Abstract

Contributed Talk - Splinter GalaxyClusters

Wednesday, 09 September 2026, 16:15   (MW-1050)

Rare Blue-core BCGs in eRASS:5 as Optical Signatures of ICM Cooling

Matthias Kluge, Esra Bulbul, Xiaoyuan Zhang
Max Planck Institute for extraterrestrial Physics

Blue cores in brightest cluster galaxies (BCGs) trace the rare conditions under which cooling intracluster gas fuels late-time star formation in cluster cores. We search for centrally blue BCGs in the relaxed eRASS:5 cluster sample at redshift 0 < z < 0.6 using eROSITA X-ray detections, eROMaPPer optical counterparts, Legacy Surveys imaging, and spectroscopy from SDSS, DESI, and HET/LRS2. We identify 382 blue-core BCGs among 11 425 systems, corresponding to an observed incidence of 3.3%, with increasing contamination at high redshift. Their main photometric signature is a central blue excess in g − r by < −0.2 mag within a ≲ 20 kpc, while their outer profiles return to the normal red-sequence envelope. Blue, Hα-emitting BCGs are preferentially found in systems with low central entropy and short cooling time, especially around S_0 ≲ 10 keV cm² and t_cool,0 ≲ 0.3 Gyr. Red, Hα-emitting BCGs occupy a less extreme but still cool-core-like regime, with most systems at S_0 ≲ 20 keV cm² and t_cool,0 ≲ 1 Gyr, significantly below the passive, non-Hα-emitting population. Hα equivalent width strongly correlates with the blue continuum excess, consistent with young stellar components produced by starbursts that add new stellar mass fractions of 0.1%–1%. However, the ionizing source is not pure star formation. Line-ratio measurements classify blue Hα-emitting BCGs as composite objects with contributions from star formation and LINER-like excitation, while red Hα emitters populate the LINER-like region more strongly. The blue-core fraction increases with optical richness from the per cent level to ∼10%, likely because richer systems more often host massive hot atmospheres with high absolute cooling losses and favorable cool-core conditions. The scarcity of robust blue BCGs without Hα argues against a dominant fading post-starburst population and instead suggests sustained fueling over timescales of ≳ 100 Myr.