Star forming and gas rich brightest cluster galaxies at z~0.4 in the Kilo-Degree Survey
Journal
Date Issued
2022
Author(s)
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Combes, Francoise
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Salomé, Patrice
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Maturi, Matteo
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Abstract
Brightest Cluster Galaxies (BCGs) are typically massive ellipticals at the
centers of clusters. They are believed to experience strong environmental
processing, and their mass assembly and star formation history are still
debated. We have selected three star forming BCGs in the equatorial field of
the Kilo-Degree Survey (KiDS) at intermediate redshifts. We have observed them
with the IRAM-30m telescope in the first three CO transitions. We remarkably
detected all BCGs at high signal-to-noise ratio ${\rm S/N}\simeq(3.8-10.2)$,
for a total of 7 detected lines out of 8, corresponding to a success rate of
$88\%$. This allows us to double the number of distant BCGs with clear
detections in at least two CO lines. We have then combined our observations
with available stellar, star formation, and dust properties of the BCGs, and we
have compared them with a sample of $\sim100$ distant cluster galaxies with
observations in CO. Our analysis yields large molecular gas reservoirs
$M_{H_2}\simeq(0.5-1.4)\times10^{11}~M_\odot$, excitation ratios $r_{31}=
L^{\prime}_{\rm CO(3\rightarrow2)}/L^{\prime}_{\rm
CO(1\rightarrow0)}\simeq(0.1-0.3)$, long depletion times $\tau_{\rm
dep}\simeq(2-4)$~Gyr, and high $M_{H_2}/M_{\rm dust}\simeq(170-300)$. The
excitation ratio $r_{31}$ of intermediate-$z$ BCGs appears to be well
correlated with the star formation rate and efficiency, which suggests that
excited gas is found only in highly star forming and cool-core BCGs. By
performing color-magnitude plots and a red sequence modeling we find that
recent bursts of star formation are needed to explain the fact that the BCGs
are measurably bluer than photometrically selected cluster members. We suggest
that a substantial amount of the molecular gas has been accreted by the KiDS
BCGs, but still not efficiently converted into stars.
Volume
667
Start page
1
Issn Identifier
0004-6361
Ads BibCode
2022A&A...667A..52C
Rights
open.access
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