Implications of a variable IMF for the interpretation of observations of galaxy populations

Abstract

We investigate the effect of a metallicity-dependent stellar initial mass\nfunction (IMF), as deduced observationally by Martin-Navarro et al. (2015c), on\nthe inferred stellar masses and star formation rates (SFRs) of a representative\nsample of 186,886 SDSS galaxies. Relative to a Chabrier IMF, for which we show\nthe implied masses to be close to minimal, the inferred masses increase in both\nthe low- and high-metallicity regimes due to the addition of stellar remnants\nand dwarf stars, respectively. The resulting galaxy stellar mass function\n(GSMF) shifts toward higher masses by 0.5 dex, without affecting the high-mass\nslope (and thus the need for effective quenching). The implied low-redshift SFR\ndensity increases by an order of magnitude. However, these results depend\nstrongly on the assumed IMF parametrisation, which is not directly constrained\nby the observations. Varying the low-end IMF slope instead of the high-end IMF\nslope, while maintaining the same dwarf-to-giant ratio, results in a much more\nmodest GSMF shift of 0.2 dex and a 10 per cent increase in the SFR density\nrelative to the Chabrier IMF. A bottom-heavy IMF during the late, metal-rich\nevolutionary stage of a galaxy would help explain the rapid quenching and the\nbimodality in the galaxy population by on the one hand making galaxies less\nquenched (due to the continued formation of dwarf stars) and on the other hand\nreducing the gas consumption timescale. We conclude that the implications of\nthe observational evidence for a variable IMF could vary from absolutely\ndramatic to mild but significant.\n