The MOSDEF Survey: Dynamical and Baryonic Masses and Kinematic Structures of Star-Forming Galaxies at $1.4 \leq z \leq 2.6$

TL;DR

This study analyzes the kinematics and masses of star-forming galaxies at redshift 1.4 to 2.6, revealing low dark matter fractions, the importance of gas mass inclusion, and insights into galaxy dynamics and evolution.

Contribution

It provides the first detailed comparison of dynamical and baryonic masses for a large sample of high-redshift galaxies, incorporating gas mass estimates and kinematic modeling.

Findings

Good agreement between dynamical and baryonic masses with low dark matter fraction.

Gas mass inclusion is critical for accurate mass comparisons.

Evidence of disk settling indicated by decreasing V/σ with increasing SSFR.

Abstract

We present H$\alpha$ gas kinematics for 178 star-forming galaxies at z~2 from the MOSFIRE Deep Evolution Field survey. We have developed models to interpret the kinematic measurements from fixed-angle multi-object spectroscopy, using structural parameters derived from CANDELS HST/F160W imaging. For 35 galaxies we measure resolved rotation with a median $(V/\sigma_{V,0})_{R_E}=2.1$. We derive dynamical masses from the kinematics and sizes and compare them to baryonic masses, with gas masses estimated from dust-corrected H$\alpha$ star formation rates (SFRs) and the Kennicutt-Schmidt relation. When assuming that galaxies with and without observed rotation have the same median $(V/\sigma_{V,0})_{R_E}$, we find good agreement between the dynamical and baryonic masses, with a scatter of $\sigma_{RMS}=0.34$dex and a median offset of $\Delta\log_{10}M=0.04$dex. This comparison implies a low dark matter fraction (8% within an effective radius) for a Chabrier initial mass function (IMF), and disfavors a Salpeter IMF. Moreover, the requirement that $M_{dyn}/M_{baryon}$ should be independent of inclination yields a median value of $(V/\sigma_{V,0})_{R_E}=2.1$ for galaxies without observed rotation. If instead we treat the galaxies without detected rotation as early-type galaxies, the masses are also in reasonable agreement ($\Delta\log_{10}M=-0.07$dex, $\sigma_{RMS}=0.37$dex). The inclusion of gas masses is critical in this comparison; if gas masses are excluded there is an increasing trend of $M_{dyn}/M_{*}$ with higher specific SFR (SSFR). Furthermore, we find indications that $V/\sigma$ decreases with increasing H$\alpha$ SSFR for our full sample, which may reflect disk settling. We also study the Tully-Fisher relation and find that at fixed stellar mass $S_{0.5}=(0.5V_{2.2}^2+\sigma_{V,0}^2)^{1/2}$ was higher at earlier times. At fixed baryonic mass, we observe the opposite trend. [abridged]