Chiara Mastropietro

Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas

We perform high-resolution N-body+SPH (smoothed particle hydrodynamics) simulations of gas-rich dwarf galaxy satellites orbiting within a Milky Way-sized halo and study for the first time the combined effects of tides and ram pressure. The structure of the galaxy models and the orbital configurations are chosen in accordance with those expected in a Lambda cold dark matter (ACDM) universe. While tidal stirring of disky dwarfs produces objects whose stellar structure and kinematics resembles that of dwarf spheroidals after a few orbits, ram pressure stripping is needed to entirely remove their gas component. Gravitational tides can aid ram pressure stripping by diminishing the overall potential of the dwarf, but tides also induce bar formation which funnels gas inwards making subsequent stripping more difficult. This inflow is particularly effective when the gas can cool radiatively. Assuming a low density of the hot Galactic corona consistent with observational constraints, dwarfs with V peak 30 km s -1 lose most or all of their gas content only if a heating source keeps the gas extended, partially counteracting the bar-driven inflow. We show that the ionizing radiation from the cosmic ultraviolet (UV) background at z > 2 can provide the required heating. In these objects, most of the gas is removed or becomes ionized at the first pericenter passage, explaining the early truncation of the star formation observed in Draco and Ursa Minor. Galaxies on orbits with larger pericenters and/or falling into the Milky Way halo at lower redshift can retain significant amounts of the centrally concentrated gas. These dwarfs would continue to form stars over a longer period of time, especially close to pericenter passages, as observed in Fornax and other dwarf spheroidal galaxies (dSphs) of the Local Group. The stripped gas breaks up into individual clouds pressure confined by the outer gaseous medium that have masses, sizes and densities comparable to the H I clouds recently discovered around M31.

Morphological evolution of discs in clusters

The recent discovery of hidden non-axisymmetric and disc-like structures in bright Virgo dwarf elliptical and lenticular galaxies (dE/dSph/dS0) indicates that they may have late-type progenitors. Using N-body simulations we follow the evolution of disc galaxies within a A cold dark matter (ACDM) cluster simulated with 10 7 particles, where the hierarchical growth and galaxy harassment are modelled self-consistently. Most of the galaxies undergo significant morphological transformation, even at the outskirts of the cluster, and move through the Hubble sequence from late-type discs to dwarf spheroidals. None of the discs is completely destroyed, therefore they cannot be the progenitors of ultracompact dwarf (UCD) galaxies. The time evolution of the simulated galaxies is compared with unsharp masked images obtained from Very Large Telescope (VLT) data and the projected kinematics of our models with the latest high-resolution spectroscopic studies from the Keck and Palomar telescopes.

The gravitational and hydrodynamical interaction between the Large Magellanic Cloud and the Galaxy

We use high-resolution N-body/smoothed particle hydrodynamic simulations to study the hydrodynamical and gravitational interaction between the Large Magellanic Cloud (LMC) and the Milky Way Galaxy. We model the dark and hot extended halo components as well as the stellar/gaseous discs of the two galaxies. Both galaxies are embedded in extended cuspy �CDM dark matter haloes. We follow the previous 4 Gyr of the LMC’s orbit such that it ends up with the correct location and orientation on the sky. Tidal forces elongate the LMC’s disc, forcing a bar and creating a strong warp and diffuse stellar halo, although very few stars become unbound. The stellar halo may account for some of the microlensing events observed towards the LMC. Ram pressure from a low-density ionized halo is then sufficient to remove

Early gas stripping as the origin of the darkest galaxies in the Universe

The known galaxies most dominated by dark matter (Draco, Ursa Minor and Andromeda IX) are satellites of the Milky Way and the Andromeda galaxies. They are members of a class of faint galaxies, devoid of gas, known as dwarf spheroidals, and have by far the highest ratio of dark to luminous matter. None of the models proposed to unravel their origin can simultaneously explain their exceptional dark matter content and their proximity to a much larger galaxy. Here we report simulations showing that the progenitors of these galaxies were probably gas-dominated dwarf galaxies that became satellites of a larger galaxy earlier than the other dwarf spheroidals. We find that a combination of tidal shocks and ram pressure swept away the entire gas content of such progenitors about ten billion years ago because heating by the cosmic ultraviolet background kept the gas loosely bound: a tiny stellar component embedded in a relatively massive dark halo survived until today. All luminous galaxies should be surrounded by a few extremely dark-matter-dominated dwarf spheroidal satellites, and these should have the shortest orbital periods among dwarf spheroidals because they were accreted early.

Simulating the Bullet Cluster

We present high resolution N-body/smoothed particle hydrodynamics (SPH) simulations of the interacting cluster 1E0657-56. The main and the subcluster are modelled using extended cuspy Acold dark matter (ACDM) dark matter haloes and isothermal β-profiles for the collisional component. The hot gas is initially in hydrostatic equilibrium inside the global potential of the clusters. We investigate the X-ray morphology and derive the most likely impact parameters, mass ratios and initial relative velocities. We find that the observed displacement between the X-ray peaks and the associated mass distribution, the morphology of the bow shock, the surface brightness and projected temperature profiles across the shock discontinuity can be well reproduced by offset 6:1 encounters where the subcluster has initial velocity (in the rest frame of the main cluster) 2.3 times the virial velocity of the main cluster dark matter halo. A model with the same mass ratio and lower velocity (1.5 times the main cluster virial velocity) matches quite well most of the observations. However, it does not reproduce the relative surface brightness between the bullet and the main cluster. Dynamical friction strongly affects the kinematics of the subcluster so that the low-velocity bullet is actually bound to the main system at the end of the simulation. We find that a relatively high concentration (c = 6) of the main cluster dark matter halo is necessary in order to prevent the disruption of the associated X-ray peak. For a selected subsample of runs we perform a detailed three-dimensional analysis following the past, present and future evolution of the interacting systems. In particular, we investigate the kinematics of the gas and dark matter components as well as the changes in the density profiles and the motion of the system in the L x -T diagram.

The first generation of Virgo cluster dwarf elliptical galaxies

In the light of the question whether most early-type dwarf (dE) galaxies in clusters formed through infall and transformation of late-type progenitors, we search for an imprint of such an infall history in the oldest, most centrally concentrated dE subclass of the Virgo cluster: the nucleated dEs that show no signatures of disks or central residual star formation. We select dEs in a (projected) region around the central elliptical galaxies, and subdivide them by their line-of-sight velocity into fast-moving and slow-moving ones. These subsamples turn out to have significantly different shapes: while the fast dEs are relatively flat objects, the slow dEs are nearly round. Likewise, when subdividing the central dEs by their projected axial ratio into flat and round ones, their distributions of line-of-sight velocities differ significantly: the flat dEs have a broad, possibly two-peaked distribution, whereas the round dEs show a narrow single peak. We conclude that the round dEs probably are on circularized orbits, while the flat dEs are still on more eccentric or radial orbits typical for an infalling population. In this picture, the round dEs would have resided in the cluster already for a long time, or would even be a cluster-born species, explaining their nearly circular orbits. They would thus be the first generation of Virgo cluster dEs. Their shape could be caused by dynamical heating through repeated tidal interactions. Further investigations through stellar population measurements and studies of simulated galaxy clusters would be desirable to obtain definite conclusions on their origin.

Effects of ram pressure on the gas distribution and star formation in the Large Magellanic Cloud

We use high resolution N-body/SPH simulations to study the hydrodynamical interaction between the Large Magellanic Cloud (LMC) and the hot halo of the Milky Way. We investigate whether ram-pressure acting on the satellite’s ISM can explain the peculiarities observed in the HI distribution and the location of the recent star formation activity. Due to the present nearly edge-on orientation of the disk with respect to the orbital motion, compression at the leading edge can explain the high density region observed in HI at the south-east border. In the case of a face-on disk (according to Mastropietro et al. 2008 the LMC was moving almost face-on before the last perigalactic passage), ram-pressure directed perpendicularly to the disk produces a clumpy structure characterized by voids and high density filaments that resemble those observed by the Parkes HI survey. As a consequence of the very recent edge-on motion, the Hα emission is mainly concentrated on the eastern side where 30 Doradus and most of the supergiant shells are located, although some Hα complexes form a patchy distribution on the entire disk. In this scenario only the youngest stellar complexes show a progression in age along the leading border of the disk.

Ram-Pressure Induced Star Formation in the LMC

We use high-resolution n-body/SPH simulations to study the hydrodynamical interaction between the Large Magellanic Cloud and the hot halo of the Milky Way. We investigate whether the ram-pressure acting on the gaseous disk of the satellite can explain the peculiarities observed in the Hi distribution and the location of the recent star formation activity.

Modeling a high velocity LMC: The formation of the Magellanic Stream

I use high resolution N-body/SPH simulations to model the new proper motion of the Large Magellanic Cloud (LMC) within the Milky Way (MW) halo and investigate the effects of gravitational and hydrodynamical forces on the formation of the Magellanic Stream (MS). Both the LMC and the MW are fully self consistent galaxy models embedded in extended cuspy ΛCDM dark matter halos. I find that ram-pressure from a low density ionized halo is sufficient to remove a large amount of gas from the LMCs disk forming a trailing Stream that extends more than 120 degrees from the Cloud. Tidal forces elongate the satellites disk but do not affect its vertical structure. No stars become unbound showing that tidal stripping is almost effectless.

Detailed kinematics and morphological features in tidally heated disks

The recent discovery of hidden non-axysimmetric and disk-like structures in bright Virgo dwarf elliptical and lenticular galaxies (dE/dSph/dS0) indicates that they may have latetype progenitors. Using N-body simulations we follow the evolution of disk galaxies within a ΛCDM cluster simulated with 10 particles, where the hierarchical growth and galaxy harassment are modeled self-consistently. Most of the galaxies undergo significant morphological transformation, even at the outskirts of the cluster, and move through the Hubble sequence from late type disks to dwarf spheroidals. The time evolution of the simulated galaxies is compared with unsharp-masked images obtained from VLT data and the projected kinematics of our models with the latest high resolution spectroscopic studies from the Keck and Palomar telescopes.