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.

No abstract

Mergers between gas--rich disks and less--massive dwarf galaxies are studied using numerical simulation. As the orbit of a satellite decays through dynamical friction, the primary disk develops large-amplitude spirals in response to its tidal forcing. While these features arise in both the stars and the gas in the disk, the non--axisymmetric structures in the gas differ slightly from those in the stars. In particular, as a consequence of the formation of strong shocks in the gas and the effects of radiative cooling, the gas response tends to lead the stellar response, enabling the stars to strongly torque the gas. These torques deprive the gas of its angular momentum, forcing a significant fraction of it into the inner regions of the disk. The radial inflows induced by these mergers accumulate large quantities of interstellar gas in the nuclear regions of the host disks. In some cases, nearly half of all the gas initially distributed throughout the disk winds up in a dense ``cloud'' several hundred parsecs in extent. The models reported here do not include star formation and, so, we cannot determine the ultimate fate of the gas. Nevertheless, given the high densities in the nuclear gas, it is plausible to identify these concentrations of dense gas in the remnants with those accompanying intense starbursts in some active galaxies. Therefore, the calculations here provide a framework for interpreting the origin of nuclear activity in otherwise quiescent disk galaxies. To the extent that galaxy formation is a chaotic process in which large structures are built up by the accretion of smaller fragments, our models may also be relevant to starbursts and the onset of nuclear activity in proto--galaxies at high redshifts.Comment: 28 pages of LaTeX, using the AASTEX macros. To appear in ApJ. 19 Postscript figures available upon request from hos@lick.ucsc.edu; postscript text also available from http://ucowww.ucsc.edu/~hos/home.htm

Motivated by recent neutral hydrogen observations with the VLA, we have undertaken an investigation into the interaction that produced the well known merger remnant NGC 7252. Through fully self-consistent N-body simulations, we are able to reproduce the kinematic character of the HI observations quite well, including the velocity reversals observed along each tidal tail. In the simulation these reversals arise from particles which have turned around in their orbit and are moving to smaller radii. The bases of the tails fall back quickly to small pericentric distances, while the more distant regions fall back more slowly to ever increasing pericentric distances. The delayed return of tidally ejected material may extend over many Gyr. The evolution of this merger is followed numerically for 800 h^-1 Myr beyond the best fit time. We find that nearly half of the present tail material, or of order 10^9 h^-2 of neutral hydrogen and 2x10^9 h^-2 of starlight, will return to within 13 h^-1 kpc of the nucleus within this time span. While the collisionless stars of the tails will continue orbiting between their inner and outer turning points, the observations show the HI gas of the tails disappearing upon its return. We discuss this result in light of the lack of central HI in the main body of this merger remnant.Comment: 28 pages of uuencoded, compressed postscript. Accepted to AJ. 9 Postscript figures available upon request from hos@lick.ucsc.edu; postscript text also available from http://ucowww.ucsc.edu/~hos/home.htm

The Next Generation Virgo Cluster Survey (NGVS) is a program that uses the 1 deg 2 MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical imaging survey of the Virgo cluster, from its core to its virial radius-covering a total area of 104 deg 2-in the u * griz bandpasses. Thanks to a dedicated data acquisition strategy and processing pipeline, the NGVS reaches a point-source depth of g ≈ 25.9 mag (10σ) and a surface brightness limit of μ g ∼ 29 mag arcsec −2 (2σ above the mean sky level), thus superseding all previous optical studies of this benchmark galaxy cluster. In this paper, we give an overview of the technical aspects of the survey, such as areal coverage, field placement, choice of filters, limiting magnitudes, observing strategies, data processing and calibration pipelines, survey timeline, and data products. We also describe the primary scientific topics of the NGVS, which include: the galaxy luminosity and mass functions; the color-magnitude relation; galaxy scaling relations; compact stellar systems; galactic nuclei; the extragalactic distance scale; the large-scale environment of the cluster and its relationship to the Local Supercluster; diffuse light and the intracluster medium; galaxy interactions and evolutionary processes; and extragalactic star clusters. In addition, we describe a number of ancillary programs dealing with "foreground" and "background" science topics, including the study of highinclination trans-Neptunian objects; the structure of the Galactic halo in the direction of the Virgo Overdensity and Sagittarius Stream; the measurement of cosmic shear, galaxy-galaxy, and cluster lensing; and the identification of distant galaxy clusters, and strong-lensing events.