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Register a new userTracing the Dynamical Mass in Galaxy Disks Using HI Velocity Dispersion and its Implications for the Dark Matter Distribution in Galaxies
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We present a method to derive the dynamical mass of face-on galaxy disks using their neutral hydrogen (HI) velocity dispersion. We have applied the method to nearby, gas rich galaxies that have extended HI gas disks and have low inclinations. The galaxy sample includes 4 large disk galaxies; NGC628, NGC6496, NGC3184, NGC4214 and 3 dwarf galaxies DDO46, DDO63 and DDO187. We have used archival data from the THINGS and LITTLE THINGS surveys to derive the HI gas distributions and SPITZER mid-infrared images to determine the stellar disk mass distributions. We examine the disk dynamical and baryonic mass ratios in the extreme outer disks where there is HI gas but no visible stellar disk. We find that for the large galaxies the disk dynamical and Hi gas mass surface densities are comparable in the outer disks. But in the smaller dwarf galaxies, for which the total HI gas mass dominates the baryonic mass i.e. M(HI)>M(stars), the disk dynamical mass is much larger than the baryonic mass. For these galaxies there must either be a very low luminosity stellar disk which provides the vertical support for the HI gas disk or there is halo dark matter associated with their disks, which is possible if the halo has an oblate shape so that the inner part of the dark matter halo is concentrated around the disk. Our results are important for explaining the equilibrium of HI disks in the absence of stellar disks, and is especially important for gas rich, dwarf galaxies that appear to have significant dark matter masses associated with their disks.
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The local velocity distribution of dark matter plays an integral role in interpreting the results from direct detection experiments. We previously showed that metal-poor halo stars serve as excellent tracers of the virialized dark matter velocity distribution using a high-resolution hydrodynamic simulation of a Milky Way--like halo. In this paper, we take advantage of the first textit{Gaia} data release, coupled with spectroscopic measurements from the RAdial Velocity Experiment (RAVE), to study the kinematics of stars belonging to the metal-poor halo within an average distance of $sim 5$ kpc of the Sun. We study stars with iron abundances [Fe/H]$ < -1.5$ and $-1.8$ that are located more than $1.5$ kpc from the Galactic plane. Using a Gaussian mixture model analysis, we identify the stars that belong to the halo population, as well as some kinematic outliers. We find that both metallicity samples have similar velocity distributions for the halo component, within uncertainties. Assuming that the stellar halo velocities adequately trace the virialized dark matter, we study the implications for direct detection experiments. The Standard Halo Model, which is typically assumed for dark matter, is discrepant with the empirical distribution by $sim6sigma$ and predicts fewer high-speed particles. As a result, the Standard Halo Model overpredicts the nuclear scattering rate for dark matter masses below $sim 10$ GeV. The kinematic outliers that we identify may potentially be correlated with dark matter substructure, though further study is needed to establish this correspondence.
We present the velocity dispersion and dynamical mass estimates for 270 galaxy clusters included in the first Planck Sunyaev-Zeldovich (SZ) source catalogue, the PSZ1. Part of the results presented here were achieved during a two-year observational program, the ITP, developed at the Roque de los Muchachos Observatory (La Palma, Spain). In the ITP we carried out a systematic optical follow-up campaign of all the 212 unidentified PSZ1 sources in the northern sky that have a declination above $-15^circ$ and are without known counterparts at the time of the publication of the catalogue. We present for the first time the velocity dispersion and dynamical mass of 58 of these ITP PSZ1 clusters, plus 35 newly discovered clusters that are not associated with the PSZ1 catalogue. Using Sloan Digital Sky Survey (SDSS) archival data, we extend this sample, including 212 already confirmed PSZ1 clusters in the northern sky. Using a subset of 207 of these galaxy clusters, we constrained the $M_{rm SZ}$--$M_{rm dyn}$ scaling relation, finding a mass bias of $(1-B) = 0.83pm0.07$(stat)$pm0.02$(sys). We show that this value is consistent with other results in the literature that were obtained with different methods (X-ray, dynamical masses, or weak-lensing mass proxies). This result cannot dissolve the tension between primordial cosmic microwave background anisotropies and cluster number counts in the $Omega_{rm M}$--$sigma_8$ plane.
We use velocity dispersion measurements of 21 individual cluster members in the core of Abell 383, obtained with MMT Hectospec, to separate the galaxy and the smooth dark halo (DH) lensing contributions. While lensing usually constrains the overall, projected mass density, the innovative use of velocity dispersion measurements as a proxy for masses of individual cluster members breaks inherent degeneracies and allows us to (a) refine the constraints on single galaxy masses and on the galaxy mass-to-light scaling relation and, as a result, (b) refine the constraints on the DM-only map, a high-end goal of lens modelling. The knowledge of cluster member velocity dispersions improves the fit by 17% in terms of the image reproduction $chi^2$, or 20% in terms of the rms. The constraints on the mass parameters improve by ~10% for the DH, while for the galaxy component, they are refined correspondingly by ~50%, including the galaxy halo truncation radius. For an L$^*$ galaxy with M$^*_B$=-20.96, for example, we obtain best fitting truncation radius r$^*_{tr}=20.5^{+9.6}_{-6.7}$ kpc and velocity dispersion $sigma^*=324pm17 km/s$. Moreover, by performing the surface brightness reconstruction of the southern giant arc, we improve the constraints on r$_{tr}$ of two nearby cluster members, which have measured velocity dispersions, by more than ~30%. We estimate the stripped mass for these two galaxies, getting results that are consistent with numerical simulations. In the future, we plan to apply this analysis to other galaxy clusters for which velocity dispersions of member galaxies are available.
We report a tight linear relation between the HI circular velocity measured at 6 $R_{rm e}$ and the stellar velocity dispersion measured within 1 $R_{rm e}$ for a sample of 16 early-type galaxies with stellar mass between $10^{10}$ and $10^{11}$ $mathrm{M}_odot$. The key difference from previous studies is that we only use spatially resolved $v_mathrm{circ}$(HI) measurements obtained at large radius for a sizeable sample of objects. We can therefore link a kinematical tracer of the gravitational potential in the dark-matter dominated outer regions of galaxies with one in the inner regions, where baryons control the distribution of mass. We find that $v_mathrm{circ}$(HI) = 1.33 $sigma_mathrm{e}$ with an observed scatter of just 12 percent. This indicates a strong coupling between luminous and dark matter from the inner- to the outer regions of early-type galaxies, analogous to the situation in spirals and dwarf irregulars. The $v_mathrm{circ}$(HI)-$sigma_mathrm{e}$ relation is shallower than those based on $v_mathrm{circ}$ measurements obtained from stellar kinematics and modelling at smaller radius, implying that vcirc declines with radius -- as in bulge-dominated spirals. Indeed, the value of $v_mathrm{circ}$(HI) is typically 25 percent lower than the maximum $v_mathrm{circ}$ derived at $sim0.2 R_mathrm{e}$ from dynamical models. Under the assumption of power-law total density profiles $rho propto r^{-gamma}$, our data imply an average logarithmic slope $langlegammarangle=2.18pm0.03$ across the sample, with a scatter of 0.11 around this value. The average slope and scatter agree with recent results obtained from stellar kinematics alone for a different sample of early-type galaxies.
Our GMRT HI observations of the ultra diffuse galaxy (UDG) UGC 2162, projected $sim$ 300 kpc from the centre of the M77 group, reveal it to a have an extended HI disk (R$_{HI}$/R$_{25}$ $sim$ 3.3) with a moderate rotational velocity (V$_{rot} sim$ 31 km/s). This V$_{rot}$ is in line with that of dwarf galaxies with similar HI mass. We estimate an M$_{dyn}$ of $sim$ 1.14 $times$ 10$^{9}$ M$_odot$ within the galaxys R$_{HI}$ $sim$ 5.2 kpc. Additionally, our estimates of M$_{200}$ for the galaxy from NFW models are in the range of 5.0 to 8.8 $times$ 10$^{10}$ M$_odot$. Comparing UGC 2162 to samples of UDGs with HI detections show it to have amongst the smallest R$_e$ with its M$_{HI}$/M$_{star}$ being distinctly higher and g -- i colour slightly bluer than typical values in those samples. We also compared HI and dark matter (DM) halo properties of UGC 2162 with dwarf galaxies in the LITTLE THINGS sample and find its DM halo mass and profile are within the range expected for a dwarf galaxy. While we were unable to to determine the origin of the galaxys present day optical form from our study, its normal HI rotation velocity in relation to its HI mass, HI morphology, environment and dwarf mass DM halo ruled out some of the proposed ultra diffuse galaxy formation scenarios for this galaxy.
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