 42.024 | +21 28 44.76 | 2021-03-16 11:57:55 | F475W |
| JDS453010 | MCG+10-21-019 | 14 36 35.851 | +57 47 49.08 | 2021-08-03 20:07:36 | F475W |
| JDS454010 | SDSS-233906.23-002615.0 | 23 39 06.236 | -00 26 15.10 | 2021-11-13 12:36:39 | F475W |
| X-Bulges: | |||||
| JDS439010 | NGC-1175 | 03 04 30.752 | +42 20 07.55 | 2019-07-18 02:34:29 | F814W |
| JDS436010 | SDSS-1237661418212229211 | 14 09 04.333 | +54 52 19.88 | 2019-09-07 11:43:00 | F814W |
| Nuclear Disks/Bars: | |||||
| JDS498010 | CGCG-245-033 | 13 12 56.707 | +47 27 23.84 | 2021-03-12 22:49:26 | F814W |
| JDS40A010 | NGC-2771 | 09 10 41.192 | +50 22 36.54 | 2021-03-14 01:09:33 | F814W |
| JDS40B010 | NGC-5945 | 15 29 45.007 | +42 55 07.13 | 2021-03-14 04:40:37 | F814W |
| JDS40F010 | MCG+09-19-030 | 11 17 43.510 | +53 47 36.25 | 2021-09-27 04:08:05 | F814W |
| Large or Prolate Bulges: | |||||
| JDS420010 | NGC-0810 | 02 05 28.560 | +13 15 05.76 | 2019-06-24 18:54:23 | F606W |
| JDS435010 | CGCG-308-012 | 06 13 05.304 | +64 33 41.04 | 2019-12-19 07:05:33 | F814W |
| JDS47T010 | UGC-10374 | 16 23 39.166 | +50 58 09.96 | 2020-03-03 13:11:57 | F814W |
| Red/Blue Pairs: | |||||
| JDS432010 | SDSS-1237678618479755694 | 22 12 06.397 | +01 46 06.37 | 2019-11-28 19:29:10 | F606W |
| JDS431010 | GAMA-636444 | 09 23 21.888 | -01 43 33.96 | 2022-01-29 18:48:26 | F606W |
| Star-forming Knots: | |||||
| JDS430010 | GAMA-302130 | 09 05 46.392 | +01 15 32.76 | 2021-10-06 04:06:12 | F606W |
| Overlapping Galaxies: | |||||
| JDS437010 | SDSS-1237661417676800323 | 14 28 14.096 | +53 14 29.00 | 2019-05-08 08:33:55 | F814W |
| JDS490010 | SDSS-115331.86+360024.2 | 11 53 31.865 | +36 00 24.27 | 2019-05-28 18:56:44 | F606W |
| JDS478010 | UGC-7064A | 12 04 44.973 | +60 40 24.56 | 2019-07-12 11:33:01 | F606W |
| JDS488010 | NGC-5021 | 13 12 06.265 | +46 11 45.75 | 2020-07-16 12:05:54 | F814W |
| JDS489010 | UGC-12281 | 22 59 14.839 | +13 36 16.74 | 2020-09-01 04:48:39 | F606W |
| JDS485010 | IC-720 | 11 42 22.336 | +08 46 11.51 | 2021-03-14 06:10:29 | F606W |
| JDS481010 | MCG+07-34-030 | 16 25 58.133 | +43 57 46.47 | 2021-10-10 11:32:38 | F606W |
| Interacting/Merging Systems: | |||||
| JDS449010 | SDSS-081913.94+591926.4 | 08 19 13.920 | +59 19 26.76 | 2019-02-22 21:23:13 | F606W |
| JDS47W010 | UGC-00240 | 00 25 10.106 | +06 29 27.17 | 2019-10-05 06:19:39 | F606W |
| JDS44M010 | VII-ZW-090 | 10 36 35.625 | +02 21 31.41 | 2020-02-27 03:33:10 | F475W |
| JDS428010 | SDSS-095346.77-012746.1 | 09 53 46.680 | -01 27 45.00 | 2020-03-03 05:50:45 | F606W |
| JDS406010 | SDSS-1237668504364187727 | 16 12 24.606 | +59 46 10.47 | 2021-03-14 03:53:56 | F475W |
| JDS444010 | CGCG-396-002 | 05 37 35.976 | +01 20 04.20 | 2021-03-18 22:36:26 | F606W |
| JDS442010 | VV-689 | 10 01 39.502 | +19 47 32.58 | 2021-04-30 17:06:39 | F606W |
| JDS441010 | IC-2431 | 09 04 34.776 | +14 35 45.96 | 2021-10-03 02:54:35 | F606W |
| Ring(ed) Galaxies, Lenses: | |||||
| JDS425010 | SDSS-1237679438812676365 | 02 03 28.727 | -06 59 49.72 | 2019-02-17 01:28:33 | F606W |
| JDS426010 | CGCG-087-009 | 07 33 17.712 | +18 17 24.36 | 2019-03-20 14:17:24 | F606W |
| JDS418010 | GAMA70579 | 12 01 43.464 | +00 10 59.16 | 2019-05-17 06:34:36 | F606W |
| JDS424010 | SDSS-133145.32+513431.2 | 13 31 45.326 | +51 34 31.22 | 2019-09-08 17:53:10 | F814W |
| JDS495010 | IC-3828 | 12 50 20.695 | +37 56 56.19 | 2020-06-13 17:27:34 | F606W |
| JDS405010 | SDSS-1237678595932094536 | 22 20 24.589 | +01 09 31.30 0 | 2020-12-16 21:52:36 | F475W |
| JDS427010 | SDSS-081740.08+042952.3 | 08 17 40.080 | +04 29 52.44 0 | 2021-01-12 21:07:04 | F606W |
| Galaxy Zoo EELRs: | |||||
| JDS403010 | SDSSJ160646.74+565139.2 | 16 06 46.740 | +56 51 39.20 | 2021-12-24 16:13:14 | F606W |
| Radio Galaxy Zoo EELRs: | |||||
| JDS46N010 | SDSS-130854.52+562155.6 | 13 08 52.460 | +56 22 42.40 | 2019-02-15 02:12:01 | F625W |
| JDS46Z010 | SDSS-075529.95+520450.6 | 07 55 26.309 | +52 03 51.25 | 2019-04-02 07:46:09 | F625W |
| JDS46U010 | SDSS-102733.29+544227.9 | 10 27 30.688 | +54 41 30.79 | 2019-06-04 03:47:14 | F625W |
| JDS46R010 | SDSS-121849.88+502617.6 | 12 18 45.908 | +50 25 30.87 | 2019-06-07 00:40:21 | F625W |
| JDS46E010 | SDSS-160344.95+524220.6 | 16 03 42.715 | +52 41 25.34 | 2019-08-20 14:58:52 | F625W |
| JDS47M010 | SDSS-010206.98+093427.6 | 01 02 03.668 | +09 35 04.12 | 2019-10-08 21:44:09 | F625W |
| JDS47G010 | PKS-0236+02 | 02 38 34.584 | +02 34 46.84 | 2019-10-16 03:00:46 | F625W |
| JDS47V010 | SDSS-101147.31+071915.2 | 10 11 50.739 | +07 19 42.52 | 2020-02-15 07:07:13 | F625W |
| JDS46H010 | SDSS-025210.17+025430.1 | 02 52 10.017 | +02 53 32.90 | 2020-03-06 11:15:15 | F625W |
| JDS47C010 | SDSS-141119.04+094225.3 | 14 11 19.04 | +09 42 25.3 | 2020-08-02 17:12:10 | F625W |
| JDS46I010 | SDSS-105426.23+573649.1 | 10 54 32.316 | +57 37 16.16 | 2020-10-17 09:26:49 | F625W |
| JDS47F010 | SDSS-082400.50+031749.4 | 08 24 04.353 | +03 18 07.73 | 2021-01-14 06:29:16 | F625W |
| JDS46C010 | B2-0832+34 | 08 35 15.391 | +34 34 01.90 | 2021-03-12 06:17:49 | F625W |
| JDS46W010 | SDSS-083512.43+175441.0 | 08 35 10.668 | +17 53 50.09 | 2021-03-19 01:55:27 | F625W |
| JDS47D010 | SDSS-123300.30+060326.1 | 12 32 58.539 | +06 04 16.38 | 2021-03-21 08:13:26 | F625W |
| JDS46P010 | 3C-458 | 23 12 48.445 | +05 17 05.36 | 2021-09-10 03:03:35 | F625W |
| JDS46Q010 | 4C+08.70 | 23 36 40.401 | +08 49 55.09 | 2021-09-12 04:19:16 | F625W |
| JDS46O010 | SDSS-141408.44+484156.0 | 14 14 08.445 | +48 41 56.00 | 2021-09-22 01:59:54 | F625W |
| Blue Elliptical Galaxies: | |||||
| JDS40O010 | CGCG-315-014 | 12 06 17.055 | +63 38 19.08 | 2019-04-15 14:34:09 | F475W |
| JDS40M010 | MKN-0888 | 16 44 30.755 | +19 56 26.73 | 2019-08-07 09:15:53 | F475W |
| JDS40P010 | SDSS-111850.04+422541.8 | 11 18 50.047 | +42 25 41.84 | 2019-12-28 23:11:30 | F475W |
| JDS40H010 | SDSS-031749.30+011337.1 | 03 17 49.304 | +01 13 37.25 | 2020-11-28 06:10:32 | F475W |
| JDS40J010 | SDSS-000907.90+142755.8 | 00 09 07.908 | +14 27 55.83 | 2021-08-06 09:06:11 | F475W |
| JDS40G010 | CGCG-432-030 | 23 47 03.791 | +14 50 30.36 | 2021-09-17 06:36:51 | F475W |
| Red Spiral Galaxies: | |||||
| JDS40T010 | UGC 3935 | 07 37 49.410 | +46 23 51.53 | 2020-10-18 00:25:12 | F606W |
| Regrowing-disk Mergers: | |||||
| JDS414010 | NGC-2292 | 06 47 40.830 | -26 45 05.00 | 2020-01-23 04:22:48 | F606W |
| JDS412010 | CFHTLS1220-215555 | 08 50 58.169 | -04 02 12.85 | 2020-03-12 02:41:03 | F606W |
| JDS410010 | UGC-4052 | 07 51 16.564 | +50 14 03.27 | 2020-05-01 11:33:05 | F606W |
| JDS408010 | SDSS-1237659936978568047 | 00 43 41.784 | +43 02 35.16 | 2020-12-19 19:47:02 | F606W |
| JDS413010 | SDSS-1237680246274064522 | 23 26 23.853 | +19 27 09.12 | 2021-09-20 01:17:10 | F606W |
| JDS409010 | SDSS-1237678439701807265 | 02 49 03.312 | +03 12 12.60 | 2021-09-21 09:16:29 | F606W |
| Post-starburst Galaxies: | |||||
| JDS464010 | CGCG-292-024 | 11 44 52.092 | +57 52 24.67 | 2018-08-09 03:21:12 | F475W |
| JDS474010 | SDSS-124354.11+163250.5 | 12 43 54.178 | +16 32 50.85 | 2019-03-19 13:30:53 | F475W |
| JDS460010 | NGC-3156 | 10 12 41.183 | +03 08 04.71 | 2020-02-24 16:44:52 | F475W |
| JDS457010 | UGCA-188 | 09 55 29.700 | +08 23 26.28 | 2020-06-12 20:44:08 | F475W |
| JDS476010 | VCC-1711 | 12 37 22.147 | +12 17 13.32 | 2021-03-21 19:20:48 | F475W |
| SDRAGNs: | |||||
| JDS45H010 | SDSS-091949.07+135910.7 | 09 19 47.195 | +13 58 22.68 | 2018-05-15 21:49:55 | F475W |
| JDS43Y010 | UGC-1797 | 02 19 58.728 | +01 55 48.72 | 2018-07-03 02:46:37 | F475W |
| JDS44C010 | SDSS-16562058+6407529 | 16 56 16.945 | +64 07 14.62 | 2018-08-24 13:52:36 | F475W |
| JDS45T010 | SDSS-112811.63+241746.9 | 11 28 09.853 | +24 18 39.94 | 2019-02-23 05:51:12 | F475W |
| JDS45Z010 | B3-1352+471 | 13 54 30.924 | +46 56 44.51 | 2019-04-28 00:35:49 | F475W |
| JDS44X010 | SDSS-132809.31+571023.3 | 13 28 03.443 | +57 10 13.25 | 2019-05-14 07:06:26 | F475W |
| JDS45J010 | SDSS-163300.85+084736.4 | 16 32 58.024 | +08 47 03.84 | 2019-07-11 23:11:15 | F475W |
| JDS44Z010 | B2-1644+38 | 16 46 25.987 | +38 31 03.31 | 2019-07-19 17:39:53 | F475W |
| JDS43V010 | SDSS-172107.89+262432.1 | 17 21 05.558 | +26 23 54.34 | 2019-08-22 16:18:28 | F475W |
| JDS47J010 | SDSS-134900.13+454256.5 | 13 49 06.051 | +45 43 03.52 | 2019-11-13 21:02:54 | F475W |
| JDS45V010 | SDSS-214110.61+082132.6 | 21 41 11.564 | +08 20 35.91 | 2019-12-11 14:04:35 | F475W |
| JDS45G010 | SDSS-081303.10+552050.7 | 08 13 00.417 | +55 21 37.26 | 2019-12-25 01:19:10 | F475W |
| JDS44D010 | SDSS-150903.21+515247.9 | 15 09 08.415 | +51 53 28.37 | 2020-01-14 13:19:06 | F475W |
| JDS44R010 | B2-0938+31A | 09 40 59.773 | +31 26 29.12 | 2020-02-13 02:39:49 | F475W |
| JDS47H010 | SDSS-095605.87+162829.9 | 09 56 01.712 | +16 28 52.37 | 2020-02-14 21:34:14 | F475W |
| JDS45A010 | IC-4234 | 13 22 58.535 | +27 07 09.45 | 2020-04-09 03:03:16 | F475W |
| JDS44J010 | SDSS-080658.46+062453.4 | 08 06 58.46 | +06 24 53.4 | 2020-05-28 23:07:53 | F475W |
| JDS44P010 | SDSS-095833.44+561937.8 | 09 58 39.333 | +56 20 16.06 | 2020-10-17 13:19:01 | F475W |
| JDS44I010 | SDSS-080259.73+115709.7 | 08 03 04.152 | +11 57 33.22 | 2021-01-10 03:57:52 | F475W |
| JDS47K010 | SDSS-113648.57+125239.7 | 11 36 48.57 | +12 52 39.7 | 2021-03-14 02:58:57 | F475W |
| JDS41L010 | B3-0852+422 | 08 55 44.151 | +42 03 44.70 | 2021-03-14 23:24:25 | F475W |
| JDS45L010 | SDSS-090305.84+432820.4 | 09 03 02.290 | +43 27 51.56 | 2021-03-15 00:59:46 | F475W |
| JDS44G010 | SDSS-082312.91+033301.3 | 08 23 11.663 | +03 32 03.79 | 2021-03-15 05:49:08 | F475W |
| JDS45B010 | SDSS-083351.28+045745.4 | 08 33 50.118 | +04 56 54.67 | 2021-03-18 00:32:22 | F475W |
| JDS45F010 | SDSS-163624.97+243230.8 | 16 36 21.753 | +24 32 46.26 | 2021-06-05 19:19:04 | F475W |
| JDS44K010 | SDSS-083224.82+184855.4 | 08 32 27.328 | +18 49 54.04 | 2021-09-28 05:30:40 | F475W |
| JDS45E010 | SDSS-130300.80+511954.7 | 13 03 00.803 | +51 19 54.70 | 2021-10-01 05:04:08 | F475W |
| JDS45I010 | SDSS-084759.90+124159.3 | 08 47 59.90 | +12 41 59.3 | 2021-10-02 04:44:49 | F475W |
| JDS44T010 | SDSS-020904.75+075004.5 | 02 09 04.750 | +07 50 04.50 | 2021-10-22 03:54:22 | F475W |
| JDS45W010 | SDSS-090147.17+164851.3 | 09 01 47.17 | +16 48 51.3 | 2021-11-13 10:05:19 | F475W |
| JDS43Z010 | B3-0911+418 | 09 14 45.528 | +41 37 14.52 | 2021-12-29 08:17:30 | F475W |
| JDS47L010 | SDSS-092605.17+465233.9 | 09 26 05.17 | +46 52 33.9 | 2021-12-29 19:23:05 | F475W |
| Green Peas: | |||||
| JDS42MO4Q/5Q | SDSS-1237651537646977181 | 15 04 57.987 | +59 54 07.27 | 2018-07-10 14:11:15 | F555W, F850LP |
| JDS42KCEQ/FQ | SDSS-1237659330316140926 | 16 33 37.941 | +37 53 14.30 | 2018-07-14 03:38:45 | F555W, F850LP |
| JDS41D010 | SDSS-1237648720155902209 | 11 49 46.471 | -01 02 17.65 | 2018-11-29 21:59:41 | F775W |
| JDS41F010 | SDSS-1237661386530882021 | 14 10 05.248 | +53 50 37.89 | 2018-12-27 01:56:41 | F775W |
| JDS40X010 | SDSS-1237651271358349509 | 10 26 15.207 | +63 33 08.49 | 2019-01-02 02:51:41 | F775W |
| JDS40Y010 | SDSS-1237657878078751164 | 08 08 16.907 | +28 14 31.14 | 2019-02-24 00:38:23 | F775W |
| JDS42IUGQ/HQ | SDSS-1237654398623023110 | 13 36 07.914 | +62 55 30.77 | 2019-04-12 02:35:01 | F555W, F850LP |
| JDS40Z010 | SDSS-1237651271899939173 | 12 05 17.538 | +66 40 29.64 | 2019-04-19 05:30:54 | F775W |
| JDS42ZV5Q/6Q | SDSS-1237667551414452597 | 10 15 41.152 | +22 27 27.52 | 2019-05-16 16:09:08 | F555W, F850LP |
| JDS42QJPQ/QQ | SDSS-1237661851469021323 | 12 14 23.180 | +45 20 40.91 | 2019-07-17 04:52:31 | F555W, F850LP |
| JDS42B010 | SDSS-1237669698364768508 | 21 08 03.059 | +05 27 07.14 | 2019-08-10 04:15:11 | F775W |
| JDS41A010 | SDSS-1237651273498755389 | 08 38 40.165 | +54 44 03.49 | 2019-10-01 17:20:01 | F775W |
| JDS43DUMQ/NQ | SDSS-1237666300559098162 | 03 53 32.464 | -00 10 28.88 | 2019-11-14 06:16:30 | F555W, F850LP |
| JDS42F010 | SDSS-1237680507722793618 | 23 19 27.467 | +33 23 24.76 | 2019-11-25 15:15:51 | F775W |
| JDS43EDKQ/LQ | SDSS-1237665369575981438 | 10 20 57.462 | +29 37 26.47 | 2020-01-26 23:25:54 | F555W, F850LP |
| JDS42WWCQ/DQ | SDSS-1237667536933945523 | 10 04 00.641 | +20 17 19.25 | 2020-02-21 03:03:38 | F555W, F850LP |
| JDS43T010 | SDSS-1237664668421849521 | 08 15 52.002 | +21 56 23.65 | 2020-04-27 20:20:00 | F850LP |
| JDS42LSNQ/OQ | SDSS-1237658491735507237 | 10 55 30.41 | +08 41 32.8 | 2020-06-03 14:22:26 | F555W, F850LP |
| JDS43M010 | SDSS-1237657632187613477 | 09 24 38.718 | +47 07 58.93 | 2020-06-10 20:58:36 | F850LP |
| JDS41Q010 | SDSS-1237662300818702524 | 13 01 28.316 | +51 04 51.18 | 2020-08-01 18:46:22 | F775W |
| JDS42NVWQ/XQ | SDSS-1237653653450064110 | 00 42 36.92 | +16 02 02.7 | 2020-11-12 16:48:32 | F555W, F850LP |
| JDS42HVJQ/KQ | SDSS-1237660343930782057 | 08 45 11.669 | +32 51 53.92 | 2020-12-09 06:05:51 | F555W, F850LP |
| JDS41G010 | SDSS-1237654383585395027 | 08 34 40.056 | +48 05 40.91 | 2021-01-11 21:16:54 | F775W |
| JDS42E010 | SDSS-1237667254538142046 | 09 51 03.165 | +24 54 35.70 | 2021-02-07 10:29:06 | F775W |
| JDS42XQMQ/NQ | SDSS-1237667211050680599 | 09 41 49.637 | +23 37 30.07 | 2021-02-14 12:28:58 | F555W, F850LP |
| JDS41U010 | SDSS-1237662640662905610 | 16 46 12.15 | +20 54 11.5 | 2021-03-13 05:34:25 | F775W |
| JDS43J010 | SDSS-1237658423543529721 | 09 05 35.161 | +04 53 34.51 | 2021-03-18 02:09:30 | F850LP |
| JDS42SAJQ/KQ | SDSS-1237648704041976118 | 12 29 33.142 | -00 18 01.68 | 2021-03-26 05:48:14 | F555W, F850LP |
| JDS42UHCQ/DQ | SDSS-1237667781210145026 | 10 04 34.733 | +17 47 35.35 | 2021-04-27 00:09:42 | F555W, F850LP |
| JDS43CVFQ/GQ | SDSS-1237679476939882858 | 01 03 21.059 | +21 32 15.91 | 2021-09-09 17:36:16 | F555W, F850LP |
| JDS43N010 | SDSS-1237655373039927410 | 16 53 04.490 | +33 39 37.74 | 2021-09-09 18:57:15 | F850LP |
| JDS43O010 | SDSS-1237663782590021909 | 00 29 38.169 | -01 12 16.05 | 2021-09-17 05:07:19 | F850LP |
| JDS41R010 | SDSS-1237657589239382254 | 09 54 22.599 | +47 51 44.02 | 2021-09-22 03:22:49 | F775W |
| JDS41S010 | SDSS-1237661382770098472 | 09 20 36.046 | +32 42 52.63 | 2021-09-29 03:39:51 | F775W |
| JDS41J010 | SDSS-1237663204918952341 | 00 44 00.266 | +00 47 24.68 | 2021-09-30 23:56:42 | F775W |
| JDS41I010 | SDSS-1237657628456386802 | 12 33 38.626 | +51 41 59.34 | 2021-10-01 01:52:15 | F775W |
| JDS42JI3Q/4Q | SDSS-1237660635458568341 | 10 27 16.725 | +43 42 02.18 | 2021-10-03 06:30:17 | F555W,F850LP |
| JDS41P010 | SDSS-1237661361304568164 | 14 46 42.608 | +40 48 44.21 | 2021-12-11 16:35:48 | F775W |
The ACS images showed some objects to be morphologically rather different than we anticipated from SDSS images. In these cases, we show them in Table 2 and in our further discussion according to the category where they fit most closely rather than the category listed in the proposal. One object, SDSS J222024.58+010931.3, turned out to be a superposition of a star and faint background galaxies that mimicked a ring structure in the SDSS data, and is not included in Table 2.
The following sections highlight some initial results from the Zoo Gems observations, and demonstrate the value of even such shallow exposures in addressing a variety of scientific questions.
5.1. Green Peas
The great majority of Green Peas (34/38, 89%) are resolved into multiple distinct components or show surrounding non-axisymmetric structure. The structures include double components, tails, and apparent disks. Four systems (SDSS J004236.92+160202.7, SDSS J092438.71+470758.9, SDSS J100400.64+201719.2, and SDSS J165304.48+333937.7) have central peaks only marginally resolved in the ACS data; a simple Gaussian comparison with star images suggests intrinsic FWHM < 1.8 pixels (0.09'' ) or 0.36 kpc at the typical z = 0.25. Some of the Green Pea systems show tidal tails or patchy spiral patterns; most have either multiple knots or show well-resolved surrounding galaxies (shown in Figure 4 for those observed in a single filter, allowing easy rejection of cosmic-ray artifacts). Analysis of the two-filter Zoo Gems data by Clarke et al. (2021) indicates that the intense starburst regions are surrounded by redder components, most likely older stellar populations. The smallest sizes we measure in these deep-red bands are comparable to the UV sizes measured (predominantly for the brightest components) by Yang et al. (2017) and Kim et al. (2021).
Figure 4. The first 15 Green Pea systems observed in single filters, as listed in Table 2. Abbreviated names showing the first four digits of the R.A. are used for convenience. Each panel is a 6 × 6'' region, typically 24 × 24 kpc, and all are shown to the same intensity scale, logarithmic above a slight negative offset. North is up, and east to the left in each case. The 2'' scale bar matches the limiting Petrosian radius from SDSS data used in the sample selection. All were observed in the F775M band except 0815, 0905, and 0924, observed in F850LP.
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Standard image High-resolution image5.2. Blue Ellipticals
Each of the six blue early-type galaxies observed in Zoo Gems shows a distinct spiral structure, which in each case is too tightly wound to have been resolved in SDSS data (Figure 5). These are not simply otherwise-normal elliptical galaxies with scattered star-forming regions, although in some cases the outer light distribution is less disk-like than the inner regions. In fact, two of these galaxies, Mkn 888 and SDSS J031749.30+011337, have a nearly pure r1/4 profile as assessed in the SDSS fracDeV parameter, with values 0.97–1 among all SDSS filters. The other four have values 0.49–0.96 in the well-measured griz bands. Among the galaxies observed, SDSS 031749.30+011337.1 and CGCG 432-030 have AGN as classified by Schawinski et al. (2009) using emission-line ratios from the SDSS spectra. Schawinski et al. (2009) reported a CO detection of SDSS 111850.04+422541.8, with a double-peaked disk-like profile and implied total molecular-gas mass near 6 × 108 M⊙. These small-scale spiral patterns are similar to those sometimes seen in recent major mergers, such as NGC 7252 (Whitmore et al. 1993), NGC 3256 (as shown in the figures by Mulia et al. (2016)), and even 2MASX J01392400+2924067 seen before coalescence of the nuclei (Koss et al. 2018), which could support the conjecture of Schawinski et al. (2009) that such mergers are one route to producing blue galaxies with elliptical-like properties. The galaxies in our sample share the radial scales of central spiral patterns with the nearby post-merger systems, as traced both by star clusters and by dust lanes. The dust spirals have radial extents 1.2–3.3 kpc, and the patterns traced by bright star-forming regions span 1.2–2.6 kpc. These are comparable to the values 1.8–6.4 kpc (dust) and 1.2–3.0 kpc (star clusters) seen in nearby merging and post-merger systems. The smaller values apply to NGC 7252, which is the oldest local merger based on comparison with simulations and ages of star clusters, and thus more comparable to our systems where merger signatures in the starlight distribution must be even more subtle.
Figure 5. Blue early-type galaxies, in the F475W filter to emphasize young stellar populations. The bottom two are classified as having spectroscopic AGN by the SDSS automated system. Each galaxy shows a tightly wound spiral pattern near the nucleus; the superimposed circles have radius 25, showing how strongly these patterns are blended together in typical survey images. Insets show SDSS composite images, 60'' square, as used by Galaxy Zoo participants in the initial classifications. Some show various levels of structure in these images, but only CGCG 432-030 might have been classified as a clear spiral from SDSS data. North is up, and east to the left, in each case.
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Standard image High-resolution imageThere is clearly more to be done in defining how these blue early-type galaxies relate to normal ellipticals, mergers, and even rejuvenated spirals. We plan to consolidate these HST images along with the new deep ground-based surveys to address this in future work.
5.3. Red Spirals
One of these, UGC 3935, has been observed. The arms include star-forming knots, while the dust arms include a spiral pattern cutting across stellar structure near the core. This object was included in the MaNGA survey's integral-field spectroscopy (Bundy et al. 2015), and in the associated and ongoing H I survey (Masters et al. 2019; Stark et al. 2021), which shows 3.7 × 1010 M⊙ of neutral hydrogen. This is close to the derived stellar mass 4.6 × 1010 M⊙ from SDSS data using the Portsmouth models (Maraston et al. 2009), so the galaxy's red color is not due purely to gas exhaustion. The HST image is compared to an SDSS color-composite in Figure 6. The arms contain blue star-forming knots, and this could be classified as a three-armed system as well.
Figure 6. Red spiral galaxy UGC 3935, comparing the usual gri composite from SDSS data with the HST ACS F606W image. The region shown is 77'' × 78'' with north at the top.
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Standard image High-resolution image5.4. Disk Structures
The range of disk structures included in Zoo Gems data is sampled in Figure 7.
Figure 7. Some of the kinds of disk structures included in Zoo Gems data. Top row: nuclear bars and bar lenses. Middle row: three-armed spirals. Bottom row: backlit spiral arms and disk with dust attenuation. All images have north at the top and east to the left; vertical white scale bars indicate 10'' in each case. The gray scale is logarithmic, with zero levels and contrast tailored to show the structures in each object. In the object names, SDSS destinations are truncated for legibility.
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Standard image High-resolution image5.4.1. Circumnuclear Disks and Bars
The two systems shown in Figure 7 both have nuclear bars and surrounding rings orbar lenses (as defined by Laurikainen et al. (2011)), which extend beyond the bar width in each case. The inner region of NGC 2771 forms a striking echo, rotated nearly 90°, of the outer bar and ring of the galaxy disk. NGC 2595 from the "unusual spiral patterns" category shows similar features. UGC 10374 has an outer pseudoring beyond the area shown. The backlit spiral in IC 720 shows a nuclear spiral and possibly a nuclear bar that were not well-resolved even in subarcsecond ground-based images.
5.4.2. Three-armed Spirals
Details in the arm structures of these systems may help understand why these do not show a preference for low-density environments, as might be expected from a straightforward analysis of spiral modes after perturbation (Elmegreen et al. 1992; Hancock 2019). In SDSS J170414.33+620234.0, among the first targets observed in this program, the enhanced angular resolution of HST images reveals quite different distributions of star-forming knots along each arm, and the arms starting from an off-center ring around the core.
5.4.3. Backlit Dust
Figure 7 shows two of the most striking backlit-galaxy systems observed in Zoo Gems. While we will present a full modeling of these systems elsewhere, it is already noteworthy that each case shows at least thin arms of attenuation farther out than the detected starlight, and nearly transparent regions between the dust arms (and within the resonance ring in the case of IC 720). The outer dust lanes illustrate in a vivid way one advantage of dust detection with this method—arbitrarily cold dust is detectable, unlike direct far-IR measurements that rely on reradiation of absorbed starlight (Domingue et al. 1999). These images also illustrate the gain in understanding structure by going from SDSS to HST angular resolution. As has been shown with WFPC2 images of two backlit spirals (Keel & White 2001), better linear resolution reduces the confusion between unresolved dust structure and slope of the reddening law, as there is less blending of areas with quite different attenuation. The Zoo Gems overlap systems provide a significant addition to the range of disk structures and backlit regions observed with Hubble's resolution.
5.5. Ring Features
Among these, SDSS 133145.32+513431.2 is especially noteworthy. This object shows a subtle annular color pattern in SDSS data, and more clearly in DES data (Abbott et al. 2018). The ACS image reveals a partial Einstein ring (Figure 8), with typical radius 40, and two main segments together spanning nearly 300° around the brightest galaxy in a group (SDSS data give z = 0.2894 for this galaxy). An inflection at its northern end is associated with an individual luminous galaxy. While the source redshift remains unknown, pending further information on the lensed source and additional foreground group members, knowing the lens redshift we can bound the group mass enclosed within the Einstein ring (radius 40 or 17.5 kpc). If the source–lens distance is equal to the lens–observer distance (0.90 Gpc), the mass within 17.5 kpc would be 9 × 1012 solar masses, dropping to 3 × 1012 for the unrealistic case of arbitrarily high redshift.
Figure 8. Deep-red F814W image of the lensed arc around the central galaxy SDSS 133145.32+513431.2 (z = 0.2894). The upward inflection to the north may indicate perturbation by the separate potential of the bright galaxy just outside the arc. The partial ring has characteristic radius 40, projecting to 17.5 kpc at the distance of the lensing system.
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Standard image High-resolution image5.6. Regrowing Disks
The brightest of these are shown in Figure 9, in comparison to two more typical advanced merging systems. The images illustrate the characteristics of two bulges, jointly surrounded by a disk with incomplete spiral patterns containing dust lanes and patchy star-forming regions.
Figure 9. Montage of merging galaxies, including two typical mergers (upper row) and two potential regrowing disks (lower row). The layout is as in Figure 7, with 10'' scale bars. The regrowing disks illustrated here span large enough areas on the sky to include the stripe of cosmic-ray events across the middle, where only a single exposure was obtained as the telescope made a dither motion to give double coverage elsewhere. For NGC 2292, the edges of the ACS combined field appear in the corners.
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After program submission, one of these objects (CFHTLS1220-215555) was seen to have a star superimposed on a single bulge as seen in Legacy Survey images (Dey et al. 2019), which were not clearly separated in SDSS data and appeared as a double bulge. This object does have the unusual combination of a dominant, off-center ring without a bar, and short spiral features.
The overlapping-galaxy system UGC 7064A may more exactly belong with these systems, on examination of the dust geometry. The eastern bulge component is surrounded by a ring including stars and dust, which may also encircle the inclined spiral disk to its west.
5.7. Reassignment of Galaxy Categories
Some objects turn out to be something quite different than we inferred from the SDSS or Legacy Survey images. For example, the components of the NGC 5021 system show obvious signs of tidal interaction (off-center nucleus, helical dust lanes, spokes), rather than being a superposition of relatively undisturbed galaxies. SDSS-1237668504364187727 (SDSS-J 161224.60+594610.4), observed as a potential reddened AGN with bluer outflow regions, looks like a multicomponent merger; a Lick Observatory spectrum, using the Kast spectrograph at the 3 m Shane telescope, shows the core to host highly reddened star formation, with relatively unreddened star formation on either side. The central region has emission-line redshift z = 0.1019.
6. Conclusions
We have described the diverse scientific cases addressed in the "Gems of the Galaxy Zoos" HST gap-filler program, and detailed the target selection and public input involved in its object list. The results so far illustrate the value of even short-exposure images at Hubble's high angular resolution, deriving additional results from the effort invested by volunteers in the Galaxy Zoo and Radio Galaxy Zoo projects. These data have revealed extended redder components around Green Pea starburst systems, small-scale spiral patterns in blue early-type galaxies, and suggested that some merging galaxies quickly reform star-forming disks. Additional uses will certainly be forthcoming. The variety of results already found from this unusually wide-ranging program may encourage the community to consider ways to achieve a similar richness of use for projects on other facilities. This includes availability of high angular resolution, which has proven crucial in such applications as substructure of Green Pea starbursts, using dust attenuation to unravel the geometry of disks and merging systems, and distinguishing key morphological features such as spiral arms in radio-AGN hosts. Many of our results illustrate the complementary roles of deep survey images and even shallow high-resolution images.
This work was enabled by the many volunteer participants in Galaxy Zoo, and especially by the beta testers and commentators on the voting interface, those who pointed to interesting objects discussed in the project Forum and Talk sites, and all who voted on target selection. We particularly note contributions by Ivan Terentiev, Chris Molloy, Victor Linares, Alexander Jonkeren, Christine MacMillan, Richard Nowell, Graham Mitchell, Claude Cornen, and Michael Peck. At STScI, program coordinator Blair Porterfield gave tips that improved the quality of our data, while John Mackenty was helpful in scheduling questions and in tracking down changes in scheduling priority during the program. We thank Julianne Dalcanton for conversations on observation setup and on coordinating object lists between two gap-filler projects. A timely Excel suggestion from Nathan Keel greatly speeded the production of Table 1.
Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org/
The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, the Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.
Facilities: HST(ACS) - Hubble Space Telescope satellite, Lick(Shane). -
Footnotes
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This research is based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 15445.
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https://www.galaxyzooforum.org/, with content frozen 9 July 2014.
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https://talk.galaxyzoo.org/ until April 2019, discussion moved to https://www.zooniverse.org/projects/zookeeper/galaxy-zoo/talk thereafter.
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As it happened, all of the galaxies in this input list were affected by an initial bug in SDSS DR7 redshifts, which applied to galaxies having strong Balmer absorption lines and gave erroneously high redshifts when the gaps between Balmer absorption lines were matched to broad emission features. This was quickly corrected in the SDSS pipeline, before the release of DR8, and did not affect our sample construction.
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One later became impossible to schedule after tighter restrictions became necessary on guide-star flux.
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Available at https://www.stsci.edu/hst/phase2-public/15445.pdf.