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

, , and . (2016)cite arxiv:1603.05281Comment: Submitted to MNRAS, 16 pages, 13 figures, 1 table.

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.

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[1603.05281] Implications of a variable IMF for the interpretation of observations of galaxy populations

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