Serendipitous ALMA detection of a distant CO-emitting X-ray bright galaxy.
TAMURA Y., SAITO T., TSURU T.G., UCHIDA H., IONO D., YUN M.S., ESPADA D. and KAWABE R.
Abstract (from CDS):
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), 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) active galactic nucleus obscured by a large hydrogen column (NH∼ 2x1023/cm2 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 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.