The Turn-Down of the Baryonic Tully-Fisher Relation at Low Galaxy Masses
  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$_{\odot}$, 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$_{bary}<\sim10^8$
M$_{\odot}$, we estimate M$_{bary}$ from infrared, optical, and HI observations
and Vmax 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$_{bary}\sim10^8$ M$_{\odot}$, corresponding to M$_{200}\sim10^{10}$
M$_{\odot}$.
