xGASS: The Role of Bulges Along and Across the Local Star-Forming Main
  Sequence
  We use our catalogue of structural decomposition measurements for the
extended GALEX Arecibo SDSS Survey (xGASS) to study the role of bulges both
along and across the galaxy star-forming main sequence (SFMS). We show that the
slope in the $sSFR$-$M_{\star}$ relation flattens by $\sim$0.1 dex per decade
in $M_{\star}$ when re-normalising $sSFR$ by disc stellar mass instead of total
stellar mass. However, recasting the $sSFR$-$M_{\star}$ relation into the
framework of only disc-specific quantities shows that a residual trend remains
against disc stellar mass with equivalent slope and comparable scatter to that
of the total galaxy relation. This suggests that the residual declining slope
of the SFMS is intrinsic to the disc components of galaxies. We further
investigate the distribution of bulge-to-total ratios ($B/T$) as a function of
distance from the SFMS ($\Delta SFR_{MS}$). At all stellar masses, the average
$B/T$ of local galaxies decreases monotonically with increasing $\Delta
SFR_{MS}$. Contrary to previous works, we find that the upper-envelope of the
SFMS is not dominated by objects with a significant bulge component. This rules
out a scenario in which, in the local Universe, objects with increased star
formation activity are simultaneously experiencing a significant bulge growth.
We suggest that much of the discrepancies between different works studying the
role of bulges originates from differences in the methodology of structurally
decomposing galaxies.
