Baryonic Tully-Fisher relation for extremely low mass Galaxies

We study Tully-Fisher (TF) relations for a sample that combines extremely faint (M_B < -14) galaxies along with bright (i.e. ~L_*) galaxies. Accurate (~10 per cent) distances, I-band photometry and B - V colours are known for the majority of the galaxies in our sample. The faint galaxies are drawn from the Faint Irregular Galaxy GMRT survey (FIGGS), and we have HI rotation velocities derived from aperture synthesis observations for all of them. For the faint galaxies, we find that even though the median HI and stellar masses are comparable, the HI mass correlates significantly better with the circular velocity indicators than the stellar mass. We also find that the velocity width at the 20 per cent level (W₂₀) correlates better with mass than the rotation velocity, although the difference is not statistically significant. The faint galaxies lie systematically below the I-band TF relation defined by bright galaxies, and also show significantly more intrinsic scatter. This implies that the integrated star formation in these galaxies has been both less efficient and also less regulated than in large galaxies. We estimate the intrinsic scatter of the faint galaxies about the I-band TF to be ~1.6 mag. We find that while the faint-end deviation is greatly reduced in Baryonic Tully-Fisher (BTF) relations, the existence of a break at the faint end of the BTF is subject to systematics such as the assumed stellar mass-to-light ratio. If we assume that there is an intrinsic BTF and try to determine the baryonic mass by searching for prescriptions that lead to the tightest BTF, we find that scaling the HI mass leads to a much more significant tightening than scaling the stellar mass-to-light ratio. The most significant tightening that we find, however, is if we scale the entire baryonic mass of the faint (but not the bright) galaxies. Such a scenario would be consistent with models where dwarf (but not large) galaxies have a large fraction of dark or `missing' baryons. In all cases, however, the minimum in the χ² curve is quite broad and the corresponding parameters are poorly constrained.