The ROSAT Brightest Cluster Sample -- III. Optical spectra of the central cluster galaxies

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]λλ6548,6584 with Hα 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 Hα‐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 Hα luminosity. Galaxies with the more luminous line emission [L(Hα)> 1041ergs−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 O stars, which also provide sufficient photoionization to produce the observed Hα luminosity. The large number of lower mass stars relative to the O‐star population suggests that this anomalous population is caused by a series of starbursts in the central galaxy. The lower Hα‐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.

The bow shock, cold fronts and disintegrating cool core in the merging galaxy group RX J0751.3+5012

“…Rasmussen & Ponman 2009 for the SW and the NE groups, respectively. Although this is not a significant difference, these results are consistent with the galaxy redshifts z = 0.0218 for UGC 4051 in the SW group and z = 0.0236 and 0.0228 for UGC 4052 E and W, respectively (Crawford et al 1999). From the global temperature of each group and the masstemperature relation (e.g.…”

“…Another large elliptical galaxy (UGC 4051) is coincident with the X-ray surface brightness peak in the SW group. The redshifts for these three large ellipticals from Crawford et al (1999) show that these galaxies are associated with the two merging groups and not a chance superposition along the line of sight. There is a hard X-ray point source spatially coincident with UGC 4051 in the SW group, which could indicate an AGN.…”

“…1 right) shows the distribution of galaxies associated with the two groups. The NE group core hosts a pair of large elliptical galaxies (UGC 4052) separated by a projected distance of only 15 kpc (Crawford et al 1999). The eastern galaxy of the pair, UGC 4052 east (E), is coincident with the X-ray surface brightness peak in the core and hosts luminous Hα line emission (L Hα ∼ 10 40 erg s −1 ; Crawford et al 1999).…”

“…The NE group core hosts a pair of large elliptical galaxies (UGC 4052) separated by a projected distance of only 15 kpc (Crawford et al 1999). The eastern galaxy of the pair, UGC 4052 east (E), is coincident with the X-ray surface brightness peak in the core and hosts luminous Hα line emission (L Hα ∼ 10 40 erg s −1 ; Crawford et al 1999). UGC 4052 west (W) is located ahead of the NE group core and appears associated with a small spur of X-ray emission or 'nose' extending ∼7 kpc (15 arcsec; Fig.…”

“…The spectra were fitted in XSPEC 12.8 (Arnaud 1996) with an absorbed single temperature APEC model version 2.0.2 (Smith et al 2001). The cluster redshift was fixed to z = 0.022 (Ebeling et al 1998;Crawford et al 1999) and abundances were measured assuming the abundance ratios of Anders & Grevesse (1989). The temperature, metallicity and normalization parameters were left free.…”