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For refcode 1999MNRAS.306..857C:
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Copyright by Royal Astronomical Society. 1999MNRAS.306..857C The ROSAT Brightest Cluster Sample D III. Optical spectra of the central cluster galaxies C. S. Crawford, S. W. Allen, H. Ebeling, A. C. Edge and A. C. Fabian Accepted 1999 February 16 Received 1998 December 21 ABSTRACT We present new spectra of dominant galaxies in X-ray-selected clusters of galaxies. which combine with our previously published spectra to form a sample of 256 dominant galaxies in 215 clusters. 177 of the clusters are members of the ROSAT Brightest Cluster Sample (BCS; Ebeling et al.), and 17 have no previous measured redshift. This is the first paper in a series correlating the properties of brightest cluster galaxies and their host clusters in the radio, optical and X-ray' wavebands. 27 per cent of the central dominant galaxies have emission-line spectra, all but five with line intensity ratios typical of cooling flow nebulae. A further 6 per cent show only[N II]{lambda}{lambda}6548, 6584 with H{alpha} in absorption. We find no evidence for an increase in the frequency of line emission with X-ray luminosity. Purely X-ray-selected clusters at low redshift have a higher probability of containing line emission. The projected separation between the optical position of the dominant galaxy and its host cluster X-ray centroid is less for the line-emitting galaxies than for those without line emission, consistent with a closer association of the central galaxy and the gravitational centre in cooling flow clusters. The more H{alpha}-luminous galaxies have larger emission-line regions and show a higher ratio of Balmer to forbidden line emission, although there is a continuous trend of ionization behaviour across four decades in H{alpha} luminosity. Galaxies with the more luminous line emission [L(H{alpha}) > 10 erg s^-1^] show a significantly bluer continuum, whereas lower luminosity and [N II]-only line emitters have continua that differ little from those of non-line-emitting dominant galaxies. Values of the Balmer decrement in the more luminous systems commonly imply intrinsic reddening of E(B - V) ~ 0.3 and, when this is corrected for, the excess blue light can be characterized by a population of massive young stars. Several of the galaxies require a large population of 0 stars, which also provide sufficient photoionization to-produce the observed H{alpha} luminosity. The large number of lower mass stars relative to the 0-star population suggests that this anomalous population is caused by a series of starbursts in the central galaxy. The lower H{alpha}-luminosity systems show a higher ionization state and few massive stars, requiring instead the introduction of a harder source of photoionization, such as turbulent mixing layers. or low-level nuclear activity. The line emission from the systems showing only [N II] is very similar to low-level LINER activity commonly found in many normal elliptical galaxies. Key words: surveys - galaxies: clusters: general - cooling flows-galaxies; elliptical and lenticular cD - galaxies: stellar content-X-rays: galaxies.
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