Observed trend in the star formation history and the dark matter fraction of galaxies at redshift $z\approx0.8$

Abstract

We study the star formation history for a sample of 154 galaxies with stellar mass $10^{10}\lesssim M_{\ast}\lesssim 10^{12} M_{\odot}$ in the redshift range $0.7 < z < 0.9$. We do this using stellar population models combined with full-spectrum fitting of good quality spectra and high resolution photometry. For a subset of 68 galaxies ($M_{\ast}\gtrsim 10^{11} M_{\odot}$) we additionally construct dynamical models. These use an axisymmetric solution to the Jeans equations, which allows for velocity anisotropy, and adopts results from abundance matching techniques to account for the dark matter content. We find that: (i) The trends in star formation history observed in the local universe are already in place by $z\sim1$: the most massive galaxies are already passive, while lower mass ones have a more extended star formation histories, and the lowest mass galaxies are actively forming stars; (ii) we place an upper limit of a factor 1.5 to the size growth of the massive galaxy population; (iii) we present strong evidence for low dark matter fractions within $1R_{\rm e}$ (median of 9 per cent and 90th percentile of 21 per cent) for galaxies with $M_{\ast} \gtrsim 10^{11} M_{\odot}$ at these redshifts; and (iv) we confirm that these galaxies have, on average, a Salpeter normalisation of the stellar initial mass function.