Héctor Velázquez

Sinking satellites and the heating of galaxy discs

We have carried out a set of self-consistent N-body simulations to study the interaction between disc galaxies and merging satellites with the aim of determining the disc kinematical changes induced by such events. We explore a region of the parameter space embracing satellites with different masses and internal structure and orbits of various eccentricities. We find that the analytic estimates of Toth & Ostriker are high; overestimating the disc heating and thickening resulting from the accretion process by a factor of about 2--3. We find that the heating and thickening of the disc differ for satellites on prograde and retrograde orbits. The former tend to heat the stellar disc while the latter primarily produce a coherent tilt. For instance, a satellite of a Milky Way type galaxy with an initial mass of 20 per cent of that of the disc and on a retrograde orbit increases the velocity ellipsoid at the solar neighbourhood by (ΔσR, Δσπ, Δσz)⊙≈ (11,9,6) km s-1 and produces a maximum increment of the vertical scalelength and the stability parameter Q, inside the solar radius, of 300 pc and 0.8, respectively, increases of about 43 and 53 per cent. The same satellite, but on a prograde orbit, leads to changes of (ΔσR, Δσπ , Δσz)⊙≈ (22,15,12) km s-1, Δ zo,⊙≈ 550 pc and Δ Q⊙≈ 1.2. Thus, disc galaxies may accrete quite massive satellites without destroying the disc, particularly, if the orbits are retrograde. We also find that a massive bulge may play a role in reducing these effects. We have quantified the importance of the responsiveness of the halo by replacing it by a rigid potential in several simulations. In these cases, the increase of the vertical scalelength is larger by a factor of 1.5--2, indicating that a self-consistent treatment is essential for obtaining realistic results. A frequent by-product of the accretion process is the formation of weak stellar warps and asymmetric discs. Finally, we have checked how well Chandrasekhars dynamical friction formula reproduces the sinking rates in several of our experiments. We find that it works well provided a suitable value is chosen for the Coulomb logarithm and the satellite mass is taken to be the mass still bound to the satellite at each moment.

On galaxy spiral arms' nature as revealed by rotation frequencies

High-resolution N-body simulations using different codes and initial condition techniques reveal two different behaviours for the rotation frequency of transient spiral arms like structures. Whereas unbarred discs present spiral arms nearly corotating with disc particles, strong barred models (bulged or bulgeless) quickly develop a bar-spiral structure dominant in density, with a pattern speed almost constant in radius. As the bar strength decreases the arm departs from bar rigid rotation and behaves similar to the unbarred case. In strong barred models, we detect in the frequency space other subdominant and slower modes at large radii, in agreement with previous studies, however, we also detect them in the configuration space. We propose that the distinctive behaviour of the dominant spiral modes can be exploited in order to constraint the nature of Galactic spiral arms by the astrometric survey Gaia and by 2D spectroscopic surveys like Calar Alto Legacy Integral Field Area Survey (CALIFA) and Mapping Nearby Galaxies at APO (MANGA) in external galaxies.

THE SPECIFIC STAR FORMATION RATE AND STELLAR MASS FRACTION OF LOW-MASS CENTRAL GALAXIES IN COSMOLOGICAL SIMULATIONS

By means of cosmological N-body + hydrodynamics simulations of galaxies in the context of the Λ cold dark matter (ΛCDM) scenario we explore the specific star formation rates (SSFR = SFR/Ms , Ms is the stellar mass) and stellar mass fractions (Fs ≡ Ms /Mh , Mh is the halo mass) for sub-M* field galaxies at different redshifts (0 z 1.5). Distinct low-mass halos (2.5 Mh /1010 M ☉ 50 at z = 0) were selected for the high-resolution re-simulations. The Hydrodynamics Adaptive Refinement Tree (ART) code was used and some variations of the sub-grid parameters were explored. Most simulated galaxies, specially those with the highest resolutions, have significant disk components and their structural and dynamical properties are in reasonable agreement with observations of sub-M* field galaxies. However, the SSFRs are 5-10 times smaller than the averages of several (compiled and homogenized here) observational determinations for field blue/star-forming galaxies at z < 0.3 (at low masses, most observed field galaxies are actually blue/star forming). This inconsistency seems to remain even at z ~ 1-1.5, although it is less drastic. The Fs of simulated galaxies increases with Mh as semi-empirical inferences show. However, the values of Fs at z 0 are ~5-10 times larger in the simulations than in the inferences; these differences increases probably to larger factors at z ~ 1-1.5. The inconsistencies reported here imply that simulated low-mass galaxies (0.2 Ms /109 M ☉ 30 at z = 0) assembled their stellar masses much earlier than observations suggest. Our results confirm the predictions found by means of ΛCDM-based models of disk galaxy formation and evolution for isolated low-mass galaxies, and highlight that our understanding and implementation of astrophysics into simulations and models are still lacking vital ingredients.

On the dynamics of the Sagittarius dwarf galaxy

We use numerical simulations to test the feasibility of the suggestion by Ibata et al. (1994) that the excess population of stars which they discovered in the Sagittarius region may be the disrupted remains of a dwarf spheroidal galaxy. We find that a Fornax-like model for the pre-disruption system can indeed reproduce the data. However, the galaxy must be on a relatively short period orbit with a pericentre of about

Tidal Radii and Destruction Rates of Globular Clusters in the Milky Way due to Bulge-Bar and Disk Shocking

10

Simulations of Galaxies Formed in Warm Dark Matter Halos of Masses at the Filtering Scale

kpc and an apocentre of about

From the Tully–Fisher relation to the Fundamental Plane through mergers

52

Detection of Satellite Remnants in the Galactic Halo with Gaia - II. A modified Great Circle Cell Method

kpc, giving a current transverse velocity of

GARROTXA COSMOLOGICAL SIMULATIONS OF MILKY WAY-SIZED GALAXIES: GENERAL PROPERTIES, HOT-GAS DISTRIBUTION, AND MISSING BARYONS

255

ON THE ORIGIN OF HIGH-ALTITUDE OPEN CLUSTERS IN THE MILKY WAY

km/s and a period of