Abstract
We investigate the effect of a metallicity-dependent stellar initial mass
function (IMF), as deduced observationally by Martín-Navarro et al. (2015c),
on the inferred stellar masses and star formation rates of a representative
sample of 186,886 SDSS galaxies. Compared to a universal Chabrier IMF, this
variable IMF implies a large increase in the star formation rates inferred from
the observed UV and IR luminosities. This extends the star formation main
sequence to higher masses and increases the total low-redshift SFR density by
an order of magnitude. Depending on the metallicity, the inferred galaxy masses
increase either because of the addition of stellar remnants or dwarf stars
relative to a Chabrier IMF, for which the implied mass is minimal. This causes
a shift of the galaxy stellar mass function (GSMF) to higher masses by 0.5 dex
and a factor 2.3 increase in the stellar mass density above 109 Msun. However,
the results depend strongly on the assumed IMF parametrisation, which is not
directly constrained by the observations. Varying the low-end IMF slope instead
of the high-end IMF slope, while maintaining the same dwarf-to-giant ratio,
results in a SFR density increase by a factor 1.1, a GSMF shift by 0.2 dex and
a factor 1.3 increase in the stellar mass density. In both cases, the steepness
of the high-end GSMF drop-off is not affected, thus a variable IMF does not
remove the need to quench high-mass galaxies. A bottom-heavy IMF during the
late, metal-rich evolutionary stage of a galaxy could help explain the rapid
quenching and the bimodality in the galaxy population by on the one hand making
galaxies less quenched (due to the continued formation of dwarf stars) and on
the other hand reducing the gas consumption timescale. We conclude that the
implications of the observational evidence for a variable IMF could vary from
absolutely dramatic to mild but significant.
Description
[1603.05281] Implications of a variable IMF for the interpretation of observations of galaxy populations
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