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Keywords:

  • galaxies: clusters: general;
  • galaxies: elliptical and lenticular, cD;
  • galaxies: evolution;
  • X-rays: galaxies: clusters

ABSTRACT

We study the properties of brightest cluster galaxies (BCGs) drawn from a catalogue of more than 69 000 clusters in the Sloan Digital Sky Survey (SDSS) Data Release 6 based on the adaptive matched filter technique. Our sample consists of more than 14 300 galaxies in the redshift range 0.1–0.3. We test the catalogue by showing that it includes well-known BCGs which lie in the SDSS footprint. We characterize the Szabo et al. catalogue content in terms of BCGs r-band luminosities and optical colours as well as their trends with redshift.

We find that the BCG luminosity distribution is close to a Gaussian with mean −22 mag and dispersion 0.54 mag. The mean has a redshift evolution broadly consistent with pure aging of the galaxies. Richer clusters tend to have brighter BCGs (mean −22.5 mag), however less dominant than in poorer systems.

In particular, we define and study the fraction of blue BCGs, namely those that are likely to be missed by either colour-based cluster surveys and catalogues, as shown by a direct comparison to maxBCG clusters that are matched in the Szabo et al. catalogue. The overall fraction of blue BCGs goes from ∼5 per cent in the redshift range 0.1–0.2 to ∼10 per cent in the redshift bin 0.2–0.3, with the average over the whole sample of ∼8 per cent. We estimate the possible contamination due to blue outliers at the 1–2 per cent level, while errors on the photometric redshift may lead to an erroneous classification of >0.5 per cent of actual red BCGs as blue. When considering only galaxies with spectroscopic redshift available and for clusters above a richness of 50 – where the catalogue is more than 85 per cent complete – our conservative estimate of the blue fraction is 1–6 per cent (at 99.6 per cent confidence). A preliminary morphological study suggests that the increase in the blue fraction at lower richnesses may have a non-negligible contribution from spiral galaxies.

Finally, we cross-matched our catalogue with the ACCEPT cluster sample, and find that blue BCGs tend to be in clusters with low entropy and short cooling times. That is, the blue light is presumably due to recent star formation associated to gas feeding by cooling flows.