A Case Study of a Companion Galaxy Outshining Its AGN Neighbour in a Distant Merger System

The study of high-redshift active galactic nuclei (AGN) and their small-scale environment is necessary to investigate the different processes that control and influence the evolution of massive galaxies. In this paper we present a case study of cid_1253 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>z</mi><mo>=</mo><mn>2.15</mn></mrow></semantics></math></inline-formula>) and its companion galaxy using archive CO(3–2) and 340 GHz continuum observations with the Atacama Large Millimeter/submillimeter Array, supplemented by multi-wavelength photometry. Previous studies treated the system as a whole, without separating its components in order to match large-beam infrared observations. Our goal is to study cid_1253 and its companion separately by re-analysing the available archive data of the system. Based on our analysis, the companion galaxy is not only more gas-rich (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>M</mi><msub><mi mathvariant="normal">H</mi><mn>2</mn></msub></msub><mo>∼</mo><msup><mn>10</mn><mn>11</mn></msup><mspace width="0.166667em"></mspace><msub><mi mathvariant="normal">M</mi><mo>⊙</mo></msub></mrow></semantics></math></inline-formula>) but also has a higher dust mass, indicative of obscured star formation. Moreover, as cid_1253 is not detected at 340 GHz, it is possible that a large fraction of the unresolved, <i>Herschel</i>-detected infrared emission is associated with the companion, rather than cid_1253. The presented case study highlights the need to be more cautious with blended sources before drawing our conclusions and the necessity of high-resolution observations.