ALBERT

Description: Cosmological hydrodynamical simulations are studied in order to analyse generic trends for the stellar, baryonic and halo mass assembly of low-mass galaxies ( M *   3 10 10 M ) as a function of their present halo mass, in the context of the cold dark matter (CDM) scenario and common subgrid physics schemes. We obtain that smaller galaxies exhibit higher specific star formation rates and higher gas fractions. Although these trends are in rough agreement with observations, the absolute values of these quantities tend to be lower than observed ones since z  ~ 2. The simulated galaxy stellar mass fraction increases with halo mass, consistently with semi-empirical inferences. However, the predicted correlation between them shows negligible variations up to high z , while these inferences seem to indicate some evolution. The hot-gas mass in z  = 0 haloes is higher than the central galaxy mass by a factor of ~1–1.5 and this factor increases up to ~5–7 at z  ~ 2 for the smallest galaxies. The stellar, baryonic and halo evolutionary tracks of simulated galaxies show that smaller galaxies tend to delay their baryonic and stellar mass assembly with respect to the halo one. The supernova feedback treatment included in this model plays a key role on this behaviour albeit the trend is still weaker than the one inferred from observations. At z  〉 2, the overall properties of simulated galaxies are not in large disagreement with those derived from observations.

Description: We present a comprehensive study of the chemical properties of the stellar haloes of Milky Way mass galaxies, analysing the transition between the inner to the outer haloes. We find the transition radius between the relative dominance of the inner-halo and outer-halo stellar populations to be ~15–20 kpc for most of our haloes, similar to that inferred for the Milky Way from recent observations. While the number density of stars in the simulated inner-halo populations decreases rapidly with distance, the outer-halo populations contribute about 20–40 per cent in the fiducial solar neighbourhood, in particular at the lowest metallicities. We have determined [Fe/H] profiles for our simulated haloes; they exhibit flat or mild gradients, in the range [–0.002, –0.01] dex kpc –1 . The metallicity distribution functions exhibit different features, reflecting the different assembly history of the individual stellar haloes. We find that stellar haloes formed with larger contributions from massive subgalactic systems have steeper metallicity gradients. Very metal-poor stars are mainly contributed to the halo systems by lower mass satellites. There is a clear trend among the predicted metallicity distribution functions that a higher fraction of low-metallicity stars are found with increasing radius. These properties are consistent with the range of behaviours observed for stellar haloes of nearby galaxies.

Description: We analyse the density profiles of the stellar halo populations in eight Milky Way mass galaxies, simulated within the cold dark matter scenario. We find that accreted stars can be well fitted by an Einasto profile, as well as any subsample defined according to metallicity. We detect a clear correlation between the Einasto fitting parameters of the low-metallicity stellar populations and those of the dark matter (DM) haloes. The correlations for stars with [Fe/H] 〈 –3 allow us to predict the shape of the dark matter profiles within residuals of ~10 per cent, in case the contribution from in situ stars remains small. Using Einasto parameters estimated for the stellar halo of the Milky Way and assuming the later formed with significant contributions from accreted low-mass satellite, our simulations predict α ~ 0.15 and r 2  ~ 15 kpc for its dark matter profile. These values, combined with observed estimations of the local DM density, yield an enclosed DM mass at ~8 kpc in the range 3.9–6.7 10 10 M , in agreement with recent observational results. These findings suggest that low-metallicity stellar haloes could store relevant information on the DM haloes. Forthcoming observations would help us to further constrain our models and predictions.

Description: We investigate the chemical and kinematic properties of the diffuse stellar haloes of six simulated Milky-Way-like galaxies from the Aquarius Project. Binding energy criteria are adopted to define two dynamically distinct stellar populations: the diffuse inner and outer haloes, which comprise different stellar subpopulations with particular chemical and kinematic characteristics. Our simulated inner- and outer-halo stellar populations have received contributions from debris stars (formed in subgalactic systems while they were outside the virial radius of the main progenitor galaxies) and endo-debris stars (those formed in gas-rich subgalactic systems inside the dark matter haloes of the main progenitor galaxy). The inner haloes possess an additional contribution from disc-heated stars, in the range ~3–30 per cent, with a mean of ~20 per cent. Disc-heated stars might exhibit signatures of kinematical support, in particular among the youngest ones. Endo-debris plus disc-heated stars define the so-called in situ stellar populations. In both the inner- and outer-halo stellar populations, we detect contributions from stars with moderate to low [α/Fe] ratios, mainly associated with the endo-debris or disc-heated subpopulations. The observed abundance gradients in the inner-halo regions are influenced by both the level of chemical enrichment and the relative contributions from each stellar subpopulation. Steeper abundance gradients in the inner-halo regions are related to contributions from the disc-heated and endo-debris stars, which tend to be found at lower binding energies than debris stars. In the case of the outer-halo regions, although [Fe/H] gradients are relatively mild, the steeper profiles arise primarily due to contributions from stars formed in more massive satellites, which sink farther into the main halo system, and tend to have higher levels of chemical enrichment and lower energies. Our findings support the existence of (at least) two distinct diffuse stellar halo populations, as suggested by a number of recent observations in the Milky Way and M31. Our results also indicate that a comparison of the range of predicted kinematics, abundance gradients and frequency of [α/Fe]-deficient stars with observations of these quantities in the Milky Way, M31 and other large spirals can both provide clues to improve the modelling of baryonic physics, and reveal detailed information about their likely history of formation and evolution.

Description: We study non-parametric morphologies of mergers events in a cosmological context, using the Illustris project. We produce mock g -band images comparable to observational surveys from the publicly available Illustris simulation idealized mock images at z = 0. We then measure non-parametric indicators: asymmetry, Gini, M 20 , clumpiness, and concentration for a set of galaxies with M * 〉 10 10 M . We correlate these automatic statistics with the recent merger history of galaxies and with the presence of close companions. Our main contribution is to assess in a cosmological framework, the empirically derived non-parametric demarcation line and average time-scales used to determine the merger rate observationally. We found that 98 per cent of galaxies above the demarcation line have a close companion or have experienced a recent merger event. On average, merger signatures obtained from the G – M 20 criterion anti-correlate clearly with the elapsing time to the last merger event. We also find that the asymmetry correlates with galaxy pair separation and relative velocity, exhibiting the larger enhancements for those systems with pair separations d 〈 50 h –1 kpc and relative velocities V 〈 350 km s –1 . We find that the G – M 20 is most sensitive to recent mergers (~0.14 Gyr) and to ongoing mergers with stellar mass ratios greater than 0.1. For this indicator, we compute a merger average observability time-scale of ~0.2 Gyr, in agreement with previous results and demonstrate that the morphologically derived merger rate recovers the intrinsic total merger rate of the simulation and the merger rate as a function of stellar mass.

Description: We present a set of hydrodynamical/ N -body controlled simulations of isolated gas-rich galaxies that self-consistently include supernova (SN) feedback and a detailed chemical evolution model, both tested in cosmological simulations. The initial conditions are motivated by the observed star-forming galaxies at z  ~ 2–3. We find that the presence of a multiphase interstellar media in our models promotes the growth of disc instability favouring the formation of clumps which, in general, are not easily disrupted on time-scales compared to the migration time. We show that stellar clumps migrate towards the central region and contribute to form a classical-like bulge with a Sérsic index, n  〉 2. Our physically motivated SN feedback has a mild influence on clump survival and evolution, partially limiting the mass growth of clumps as the energy released per SN event is increased, with the consequent flattening of the bulge profile. This regulation does not prevent the building of a classical-like bulge even for the most energetic feedback tested. Our SN feedback model is able to establish self-regulated star formation, producing mass-loaded outflows and stellar age spreads comparable to observations. We find that the bulge formation by clumps may coexist with other channels of bulge assembly such as bars and mergers. Our results suggest that galactic bulges could be interpreted as composite systems with structural components and stellar populations storing archaeological information of the dynamical history of their galaxy.