Stars, dust, and the growth of ultraviolet-selected sub-L[superscript ∗] galaxies at redshift z ∼ 2

Abstract

This work concerns the physical properties of very faint (R[subscript lim] = 28 AB mag; M[subscript stars,lim] ∼ 10[superscript 8] M[subscript circled dot]), ultraviolet-selected (UV-selected) sub-L[superscript ∗] BX galaxies at z ∼ 2.3. Stellar masses, dust content and dust-corrected star formation rates are constrained using broad-band spectral energy distribution fitting, resulting in a number of insights into the nature of these low-mass systems. First, a correlation between rest-frame UV luminosity and galaxy stellar mass appears to exist in BX galaxies and its presence suggests that many sub-L[superscript ∗] galaxies at this redshift may have approximately constant, rather than highly variable, star formation histories. A nearly linear relation between stellar mass and star formation rate is also found, hinting that the rate at which a sub-L[superscript ∗] BX galaxy forms its stars is directly related to the mass of stars that it has already formed. A possible explanation for this phenomenon lies in a scenario in which new gas that falls on to the galaxy’s host halo along with accreting dark matter is the main source of fuel for ongoing star formation. The instantaneous efficiency of star formation is low in this scenario, of the order of 1 per cent. Turning to bulk quantities, it is found that the low-mass end of the stellar mass function at z ∼ 2.3 is steeper than expected from extrapolations of shallower surveys, resulting in a stellar mass density at z ∼ 2.3 that is ∼25 per cent of the present-day value; this value is ∼1.5–2 times higher than that given by extrapolations of most of the shallower surveys, suggesting that the build-up of stellar mass in the Universe has proceeded somewhat more rapidly than previously thought. With spectral energy distribution fitting results in hand, an update to the Keck Deep Fields z ∼ 2 UV luminosity function finds a steeper faint-end slope than previously reported, [alpha] = −1.47, though this is not as steep as that found by Reddy & Steidel. Finally, it is also found that sub-L[superscript ∗] galaxies at z ∼ 2 carry very small amounts of dust compared to their more luminous cousins, so that while only ∼20 per cent of 1700-[Angstrom] photons escape from a typical M[superscript ∗] galaxy, more than half make it out of an M[superscript ∗]+3 one. This paucity of dust highlights the fact that sub-L[superscript ∗] galaxies are not simple scaled copies of their more luminous cousins. Assuming that absorption by neutral hydrogen is not stronger in sub-L[superscript ∗] galaxies than in their more luminous counterparts, it also means that sub-L[superscript ∗] galaxies are important contributors to keeping the Universe ionized at z ∼ 2.