Rare Blue-core BCGs as Optical Signatures of ICM Cooling
Abstract
Blue cores in brightest cluster galaxies (BCGs) trace recent central activity associated with cooling cluster atmospheres. We search for blue cores at z<0.6 in five eROSITA all-sky surveys using X-ray data, optical imaging, and optical spectroscopy. We identify 379 blue-core BCGs among 11352 systems (3.3%), although contamination increases toward higher redshift. Of these, 98 (0.86%) are X-ray bright. Their defining signature is a central g−r color at least 0.2 mag bluer than the reference population within a=20 kpc before returning to the normal BCG envelope. Ha emission equivalent width correlates strongly with the blue continuum excess, consistent with young stellar components comprising ~0.1-1% of pre-existing stellar mass. Line ratios indicate widespread LINER-like excitation, with a larger star-forming contribution in blue Ha emitters but substantial non-stellar excitation. This activity is closely linked to the thermodynamics of the host-cluster gas. At 0.1R500, cooling times decrease from BCGs without significant Ha emission to red and then blue Ha emitters. Blue BCGs preferentially occur in clusters with LX,300>1044 erg s−1; red Ha emitters also favor higher X-ray luminosities. The blue-core fraction rises with optical richness. For systems with tcool(0.1R500)<20 Gyr, doubling richness increases the odds of hosting a blue-core BCG by 2.45±0.39. This indicates that cluster scale carries information beyond cooling time, plausibly through the amount of gas available for cooling and condensation. The scarcity of blue BCGs without significant Ha emission disfavors isolated short bursts followed by long Ha-quiet fading phases. It can instead be explained by sustained or closely recurrent star formation over timescales much longer than 100 Myr, or by Ha persisting through additional excitation mechanisms as the young stellar component fades.
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BibTeX
@misc{kluge2026rare,
title = {{Rare Blue-core BCGs as Optical Signatures of ICM Cooling}},
author = {M. Kluge and E. Bulbul and X. Zhang and M. McDonald and E. Artis and F. Balzer and R. Bender and Z. Ding and L. Fiorino and N. Malavasi and A. Merloni and T. Mistele and K. Nandra and M. Ramos-Ceja and M. Salvato and J. S. Sanders and S. Zelmer},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37274},
eprint = {2610.37274},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37274},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Rare Blue-core BCGs as Optical Signatures of ICM Cooling
AU - M. Kluge
AU - E. Bulbul
AU - X. Zhang
AU - M. McDonald
AU - E. Artis
AU - F. Balzer
AU - R. Bender
AU - Z. Ding
AU - L. Fiorino
AU - N. Malavasi
AU - A. Merloni
AU - T. Mistele
AU - K. Nandra
AU - M. Ramos-Ceja
AU - M. Salvato
AU - J. S. Sanders
AU - S. Zelmer
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37274
UR - https://arxiv.org/abs/2610.37274
AB - Blue cores in brightest cluster galaxies (BCGs) trace recent central activity associated with cooling cluster atmospheres. We search for blue cores at z<0.6 in five eROSITA all-sky surveys using X-ray data, optical imaging, and optical spectroscopy. We identify 379 blue-core BCGs among 11352 systems (3.3%), although contamination increases toward higher redshift. Of these, 98 (0.86%) are X-ray bright. Their defining signature is a central g−r color at least 0.2 mag bluer than the reference population within a=20 kpc before returning to the normal BCG envelope. Ha emission equivalent width correlates strongly with the blue continuum excess, consistent with young stellar components comprising ~0.1-1% of pre-existing stellar mass. Line ratios indicate widespread LINER-like excitation, with a larger star-forming contribution in blue Ha emitters but substantial non-stellar excitation. This activity is closely linked to the thermodynamics of the host-cluster gas. At 0.1R_500, cooling times decrease from BCGs without significant Ha emission to red and then blue Ha emitters. Blue BCGs preferentially occur in clusters with L_X,300>10^44 erg s^−1; red Ha emitters also favor higher X-ray luminosities. The blue-core fraction rises with optical richness. For systems with t_cool(0.1R_500)<20 Gyr, doubling richness increases the odds of hosting a blue-core BCG by 2.45±0.39. This indicates that cluster scale carries information beyond cooling time, plausibly through the amount of gas available for cooling and condensation. The scarcity of blue BCGs without significant Ha emission disfavors isolated short bursts followed by long Ha-quiet fading phases. It can instead be explained by sustained or closely recurrent star formation over timescales much longer than 100 Myr, or by Ha persisting through additional excitation mechanisms as the young stellar component fades.
ER -