SERENDIPITOUS ALMA DETECTION OF A DISTANT CO-EMITTING X-RAY BRIGHT GALAXY

Abstract

We report the detection of a distant star-forming galaxy, ALMA J010748.3−173028, which is identified by a 13σ emission line at 99.75 GHz (SΔv = 3.1 Jy km s−1), behind the nearby merging galaxies VV114 using the Atacama Large Millimeter/submillimeter Array (ALMA) Band 3. We also find an 880 μm counterpart with ALMA Band 7 (S880μm = 11.2 mJy). A careful comparison of the intensities of the line and the continuum suggests that the line is a redshifted 12CO transition. A photometric redshift analysis using the infrared to radio data favors a CO redshift of z = 2.467, although z = 3.622 is acceptable. We also find a hard X-ray counterpart, suggesting the presence of a luminous (LX ∼ 1044 erg s−1) active galactic nucleus obscured by a large hydrogen column (NH ∼ 2 × 1023 cm−2 if z = 2.47). A cosmological simulation shows that the chance detection rate of a CO-emitting galaxy at z > 1 with ⩾1 Jy km s−1 is ∼10−3 per single ALMA field of view and 7.5 GHz bandwidth at 99.75 GHz. This demonstrates that ALMA has sufficient sensitivity to find an emission-line galaxy such as ALMA J010748.3−173028 even by chance, although the likelihood of stumbling across such a source is not high.