Is the Butcher-Oemler Effect a Function of the Cluster Redshift? S
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server Submitted to The Astrophysical Journal Preprint typeset using LATEX style emulateapj IS THE BUTCHER-OEMLER EFFECT A FUNCTION OF THE CLUSTER REDSHIFT? S. Andreon Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy E-mail: [email protected] and S. Ettori Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK E-mail: [email protected] Submitted to The Astrophysical Journal ABSTRACT Using PSPC Rosat data, we measure x–ray surface brightness profiles, size and luminosity of the Butcher–Oemler (BO) sample of clusters of galaxies. The cluster x-ray size, as measured by the Petrosian rη=2 radius, does not change with redshift and is independent from x-ray luminosity. On the other hand, the x–ray luminosity increases with redshift. Considering that fair samples show no-evolution, or negative luminosity evolution, we conclude that the BO sample is not formed from the same class of objects observed at different look–back times. This is in conflict with the usual interpretation of the Butcher–Oemler as an evolutionary (or redshift–dependent) effect, based on the assumption that we are comparing the same class of objects at different redshifts. Other trends present in the BO sample reflect selection criteria rather than differences in look-back time, as independently confirmed by the fact that trends loose strength when we enlarge the sample with x–ray selected sample of clusters. The variety of optical sizes and shapes of the clusters in the Butcher–Oemler sample, and the Malmquist-like bias, are the reasons for these selection effects that mimic the trends usually interpreted as changes due to evolution. Subject headings: Galaxies: evolution — galaxies: clusters: general — X-rays: general 1. INTRODUCTION important role played by the cluster environment: evolu- tion is strong in clusters and negligible in groups. However Galaxies in distant (z 0:4) clusters differ from those in the nearby systems (e.g.∼ Butcher & Oemler 1984; Dressler this idea have been questioned: Rakos & Schombert (1995) show that it is difficult to fade the majority of the clus- & Gunn 1992). There is a blueing of galaxy color with redshift, also known as Butcher-Oemler effect (hereafter ter population at z 0:7 to make their blue population as scarce as in present–day∼ clusters. Andreon, Davoust & BO effect, Butcher & Oemler 1977, 1984). At z 0:4 Heim (1997) and Ellis et al. (1997) show that cluster ellip- there is a population of almost normal late–type galax-∼ ies that by the present epoch has disappeared, faded or ticals and lenticulars are old galaxies already at z 0:4, and that the majority of them cannot be the end∼ prod- has been disrupted (Dressler et al. 1997). Distant clusters uct of the blue galaxy population. Recently, this result contain galaxies with disturbed morphologies and peculiar spectra. The occurrence of these peculiarities varies from has been extended with clusters up to z 0:9 (Stanford, Eisenhardt & Dickinson 1998). ∼ cluster to cluster and, on average, increases with redshift. Andreon et al. (1997) have made a detailed comparison In general, the change of galaxy properties is explained as the effect of some kind of evolution. of the properties of galaxies in the nearby Coma cluster and the distant cluster, Cl 0939+47. They found that the Oemler, Dressler & Butcher (1997) proposed a physi- spiral population of these two clusters appears too differ- cal reason to explain why most of the clusters showing a BO effect are at high redshift and almost none at the ent in spatial, color and surface brightness distributions to be the same galaxy population observed at two different present epoch: clusters at z 0:4 are much more excep- epochs. The Coma cluster is therefore unlikely to be rep- tional objects than present–day∼ clusters and they are ob- served in the act of growing by merger of smaller clumps, resentative of the end of the evolution path of Cl 0939+47. in agreement with a hierarchical growth of structures as In order to quantify the effect of evolution on the prop- described, for example, by Kauffmann (1995). Further- erties of a given class of objects, it is required that the more, this scenario permits the existence of dynamically observed class is the same at different times. In the case young local clusters, such as the spiral rich Abell 1367 and of the evolution of galaxies in clusters, it is necessary that 2151 clusters, and evolved clusters at high redshift, such as the high redshift clusters studied are the ancestors of the Cl 0024+16. In the Oemler et al. (1997) interpretation of investigated present–day clusters. the BO effect, clusters at higher redshift are dynamically The goal of this paper is to test whether the distant younger, on average, than the nearby ones, because we are and nearby clusters of the BO sample are really the same looking at the epoch of an enhanced cluster formation. population seen at different epochs or two different popu- Allington-Smith et al. (1993) showed that galaxies in lations; in other words, we want to check if we are com- groups do not evolve (except passively) and suggest that paring unripe apples to ripe oranges in understanding how the BO effect should be interpreted as an evidence of the fruit ripens!. We achieve this goal by means of the X–ray 1 2 Is the Butcher-Oemler effect a function of z? properties of the clusters of galaxies, whose evolution is 3.1. Data reduction known. PSPC images in the hard band 0.5–2 keV with 15 arc- The paper is organized as follows: in the next section, sec pixel size have been extracted from the public archive we present our sample of clusters. In Section 3, we discuss at the Max–Planck–Institut f¨ur extraterrestrische Physik the x–ray analysis of their images. The observed trends (MPE) or at the Goddard Space Flight Center (according and their relevance on the BO effect are presented in Sec- to their availability). Table 2 lists x–ray related quantities. tion 4 and 5, respectively. In Section 6, we summarize our We correct our images for exposure variation and telescope main results. 1 vignetting using the distributed exposure maps. All pixels In the following analysis, we adopt H0 =50kms− contaminated by other objects or occulted by ribs have Mpc 1 and =05. The conversion to the physical di- − q0 : been flagged and excluded from the following analysis. mension is done through the equation (Sandage 1988): In order to measure the x–ray radial profiles, bright- nesses are computed in elliptical annuli of semi-major axis z +1 √z+1 3 increasing in geometrical progression of base √2 in order r(kpc) = 2:91 10 θ − 2 ; (1) × H0 (1 + z) to take the S/N approximatively constant along the radius. Ellipticity, position angle (PA hereafter) and center for where θ is the angular radius in arcsec. the cluster emission have been derived paying attention to 2. OUR SAMPLE: THE BUTCHER & OEMLER (1985) the observational data available for distant clusters. For SAMPLE example, we keep fixed the center, even if isophotes center The clusters most frequently compared for measuring moves, since in distant clusters we seldom have data of the BO effect are listed in Butcher & Oemler (1985). This good enough quality to measure the displacement of the list is the master list for many studies (e.g. Dressler & center of the various isophotes. When data are not good Gunn 1992; Oemler, Dressler & Butcher 1997; half of the enough to estimate the ellipticity or the position angle, we sample by Smail et al. 1997; Dressler et al. 1998 is drawn adopt circular apertures. from the BO sample, etc.). The details of the data reduction are as follows: Our sample is made from the BO sample plus Cl – The equivalent radius of each ellipse is that of a circle 0939+47, a cluster at z 0:4 which is frequently stud- of the same area, i.e. r = √ab,wherea; b are the major ied in the context of the∼ BO effect. This sample, which and minor axis of the ellipse. consist of 33(+1) clusters, is not complete in any sense (in – The axis lengths of an elliptical annulus of finite width cluster richness, in z, etc.). Figure 1 reproduces Figure are at half way from the internal and external edges. 3 in Butcher & Oemler (1984), with the addition of Cl – The brightness in an annulus is computed as the ra- 0939+47. The BO effect is evident from the increase of tio of the intensity measured in unflagged pixels and their the fraction of blue galaxies with the redshift. total area. We have x–ray data for 30 of the 34 clusters. Position – We assume that flagged pixels have the same bright- Sensitive Proportional Counter (PSPC) images are avail- ness as unflagged ones in the same annulus. The flux in- able for 25 of these. We call this subsample “HQ” (high side an ellipse is the sum, over the internal annulus, of quality) sample. Five more clusters, as well as many of the product of the brightness computed in each annulus the clusters observed by Rosat, have been observed with and the total area of the annulus (calculated including the previous x–ray missions. We use these old data when nec- flagged pixels). essary. Before we proceed further in the data analysis we need Table 1 presents the whole sample in order of increas- to verify two assumptions: the computed profiles are in- ing redshift.