Using numerical simulation, we study the development of gaseous inflows and triggering of starburst activity in mergers of comparable-mass disk galaxies. In all encounters studied, the galaxies experience strong gaseous inflows and moderate to intense starburst activity. We find that galaxy structure plays a dominant role in regulating activity. The gaseous inflows are strongest when galaxies with dense central bulges are in the final stages of merging, while inflows in bulgeless galaxies are weaker and occur earlier in the interaction. Orbital geometry plays only a relatively modest role in the onset of collisionally-induced activity. Through an analysis of the torques acting on the gas, we show that these inflows are generally driven by gravitational torques from the host galaxy (rather than the companion), and that dense bulges act to stabilize galaxies against bar modes and inflow until the galaxies merge, at which point rapidly varying gravitational torques drive strong dissipation and inflow of gas in the merging pair. The strongest inflows (and associated starburst activity) develop in co-planar encounters, while the activity in inclined mergers is somewhat less intense and occurs slightly later during the merger. The starbursts which develop in mergers of galaxies with central bulges represent an increase in the star formation rate of two orders of magnitude over that in isolated galaxies. We find that the gaseous and stellar morphology and star-forming properties of these systems provide a good match to those of observed ultraluminous infrared galaxies. Our results imply that the internal structure of the merging galaxies, rather than orbital geometry, may be the key factor in producing ultraluminous infrared galaxies.Comment: 40 pages of LaTeX using AASTeX; 15 Postscript figures available via http://www.ucolick.org/~hos/preprints/majmerge/abstract.html with MPEG movies at http://www.ucolick.org/~hos/models/models.html To appear in the Astrophysical Journa

We present deep optical imaging of the inner 1.5 x 1.5 degrees of the Virgo cluster to search for diffuse intracluster light (ICL). Our image reaches a 1 sigma depth of mu_v=28.5 mag/arcsec^2 -- 1.5 mag/arcsec^2 deeper than previous surveys -- and reveals an intricate web of diffuse intracluster light. We see several long (>100 kpc) tidal streamers, as well as a myriad of smaller-scale tidal tails and bridges between galaxies. The diffuse halo of M87 is traced out to nearly 200 kpc, appearing very irregular on these scales, while significant diffuse light is also detected around the M84/M86 pair. Several galaxies in the core are embedded in common envelopes, suggesting they are true physical subgroups. The complex substructure of Virgo's diffuse ICL reflects the hierarchical nature of cluster assembly, rather than being the product of smooth accretion around a central galaxy.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter

We perform fully self-consistent stellar dynamical simulations of the accretion of a companion ("satellite") galaxy by a large disk galaxy to investigate the interaction between the disk, halo, and satellite components of the system during a merger. Our fiducial encounter begins with a satellite in a prograde, circular orbit inclined thirty degrees with respect to the disk plane at a galactocentric distance of six disk scalelengths. The satellite's mass is 10% of the disk's mass and its half-mass radius is about 1.3 kpc. The system is modelled with 500 000 particles, sufficient to mitigate numerical relaxation noise over the merging time. The satellite sinks in only ∼ 1 Gyr and a core containing ∼ 45% of its initial mass reaches the centre of the disk. With so much of the satellite's mass remaining intact, the disk sustains significant damage as the satellite passes through. At the solar circle we find that the disk thickens ∼ 60%, the velocity dispersions increase by ∆σ ≃ (10, 8, 8) km/s to (σ R , σ φ , σ z ) ≃ (48, 42, 38) km/s, and the asymmetric drift is unchanged at ∼ 18 km/s. Although the disk is not destroyed by these events (hence "minor" mergers), its final state resembles a disk galaxy of earlier Hubble type than its initial state, thicker and hotter, with the satellite's core enhancing the bulge. Thus minor mergers continue to be a promising mechanism for driving galaxy evolution.

We use numerical simulation to investigate the triggering of starbursts in merging disk galaxies. The properties of the merger-driven starbursts are sensitive to the structure of the progenitor galaxies; speci cally, to the amount of material in a dense central bulge. Galaxies without bulges develop bars shortly after their rst close passage, driving signi cant gas in ow and subsequent starbursts in the centers of the galaxies. These starbursts signi cantly deplete the star-forming gas, so that only relatively weak starbursts arise during the nal merger. By contrast, models of galaxies with central bulges show that a bulge acts to stabilize the galaxies against in ow and starbursts until the galaxies actually merge. At this time, strong dissipation leads to the formation of a massive central gas mass and an ensuing star formation rate two orders of magnitude greater than that in our isolated disk models. These starbursts are very short in duration, typically 50 Myr, suggesting that the rarity of ultraluminous infrared galaxies is a result of their being in a very short evolutionary phase, rather than special and rare formation conditions. The fact that these mergers display many of the properties of ultraluminous infrared galaxies { tidal features, double nuclei, massive compact gas concentrations, and extreme levels of starburst activity { suggests that merger-driven starbursts can explain the emission from many ultraluminous infrared galaxies without an active nucleus.

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