Wide-field Imaging of the Environments of LITTLE THINGS Dwarf Irregular Galaxies

LITTLE THINGS¹³ (Local Irregulars That Trace Luminosity Extremes, The H i Nearby Galaxy Survey; Hunter et al. 2012a) is a multiwavelength survey of 37 dwarf irregular (dIrr) galaxies and 4 blue compact dwarfs (BCD) aimed at understanding what drives star formation in tiny systems. The LITTLE THINGS galaxies were chosen to be nearby (≤10.3 Mpc), contain gas so they could be forming stars, and cover a large range in dIrr galactic properties.

In order to concentrate on internal processes, the LITTLE THINGS galaxies were also chosen to be non-interacting. That is, they are not companions of giant galaxies and show no obvious signs of currently undergoing an interaction with another galaxy. One of the LITTLE THINGS data sets includes H i emission maps over a moderately large field of view (FOV; 30'), and these data did not reveal any nearby companions containing gas.

However, in 2012 several groups announced the discovery of a stellar dwarf companion to the irregular galaxy NGC 4449 (Martínez-Delgado et al. 2012; Rich et al. 2012). The companion had not shown up in the H i maps because it has been stripped of its gas by NGC 4449. Although the optical companion was noticed previously on Digitized Sky Survey (POSS-II) plates (Karachentsev et al. 2007), its significance was not realized and it was not caught in subsequent imaging of NGC 4449 because it was outside of the FOV of the typical astronomical imagers. Thus, it became clear that wide-field imaging of the LITTLE THINGS sample to search for stellar companions would be important in determining the current isolated status of the LITTLE THINGS galaxies.

At the Society for Astronomical Sciences conference in 2012 May, Lowell Observatory launched the Lowell Amateur Research Initiative (LARI). LARI is designed to bring amateur and professional astronomers together in collaborative research projects. This was an excellent opportunity to obtain the needed images of the galactic environments of LITTLE THINGS, and so this became one of the LARI projects (for another example of a survey of dwarf galaxies by amateurs, see Henkel et al. 2017). In this paper we present the results of imaging 36 of the LITTLE THINGS galaxies by 10 dedicated amateur astronomers with an expertise in deep sky imaging.

The observing team's telescope and camera facilities, image FOV, and pixel scales are given in Table 1 and the observations are listed in Table 2. One observer, Leshin, used two different telescopes with different FOVs, and both are listed in Table 1, and in Table 2 the images obtained with the larger FOV are labeled as "Leshin-TEC" under "Observer." The distances to the galaxies and the FOVs in kpc at the galaxy are also given in Table 2. The FOV ranges from 5 to 166 kpc with a median of 41 kpc. For comparison the dwarf companion to NGC 4449 is 10 kpc from NGC 4449, measured center to center. We also give the size of the FOV in units of R₂₅, the radius of the galaxy at a B-band surface brightness of 25 mag arcsec⁻².

The images are shown in Figure 1. Most images were centered on or near the LITTLE THINGS galaxy and a single image was taken. However, NGC 4214 and NGC 4163 were imaged by Leshin in a mosaic because both LITTLE THINGS galaxies are fairly near each other. Also, Haro 36 was placed at the northern edge of the FOV in order to include NGC 4707, a galaxy that had been suggested as interacting with Haro 36, at the southern edge. In addition some galaxies were observed twice with different FOVs and depths: NGC 2366, NGC 4214/NGC 4163, WLM, UGC 8508, and Haro 36. Both of the sets of images are included here.

Zoom In Zoom Out Reset image size

View extended figure (8 panels)

In order to maximize throughput, all of the observations were made through a luminance filter that cuts off light in the UV (below about 3900 Å) and the IR (above about 7000 Å). Most of the images have been flat-fielded, and some were flat-fielded using the images themselves (DDO 52, DDO 133, DDO 154, DDO 155, IC 10, NGC 2366-Wheelband, UGC 8508-Clark, and Mrk 178). Images of DDO 216, NGC 1569, and possibly DDO 63 also contain Milky Way cirrus that is seen as large-scale filamentary structure across the FOV. We have rotated the images so that north is up and east is to the left.

We have calibrated the photometry of each image to the Johnson V filter using V-band images obtained of the galaxies by Hunter & Elmegreen (2006). We calculated a limiting magnitude as 3×rms, where the rms is the standard deviation of the background in the image. We then calculated a surface brightness limit as this magnitude per arcsec⁻². The limiting magnitude and surface brightness of each image are given in Table 2. The limiting magnitudes range from 19.7 mag arcsec⁻² to 28.3 mag arcsec⁻², with a median value of 25.9 mag arcsec⁻². For comparison the central surface brightness of the dwarf companion to NGC 4449 is about 25.5 mag arcsec⁻² in V (Rich et al. 2012).

We examined each image for extended objects that were likely to be galactic in nature, and used the NASA/IPAC Extragalactic Database (NED) to determine the radial velocity of the cataloged galaxies. Most of the objects that we found in the images were cataloged galaxies at significantly higher radial velocities than our object galaxies, placing them in the background. In most cases, we have not identified these background objects in Figure 1.

Galaxies of special note:

•   
  DDO 154. DDO 154 is also known as NGC 4789A, which implies that it is associated with NGC 4789. However, NGC 4789 has a recessional velocity of 8365 km s⁻¹, while the velocity of DDO 154 is 374 km s⁻¹, so they are not physically close. Many of the other galaxies in the FOV of DDO 154 are at velocities of 6000–8000 km s⁻¹.
•   
  NGC 3738. NGC 3756 is relatively nearby in angular separation. However, its distance is placed at about 19.7 Mpc by various studies listed by NED. NGC 3738, on the other hand, is placed at 4.9 Mpc by Karachentsev et al. (2003).
•   
  Haro 36. Haro 36 and NGC 4707 are only 302 apart on the sky. However, Haro 36 is at a radial distance of approximately 9.3 Mpc (Hunter et al. 2012a), and NGC 4707 is measured from the tip of the red giant branch to be at a distance of 6.5 Mpc (Tully et al. 2013). Thus, unless one of the distances of these objects is wrong, the two galaxies are likely of order 2.8 Mpc apart and are unlikely to have gravitationally interacted.
•   
  Mrk 178. The object 33 to the northwest from Mrk 178 is UGC 6538, which has a radial velocity of 3077 km s⁻¹, while the radial velocity of Mrk 178 is 250 km s⁻¹. In the FOV of our image there are also 32 objects with redshifts from −98 to 61 km s⁻¹, but these objects are identified as stars. There are also other objects with very high redshifts in the NED catalog, and other faint galaxies without radial velocities that look like background spirals or ellipticals.
•   
  NGC 4214/NGC 4163. There are two LITTLE THINGS galaxies, NGC 4163 and NGC 4214, that are close to each other and to the faint dwarf UGCA 276. NGC 4163 is at a radial distance of 2.87 ± 0.03 Mpc (Tully et al. 2013), NGC 4214 is at 2.94 ± 0.18 Mpc (Maíz-Apellániz et al. 2002), and UGCA 276 is at 2.96 ± 0.10 (Jacobs et al. 2009). NGC 4214 and UGCA 276 are 106 apart on the sky and at the same distance, so their angular separation is about 9.1 ± 0.3 kpc. The uncertainty in the distances give an uncertainty in the radial separation of 200 kpc. NGC 4163 and UGCA 276 are 342 apart on the sky for a separation of 29 ± 1 kpc and they are 90 ± 104 kpc apart in radial distance, so their separation is about 95 ± 104 kpc. Although we see no obvious signs of an interaction in the optical or H i, the distances between them are small enough that it is plausible that gravitational effects between these three galaxies occurred at some time in the past.
•   
  IC 10. IC 10 was imaged as part of this program, but no optical companions were found in the FOV. However, an H i survey of a large region around IC 10 has revealed an H i blob at the end of a long filament (Nidever et al. 2013). This structure could be what is left of an interacting companion or could be a filament resulting from a merger of IC 10 with another dwarf (Ashley et al. 2014). The H i object is off the image centered on IC 10 and shown in Figure 1. Another image was taken centered on the H i blob, also shown in Figure 1, but no stellar counterpart was detected.

We obtained images in the optical broad band of 36 of the LITTLE THINGS nearby dwarf irregular and BCD galaxies. The purpose was to search for previously unknown optical companion galaxies that might be interacting with these galaxies that were chosen to be fairly isolated. No objects had been found in H i radio interferometric imaging, but optical companions without gas could not be ruled out. The concern was that galaxy–galaxy gravitational interactions would introduce external influences to the star formation processes. We found no new stellar companions to these galaxies. However, we note that NGC 4163 and NGC 4214, two of the LITTLE THINGS galaxies, appear to lie within 100 kpc of each other and the tiny dwarf UGCA 276, which is close enough for interactions to be possible, although we did not detect any stellar bridges between the galaxies.

We thank John Menke, Bruce Koehn, Michael Beckage, Klaus Brasch, and Sue Durling for starting LARI, setting up the web site, and vetting applications. We are also grateful to Mattie Harrington, Klaus Brasch, Michael West, and Michael Beckage for ongoing efforts to run and advertise the LARI program. C.M. appreciates the 2017 MIT Field Camp at Lowell Observatory for supporting her work on this project and Dr. Amanda Bosh for running the program. We also appreciate the constructive suggestions of the anonymous referee. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.