Can galaxy growth be sustained through HI-rich minor mergers?

¹ Sorbonne Universités, UPMC Université Paris VI, Institut d’Astrophysique de Paris, 75014 Paris, France
² CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
³ GEPI, Observatoire de Paris, UMR 8111, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
e-mail: wim.vandriel@obspm.fr
⁴ Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU USR 704, Université d’Orléans OSUC, route de Souesmes, 18330 Nançay, France
⁵ Arecibo Observatory, National Astronomy and Ionosphere Center, Arecibo, PR 00612, USA

Received: 26 January 2015
Accepted: 10 October 2015

Abstract

Local galaxies with specific star-formation rates (star-formation rate per unit mass; sSFR ~ 0.2–10 Gyr^-1) that are as high as distant galaxies (z ≈ 1–3), are very rich in Hi. Those with low stellar masses, M_⋆ = 10⁸⁻⁹ M_⊙, for example, have M_HI/M_⋆ ≈ 5–30. Using continuity arguments, whereby the specific merger rate is hypothesized to be proportional to the specific star-formation rate, along with Hi gas mass measurements for local galaxies with high sSFR, we estimate that moderate-mass galaxies, M_⋆ = 10^9−10.5 M_⊙, can acquire enough gas through minor mergers (stellar mass ratios ~4–100) to sustain their star formation rates at z ~ 2. The relative fraction of the gas accreted through minor mergers declines with increasing stellar mass, and for the most massive galaxies considered, M_⋆ = 10^10.5−11 M_⊙, this accretion rate is insufficient to sustain their star formation. We checked our minor merger hypothesis at z = 0 using the same methodology, but now with relations for local normal galaxies, and find that minor mergers cannot account for their specific growth rates, in agreement with observations of Hi-rich satellites around nearby spirals. We discuss a number of attractive features, such as a natural downsizing effect, in using minor mergers with extended Hi disks to support star formation at high redshift. The answer to the question posed by the title, “Can galaxy growth be sustained through Hi-rich minor mergers?”, is “maybe”, but only for relatively low-mass galaxies and at high redshift.