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Register a new userLinear relation between HI circular velocity and stellar velocity dispersion in early-type galaxies, and slope of the density profiles
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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.
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We analyzed a sample of high and low surface brightness (HSB and LSB) disc galaxies and elliptical galaxies to investigate the correlation between the circular velocity (Vc) and the central velocity dispersion (sigma). We better defined the previous Vc-sigma correlation for HSB and elliptical galaxies, especially at the lower end of the sigma values. Elliptical galaxies with Vc based on dynamical models or directly derived from the HI rotation curves follow the same relation as the HSB galaxies in the V-sigma plane. On the contrary, the LSB galaxies follow a different relation, since most of them show either higher Vc (or lower sigma) with respect to the HSB galaxies. This argues against the relevance of baryon collapse in the radial density profile of the dark matter haloes of LSB galaxies. Moreover, if the Vc-sigma relation is equivalent to one between the mass of the dark matter halo and that of the supermassive black hole, these results suggest that the LSB galaxies host a supermassive black hole with a smaller mass compared to HSB galaxies of equal dark matter halo. On the other hand, if the fundamental correlation of SMBH mass is with the halo Vc, then LSBs should have larger black hole masses for given bulge sigma.
In order to investigate the correlation between the circular velocity Vc and the central velocity dispersion of the spheroidal component sigma_c, we analyzed these quantities for a sample of 40 high surface brightness disc galaxies (hereafter HSB), 8 giant low surface brightness spiral galaxies (hereafter LSB), and 24 elliptical galaxies characterized by flat rotation curves. We find that the Vc-sigma_c relation is descri ed by a linear law out to velocity dispersions as low as sigma_c~50km/s, while in previous works a power law was adopted for galaxies with sigma_c>80k/ms. Elliptical galaxies with Vc based on dynamical models or directly derived from the HI rotation curves follow the same relation as the HSB galaxies in the Vc-sigma_c plane. On the contrary, the LSB galaxies follow a different relation, since most of them show either higher Vc (or lower sigma_c) with respect to the HSB galaxies. This argues against the relevance of baryon collapse in the radial density profile of the dark matter haloes of LSB galaxies. (abridged)
Based on our recent work on tidal tails of star clusters (Kuepper et al. 2009) we investigate star clusters of a few 10^4 Msun by means of velocity dispersion profiles and surface density profiles. We use a comprehensive set of $N$-body computations of star clusters on various orbits within a realistic tidal field to study the evolution of these profiles with time, and ongoing cluster dissolution From the velocity dispersion profiles we find that the population of potential escapers, i.e. energetically unbound stars inside the Jacobi radius, dominates clusters at radii above about 50% of the Jacobi radius. Beyond 70% of the Jacobi radius nearly all stars are energetically unbound. The velocity dispersion therefore significantly deviates from the predictions of simple equilibrium models in this regime. We furthermore argue that for this reason this part of a cluster cannot be used to detect a dark matter halo or deviations from Newtonian gravity. By fitting templates to the about 10^4 computed surface density profiles we estimate the accuracy which can be achieved in reconstructing the Jacobi radius of a cluster in this way. We find that the template of King (1962) works well for extended clusters on nearly circular orbits, but shows significant flaws in the case of eccentric cluster orbits. This we fix by extending this template with 3 more free parameters. Our template can reconstruct the tidal radius over all fitted ranges with an accuracy of about 10%, and is especially useful in the case of cluster data with a wide radial coverage and for clusters showing significant extra-tidal stellar populations. No other template that we have tried can yield comparable results over this range of cluster conditions. All templates fail to reconstruct tidal parameters of concentrated clusters, however. (abridged)
The distribution of early-type galaxy velocity dispersions, phi(sigma), is measured using a sample drawn from the SDSS database. Its shape differs significantly from that which one obtains by simply using the mean correlation between luminosity, L, and velocity dispersion, sigma, to transform the luminosity function into a velocity function: ignoring the scatter around the mean sigma-L relation is a bad approximation. An estimate of the contribution from late-type galaxies is also made, which suggests that phi(sigma) is dominated by early-type galaxies at velocities larger than ~ 200 km/s.
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.
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