---
abstract: 'We have used the GALEX ultraviolet telescope to study stellar populations and star formation morphology in a well-defined sample of more than three dozen nearby optically-selected pre-merger interacting galaxy pairs. We have combined the GALEX NUV and FUV images with broadband optical maps from the Sloan Digitized Sky Survey to investigate the ages and extinctions of the tidal features and the disks. We have identified a few new candidate tidal dwarf galaxies in this sample, as well as other interesting morphologies such as accretion tails, ‘beads on a string’, and ‘hinge clumps’. In only a few cases are strong tidal features seen in HI maps but not in GALEX.'
author:
- 'Beverly J. Smith$^1$, Mark L. Giroux$^1$, Curtis Struck$^2$, Mark Hancock$^{3}$, and Sabrina Hurlock$^1$'
title: 'Tidal Dwarf Galaxies, Accretion Tails, and ‘Beads on a String’ in the ‘Spirals, Bridges, and Tails’ Interacting Galaxy Survey'
---

Introduction
============

Tidal disturbances have played an important role in reshaping galaxies and triggering star formation over cosmic time. This is confirmed by H$\alpha$, far-infrared, and mid-infrared studies showing that the mass-normalized star formation rates of pre-merger optically-selected interacting galaxies are enhanced by a factor of two on average compared to normal spirals [@bushouse87; @kennicutt87; @bushouse88; @smith07].

With the advent of the Galaxy Evolution Explorer (GALEX), a new window on star formation in galaxies is now available. The addition of UV helps to break the age$-$extinction degeneracy in population synthesis modeling (e.g., [@smith08]). Furthermore, since the UV traces somewhat older and lower mass stars ($\le$400 Myrs; O to early-B stars) than H$\alpha$ ($\le$10 Myrs; early- to mid-O stars), it provides a measure of star formation over a longer timescale than H$\alpha$ studies. GALEX imaging has shown that some tidal features in interacting galaxies are quite bright in the UV (e.g., [@neff05]). In some cases, tidal features previously thought to be purely gaseous have been detected by GALEX (e.g., [@hancock07]). In other systems, GALEX images have been used to identify new tidal features (e.g., [@boselli05]).

To address these issues, we have used the GALEX telescope to image more than three dozen strongly interacting galaxies in the UV (the ‘Spirals, Bridges, and Tails’ (SB&T) sample). These galaxies were selected from the Arp (1966) Atlas using the following criteria: 1) They are relatively isolated binary systems; we eliminated merger remnants, close triples, and multiple systems in which the galaxies have similar optical brightnesses. 2) They are tidally disturbed. 3) They have radial velocities less than 10,350 km/s. 4) Their total angular size is $>$ 3$'$, to allow for good resolution with GALEX.

Each galaxy was imaged for $\ge$1500 seconds in the FUV and NUV broadband filters of GALEX, which have effective bandpasses of 1350 $-$ 1705Å  and 1750 $-$ 2800Å, respectively. Some of the galaxies that fit our selection criteria were previously observed by guaranteed time projects. For these galaxies, we used the archival GALEX images. The circular GALEX field of view has a diameter of 1.2 degrees. The pixel size is 15, and the spatial resolution $\sim$ 5$''$. About 2/3rds of our galaxies have broadband optical images available from the Sloan Digitized Sky Survey (SDSS), while 3/4 have broadband Spitzer infrared images available [@smith07]. About half have published 21 cm HI maps.

Morphologies
============

The SB&T galaxies have a large range of collisional morphologies, including M51-like systems, wide pairs with long tails and/or bridges, wide pairs with short tails, close pairs with long tails, and close pairs with short tails. In the current paper, we discuss unusual tidal morphologies in a subset of the galaxies. In @giroux10, we present an Atlas of UV images of additional SB&T galaxies. The full survey is described in detail in @smith10. For four of the galaxies in the SB&T sample, we have already published the GALEX images as part of detailed studies of the individual galaxies, and compared with numerical simulations of the interaction [@hancock07; @hancock09; @hancock10; @smith08; @peterson09; @peterson10].

There is a large variety of star formation morphologies within the tidal features in this sample. In many cases, the tidal features are quite bright in the UV. This is illustrated by Arp 72 (Figure 1a), whose eastern tail is very prominent in the GALEX images, and has very blue UV/optical colors. Arp 72 is also a good example of the so-called ‘beads on a string’ morphology, in which regularly-spaced clumps of star formation are seen along spiral arms and tidal features. These clumps are generally spaced about 1 kpc apart, the characteristic scale for gravitational collapse of molecular clouds [@elmegreen96]. Such beads are seen in many other systems in our sample, including the northern tail of the western galaxy in Arp 65 (Figure 1b), Arp 82 [@hancock07], and Arp 285 [@smith08].

In a few systems, we see luminous star forming regions at the base of a tidal feature. We call these features ‘hinge clumps’ [@hancock09]. These lie near the intersection of the spiral density wave in the inner disk and the material wave in the tail. These may form when dense material in the inner disk gets pulled out into a tail. This lowers the shear, which may allow more massive clouds to gravitationally collapse. Hinge clumps are visible at the eastern end of the Arp 72 bridge (Figure 1a) and the base of the northern tail of Arp 65 (Figure 1b). Hinge clumps are also seen in Arp 82 [@hancock07] and Arp 305 [@hancock09].

Our sample also includes some candidate ‘tidal dwarf galaxies’ (TDGs), massive concentrations of young stars near the tips of tidal features. The prototypical TDGs in Arp 244 and Arp 245 [@mirabel92; @duc00] are included in the SB&T sample, along with the bridge TDG in Arp 305 [@hancock09; @hancock10]. Another possible TDG is seen in Arp 202 (Figure 2), an interaction between an edge-on disk galaxy and a smaller irregularly-shaped galaxy to the south. A long clumpy tail is visible to the west of the southern galaxy. The tip of this tail is particularly prominent in the GALEX images, and has very blue UV/optical colors. Our optical spectrum shows that this clump is at the same redshift as Arp 202. This source was not detected in our Spitzer 8 $\mu$m map [@smith07] or in our SARA H$\alpha$ map, suggesting that it is in a post-starburst stage.

Another SB&T system that may have TDGs is Arp 181 (Figure 3). A clump is visible near the end of the western tail in the GALEX and SDSS images, with very blue optical/UV colors. However, no optical spectrum is available, thus it is unclear whether it is at the same redshift as Arp 181. Further west, another galaxy is visible, without any obvious link to the tail. Our optical spectrum shows that it is at the same redshift as Arp 181. In the SDSS image it looks like a spiral galaxy or a disturbed disk with short tidal tails. It is extremely blue in NUV $-$ g, and is detected at 8 $\mu$m [@smith07]. This may be either a pre-existing dwarf galaxy or a recently detached TDG.

The SB&T sample also contains numerous examples of accretion from one galaxy to another. One of the best-studied examples is the northern tail of Arp 285, which was likely produced from material accreted from the southern galaxy [@toomre72; @smith08]. According to our numerical simulations, the material in this tail fell into the gravitational potential of the northern galaxy, overshot that potential, and is now gravitationally collapsing and forming stars [@smith08]. We call such features ‘accretion tails’, to distinguish them from classical tidal features. The inner western tail of Arp 284 was likely produced by the same process [@struck03]. Another system which may have an accretion tail is Arp 105 (Figure 4). The spiral in this system has a long tail extending to the north, previously classified as a TDG [@duc97]. The spiral is connected by a bridge to an elliptical galaxy to the south. South of the elliptical is a bright star formation knot [@stockton72]. Both the northern TDG candidate and the southern knot of star formation are luminous in HI maps [@duc97]. In the GALEX images, the spiral and the northern TDG are quite bright in the UV, but the highest UV surface brightness is found in the knot of star formation south of the elliptical. We suggest, based on analogy to Arp 285 [@smith08] and proximity to the elliptical, that the southern star forming region in Arp 105 is an accretion tail, rather than simply a classical tidal tail coincidently seen in projection behind the elliptical.

Another system with possible mass transfer is Arp 269 (Figure 5a), an unequal-mass pair of galaxies with a bridge. In both the GALEX and SDSS images, an off-center group of blue star forming regions is visible in the smaller galaxy NGC 4485, as well as along the bridge. As noted by @elmegreen98, several of these knots of star formation lie in a tail-like structure southwest of NGC 4485. @clemens00 suggest that NGC 4485 passed through the disk of the larger galaxy NGC 4490, and ram pressure caused an offset in the location of the interstellar gas in NGC 4485, and thus the observed offset in star formation. We suggest an alternative possibility, that the star formation was triggered by gas flow from NGC 4490 along the bridge. Thus this may be an example of accretion from one galaxy to another.

Our sample contains only a few tidal features with high HI column densities that are not detected in our GALEX maps or published optical maps. One of these systems is Arp 269. In the @clemens98a HI maps, two large plume-like features extend 10$'$ ($\sim$20 kpc) to the north and south of the pair. Neither of these plumes is strongly detected in the GALEX or SDSS images, although smoothed SDSS images show a possible hint of the southern plume. This is surprising in light of the relatively high N$_{HI}$ in the inner 5$'$ (10 kpc) sections of these plumes of 4 $\times$ 10$^{20}$ cm$^{-2}$. It was suggested by @clemens98a that these HI features were produced by SN-driven outflow from the main galaxy NGC 4490. Instead, we suggest that the HI plumes are simply gas-rich tidal features. Deeper optical and UV imaging is needed to search for a stellar component to these features.

Another system with a gas-rich tidal feature without a GALEX counterpart is Arp 84 (‘The Heron’), a pair of unequal mass spiral galaxies connected by a bridge (Figure 5b). In HI maps, a large gaseous plume extends to the south of the main galaxy [@kaufman99]. In spite of its high N$_{HI}$ $\sim$ 4 $\times$ 10$^{20}$ cm$^{-2}$ and narrow HI line width of $\sim$100 km s$^{-1}$, no diffuse UV emission is seen in this feature, although a few UV-bright clumps are present. No redshifts are available at present for these clumps. Arp 84 also exhibits ‘beads on a string’ along the inner edge of the northern tail of the smaller galaxy. The most northern knot along this ‘string’ is bright in the GALEX image, as well as at 8 $\mu$m [@smith07] and H$\alpha$ [@kaufman99].

Summary
=======

In this paper, we present GALEX UV images of a subset of the SB&T interacting galaxy pairs. We identify examples of ‘beads on a string’, accretion from one galaxy to another, and candidate tidal dwarf galaxies. There are only a few tidal features that are bright in HI maps that are not detected in GALEX.

This research was supported by GALEX grant\
GALEXGI04-0000-0026, NASA LTSA grant NAG5-13079, and Spitzer grant RSA 1353814. We thank the Lick Observatory staff for their support of our optical spectroscopy campaign.

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