Keywords

We measure the significance of thermally pulsing asymptotic giant branch (TP-AGB) stars via the spectral energy distributions (SEDs) of a sample of post-starburst (PSB) galaxies at z = 0.2–0.7. Using ground- and space-based photometry from the 3D-HST catalog, as well as associated near-infrared (NIR) Hubble Space Telescope (HST) slitless grism spectroscopy, we evaluate the importance of TP-AGB stars in the SEDs of 177 PSB galaxies by fitting simple stellar populations with different levels of TP-AGB contributions. The grism spectra, despite their low resolution of R ∼ 100, enable the detection of molecular features specific to TP-AGB stars and thus improve constraints on their contribution. A majority (∼70%) of galaxies in the PSB sample show features indicative of TP-AGB stars, while the remainder does not and they are well fit by Bruzual & Charlot TP-AGB light models. Stacked spectra of sources classified to be the best fit by TP-AGB heavy/mild models reveal strong detections of NIR molecular features associated with TP-AGB stars. Additionally, we observe a tentative trend with redshift where more TP-AGB heavy galaxies are observed in the higher redshift PSB galaxy population. Finally, neglecting the contribution of TP-AGB stars can yield an over-prediction of stellar masses measured in the K-band ranging from 0.13–0.23 dex.

Narrow-line Seyfert 1 galaxies (NLS1s) are a class of active galactic nuclei that are known to be one of the few sources of γ-rays, which originate in a relativistic beamed jet. Because of their relatively large distance, a poorly investigated aspect of these jetted NLS1s is their environment, and in particular, their host galaxy. In this work, we present the results of a morphological analysis of the host galaxy of the jetted NLS1 IRAS 20181-2244 observed with the 6.5 m Baade Telescope of the Las Campanas Observatory. The GALFIT analysis run on the Ks image, along with additional spectroscopic observations performed with the Nordic Optical Telescope, clearly revealed the presence of an interacting system of two galaxies. The data suggest that this NLS1 is hosted by a late-type galaxy, although the result is not conclusive. This analysis, along with other results in the literature, might suggest that two populations of jetted NLS1 exist. Further morphological studies are needed to confirm or disprove this hypothesis.

We present new HST F275W, F475W, and F814W imaging of the region of the Coma cluster around D100, a spiral galaxy with a remarkably long and narrow (60 × 1.5 kpc) ram pressure–stripped gas tail. We find blue sources coincident with the Hα tail, which we identify as young stars formed in the tail. We also determine they are likely to be unbound stellar complexes with sizes of ∼50–100 pc, likely to disperse as they age. From a comparison of the colors and magnitudes of the young stellar complexes with simple stellar population models, we find ages ranging from ∼1 to 50 Myr and masses ranging from 10³ to ∼10⁵ M_⊙. We find that the overall rate and efficiency of star formation are low, ∼6.0 × 10⁻³ M_⊙ yr⁻¹ and ∼6 × 10⁻¹² yr⁻¹, respectively. The total Hα flux of the tail would correspond to a star formation rate 7 times higher, indicating that some other mechanism for Hα excitation is dominant. From an analysis of the colors, we track the progression of outside-in star formation quenching in the main body of D100, as well as its apparent companion, S0 D99. Finally, we observe that the dust extinction in the base of the tail has an outer envelope with remarkably smooth and straight edges and linear filamentary substructure strongly suggestive of magnetic fields. These features and the small amount of tail broadening strongly suggest gas cooling restricting broadening and the influence of magnetic fields inhibiting turbulence.

Ring galaxies are fascinating laboratories: a catastrophic impact between two galaxies (one not much smaller than the other) has produced fireworks, especially in the larger one, when hit roughly perpendicularly to the plane. We analyze the point sources produced by the starburst episode following the impact in the rings of seven galaxies and determine their X-ray luminosity function (XLF). In total, we detect 63 sources, of which 50 have luminosity L_X ≥ 10³⁹ erg s⁻¹, classifying them as ultraluminous X-ray sources (ULXs). We find that the total XLF is not significantly different from XLFs derived for other kinds of galaxies, with a tendency of having a larger fraction of high X-ray luminosity objects. Both the total number of ULXs and the number of ULXs per unit star formation rate are found in the upper envelope of the more normal galaxies distribution. Further analysis would be needed to address the issue of the nature of the compact component in the binary system.

Tens of early-type galaxies have been recently reported to possess prolate rotation of the stellar component, i.e., a significant amount of rotation around the major axis, including two cases in the Local Group. Although expected theoretically, this phenomenon is rarely observed and remains elusive. We study its origin using the population of well-resolved galaxies in the Illustris cosmological simulation. We identify 59 convincing examples of prolate rotators at the present time, more frequently among more massive galaxies, with the number varying very little with redshift. We follow their evolution back in time using the main progenitor branch galaxies of the Illustris merger trees. We find that the emergence of prolate rotation is strongly correlated with the time of the last significant merger that the galaxy experienced, although other evolutionary paths leading to prolate rotation are also possible. The transition to prolate rotation most often happens around the same time as the transition to prolate shape of the stellar component. The mergers leading to prolate rotation have slightly more radial orbits and higher mass ratios, and they occur at more recent times than mergers in the reference sample of twin galaxies we construct for comparison. However, they cover a wide range of initial conditions in terms of the mass ratio, merger time, radiality of the progenitor orbits, and relative orientations of progenitor spins with respect to the orbital angular momenta. About half of our sample of prolate rotators was created during gas-rich mergers, and the newly formed stars usually support prolate rotation.

We developed and applied a computer analysis method to detect ring galaxy candidates in the first data release of the Panoramic Survey Telescope and Rapid Response System (PanSTARRS). The method works by applying a low-pass filter, followed by dynamic global thresholding, to search for closed regions in the binary mask of each galaxy image. Applying the method to ∼3 × 10⁶ PanSTARRS galaxy images produced a catalog of 185 ring galaxy candidates based on their visual appearance.

We use the Atacama Large Millimeter Array (ALMA) to detect and image CO (1-0) emission from Minkowski's Object, a dwarf galaxy in the cluster Abell 194 that is interacting with a radio jet from a nearby elliptical galaxy. The ALMA observations, which are the first to detect molecular gas in Minkowski's Object, also image the high-frequency continuum emission from the radio jet, allowing us to study the interaction in detail. We estimate the range in the mass of molecular gas in Minkowski's Object assuming two different values of the ratio of the molecular gas mass to the CO luminosity, ${\alpha }_{\mathrm{CO}}$. For the Milky Way value of ${\alpha }_{\mathrm{CO}}=4.6\,{M}_{\odot }{({\rm{K}}\mathrm{km}{{\rm{s}}}^{-1}{\mathrm{pc}}^{2})}^{-1}$ we obtain a molecular gas mass of ${M}_{{{\rm{H}}}_{2}}=3.0\times {10}^{7}\,{M}_{\odot }$, 6% of the H I gas mass. We also use the prescription of Narayanan et al. (2012) to estimate an ${\alpha }_{\mathrm{CO}}=27\,{M}_{\odot }{({\rm{K}}\mathrm{km}{{\rm{s}}}^{-1}{\mathrm{pc}}^{2})}^{-1}$, in which case we obtain ${M}_{{{\rm{H}}}_{2}}=1.8\times {10}^{8}\,{M}_{\odot }$, 36% of the H I mass. The observations are consistent with previous claims of star formation being induced in Minkowski's Object via the passage of the radio jet, and it therefore being a rare local example of positive feedback from an active galactic nucleus. In particular, we find highly efficient star formation, with gas depletion timescales $\sim 5\times {10}^{7}\mbox{--}3\times {10}^{8}$ year (for assumed values of ${\alpha }_{\mathrm{CO}}=4.6$ and $27\,{M}_{\odot }{({\rm{K}}\mathrm{km}{{\rm{s}}}^{-1}{\mathrm{pc}}^{2})}^{-1}$, respectively) in the upstream regions of Minkowski's Object that were struck first by the jet, and less efficient star formation downstream. We discuss the implications of this observation for models of jet-induced star formation and radio-mode feedback in massive galaxies.

We report the discovery of a mysterious giant Hα blob that is ∼8 kpc away from the main MaNGA target 1-24145, one component of a dry galaxy merger, and has been identified in the first-year SDSS-IV MaNGA data. The size of the Hα blob is ∼3–4 kpc in radius, and the Hα distribution is centrally concentrated. However, there is no optical continuum counterpart in the deep broadband images reaching ∼26.9 mag arcsec⁻² in surface brightness. We estimate that the masses of the ionized and cold gases are $3.3\times {10}^{5}$ ${M}_{\odot }$ and $\lt 1.3\times {10}^{9}$ ${M}_{\odot }$, respectively. The emission-line ratios indicate that the Hα blob is photoionized by a combination of massive young stars and AGNs. Furthermore, the ionization line ratio decreases from MaNGA 1-24145 to the Hα blob, suggesting that the primary ionizing source may come from MaNGA 1-24145, likely a low-activity AGN. Possible explanations for this Hα blob include the AGN outflow, the gas remnant being tidally or ram-pressure stripped from MaNGA 1-24145, or an extremely low surface brightness galaxy. However, the stripping scenario is less favored according to galaxy merger simulations and the morphology of the Hα blob. With the current data, we cannot distinguish whether this Hα blob is ejected gas due to a past AGN outburst, or a special category of "ultra-diffuse galaxy" interacting with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA 1-24145.

During a systematic search for supermassive black holes (SMBHs) not in galactic nuclei, we identified the compact, symmetric radio source B3 1715+425 with an emission-line galaxy offset $\approx 8.5\,\mathrm{kpc}$ from the nucleus of the brightest cluster galaxy (BCG) in the redshift z = 0.1754 cluster ZwCl 8193. B3 1715+425 is too bright (brightness temperature ${T}_{{\rm{b}}}\sim 3\times {10}^{10}\,{\rm{K}}$ at observing frequency $\nu =7.6\,\mathrm{GHz}$) and too luminous (1.4 GHz luminosity ${L}_{1.4\mathrm{GHz}}\sim {10}^{25}\,{\rm{W}}\,{\mathrm{Hz}}^{-1}$) to be powered by anything but an SMBH, but its host galaxy is much smaller ($\sim 0.9\,\mathrm{kpc}\times 0.6\,\mathrm{kpc}$ full width between half-maximum points) and optically fainter (R-band absolute magnitude ${M}_{{\rm{r}}}\approx -18.2$) than any other radio galaxy. Its high radial velocity ${v}_{{\rm{r}}}\approx 1860\,\mathrm{km}\,{{\rm{s}}}^{-1}$ relative to the BCG, continuous ionized wake extending back to the BCG nucleus, and surrounding debris indicate that the radio galaxy was tidally shredded passing through the BCG core, leaving a nearly naked SMBH fleeing from the BCG with space velocity $v\gtrsim 2000\,\mathrm{km}\,{{\rm{s}}}^{-1}$. The radio galaxy has mass $M\lesssim 6\times {10}^{9}\,{M}_{\odot }$ and infrared luminosity ${L}_{\mathrm{IR}}\sim 3\times {10}^{11}\,{L}_{\odot }$ close to its dust Eddington limit, so it is vulnerable to further mass loss from radiative feedback.

The Next Generation Virgo Cluster Survey is a deep (with a 2σ detection limit μ_g = 29 mag arcsec⁻² in the g-band) optical panchromatic survey targeting the Virgo cluster from its core to virial radius, for a total areal coverage of 104 square degrees. As such, the survey is well suited for the study of galaxies' outskirts, haloes, and low surface brightness features that arise from dynamical interactions within the cluster environment. We report the discovery of extremely faint (μ_g > 25 mag arcsec⁻²) shells in three Virgo cluster early-type dwarf galaxies: VCC 1361, VCC 1447, and VCC 1668. Among them, VCC 1447 has an absolute magnitude M_g = −11.71 mag and is the least massive galaxy with a shell system discovered to date. We present a detailed study of these low surface brightness features. We detect between three and four shells in each of our galaxies. Within the uncertainties, we find no evidence of a color difference between the galaxy main body and shell features. The observed arcs of the shells are located up to several effective radii of the galaxies. We further explore the origin of these low surface brightness features with the help of idealized numerical simulations. We find that a near equal mass merger is best able to reproduce the main properties of the shells, including their quite symmetric appearance and their alignment along the major axis of the galaxy. The simulations provide support for a formation scenario in which a recent merger, between two near-equal mass, gas-free dwarf galaxies, forms the observed shell systems.