Star formation activities in early-type brightest cluster galaxies
  We identify a total of 120 early-type Brightest Cluster Galaxies (BCGs) at
0.1<z<0.4 in two recent large cluster catalogues selected from the Sloan
Digital Sky Survey (SDSS). They are selected with strong emission lines in
their optical spectra, with both H{\alpha} and [O II]{\lambda}3727 line
emission, which indicates significant ongoing star formation. They constitute
about ~ 0.5% of the largest, optically-selected, low-redshift BCG sample, and
the fraction is a strong function of cluster richness. Their star formation
history can be well described by a recent minor and short starburst
superimposed on an old stellar component, with the recent episode of star
formation contributing on average only less than 1 percent of the total stellar
mass. We show that the more massive star-forming BCGs in richer clusters tend
to have higher star formation rate (SFR) and specific SFR (SFR per unit galaxy
stellar mass). We also compare their statistical properties with a control
sample selected from X-ray luminous clusters, and show that the fraction of
star-forming BCGs in X-ray luminous clusters is almost one order of magnitude
larger than that in optically-selected clusters. BCGs with star formation in
cooling flow clusters usually have very flat optical spectra and show the most
active star formation, which may be connected with cooling flows.
