The Effects of Gas on Morphological Transformation in Mergers: Implications for Bulge and Disk Demographics

TL;DR

This paper demonstrates that considering gas effects in galaxy mergers is crucial for accurately modeling galaxy morphology, especially the formation of bulges, and aligns models with observed galaxy properties across cosmic time.

Contribution

It introduces the importance of gas physics in merger models, showing that including gas suppresses bulge formation in low-mass, gas-rich galaxies, improving agreement with observations.

Findings

Ignoring gas leads to overproduction of bulges in models.

Including gas effects results in more realistic bulge-to-total ratios.

Models with gas effects match observed morphology-mass relations and redshift evolution.

Abstract

Transformation of disks into spheroids via mergers is a well-accepted element of galaxy formation models. However, recent simulations have shown that bulge formation is suppressed in increasingly gas-rich mergers. We investigate the global implications of these results in a cosmological framework, using independent approaches: empirical halo-occupation models (where galaxies are populated in halos according to observations) and semi-analytic models. In both, ignoring the effects of gas in mergers leads to the over-production of spheroids: low and intermediate-mass galaxies are predicted to be bulge-dominated (B/T~0.5 at <10^10 M_sun), with almost no bulgeless systems), even if they have avoided major mergers. Including the different physical behavior of gas in mergers immediately leads to a dramatic change: bulge formation is suppressed in low-mass galaxies, observed to be gas-rich (giving B/T~0.1 at <10^10 M_sun, with a number of bulgeless galaxies in good agreement with observations). Simulations and analytic models which neglect the similarity-breaking behavior of gas have difficulty reproducing the strong observed morphology-mass relation. However, the observed dependence of gas fractions on mass, combined with suppression of bulge formation in gas-rich mergers, naturally leads to the observed trends. Discrepancies between observations and models that ignore the role of gas increase with redshift; in models that treat gas properly, galaxies are predicted to be less bulge-dominated at high redshifts, in agreement with the observations. We discuss implications for the global bulge mass density and future observational tests.