Scaling Laws for Dark Matter Halos in Late-Type and Dwarf Spheroidal Galaxies
John Kormendy, K. C. Freeman
备注：
6 pages, 4 figures, requires iau.cls; to be published in IAU Symposium 311, Galaxy Masses as Constraints of Formation Models, ed. M. Cappellari & S. Courteau, Cambridge Univ. Press, in press (2014)
Dark matter (DM) halos of Sc-Im galaxies satisfy scaling laws analogous to the fundamental plane relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii, higher central densities, and smaller central velocity dispersions. If dwarf spheroidal (dSph) and dwarf Magellanic irregular (dIm) galaxies lie on the extrapolations of these correlations, then we can estimate their baryon loss relative to that of brighter Sc-Im galaxies. We find that, if there had been no such enhanced baryon loss, then typical dSph and dIm galaxies would be brighter in absolute magnitude by 4 and 3.5 mag, respectively. Instead, these galaxies lost or retained as gas (in dIm galaxies) baryons that could have formed stars. Also, typical dSph and dIm galaxies have DM halos that are more massive than we thought, with velocity dispersions of about 30 km/s or circular-orbit rotation velocities of V_circ ~ 42 km/s. Comparison of DM and visible matter correlations confirms that, at V-band absolute magnitudes fainter than -18, dSph and dIm galaxies form a sequence of decreasing baryon-to-DM mass ratios in smaller dwarfs. We show explicitly that galaxy baryon content goes to (almost) zero at halo V_circ = 42 +- 4 km/s, in agreement with what we found from our estimate of baryon depletion. Our results suggest that there may be a large population of DM halos that are essentially dark and undiscovered. This helps to solve the problem that the fluctuation spectrum of cold DM predicts more dwarfs than we observe.
Dark matter (DM) halos of Sc-Im and dwarf spheroidal (dSph) galaxies satisfy scaling laws: halos in lower-luminosity galaxies have smaller core radii, higher central densities, and smaller velocity dispersions. These results are based on maximum-disk rotation curve decompositions for giant galaxies and Jeans equation analysis for dwarfs. (1) We show that spiral, Im, and Sph galaxies with absolute magnitudes M_V > -18 form a sequence of decreasing baryon-to-DM surface density with decreasing luminosity. We suggest that this is a sequence of decreasing baryon retention vs. supernova-driven losse…
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