Physical Characterization of Serendipitously Uncovered Millimeter-wave Line-emitting Galaxies at z 2.5 behind the Local Luminous Infrared Galaxy VV 114

Abstract

We present a detailed investigation of millimeter-wave line emitters ALMA J010748.3-173028 (ALMA-J0107a) and ALMA J010747.0-173010 (ALMA-J0107b), which were serendipitously uncovered in the background of the nearby galaxy VV 114 with spectral scan observations at λ = 2-3 mm. Via Atacama Large Millimeter/submillimeter Array (ALMA) detection of CO(4-3), CO(3-2), and [C I](1-0) lines for both sources, their spectroscopic redshifts are unambiguously determined to be z = 2.4666 ± 0.0002 and z = 2.3100 ± 0.0002, respectively. We obtain the apparent molecular gas masses M_gas of these two line emitters from [C I] line fluxes as (11.2 ± 3.1) × 10¹⁰ M_⊙ and (4.2 ± 1.2) × 10¹⁰ M_⊙, respectively. The observed CO(4-3) velocity field of ALMA-J0107a exhibits a clear velocity gradient across the CO disk, and we find that ALMA-J0107a is characterized by an inclined rotating disk with a significant turbulence, that is, a deprojected maximum rotation velocity to velocity dispersion ratio ${v}_{\max }/{\sigma }_{v}$ of 1.3 ± 0.3. We find that the dynamical mass of ALMA-J0107a within the CO-emitting disk computed from the derived kinetic parameters, (1.1 ± 0.2) × 10¹⁰ M_⊙, is an order of magnitude smaller than the molecular gas mass derived from dust continuum emission, (3.2 ± 1.6) × 10¹¹ M_⊙. We suggest this source is magnified by a gravitational lens with a magnification of μ ≳ 10, which is consistent with the measured offset from the empirical correlation between CO-line luminosity and width.