I discuss recent work in which we construct models of poststarburst galaxies by combining fully three-dimensional hydrodynamic simulations of galaxy mergers with radiative transfer calculations of dust attenuation. The poststarburst signatures can occur shortly after a bright starburst phase in gas-rich mergers, and thus offer a unique opportunity to study the formation of bulges and the effects of feedback. Several additional applications of spatially-resolved spectroscopic models of interacting galaxies include multi-wavelength studies of AGN/starburst diagnostics, mock integral field unit data to interpret the evolution of ULIRGs, and the ‘Green Valley’.

Optical spectra of simulated major gas-rich galaxy mergers can be found at http://www.cfa.harvard.edu/~gsnyder

The evolution of galaxies is greatly influenced by their interactions. As part of a program to study interacting galaxies, we have measured and modeled the spectral energy distributions (SEDs) from the ultraviolet (UV) to the far-infrared (FIR). We describe the constraints imposed on star formation histories by these SEDs, and the variations therein seen across the interaction sequence, and we compare the results of different star formation rate prescriptions applied to the data. The sample itself is based on the Spitzer Interacting Galaxy Survey (SIGS) of 111 galaxies in 50 systems, a project designed to probe a range of galaxy interaction parameters in the infrared. Our SEDs combine the Spitzer results with multiwavelength data from other missions, in particular GALEX and Herschel. The subset presented here is the sample for which FIR Herschel observations are currently publicly available.

The Arecibo Legacy Fast ALFA survey is in the process of yielding a complete HI dataset of the Virgo Cluster and its environs (Giovanelli et al. 2007, Kent et al., in preparation). Assuming a distance to Virgo of 16.7 Mpc, the minimum detectable HI mass by ALFALFA is of order 2 × 107 M⊙. A number of the HI detections appear to have interesting properties. Some appear associated with, but offset from, low surface brightness optical counterparts; others, at larger spatial offsets, may be tidally related to optical counterparts. Yet another class includes detections which are not identifiable with any optical counterparts. We present the ALFALFA results on these objects in the Virgo region, as well as followup aperture synthesis observations obtained with the VLA.

Debris sent into the intergalactic medium during tidal collisions has received much attention as it can tell us about several fundamental properties of galaxies, in particular their missing mass, both in the form of cosmological Dark Matter and so-called Lost Baryons.

High velocity encounters, which are common in clusters of galaxies, are able to produce faint tidal debris that may appear as star–less, free floating HI clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous, dark matter (DM) dominated, objects which for some reason never managed to form stars. VirgoHI21, in the Virgo Cluster, is by far the most spectacular and most discussed Dark Galaxy candidate so far detected in HI surveys. We show here that it is most likely made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC 4254 during its fly–by at about 1000 km s−1 by a massive intruder. Our numerical model of the collision is able to reproduce the main characteristics of the system: in particular the absence of stars, and its prominent velocity gradient. Originally attributed to the gas being in rotation within a massive dark matter halo, we find it instead to be consistent with a combination of simple streaming motion plus projection effects (Duc & Bournaud, 2007).

Based on our multi-wavelength and numerical studies of galaxy collisions, we discuss several ways to identify a tidal origin in a Dark Galaxy candidate such as optical and millimetre–wave observations to reveal a high metallicity and CO lines, and more importantly, kinematics indicating the absence of a prominent Dark Matter halo. We illustrate the method using another HI system in Virgo, VCC 2062, which is most likely a Tidal Dwarf Galaxy (Duc et al., 2007).

Now, whereas tidal debris should not contain any dark matter from the halo of their parent galaxies, it may exhibit missing mass in the form of dark baryons, unaccounted for by classical observations, as recently found in the collisional ring of NGC 5291 (Bournaud et al., 2007) and probably in the TDG VCC 2062. These “Lost Baryons” must originally have been located in the disks of their parent galaxies.

We have observed over 100 interacting galaxies (IGs) with Spitzer IRAC as part of a detailed study of the their properties (see also Zezas, et al. in this volume). The majority of sources are nearby. Precise IRAC imaging photometry of these galaxies finds a wide range of colors in these systems, and correlative spectroscopic data suggests the colors are reflected by composition that is in turn reflective of age; orientation also plays a role. We have also observed a distant pair of interacting galaxies at z=3.01, a HyLIG, EGS20 J1418+5236 (Huagg et al. 2006), and find that, like the nearby systems, it has similar processes at work albeit in a hyperactive way.

To overcome limitations of some previous studies of IGs, we observed two complementary nearby samples, one selected on the basis of galaxy separation, and the other selected on a morphological basis. Figure 1 below is a summary IRAC [3.6um]-[4.5um] versus [5.8um]-[8.0um] color-color plot of these IGs. Five generic regions emerge: ellipticals near (0,-0.1), early-stage IGS around (1.5, -0.1), Seyfert 1s around (1.3, 0.5), Seyfert 2s around (1.5, 1.2), and LINERS/HII galaxies centered at 2.2, 0.2). These systems are each characterized by spectral dust features in the IRAC bands that are strong enough to influence and]or mirror the underlying stellar continuum colors. The arrows suggest possible evolutionary development.