Ram Pressure Stripping of Spiral Galaxies in Clusters

We use 3-dimensional SPH/N-BODY simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters. We find that the analytic expectation of Gunn & Gott (1972) relating the gravitational restoring force provided by the disk to the ram pressure force, provides a good approximation to the radius that gas will be stripped from a galaxy. However, at small radii it is also important to consider the potential provided by the bulge component. A spiral galaxy passing through the core of a rich cluster such as Coma, will have its gaseous disk truncated to $\sim 4$ kpc, thus losing $\sim 80$ of its diffuse gas mass. The timescale for this to occur is a fraction of a crossing time $\sim 10^7$ years. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intra-cluster medium will lose significantly less gas. We conclude that ram-pressure alone is insufficient to account for the rapid and widespread truncation of star-formation observed in cluster galaxies, or the morphological transformation of Sab's to S0's that is necessary to explain the Butcher-Oemler effect.Comment: 8 pages, 7 figures, to be published in MNRAS. Levels added/corrected on figures 3, 4 and

Stars beyond Galaxies: The Origin of Extended Luminous Halos around Galaxies

(Abridged) We use numerical simulations to investigate the origin and structure of the luminous halos that surround isolated galaxies. These stellar structures extend out to several hundred kpc away from a galaxy, and consist of stars shed by merging subunits during the many accretion events that characterize the hierarchical assembly of galaxies. Such origin suggests that outer luminous halos are ubiquitous and that they should appear as an excess of light over extrapolations of the galaxy's inner profile beyond its traditional luminous radius. The mass profile of the accreted stellar component is well approximated by a model where the logarithmic slope steepens monotonically with radius; from -3 at the luminous edge of the galaxy to -4 or steeper near the virial radius of the system. Such spatial distribution is consistent with that of Galactic and M31 globular clusters, suggesting that many of the globulars were brought in by accretion events, in a manner akin to the classic Searle-Zinn scenario. The outer stellar spheroid is supported by a velocity dispersion tensor with a substantial and radially increasing radial anisotropy. These properties distinguish the stellar halo from the dark matter component, which is more isotropic in velocity space, as well as from some tracers of the outer spheroid such as satellite galaxies. Most stars in the outer halo formed in progenitors that have since merged with the central galaxy; very few stars in the halo are contributed by satellites that survive as self-bound entities at the present. These features are in reasonable agreement with recent observations of the outer halo of the MW, of M31, and of other isolated spirals, and suggest that all of these systems underwent an early period of active merging, as envisioned in hierarchical models of galaxy formation.Comment: Submitted to MNRAS, 13 pages, 12 figure

An Alternative Origin for Hypervelocity Stars

Halo stars with unusually high radial velocity ("hypervelocity" stars, or HVS) are thought to be stars unbound to the Milky Way that originate from the gravitational interaction of stellar systems with the supermassive black hole at the Galactic center. We examine the latest HVS compilation and find peculiarities that are unexpected in this black hole-ejection scenario. For example, a large fraction of HVS cluster around the constellation of Leo and share a common travel time of $\sim 100$-200 Myr. Furthermore, their velocities are not really extreme if, as suggested by recent galaxy formation models, the Milky Way is embedded within a $2.5\times 10^{12} h^{-1} M_{\odot}$ dark halo with virial velocity of $\sim 220$ km/s. In this case, the escape velocity at $\sim 50$ kpc would be $\sim 600$ km/s and very few HVS would be truly unbound. We use numerical simulations to show that disrupting dwarf galaxies may contribute halo stars with velocities up to and sometimes exceeding the nominal escape speed of the system. These stars are arranged in a thinly-collimated outgoing ``tidal tail'' stripped from the dwarf during its latest pericentric passage. We speculate that some HVS may therefore be tidal debris from a dwarf recently disrupted near the center of the Galaxy. In this interpretation, the angular clustering of HVS results because from our perspective the tail is seen nearly ``end on'', whereas the common travel time simply reflects the fact that these stars were stripped simultaneously from the dwarf during a single pericentric passage. This proposal is eminently falsifiable, since it makes a number of predictions that are distinct from the black-hole ejection mechanism and that should be testable with improved HVS datasets.Comment: 4 pages, 4 figures. Replacement to match version accepted to ApJ

A Sagittarius-Induced Origin for the Monoceros Ring

The Monoceros ring is a collection of stars in nearly-circular orbits at roughly 18 kpc from the Galactic center. It may have originated (i) as the response of the disc to perturbations excited by satellite companions or (ii) from the tidal debris of a disrupted dwarf galaxy. The metallicity of Monoceros stars differs from that of disc stars at comparable Galactocentric distances, an observation that disfavours the first scenario. On the other hand, circular orbits are difficult to accommodate in the tidal-disruption scenario, since it requires a satellite which at the time of disruption was itself in a nearly circular orbit. Such satellite could not have formed at the location of the ring and, given its low mass, dynamical friction is unlikely to have played a major role in its orbital evolution. We search cosmological simulations for low-mass satellites in nearly-circular orbits and find that they result, almost invariably, from orbital changes induced by collisions with more massive satellites: the radius of the circular orbit thus traces the galactocentric distance of the collision. Interestingly, the Sagittarius dwarf, one of the most luminous satellites of the Milky Way, is in a polar orbit that crosses the Galactic plane at roughly the same Galactocentric distance as Monoceros. We use idealized simulations to demonstrate that an encounter with Sagittarius might well have led to the circularization and subsequent tidal demise of the progenitor of the Monoceros ring.Comment: 6 pages, 4 figures, to match version published in MNRAS Letters (http://onlinelibrary.wiley.com/doi/10.1111/j.1745-3933.2011.01035.x/abstract

Simulaciones numéricas hidrodinámicas

Las simulaciones numéricas son una de las herramientas más adecuadas y poderosas para el estudio de la formación y evolución de estructuras no lineales en el universo. Estas estructuras se forman a partir de pequeñas fluctuaciones cuánticas presentes en el universo primitivo que crecen por inestabilidad gravitacional hasta llegar a formar objetos tales como galaxias, grupos, cúmulos, filamentos, etc. La correcta descripción de estos fenómenos físicos depende tanto del modelo matemático utilizado como de las limitaciones debido a la resolución numérica.Asociación Argentina de Astronomí

Counterrotating Stars in Simulated Galaxy Disks

Counterrotating stars in disk galaxies are a puzzling dynamical feature whose origin has been ascribed to either satellite accretion events or to disk instabilities triggered by deviations from axisymmetry. We use a cosmological simulation of the formation of a disk galaxy to show that counterrotating stellar disk components may arise naturally in hierarchically-clustering scenarios even in the absence of merging. The simulated disk galaxy consists of two coplanar, overlapping stellar components with opposite spins: an inner counterrotating bar-like structure made up mostly of old stars surrounded by an extended, rotationally-supported disk of younger stars. The opposite-spin components originate from material accreted from two distinct filamentary structures which at turn around, when their net spin is acquired, intersect delineating a "V"-like structure. Each filament torques the other in opposite directions; the filament that first drains into the galaxy forms the inner counterrotating bar, while material accreted from the other filament forms the outer disk. Mergers do not play a substantial role and most stars in the galaxy are formed in situ; only 9% of all stars are contributed by accretion events. The formation scenario we describe here implies a significant age difference between the co- and counterrotating components, which may be used to discriminate between competing scenarios for the origin of counterrotating stars in disk galaxies.Comment: 7 pages, 7 figures. Accepted for publication in MNRA

Tidal Torques and the Orientation of Nearby Disk Galaxies

We use numerical simulations to investigate the orientation of the angular momentum axis of disk galaxies relative to their surrounding large scale structure. We find that this is closely related to the spatial configuration at turnaround of the material destined to form the galaxy, which is often part of a coherent two-dimensional slab criss-crossed by filaments. The rotation axis is found to align very well with the intermediate principal axis of the inertia momentum tensor at this time. This orientation is approximately preserved during the ensuing collapse, so that the rotation axis of the resulting disk ends up lying on the plane traced by the protogalactic material at turnaround. This suggests a tendency for disks to align themselves so that their rotation axis is perpendicular to the minor axis of the structure defined by surrounding matter. One example of this trend is provided by our own Galaxy, where the Galactic plane is almost at right angles with the supergalactic plane (SGP) drawn by nearby galaxies; indeed, the SGP latitude of the North Galactic Pole is just 6 degrees. We have searched for a similar signature in catalogs of nearby disk galaxies, and find a significant excess of edge-on spirals (for which the orientation of the disk rotation axis may be determined unambiguously) highly inclined relative to the SGP. This result supports the view that disk galaxies acquire their angular momentum as a consequence of early tidal torques acting during the expansion phase of the protogalactic material.Comment: 5 pages, 2 figures, accepted for publication in ApJ

Mergers and the outside-in formation of dwarf spheroidals

We use a cosmological simulation of the formation of the Local Group to explore the origin of age and metallicity gradients in dwarf spheroidal galaxies. We find that a number of simulated dwarfs form "outside-in", with an old, metal-poor population that surrounds a younger, more concentrated metal-rich component, reminiscent of dwarf spheroidals like Sculptor or Sextans. We focus on a few examples where stars form in two populations distinct in age in order to elucidate the origin of these gradients. The spatial distributions of the two components reflect their diverse origin; the old stellar component is assembled through mergers, but the young population forms largely in situ. The older component results from a first episode of star formation that begins early but is quickly shut off by the combined effects of stellar feedback and reionization. The younger component forms when a late accretion event adds gas and reignites star formation. The effect of mergers is to disperse the old stellar population, increasing their radius and decreasing their central density relative to the young population. We argue that dwarf-dwarf mergers offer a plausible scenario for the formation of systems with multiple distinct populations and, more generally, for the origin of age and metallicity gradients in dwarf spheroidals.Comment: 10 pages, 8 figures, Accepted for publication in MNRA