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Register a new userOn the relation between circular velocity and central velocity dispersion in high and low surface brightness galaxies
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Added by
Alessandro PIzzella
Publication date
2005
fields
Physics
and research's language is
English
Authors
A. Pizzella
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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)
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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.
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.
We investigate the relation between the asymptotic circular velocity, V_c, and the central stellar velocity dispersion, sigma_c, in galaxies. We consider a new sample of high surface brightness spiral galaxies (HSB), low surface brightness spiral galaxies (LSB), and elliptical galaxies with HI-based V_c measurements. We find that: 1) elliptical galaxies with HI measurements fit well within the relation; 2) a linear law can reproduce the data as well as a power law (used in previous works) even for galaxies with sigma_c < 70 km/s; 3) LSB galaxies, considered for the first time with this respect, seem to behave differently, showing either larger V_c values or smaller sigma_c values.
We present Halpha rotation curves for a sample of 15 dwarf and LSB galaxies. From these, we derive limits on the slopes of the central mass distributions. Assuming the density distributions of dark matter halos follow a power-law at small radii, rho(r)~r^(-alpha), we find inner slopes in the range 0<alpha<1 for most galaxies. In general, halos with constant density cores (alpha=0) provide somewhat better fits, but the majority of our galaxies (~75%) are also consistent with alpha=1, provided that the R-band mass-to-light ratios are smaller than about 2. Halos with alpha=1.5, however, are ruled out in virtually every case. To investigate the robustness of these results we discuss and model several possible causes of systematic errors including non-circular motions, slit width, seeing, and slit alignment errors. Taking the associated uncertainties into account, we conclude that even for the 25% of the cases where alpha=1 seems inconsistent with the rotation curves, we cannot rule out cusp slopes this steep. Inclusion of literature samples similar to the one presented here leads to the same conclusion when possible systematic errors are taken into account. In the ongoing debate on whether the rotation curves of dwarf and LSB galaxies are consistent with predictions for a CDM universe, we argue that our sample and the literature samples discussed in this paper provide insufficient evidence to rule out halos with alpha=1. At the same time, we note that none of the galaxies in these samples require halos with steep cusps, as most are equally well or better explained by constant density cores. (abridged)
We present an updated investigation of the relation between large scale disk circular velocity, v_c, and bulge velocity dispersion, sigma_c. New bulge velocity dispersions are measured for a sample of 11 low surface brightness (LSB) and 7 high surface brightness (HSB) spiral galaxies for which v_c is known from published optical or HI rotation curves. We find that, while LSB galaxies appear to define the upper envelope of the region occupied by HSB galaxies (having relatively larger v_c for any given sigma_c), the distinction between LSB and HSB galaxies in the v_c-sigma_c plane becomes less pronounced for sigma_c <= 80 km/s. We conclude that either the scatter of the v_c-sigma_c relation is a function of v_c (and hence galaxy mass) or that the character of the v_c-sigma_c relation changes at v_c ~ 80 km/s. Some inplications of our findings are discussed.
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