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Description
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~0.2-0.9 that are drawn from the Cluster Lensing And Supernova survey with Hubble (CLASH) survey. Our sample includes RX1532, which is our primary target, being among the BCGs with the highest star formation rate (SFR>~100M_{sun}_/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_H2_=(8.7+/-1.1)10^10^M_{sun}_, and a high level of excitation r31=0.75+/-0.12. A morphological analysis of the Hubble Space Telescope I-band image of RX1532 reveals the presence of clumpy substructures both within and outside the half-light radius re=(11.6+/-0.3)kpc, similarly to those found independently both in ultraviolet and in H 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_H2_=2x10^10-11^M_{sun}_. For the remaining 13 BCGs we set robust upper limits of M_H2_/M*<~0.1, 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_H2_/M*=0.40+/-0.05) 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_H2_>~10^10^M_{sun}_ 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.
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