Temporally and Spatially Extended Star Formation in the Brightest Cluster Galaxy of MACS\,J0329.7\(-\)0211 at \(z=0.45\): Implications for Stellar Growth

Abstract

Brightest cluster galaxies (BCGs), particularly those at the centers of cool-core clusters, can exhibit star formation over spatial extents of up to \(\gtrsim\)100\,kpc at inferred rates of up to \(\gtrsim100\rm\,M_\sun\,yr^{-1}\). Is their star formation also extended over time, as might be expected if fuelled by cooling of the surrounding hot intracluster gas -- a residual cooling flow -- as demonstrated hitherto only for the BCG in the Perseus cluster? Here, to infer the formation history of relatively young stars in the BCG of MACS\,J0329.7\(-\)0211, we fit model single-stellar-populations to the spectral energy distributions (spanning near-UV to near-IR) measured along different sightlines towards its young stellar population. Employing a Markov Chain Monte Carlo method, we show that star formation in this BCG has persisted at a relatively constant rate of \(\sim2{\rm\,M_\sun\,yr^{-1}}\) (factors of 10--40 below the rates previously inferred using simpler methods and/or ad hoc assumptions) over the past \(\sim\)400\,Myr, beyond which any star formation falls below the observational detection threshold. Such persistent star formation from a residual cooling flow can contribute up to \(\sim\)10\% of the original stellar mass of this BCG if its progenitor was among the most massive red nuggets known at \(z\sim\)2 having masses of \(\sim1\times10^{11}\rm\,M_\sun\), but only a few percent of its overall growth in stellar mass to \(\sim8\times10^{11}\rm\,M_\sun\) at \(z=0.45\). Although constituting only a minor pathway for the stellar growth of this BCG, persistent star formation from a residual cooling flow can nevertheless contribute significantly to the enormous number of globular clusters found around BCGs in the local Universe.