2008/07/31 by A. Pipino, S. Kaviraj, Sugata Kaviraj +7 · 1 citation
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysical Phenomena and Observations #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.1111/j.1365-2966.2009.14534.x
11 pages, 5 figures, added references, corrected typos, matches MNRAS accepted version
arxiv created 2009/01/28 · openalex publication_date 2009/03/26 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We present a joint analysis of near-ultraviolet (NUV) data from the GALEX (Galaxy Evolution Explorer) mission and (optical) colour profiles for a sample of seven brightest cluster galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG, which has a blue rest-frame UV colour, also shows a blue core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. Furthermore, the UV enhancement cannot be from an active galactic nuclei (AGN) because the spatial extent of the blue cores is significantly larger than the possible contamination region due to a massive black hole. The recent star formation in the blue BCGs typically has an age less than 200 Myr and contributes mass fractions of less than a per cent. Although the sample studied here is small, we demonstrate, for the first time, a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV enhancement observed using GALEX. The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster's X-ray properties, strongly suggests that the star formation is fuelled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and once this flow starts, it is nearly always active. Collectively, these results suggest that AGN feedback in present-day BCGs, while important, cannot be as efficient as suggested by the recent theoretical model by proposed by De Lucia et al.