Star forming brightest cluster galaxies at $z\sim0.4$ in KiDS. Further studies of cold gas and stellar properties

Abstract

Brightest cluster galaxies (BCGs) are among the most massive galaxies in the Universe. Their star formation (SF) history and stellar mass assembly are debated. Recent studies suggest the presence of an emerging population of intermediate-$z$ star forming and gas-rich BCGs, where the molecular gas reservoirs are impacted by strong environmental processing. We have selected three among the most star-forming $z\sim0.4$ BCGs in the Kilo-Degree Survey (KiDS), and observed them with the IRAM 30m telescope in the first three CO transitions. We found double-horn CO(1$\rightarrow$0) and CO(3$\rightarrow$2) emission for the KiDS 1433 BCG, yielding a large molecular gas reservoir with $M_{H_2}=(5.9\pm1.2)\times10^{10}~M_\odot$ and a high gas-to-stellar mass ratio $M_{H_2}/M_\star=(0.32^{+0.12}_{-0.10})$. We increase the limited sample of distant BCGs with detections in multiple CO transitions. The double-horn emission for the KiDS 1433 BCG implies a low gas concentration, while a modeling of the spectra yields an extended molecular gas reservoir, with a characteristic radius of $\sim$(5-7) kpc, which is reminiscent of a mature extended-disk phase observed in some local BCGs. For the other two BCGs we are able to set upper limits of $M_{H_2}/M_\star<0.07$ and $<0.23$, which are among the lowest for distant BCGs. We then combined our observations with available stellar, SF, and dust properties of the targeted BCGs, and compared them with $\sim100$ distant cluster galaxies, including additional intermediate-$z$ BCGs, with observations in CO from the literature. The molecular gas properties of star forming BCGs are heterogeneous. On one side, gas-rich BCGs show extended gas reservoirs, which sustain the significant SF activity, which is reminiscent of recent gas infall. Conversely, the existence of similarly star forming, but gas-poor, BCGs suggest that gas depletion precedes SF quenching.