Precise Tully-Fisher Relations without Galaxy Inclinations
  Power-law relations between tracers of baryonic mass and rotational
velocities of disk galaxies, so-called Tully-Fisher relations (TFRs), offer a
wealth of applications in galaxy evolution and cosmology. However, measurements
of rotational velocities require galaxy inclinations, which are difficult to
measure, thus limiting the range of TFR studies. This work introduces a maximum
likelihood estimation (MLE) method for recovering the TFR in galaxy samples
with limited or no information on inclinations. The robustness and accuracy of
this method is demonstrated using virtual and real galaxy samples.
Intriguingly, the MLE reliably recovers the TFR of all test samples, even
without using any inclination measurements - that is, assuming a
sin(i)-distribution for galaxy inclinations. Explicitly, this 'inclination-free
MLE' recovers the three TFR parameters (zero-point, slope, scatter) with
statistical errors only about 1.5-times larger than the best estimates based on
perfectly known galaxy inclinations with zero uncertainty. Thus, given
realistic uncertainties, the inclination-free MLE is highly competitive. If
inclination measurements have mean errors larger than 10 degrees, it is better
not to use any inclinations, than to consider the inclination measurements to
be exact. The inclination-free MLE opens interesting perspectives for future HI
surveys by the SKA and its pathfinders.
