We report the discovery of a stellar overdensity 8 degrees north of the centre of the Small Magellanic Cloud (SMC; Small Magellanic Cloud Northern Over-Density; SMCNOD), using data from the first 2 yr of the Dark Energy Survey (DES) and the first year of the MAGellanic SatelLITEs Survey (MagLiteS). The SMCNOD is indistinguishable in age, metallicity and distance from the nearby SMC stars, being primarily composed of intermediate-age stars (6 Gyr, Z=0.001), with a small fraction of young stars (1 Gyr, Z=0.01). The SMCNOD has an elongated shape with an ellipticity of 0.6 and a size of similar to 6 degrees x 2 degrees. It has an absolute magnitude of M-V congruent to -7.7, r(h) = 2.1 kpc, and mu v(r < r(h)) = 31.2 mag arcsec(-2). We estimate a stellar mass of similar to 10(5) M-circle dot, following a Kroupa mass function. The SMCNOD was probably removed from the SMC disc by tidal stripping, since it is located near the head of the Magellanic Stream, and the literature indicates likely recent Large Magellanic Cloud-SMC encounters. This scenario is supported by the lack of significant H-1 gas. Other potential scenarios for the SMCNOD origin are a transient overdensity within the SMC tidal radius or a primordial SMC satellite in advanced stage of disruption.

Compact groups provide an environment to study the growth of galaxies amid multiple prolonged interactions. With their dense galaxy concentrations and relatively low velocity dispersions, compact groups mimic the conditions of hierarchical galaxy assembly. Compact group galaxies are known to show a bimodality in Spitzer IRAC infrared color space: galaxies are preferentially either quiescent with low specific star formation rates (SSFRs) or prolifically forming stars-galaxies with moderate levels of specific star formation are rare. Previous Spitzer IRAC studies identifying this "canyon" have been limited by small number statistics. We utilize whole-sky Wide-field Infrared Survey Explorer (WISE) data to study 163 compact groups, thereby tripling our previous sample and including more galaxies with intermediate mid-IR colors indicative of moderate SSFRs. We define a distinct WISE. mid-IR color space (log [f(12)/f(4.6)]) versus (log [f(22)/f(3.4)]) that we use to identify canyon galaxies from the larger sample. We confirm that compact group galaxies show a bimodal distribution in the mid-infrared and identify 37 canyon galaxies with reliable photometry and intermediate mid-IR colors. Morphologically, we find that the canyon harbors a large population of both Sa-Sbc and E/S0 type galaxies, and that they fall on the optical red sequence rather than the green valley. Finally, we provide a catalog of WISE. photometry for 567 of 652 galaxies selected from the sample of 163 compact groups.

Interacting galaxies are known to have higher global rates of star formation on average than normal galaxies, relative to their stellar masses. Using UV and IR photometry combined with new and published Hα images, we have compared the star formation rates (SFRs) of ∼700 star forming complexes in 46 nearby interacting galaxy pairs with those of regions in 39 normal spiral galaxies. The interacting galaxies have proportionally more regions with high SFRs than the spirals. The most extreme regions in the interacting systems lie at the intersections of spiral/tidal structures, where gas is expected to pile up and trigger star formation. Published Hubble Space Telescope images show unusually large and luminous star clusters in the highest luminosity regions. The SFRs of the clumps correlate with measures of the dust attenuation, consistent with the idea that regions with more interstellar gas have more star formation. For the clumps with the highest SFRs, the apparent dust attenuation is consistent with the Calzetti starburst dust attenuation law. This suggests that the high luminosity regions are dominated by a central group of young stars surrounded by a shell of clumpy interstellar gas. In contrast, the lower luminosity clumps are bright in the UV relative to Hα, suggesting either a high differential attenuation between the ionized gas and the stars, or a post-starburst population bright in the UV but faded in Hα. The fraction of the global light of the galaxies in the clumps is higher on average for the interacting galaxies than for the spirals. Thus either star formation in interacting galaxies is "clumpier" on average, or the star forming regions in interacting galaxies are more luminous, dustier, or younger on average.

We present deep optical images of the Large and Small Magellanic Clouds (LMC and SMC) using a low cost telephoto lens with a wide field of view to explore stellar substructure in the outskirts of the stellar disk of the LMC (< 10 degrees from the LMC center). These data have higher resolution than existing star count maps, and highlight the existence of stellar arcs and multiple spiral arms in the northern periphery, with no comparable counterparts in the south. We compare these data to detailed simulations of the LMC disk outskirts, following interactions with its low mass companion, the SMC. We consider interaction in isolation and with the inclusion of the Milky Way tidal field. The simulations are used to assess the origin of the northern structures, including also the low density stellar arc recently identified in the Dark Energy Survey data by Mackey et al. at similar to 15 degrees. We conclude that repeated close interactions with the SMC are primarily responsible for the asymmetric stellar structures seen in the periphery of the LMC. The orientation and density of these arcs can be used to constrain the LMC's interaction history with and impact parameter of the SMC. More generally, we find that such asymmetric structures should be ubiquitous about pairs of dwarfs and can persist for 1-2 Gyr even after the secondary merges entirely with the primary. As such, the lack of a companion around a Magellanic Irregular does not disprove the hypothesis that their asymmetric structures are driven by dwarf-dwarf interactions.

We construct UV/optical/IR spectral energy distributions for 29 star forming regions in the interacting galaxy Arp 107, using GALEX UV, Sloan Digitized Sky Survey optical, and Spitzer infrared images. In an earlier study utilizing only the Spitzer data, we found a sequence in the mid-infrared colors of star-forming knots along the strong tidal arm in this system. In the current study, we find sequences in the UV/optical colors along the tidal arm that mirror those in the mid-infrared, with blue UV/optical colors found for regions that are red in the mid-infrared, and vice versa. With single-burst stellar population synthesis models, we find a sequence in the average stellar age along this arm, with younger stars preferentially located further out in the arm. Models that allow two populations of different ages and dust attenuations suggest that there may be both a young component and an older population present in these regions. Thus the observed color sequences may be better interpreted as a sequence in the relative proportion of young and old stars along the arm, with a larger fraction of young stars near the end. Comparison with star forming regions in other interacting galaxies shows that the Arp 107 regions are relatively quiescent, with less intense star formation than in many other systems.

We present new radio continuum, 21 cm H I, and 2.6 mm CO data for the peculiar radio galaxy NGC 4410A and its companion NGC 4410B and compare with available optical and X-ray maps. Our radio continuum maps show an asymmetric double-lobed structure, with a high surface brightness lobe extending 3′.6 (∼100 kpc) to the southeast and a 6′.2 (∼180 kpc) low surface brightness feature in the north-west. Molecular gas is abundant in NGC 4410A, with MH2 ∼ 4 × 109 M⊙ (using the standard Galactic conversion factor) but is undetected in NGC 4410B. H I is less abundant, with MHI ∼ 109 M⊙ for the pair. Our H I map shows a 3 × 108 M⊙ H I tail extending 1′.7 (50 kpc) to the southeast of the pair, coincident with a faint optical tail and partially overlapping with the southeastern radio lobe. The H I tail is anticoincident with a 2′ (56 kpc) long X-ray structure aligned with a stellar bridge that connects the pair to a third galaxy. If this X-ray emission is associated with the group, we infer (3-8) × 108 M⊙ of hot gas in this feature. This may be either intracluster gas or shocked gas associated with the bridge. Our detection of abundant interstellar gas in this pair, including an H I-rich tidal tail near the south-eastern radio lobe, suggests that the observed distortions in this lobe may have been caused by the interstellar medium in this system. The gravitational interaction of the two galaxies and the subsequent motion of the interstellar medium in the system relative to the jet may have produced sufficient ram pressure to bend and distort the radio jet. An alternative hypothesis is that the jet was distorted by ram pressure due to an intracluster medium, although the small radial velocity of NGC 4410A relative to the group and the lack of diffuse X-ray emission in the group makes this less likely unless the group is not virialized or is in the process of merging with another group. Using our VLA data, we also searched for H I counterparts to the other 10 known members of the NGC 4410 group and CO from three other galaxies in the inner group. In our velocity range of 6690-7850 km s-1, we detected six other galaxies above our H I sensitivity limits of 2 × 108 M⊙ for the inner group and 4 × 108 M⊙ for the outer group. The total H I in the group is 1.4 × 1010 M⊙, 80% of which arises from four galaxies in the outer group. Three of these galaxies (VCC 822, VCC 831, and VCC 847) are spirals with MHI/LB ratios typical of field galaxies, while FGC 170A appears to be a gas-rich dwarf galaxy (MB ∼ -18, MHI ∼ 3 × 109 M⊙). In the inner group, the SBa galaxy NGC 4410D (VCC 934) was detected in H I and CO (MHI, ∼ 5 × 108 M⊙ and MH2 ∼ 8 × 108 M⊙) and has a 1′ (28 kpc) long H I tail that points toward the nearby disk galaxy NGC 4410F. NGC 4410F was also detected in H I (MHI ∼ 4 × 108 M⊙). The galaxies in the inner group appear to be somewhat deficient in H I compared to their blue luminosities, suggesting phase changes driven by galaxy-galaxy or galaxy-intracluster medium encounters.

We construct UV/optical/IR spectral energy distributions for 29 star forming regions in the interacting galaxy Arp 107, using GALEX UV, Sloan Digitized Sky Survey optical, and Spitzer infrared images. In an earlier study utilizing only the Spitzer data, we found a sequence in the mid-infrared colors of star-forming knots along the strong tidal arm in this system. In the current study, we find sequences in the UV/optical colors along the tidal arm that mirror those in the mid-infrared, with blue UV/optical colors found for regions that are red in the mid-infrared, and vice versa. With single-burst stellar population synthesis models, we find a sequence in the average stellar age along this arm, with younger stars preferentially located further out in the arm. Models that allow two populations of different ages and dust attenuations suggest that there may be both a young component and an older population present in these regions. Thus the observed color sequences may be better interpreted as a sequence in the relative proportion of young and old stars along the arm, with a larger fraction of young stars near the end. Comparison with star forming regions in other interacting galaxies shows that the Arp 107 regions are relatively quiescent, with less intense star formation than in many other systems.

We present analytic models for the formation and evolution of tidal tails and related structures following single or multiple impulsive disturbances in galaxy collisions. Since the epicyclic approximation is not valid for large radial excursions, we use orbital equations of the form we call p-ellipses (a class of precessing ellipses). These have been shown to provide accurate representations of orbits in logarithmic and power-law halo potentials. In the simplest case of an impulsive collision yielding a purely tidal disturbance the resulting tidal tails have simple structure. Scalings for their maximum lengths and other characteristics as non-linear functions of the tidal amplitude and the exponent of the power-law potentials are described. The analytic model shows that azimuthal caustics (orbit crossing zones of high density also seen in numerical models) are produced generically in these tails at a fixed azimuth relative to the point of closest approach. Long tails, with high-order caustics at their base, and ocular waveforms are also produced at larger amplitudes. The analysis is then extended to non-linear disturbances and multiple encounters, which break the symmetries of purely tidal perturbations. The p-ellipse orbital solutions are similar to those in the linear tidal case. However, as the strength of the non-linear terms is varied the structure of the resulting forms varies from symmetric tails to one-armed plumes. Cases with two or more impulse disturbances are also considered as the simplest analytic models distinguishing between prograde and retrograde encounters. The model shows explicitly how tail growth differs in the two cases. In the prograde case a specific mechanism for the formation of tidal dwarf galaxies at the end of tails is suggested as a consequence of resonance effects in multiple or prolonged encounters. Qualitative comparisons to Arp Atlas systems suggest that the limiting analytic cases are realized in real systems. For example, we identify a few Arp systems which have multiple tidal strands meeting near the base of long tails. These may be swallowtail caustics, where dissipative gas streams are converging and triggering star formation. Ultraviolet and optical images reveal luminous knots of young stars at these 'hinge clump' locations.

We compare the UV-optical colors of a well-defined set of optically selected pre-merger interacting galaxy pairs with those of normal spirals. The shorter wavelength colors show a larger dispersion for the interacting galaxies than for the spirals. This result can best be explained by higher star formation rates on average in the interacting galaxies, combined with higher extinctions on average. This is consistent with earlier studies which found that the star formation in interacting galaxies tends to be more centrally concentrated than in normal spirals, perhaps due to gas being driven into the center by the interaction. As noted in earlier studies, there is a large variation from galaxy to galaxy in the implied star formation rates of the interacting galaxies, with some galaxies having enhanced rates but others being fairly quiescent.

We have used the Galaxy Evolution Explorer (GALEX) ultraviolet telescope to study stellar populations and star formation morphology in a well-defined sample of 42 nearby optically selected pre-merger interacting galaxy pairs. Galaxy interactions were likely far more common in the early universe than in the present; thus our study provides a nearby well-resolved comparison sample for high-redshift studies. We have combined the GALEX near-ultraviolet (NUV) and far-ultraviolet images with broadband optical maps from the Sloan Digitized Sky Survey to investigate the ages and extinctions of the tidal features and the disks. The distributions of the UV/optical colors of the tidal features and the main disks of the galaxies are similar; however, the tidal features are bluer on average in NUV - g when compared with their own parent disks; thus tails and bridges are often more prominent relative to the disks in UV images compared to optical maps. This effect is likely due to enhanced star formation in the tidal features compared to the disks rather than reduced extinction; however, lower metallicities may also play a role. We have identified a few new candidate tidal dwarf galaxies in this sample. Other interesting morphologies such as accretion tails and "beads on a string" are also seen in these images. We also identify a possible "Taffy" galaxy in our sample, which may have been produced by a head-on collision between two galaxies. In only a few cases are strong tidal features seen in H I maps but not in GALEX.