Dark Matter Fraction in z~1 Star-Forming Galaxies

TL;DR

This study measures dark matter fractions in 225 star-forming galaxies at redshift ~1, revealing most are dark matter dominated in their outer regions and exhibit diverse dark matter properties compared to local galaxies.

Contribution

First comprehensive analysis of dark matter fractions in a large sample of z~1 star-forming galaxies using rotation curves and scaling relations.

Findings

Most galaxies have >72% dark matter in outer disks.

Only ~5% have low (<20%) dark matter fractions within 2-3 R_e.

Significant scatter in dark matter fraction at fixed stellar mass.

Abstract

We present a observational study of the dark matter fraction in 225 rotation supported star-forming galaxies at $z\approx 0.9$ having stellar mass range: $ 9.0 \leq log(M_* \ \mathrm{M_\odot}) \leq 11.0$ and star formation rate: $0.49 \leq log \left(SFR \ \mathrm{[M_{\odot}\ yr^{-1}]} \right) \leq 1.77$. This is a sub sample of KMOS redshift one spectroscopic survey (KROSS) previously studied by \citet{GS20}. The stellar masses ($M_*$) of these objects were previously estimated using mass-to-light ratios derived from fitting the spectral energy distribution of the galaxies. Star formation rates were derived from the H$_\alpha$ luminosities. The total gas masses ($M_{gas}$) are determined by scaling relations of molecular and atomic gas \citep[][respectively] {Tacconi2018, Lagos2011}. The dynamical masses ($M_{dyn}$) are directly derived from the rotation curves (RCs) at different scale lengths (effective radius: $R_e$, $\sim 2 \ R_e$ and $\sim 3 \ R_e$) and then the dark matter fractions ($f_{ DM }=1-M_{bar}/M_{dyn}$) at these radii are calculated. We report that at $z\sim 1$ only a small fraction ($\sim 5\%$) of our sample has a low ($< 20\%$) DM fraction within $\sim$ 2-3 $R_e$. The majority ($> 72\%$) of SFGs in our sample have dark matter dominated outer disks ($\sim 5-10$ kpc) in agreement with local SFGs. Moreover, we find a large scatter in the fraction of dark matter at a given stellar mass (or circular velocity) with respect to local SFGs, suggesting that galaxies at $z \sim 1$, a) span a wide range of stages in the formation of stellar disks, b) have diverse DM halo properties coupled with baryons.