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arXiv · 0901.4111

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

Philip F. Hopkins·Rachel S. Somerville·Thomas J. Cox·Lars Hernquist·Shardha Jogee·Dusan Keres·Chung-Pei Ma·Brant Robertson·Kyle Stewart
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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.

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Philip F. Hopkins, Rachel S. Somerville, Thomas J. Cox, Lars Hernquist, Shardha Jogee, Dusan Keres, Chung-Pei Ma, Brant Robertson, Kyle Stewart. 2009-09-10. The Effects of Gas on Morphological Transformation in Mergers: Implications for Bulge and Disk Demographics. https://doi.org/10.1111/j.1365-2966.2009.14983.x

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