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Register a new userExplaining the observed velocity dispersion of dwarf galaxies by baryonic mass loss during the first collapse
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Matthias Gritschneder Dr
Publication date
2013
fields
Physics
and research's language is
English
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In the widely adopted LambdaCDM scenario for galaxy formation, dwarf galaxies are the building blocks of larger galaxies. Since they formed at relatively early epochs when the background density was relatively high, they are expected to retain their integrity as satellite galaxies when they merge to form larger entities. Although many dwarf spheroidal galaxies (dSphs) are found in the galactic halo around the Milky Way, their phase space density (or velocity dispersion) appears to be significantly smaller than that expected for satellite dwarf galaxies in the LambdaCDM scenario. In order to account for this discrepancy, we consider the possibility that they may have lost a significant fraction of their baryonic matter content during the first infall at the Hubble expansion turnaround. Such mass loss arises naturally due to the feedback by relatively massive stars which formed in their centers briefly before the maximum contraction. Through a series of N-body simulations, we show that the timely loss of a significant fraction of the dSphs initial baryonic matter content can have profound effects on their asymptotic half-mass radius, velocity dispersion, phase-space density, and the mass fraction between residual baryonic and dark matter.
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We present the study of a large sample of early-type dwarf galaxies in the Coma cluster observed with DEIMOS on the Keck II to determine their internal velocity dispersion. We focus on a subsample of 41 member dwarf elliptical galaxies for which the velocity dispersion can be reliably measured, 26 of which were studied for the first time. The magnitude range of our sample is $-21<M_R<-15$ mag. This paper (paper I) focuses on the measurement of the velocity dispersion and their error estimates. The measurements were performed using {it pPXF (penalised PiXel Fitting)} and using the Calcium triplet absorption lines. We use Monte Carlo bootstrapping to study various sources of uncertainty in our measurements, namely statistical uncertainty, template mismatch and other systematics. We find that the main source of uncertainty is the template mismatch effect which is reduced by using templates with a range of spectral types. Combining our measurements with those from the literature, we study the Faber-Jackson relation ($Lproptosigma^alpha$) and find that the slope of the relation is $alpha=1.99pm0.14$ for galaxies brighter than $M_Rsimeq-16$ mag. A comprehensive analysis of the results combined with the photometric properties of these galaxies is reported in paper II.
We perform a systematic Bayesian analysis of rotation vs. dispersion support ($v_{rm rot} / sigma$) in $40$ dwarf galaxies throughout the Local Volume (LV) over a stellar mass range $10^{3.5} M_{rm odot} < M_{star} < 10^8 M_{rm odot}$. We find that the stars in $sim 80%$ of the LV dwarf galaxies studied -- both satellites and isolated systems -- are dispersion-supported. In particular, we show that $6/10$ *isolated* dwarfs in our sample have $v_{rm rot} / sigma < 1.0$. All have $v_{rm rot} / sigma lesssim 2.0$. These results challenge the traditional view that the stars in gas-rich dwarf irregulars (dIrrs) are distributed in cold, rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs) are kinematically distinct in having dispersion-supported stars. We see no clear trend between $v_{rm rot} / sigma$ and distance to the closest $rm L_{star}$ galaxy, nor between $v_{rm rot} / sigma$ and $M_{star}$ within our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic zoom-in simulations of isolated dwarf galaxies ($10^9 M_{odot} < M_{rm vir} < 10^{10} M_{rm odot}$) and show that the simulated *isolated* dIrr galaxies have stellar ellipticities and stellar $v_{rm rot} / sigma$ ratios that are consistent with the observed population of dIrrs *and* dSphs without the need to subject these dwarfs to any external perturbations or tidal forces. We posit that most dwarf galaxies form as puffy, dispersion-dominated systems, rather than cold, angular momentum-supported disks. If this is the case, then transforming a dIrr into a dSph may require little more than removing its gas.
We study the structure of spatially resolved, line-of-sight velocity dispersion for galaxies in the Epoch of Reionization (EoR) traced by [CII] $158murm{m}$ line emission. Our laboratory is a simulated prototypical Lyman-break galaxy, Freesia, part of the SERRA suite. The analysis encompasses the redshift range 6 < z < 8, when Freesia is in a very active assembling phase. We build velocity dispersion maps for three dynamically distinct evolutionary stages (Spiral Disk at z=7.4, Merger at z=8.0, and Disturbed Disk at z=6.5) using [CII] hyperspectral data cubes. We find that, at a high spatial resolution of 0.005 ($simeq 30 pc$), the luminosity-weighted average velocity dispersion is $sigma_{rm{CII}}$~23-38 km/s with the highest value belonging to the highly-structured Disturbed Disk stage. Low resolution observations tend to overestimate $sigma_{rm CII}$ values due to beam smearing effects that depend on the specific galaxy structure. For an angular resolution of 0.02 (0.1), the average velocity dispersion is 16-34% (52-115%) larger than the actual one. The [CII] emitting gas in Freesia has a Toomre parameter $mathcal{Q}$~0.2 and a rotational-to-dispersion ratio of $v_{rm c}/sigma$~ 7 similar to that observed in z=2-3 galaxies. The primary energy source for the velocity dispersion is due to gravitational processes, such as merging/accretion events; energy input from stellar feedback is generally subdominant (< 10%). Finally, we find that the resolved $sigma_{rm{CII}} - {Sigma}_{rm SFR}$ relation is relatively flat for $0.02<{Sigma}_{rm SFR}/{{rm M}_{odot}} mathrm{yr}^{-1} {mathrm kpc}^{-2} < 30$, with the majority of data lying on the derived analytical relation $sigma propto Sigma_{rm SFR}^{5/7}$. At high SFR, the increased contribution from stellar feedback steepens the relation, and $sigma_{rm{CII}}$ rises slightly.
Cold Dark Matter (CDM) simulations predict a central cusp in the mass distribution of galaxies. This prediction is in stark contrast with observations of dwarf galaxies which show a central plateau or core in their density distribution. The proposed solutions to this core-cusp problem can be classified into two types. Either they invoke feedback mechanisms produced by the baryonic component of the galaxies, or they assume the properties of the dark matter (DM) particle to depart from the CDM hypothesis. Here we propose an alternative yet complementary explanation. We argue that cores are unavoidable in the self-gravitating systems of maximum entropy resulting from non-extensive statistical mechanics. Their structure follows from the Tsallis entropy, suitable for systems with long-range interactions. Strikingly, the mass density profiles predicted by such thermodynamic equilibrium match the observed cores without any adjustment or tuning. Thus, the principle of maximum Tsallis entropy explains the presence of cores in dwarf galaxies.
Using the DIANOGA hydrodynamical zoom-in simulation set of galaxy clusters, we analyze the dynamics traced by stars belonging to the Brightest Cluster Galaxies (BCGs) and their surrounding diffuse component, forming the intracluster light (ICL), and compare it to the dynamics traced by dark matter and galaxies identified in the simulations. We compute scaling relations between the BCG and cluster velocity dispersions and their corresponding masses (i.e. $M_mathrm{BCG}^{star}$- $sigma_mathrm{BCG}^{star}$, $M_{200}$- $sigma_{200}$, $M_mathrm{BCG}^{star}$- $M_{200}$, $sigma_mathrm{BCG}^{star}$- $sigma_{200}$), we find in general a good agreement with observational results. Our simulations also predict $sigma_mathrm{BCG}^{star}$- $sigma_{200}$ relation to not change significantly up to redshift $z=1$, in line with a relatively slow accretion of the BCG stellar mass at late times. We analyze the main features of the velocity dispersion profiles, as traced by stars, dark matter, and galaxies. As a result, we discuss that observed stellar velocity dispersion profiles in the inner cluster regions are in excellent agreement with simulations. We also report that the slopes of the BCG velocity dispersion profile from simulations agree with what is measured in observations, confirming the existence of a robust correlation between the stellar velocity dispersion slope and the cluster velocity dispersion (thus, cluster mass) when the former is computed within $0.1 R_{500}$. Our results demonstrate that simulations can correctly describe the dynamics of BCGs and their surrounding stellar envelope, as determined by the past star-formation and assembly histories of the most massive galaxies of the Universe.
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