The kiloparsec-scale gas kinematics in two star-forming galaxies at $z\sim 1.47$ seen with ALMA and VLT-SINFONI

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) CO($J=2-1$) observations of two main-sequence star-forming galaxies at $z\sim 1.47$ taken from the High-Z Emission Line Survey (HiZELS). These two systems have been previously reported to be molecular gas rich $f_{{\mathrm{H}}_2}\equiv M_{{\mathrm{H}}_2}/(M_{{\mathrm{H}}_2}+M_{\star })\sim 0.8$. We carried out a follow-up study to resolve, at $\sim$kpc-scales, the CO emission. These new observations are combined with our earlier ALMA observations (sensitive to diffuse CO emission) and compared with our previous H$\alpha$-based study at matched spatial resolution. One target is marginally resolved in CO(2-1), showing complex dynamics with respect to the ionised gas traced by H$\alpha$. While the other source is spatially resolved, enabling a detailed exploration of its internal dynamical properties. In this system, both gaseous phases show similar spatial extension, rotation velocities and velocity dispersions ($V_{\mathrm{rot}}\sim {\sigma }_v\sim 100$\,km\,s${}^{-1}$) suggesting a rotational velocity to velocity dispersion ratio consistent with unity. By comparing the ionized and molecular gas tracers through the use of a two-dimensional kinematic model, we estimate a median depletion time ${\tau }_{\mathrm{dep}}=2.3\pm 1.2$\,Gyr for the galaxy as a whole. This value is in agreement with the average ${\tau }_{\mathrm{dep}}$ value observed in local star-forming galaxies at similar spatial scales. Using a thick-disk dynamical modelling, we derive a dynamical mass $M_{\mathrm{dyn}}=(1.59\pm 0.19)\times 10^{11}$\,$M_{\odot }$ within $\approx 6$\,kpc. This suggests a dark matter fraction ($f_{\mathrm{DM}}\equiv M_{\mathrm{DM}}/M_{\mathrm{dyn}}$) of $0.59\pm 0.10$, in agreement with the average $f_{\mathrm{DM}}$ value derived from stacked rotation curve analysis of galaxies at similar redshift range.

15 Pages, 9 Figures and 4 Tables. Accepted for publication in MNRAS

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