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

Molina, Juan and Ibar, Edo and Smail, Ian and Swinbank, A. M. and Villard, E. and Escala, A. and Sobral, David and Hughes, T. M. (2019) The kiloparsec-scale gas kinematics in two star-forming galaxies at z~1.47 seen with ALMA and VLT-SINFONI. Monthly Notices of the Royal Astronomical Society, 487 (4). 4856–4869. ISSN 0035-8711

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Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) CO(J = 2 − 1) observations of two main-sequence star-forming galaxies at z ∼ 1.47 taken from the High-Z Emission Line Survey (HiZELS). These two systems have been previously reported to be molecular gas rich fH2 ≡ MH2 /(MH2 + M⋆) ∼ 0.8. We carried out a follow-up study to resolve, at ∼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 Hα-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 Hα. 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 (Vrot ∼ σv ∼ 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 τdep = 2.3 ± 1.2 Gyr for the galaxy as a whole. This value is in agreement with the average τdep value observed in local star-forming galaxies at similar spatial scales. Using a thick-disk dynamical modelling, we derive a dynamical mass M = (1.59 ± 0.19) × 10^11 M within ≈ 6 kpc. This suggests a dark matter fraction fDM ≡ MDM/Mdyn of 0.59 ± 0.10, in agreement with the average fDM value derived from stacked rotation curve analysis of galaxies at similar redshift range.