Molecular gas in CLASH brightest cluster galaxies at $z\sim0.2-0.9$
  Brightest cluster galaxies (BCGs) are excellent laboratories to study galaxy
evolution in dense Mpc-scale environments. We have observed in CO(1-0),
CO(2-1), CO(3-2), or CO(4-3), with the IRAM-30m, 18 BCGs at $z\sim0.2-0.9$ that
are drawn from the CLASH survey. Our sample includes RX1532, which is our
primary target, being among the BCGs with the highest star formation rate
(SFR$\gtrsim100~M_\odot$/yr) in the CLASH sample. We unambiguously detected
both CO(1-0) and CO(3-2) in RX1532, yielding a large reservoir of molecular
gas, $M_{H_2}=(8.7\pm1.1)\times10^{10}~M_\odot$, and a high level of excitation
$r_{31}=0.75\pm0.12$. A morphological analysis of the HST I-band image of
RX1532 reveals the presence of clumpy substructures both within and outside the
half-light radius $r_e=(11.6\pm0.3)$ kpc, similarly to those found
independently both in ultraviolet and in H$_\alpha$ in previous work. We
tentatively detected CO(1-0) or CO(2-1) in four other BCGs, with molecular gas
reservoirs in the range $M_{H_2}=2\times10^{10-11} M_\odot$. For the remaining
13 BCGs we set robust upper limits of $M_{H_2}/M_\star\lesssim0.1$, which are
among the lowest molecular gas to stellar mass ratios found for distant
ellipticals and BCGs. By comparison with distant cluster galaxies observed in
CO our study shows that RX1532 ($M_{H_2}/M_\star = 0.40\pm0.05$) belongs to the
rare population of star forming and gas-rich BCGs in the distant universe. By
using available X-ray based estimates of the central intra-cluster medium
entropy, we show that the detection of large reservoirs of molecular gas
$M_{H_2}\gtrsim10^{10}~M_\odot$ in distant BCGs is possible when the two
conditions are met: i) high SFR and ii) low central entropy, which favors the
condensation and the inflow of gas onto the BCGs themselves, similarly to what
has been previously found for some local BCGs.
