James Webb Space Telescope Feed Post

Harvard ADS: Reconciling M/L Ratios Across Cosmic Time: a Concordance IMF for Massive Galaxies

Paper abstract: The stellar initial mass function (IMF) is thought to be bottom-heavy in the cores of the most massive galaxies, with an excess of low mass stars compared to the Milky Way. However, studies of the kinematics of quiescent galaxies at 2 < z < 5 find M/L ratios that are better explained with lighter IMFs. Light IMFs have also been proposed for the unexpected populations of luminous galaxies that JWST has uncovered at z>7, to reduce tensions with galaxy formation models. Here we explore IMFs that are simultaneously bottom-heavy, with a steep slope at low stellar masses, and top-heavy, with a shallow slope at high masses. We derive a form of the IMF for massive galaxies that is consistent with measurements in the local universe and yet produces relatively low M/L ratios at high redshift. This `concordance' IMF has slopes \gamma_1=2.40\pm0.09, \gamma_2=2.00\pm0.14, and \gamma_3=1.85\pm0.11 in the regimes 0.1-0.5 Msun, 0.5-1 Msun, and >1 Msun respectively. The IMF parameter \alpha, the mass excess compared to a Milky Way IMF, ranges from \log(\alpha)~+0.3 for present-day galaxies to \log(\alpha)~-0.1 for their star forming progenitors. The concordance IMF applies only to the central regions of the most massive galaxies, with velocity dispersions ~300 km/s, and their progenitors. However, it can be generalized using a previously-measured relation between \alpha and \sigma. We arrive at the following modification to the Kroupa (2001) IMF for galaxies with \sigma\gtrsim 160 km/s: \gamma_1~1.3+4.3\log\sigma_{160}; \gamma_2~2.3-1.2\log\sigma_{160}; and \gamma_3~2.3-1.7\log\sigma_{160}, with \sigma_{160}=\sigma/160 km/s. If galaxies grow primarily inside-out, so that velocity dispersions are relatively stable, these relations should also hold at high redshift.