xGASS: The Impact of Photometric Bulges on the Scatter of HI Scaling Relations

TL;DR

This study analyzes how photometric bulges influence the scatter in HI scaling relations in galaxies, finding that bulge presence minimally affects global HI content once stellar mass and star formation rate are accounted for.

Contribution

It provides a detailed structural decomposition of galaxies using Bayesian light profile fitting to assess bulge effects on HI content, revealing minimal impact of bulges on HI reservoirs.

Findings

Bulge-dominated galaxies have lower HI masses at fixed stellar mass.

Trends are more evident when separating by luminosity.

Bulge presence has little effect on global HI content after controlling for stellar mass and SFR.

Abstract

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.