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Register a new userSimulating the evolution of disc galaxies in a group environment. I. The influence of the global tidal field
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\\'Alvaro Villalobos
Publication date
2012
fields
Physics
and research's language is
English
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We present the results of a series of numerical simulations aimed to study the evolution of a disc galaxy within the global tidal field of a group environment. Both the disc galaxy and the group are modelled as multi-component, collision-less, N-body systems, composed by both dark matter and stars. In our simulations, the evolution of disc galaxies is followed as their orbits sink towards the group centre, under the effect of dynamical friction. We explore a broad parameter space, covering several aspects of the galaxy-group interaction that are potentially relevant to galaxy evolution. Namely, prograde and retrograde orbits, orbital eccentricities, disc inclination, role of a central bulge in discs, internal disc kinematics, and galaxy-to-group mass ratios. We find that significant disc transformations occur only after the mean density of the group, measured within the orbit of the galaxy, exceeds ~0.3-1 times the central mean density of the galaxy. The morphological evolution of discs is found to be strongly dependent on the initial inclination of the disc with respect to its orbital plane. That is, discs on face-on and retrograde orbits are shown to retain longer their disc structures and kinematics, in comparison to prograde discs. This suggests that after interacting with the global tidal field alone, a significant fraction of disc galaxies should be found in the central regions of groups. Prominent central bulges are not produced, and pre-existing bulges are not enhanced in discs after the interaction with the group. Assuming that most S0 are formed in group environments, this implies that prominent bulges should be formed mostly by young stars, created only after a galaxy has been accreted by a group. Finally, contrary to some current implementations of tidal stripping in semi-analytical models of galaxy evolution, we find that more massive galaxies suffer more tidal stripping.
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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.
The formation mechanism of tidal dwarf galaxies means they are expected to contain little or no dark matter. As such, they might be expected to be very sensitive to their environment. We investigate the impact of ram pressure on tidal dwarf galaxies in a parameter study, varying dwarf galaxy properties and ram pressures. We submit model tidal dwarf galaxies to wind-tunnel style tests using a toy ram pressure model. The effects of ram pressure are found to be substantial. If tidal dwarf galaxies have their gas stripped, they may be completely destroyed. Ram pressure drag causes acceleration of our dwarf galaxy models, and this further enhances stellar losses. The dragging can also cause stars to lie in a low surface brightness stellar stream that points in the opposite direction to the stripped gas, in a manner distinctive from tidal streams. We investigate the effects of ram pressure on surface density profiles, the dynamics of the stars, and discuss the consequences for dynamical mass measurements.
The mean alpha-to-iron abundance ratio ([$alpha$/Fe]) of galaxies is sensitive to the chemical evolution processes at early time, and it is an indicator of star formation timescale ($tau_{{rm SF}}$). Although the physical reason remains ambiguous, there is a tight relation between [$alpha$/Fe] and stellar velocity dispersion ($sigma$) among massive early-type galaxies (ETGs). However, no work has shown convincing results as to how this relation behaves at low masses. We assemble 15 data sets from the literature and build a large sample that includes 192 nearby low-mass ($18<sigma<80$~kms) ETGs. We find that the [$alpha$/Fe]-$sigma$ relation generally holds for low-mass ETGs, except in extreme environments. Specifically, in normal galaxy cluster environments, the [$alpha$/Fe]-$sigma$ relation and its intrinsic scatter are, within uncertainties, similar for low-mass and high-mass ETGs. However, in the most massive relaxed galaxy cluster in our sample, the zero point of the relation is higher and the intrinsic scatter is significantly larger. By contrast, in galaxy groups the zero point of the relation offsets in the opposite direction, again with substantial intrinsic scatter. The elevated [$alpha$/Fe] of low-mass ETGs in the densest environments suggests that their star formation was quenched earlier than in high-mass ETGs. For the low-mass ETGs in the lowest density environments, we suggest that their more extended star formation histories suppressed their average [$alpha$/Fe]. The large scatter in [$alpha$/Fe] may reflect stochasticity in the chemical evolution of low-mass galaxies.
We present a sample of dwarf galaxies that suffer ongoing disruption by the tidal force of nearby massive galaxies. Analysing structural and stellar population properties using the archival imaging and spectroscopic data from the Sloan Digital Sky Survey (SDSS), we find that they are likely a `smoking gun example of the formation of early-type dwarf galaxies (dEs) in the galaxy group environment through the tidal stirring. Inner cores of these galaxies are fairly intact and the observed light profiles are well fitted with the Sersic functions, while the tidally stretched stellar halos are prominent in the outer parts. They are all located within the 50 kpc sky-projected distance from the center of host galaxies and no dwarf galaxies have relative line-of-sight velocity larger than 205 km/s to their hosts. We derive the Composite Stellar Population (CSP) properties these galaxies by fitting the SDSS optical spectra to a multiple-burst composite stellar population model. We find that these galaxies accumulate a significant fraction of stellar mass within the last 1 Gyr, while they contain a majority stellar population of intermediate age of 2 to 4 Gyr. With these evidences, we argue that tidal stirring, particularly through the galaxy-galaxy interaction, might have an important role in the formation and evolution of dEs in the group environment, where the influence of other gas stripping mechanism might be limited.
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).
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