Speaker
Description
I will summarize recent results from the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. To date, SPARC is the largest collection of galaxies with both high-quality HI rotation curves and NIR surface photometry. I will describe two classic methods to probe the connection between galaxies and dark matter (DM) halos: (I) fitting rotation curves with a prescribed DM halo profile, and (II) studying empirical relations between dynamic and photometric quantities, without a-priori assumptions on DM halos.
The first approach leads to the following results: (1) cored DM profiles provide better fits than cuspy DM profiles, (2) the best-fit parameters from cored profiles naturally recover LCDM scaling relations (like the $M_{\rm star}-M_{\rm halo}$ relation) whereas cuspy profiles do not, and (3) the characteristic volume density of DM halos seem to be remarkably constant over 5 dex in luminosity.
The second approach enables us to identify and characterize three empirical laws of galactic rotation: (1) at large radii the mean velocity along the flat part of the rotation curve correlates with the baryonic mass with a slope close to 4 (baryonic Tully-Fisher relation); (2) at small radii the dynamical mass surface density from the rising part of the rotation curve correlates with the baryonic surface density (central density relation); and (3) at any radii the observed centripetal acceleration correlates with the expected baryonic acceleration (radial acceleration relation).
These relations are remarkably tight by astronomical standards, being consistent with no intrinsic scatter beyond the observational errors. They imply that the amount and distribution of baryons fully specify the amount and distribution of DM, and vice versa. Therefore, whilst the first approach is useful to link galaxy dynamics to cosmological expectations, it is effectively over-constraining the observational problem: in general there is no freedom in fitting arbitrary DM halos to HI rotation curves.
| Wish list question? | 4. How can HI dynamics be used to probe the connection between galaxies and their dark matter halos over cosmic time? |
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