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Register a new userThe Halo Shape and Evolution of Polar Disc Galaxies
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We examine the properties and evolution of a simulated polar disc galaxy. This galaxy is comprised of two orthogonal discs, one of which contains old stars (old stellar disc), and the other, containing both younger stars and the cold gas (polar disc) of the galaxy. By exploring the shape of the inner region of the dark matter halo, we are able to confirm that the halo shape is a oblate ellipsoid flattened in the direction of the polar disc. We also note that there is a twist in the shape profile, where the innermost 3 kpc of the halo flattens in the direction perpendicular to the old disc, and then aligns with the polar disc out until the virial radius. This result is then compared to the halo shape inferred from the circular velocities of the two discs. We also use the temporal information of the simulation to track the systems evolution, and identify the processes which give rise to this unusual galaxy type. We confirm the proposal that the polar disc galaxy is the result of the last major merger, where the angular moment of the interaction is orthogonal to the angle of the infalling gas. This merger is followed by the resumption of coherent gas infall. We emphasise that the disc is rapidly restored after the major merger and that after this event the galaxy begins to tilt. A significant proportion of the infalling gas comes from filaments. This infalling gas from the filament gives the gas its angular momentum, and, in the case of the polar disc galaxy, the direction of the gas filament does not change before or after the last major merger.
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We present a modified version of the L-GALAXIES 2020 semi-analytic model of galaxy evolution, which includes significantly increased direct metal enrichment of the circumgalactic medium (CGM) by supernovae (SNe). These more metal-rich outflows do not require increased mass-loading factors, in contrast to some other galaxy evolution models. This modified L-GALAXIES 2020 model is able to simultaneously reproduce the gas-phase metallicity $(Z_{rm g})$ and stellar metallicity $(Z_{*})$ radial profiles observed in nearby disc galaxies by MaNGA and MUSE, as well as the observed mass - metallicity relations for gas and stars at $z=0$ and their evolution back to $zsim{}2-3$. A direct CGM enrichment fraction of $sim{}90%$ for SNe-II is preferred. We find that massive disc galaxies have slightly flatter $Z_{rm g}$ profiles than their lower-mass counterparts in L-GALAXIES 2020, due to more efficient enrichment of their outskirts via inside-out growth and metal-rich accretion. Such a weak, positive correlation between stellar mass and $Z_{rm g}$ profile slope is also seen in our MaNGA-DR15 sample of 571 star-forming disc galaxies. Although, below ${rm log}(M_{*}/{rm M}_{odot})sim{}10.0$ this observational result is strongly dependent on the metallicity diagnostic and morphological selection chosen. In addition, a lowered maximum SN-II progenitor mass of $25{rm M}_{odot}$, reflecting recent theoretical and observational estimates, can also provide a good match to observed metallicity profiles at $z=0$ in L-GALAXIES 2020. However, this model version fails to reproduce an evolution in $Z_{rm g}$ at fixed mass over cosmic time, or the magnesium abundances observed in the intracluster medium (ICM).
Minor accretion events with mass ratio M_sat : M_host ~ 1:10 are common in the context of LCDM cosmology. We use high-resolution simulations of Galaxy-analogue systems to show that these mergers can dynamically eject disk stars into a diffuse light component that resembles a stellar halo both spatially and kinematically. For a variety of orbital configurations, we find that ~3-5e8 M_sun of primary stellar disk material is ejected to a distance larger than 5 kpc above the galactic plane. This ejected contribution is similar to the mass contributed by the tidal disruption of the satellite galaxy itself, though it is less extended. If we restrict our analysis to the approximate solar neighborhood in the disk plane, we find that ~1% of the initial disk stars in that region would be classified kinematically as halo stars. Our results suggest that the inner parts of galactic stellar halos contain ancient disk stars and that these stars may have been liberated in the very same events that delivered material to the outer stellar halo.
We study the evolution of disc galaxies in group environments under the effect of both the global tidal field and close-encounters between galaxies, using controlled N-body simulations of isolated mergers. We find that close-range encounters between galaxies are less frequent and less damaging to disc galaxies than originally expected, since they mostly occur when group members have lost a significant fraction of their initial mass to tidal stripping. We also find that group members mostly affect disc galaxies indirectly by modifying their common global tidal field. Different initial orbital parameters of group members introduce a significant scatter in the evolution of general properties of disc galaxies around a median evolution that is similar to when only the effect of the global tidal field is included. Close-encounters introduce a high variability in the properties of disc galaxies, even slowing their evolution in some cases, and could wash out correlations between galaxy properties and the group total mass. The combined effect of the global tidal field and close-encounters appears to be inefficient at forming/enhancing central stellar bulges. This implies that bulges of S0 galaxies should be mostly composed by young stars, which is consistent with recent observations.
Context. Galactic structure studies can be used as a path to constrain the scenario of formation and evolution of our Galaxy. The dependence with the age of stellar population parameters would be linked with the history of star formation and dynamical evolution. Aims. We aim to investigate the structures of the outer Galaxy, such as the scale length, disc truncation, warp and flare of the thin disc and study their dependence with age by using 2MASS data and a population synthesis model (the so-called Besanc{c}on Galaxy Model). Methods. We have used a genetic algorithm to adjust the parameters on the observed colour-magnitude diagrams at longitudes 80 deg <= l <= 280 deg for |b| <= 5.5 deg. We explored parameter degeneracies and uncertainties. Results. We identify a clear dependence of the thin disc scale length, warp and flare shapes with age. The scale length is found to vary between 3.8 kpc for the youngest to about 2 kpc for the oldest. The warp shows a complex structure, clearly asymmetrical with a node angle changing with age from approximately 165 deg for old stars to 195 deg for young stars. The outer disc is also flaring with a scale height that varies by a factor of two between the solar neighbourhood and a Galactocentric distance of 12 kpc. Conclusions. We conclude that the thin disc scale length is in good agreement with the inside-out formation scenario and that the outer disc is not in dynamical equilibrium. The warp deformation with time may provide some clues to its origin.
Galaxy scaling laws, such as the Tully-Fisher, mass-size and Fall relations, can provide extremely useful clues on our understanding of galaxy formation in a cosmological context. Some of these relations are extremely tight and well described by one single parameter (mass), despite the theoretical existence of secondary parameters such as spin and concentration, which are believed to impact these relations. In fact, the residuals of these scaling laws appear to be almost uncorrelated with each other, posing significant constraints on models where secondary parameters play an important role. Here, we show that a possible solution is that such secondary parameters are correlated amongst themselves, in a way that removes correlations in observable space. In particular, we focus on how the existence of an anti-correlation between the dark matter halo spin and its concentration -- which is still debated in simulations -- can weaken the correlation of the residuals of the Tully-Fisher and mass-size relations. Interestingly, using simple analytic galaxy formation models, we find that this happens only for a relatively small portion of the parameter space that we explored, which suggests that this idea could be used to derive constraints to galaxy formation models that are still unexplored.
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