The Turn-Down of the Baryonic Tully-Fisher Relation and Changing Baryon Fractions at Low Galaxy Masses

Abstract

The ratio of baryonic to dark matter in present-day galaxies constrains galaxy formation theories and can be determined empirically via the baryonic Tully-Fisher relation (BTFR), which compares a galaxy's baryonic mass (M bary ) to its maximum rotation velocity (V max ). The BTFR is well-determined at M bary > 10 8 M , but poorly constrained at lower masses due to small samples and the challenges of measuring rotation velocities in this regime. For 25 galaxies with high-quality data and M bary 10 8 M , we estimate M bary from infrared, optical, and HI observations and V max from the Hi gas rotation. Many of the V max values are lower limits because the velocities are still rising at the edge of the detected HI disks; consequently, most of our sample has lower velocities than expected from extrapolations of the BTFR at higher masses. To estimate V max , we map each galaxy to a dark matter halo assuming density profiles with and without cores, and find that the cored profiles match the data better. When we compare the V max values derived from the cored density profiles to our M bary measurements, we find a turndown of the BTFR at low masses that is consistent with CDM predictions and implying baryon fractions of 1-10% of the cosmic value. Although we are limited by the sample size and assumptions inherent in mapping measured rotational velocities to theoretical rotation curves, our results suggest that the galaxy formation efficiency drops at masses below M bary ∼ 10 8 M , corresponding to M 200 ∼ 10 10 M .

Astrophysical Tests of Dark Matter Self-Interactions

“…Simulations with baryons indicate that the subhalo density profiles are much flatter than in dark-matter-only CDM simulations. A flatter density profile goes a long way toward matching the central density, as measured observationally, to the total halo mass [466,467,468]. Moreover, including the disk of the MW is known to have an impact on the satellites that have pericenters close to the disk [469,467].…”

“…There is evidence that the overprediction of dynamical mass in ΛCDM is in fact more general than implied solely by these relations [522]. On the other hand, relaxing assumptions about the density profile of the dark-matter halos, allowing for cored halos, leads to better consistency between the halo masses preferred by rotation curves and those implied by abundance matching [523,524,468].…”

“…The evidence for cores in many spiral galaxies is strong, particularly those of low surface brightness (e.g. [41,504,505,506,468]). This evidence is sufficiently ubiquitous to lead some to suggest a universal cored density profile with a well-defined relation between core density and size [507,508,509].…”