xGASS: The Impact of Photometric Bulges on the Scatter of HI Scaling
  Relations
  We present a structural decomposition analysis of the galaxies in the
extended GALEX Arecibo SDSS Survey (xGASS) using (gri) images from the Sloan
Digital Sky Survey. Utilising the 2D Bayesian light profile fitting code
ProFit, we fit single- and double-component models taking advantage of a robust
Markov chain Monte Carlo optimisation algorithm in which we assume a Sersic
profile for single-component models and a combination of a Sersic bulge and
near-exponential disc (0.5 < n < 1.5) for double-component models. We
investigate the effect of bulges on the atomic hydrogen (HI) content in
galaxies by revisiting the HI-to-stellar mass scaling relations with the
bulge-to-total ratio measured in the ProFit decompositions. We show that, at
both fixed total and disc stellar mass, more bulge-dominated galaxies have
systematically lower HI masses, implying that bulge-dominated galaxies with
large HI reservoirs are rare in the local Universe. We see similar trends when
separating galaxies by a bulge-to-total ratio based either on luminosity or
stellar mass, however, the trends are more evident with luminosity.
Importantly, when controlling for both stellar mass and star formation rate,
the separation of atomic gas content reduces to within 0.3 dex between galaxies
of different bulge-to-total ratios. Our findings suggest that the presence of a
photometric bulge has little effect on the global HI gas reservoirs of local
galaxies.
