Physical Characterization of Serendipitously Uncovered Millimeter-wave Line-emitting Galaxies at z~2.5 behind the Local Luminous Infrared Galaxy VV114

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 VV114 with spectral scan observations at $\lambda$ = 2 - 3 mm. Via Atacama Large Millimeter/submillimeter Array (ALMA) detection of CO(4-3), CO(3-2), and [CI](1-0) lines for both sources, their spectroscopic redshifts are unambiguously determined to be $z=2.4666\pm 0.0002$ and $z=2.3100\pm 0.0002$, respectively. We obtain the apparent molecular gas masses $M_{\mathrm{gas}}$ of these two line emitters from [CI] line fluxes as $(11.2\pm 3.1)\times 10^{10}{M}_{\odot }$ and $(4.2\pm 1.2)\times 10^{10}{M}_{\odot }$, 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 $1.3\pm 0.3$. We find that the dynamical mass of ALMA-J0107a within the CO-emitting disk computed from the derived kinetic parameters, $(1.1\pm 0.2)\times 10^{10}{M}_{\odot }$, is an order of magnitude smaller than the molecular gas mass derived from dust continuum emission, $(3.2\pm 1.6)\times 10^{11}{M}_{\odot }$. We suggest this source is magnified by a gravitational lens with a magnification of $\mu \gtrsim 10$, which is consistent with the measured offset from the empirical correlation between CO-line luminosity and width.

19 pages, 11 figures, accepted for publication in ApJ

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