Mon. Not. R. Astron. Soc. 000, 000–000 (2002) Printed 28 October 2018 (MN LATEX style file v2.2) AGN-selected clusters as revealed by weak lensing Margrethe Wold1,2, Mark Lacy2, H˚akon Dahle3, Per B. Lilje4, Susan E. Ridgway5 1Stockholm Observatory, SCFAB, Roslagstullsbacken 21, SE-106 91 Stockholm, Sweden 2SIRTF Science Center, Caltech, MS 220-6, 1200 California Bl., Pasadena, CA 91125, U.S.A. 3NORDITA, Blegdamsvej 17, DK-2100 Copenhagen, Denmark 4Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern, N-0315 Oslo, Norway 5Department of Physics & Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218-2686, U.S.A. ABSTRACT As a pilot study to investigate the distribution of matter in the fields of powerful AGN, we present weak lensing observations of deep fields centered on the radio-quiet quasar E1821+643, the radio galaxy 3C 295, and the two radio-loud quasars, 3C 334 and 3C 254. The host clusters of E1821+643 and 3C 295 are comfortably detected via their weak lensing signal, and we report the detection of a cluster-sized mass concentration in the field of the z = 0.734 quasar 3C 254. The data for the 3C 334 field are so far inconclusive. We find that the clusters are massive and have smooth mass distributions, although one shows some evidence of past merger activity. The mass-to-light ratios are found to be moderately high. We discuss the results in light of the cooling flow and the merger/interaction scenarios for triggering and fuelling AGN in clusters, but find that the data do not point unambiguously to neither of the two. Instead, we speculate that sub-cluster mergers may be responsible for generating close encounters or mergers of galaxies with the central cluster member, which could trigger the AGN. Key words: galaxies:clusters – galaxies:active – galaxies: interactions – clus- ters:gravitational lensing 1 INTRODUCTION merger/interaction scenario when they made multi-object arXiv:astro-ph/0205336v1 20 May 2002 spectroscopy of galaxies associated with optically luminous The causes of the dramatic cosmic evolution of AGN (e.g. quasars. They found that the richest quasar host clusters Dunlop & Peacock 1990; Hartwick & Schade 1990) remain have abnormally low velocity dispersions compared to those unclear, we have little knowledge of how and why AGN form, of clusters in general with similar optical richness. This is how quasar activity is triggered and sustained in a galaxy, consistent with the idea that galaxy-galaxy interactions are and what extinguishes the activity. The findings that AGN related to the quasar phenomenon, (e.g. Hutchings, Cramp- are often associated with other galaxies is believed to in- ton & Campbell 1984), since in dynamically young clusters dicate that interactions between galaxies may play a role, where the galaxies have low velocities, the encounters be- since interactions are more likely to occur in a dense en- tween galaxies will be disruptive and perhaps trigger or fuel vironment. Host galaxies and their close companions have quasar activity in one of the interacting galaxies. been found to display disturbed morphology (e.g. Bahcall et al. 1997; Canalizo & Stockton 2001), also taken as a sign Further support for this was found by Yee & Green that interactions have taken place. Even in cases where com- (1987) and Ellingson et al. (1991b) in their studies of quasar panion galaxies appear undisturbed in the optical, radio im- environments. Optically luminous quasars were found more & ages taken at redshifted 21cm (of nearby AGN) show long frequently in rich environments at z 0.6 than at lower bridges or tidal tails of HI gas connecting the AGN with a redshifts, suggesting that by z ∼ 0.3–0.4, the quasars in rich close neighbour (Lim & Ho 1999). clusters have faded. Since the time elapsed between z ∼ 0.4 and z ∼ 0.6 corresponds to the dynamical time scales of clus- These observations have raised the question of the ex- ter cores, they proposed that dynamical changes in the clus- istence and the nature of a physical link between the AGN ter environments were responsible for the fading of quasars, and its environment. Two very different scenarios for this perhaps as a result of lack of gaseous fuel after the clusters link have been suggested, the merger/interaction and the virialize. Accordingly, highly luminous AGN should there- cooling flow scenario. fore not exist in rich clusters at low redshifts, but there are Ellingson, Green & Yee (1991a) found evidence for the several examples which contradict this, demonstrating that c 2002 RAS 2 Wold et al. the requirement of non-virialized clusters is not sufficient. methods. The weak lensing technique allows us to probe the One example is Cygnus A, a powerful FRII radio galaxy in distribution of total mass (dark + luminous) in the AGN a rich cluster at z = 0.056. Two other examples, which we fields, and the mass distributions can be used to test the study in this paper, are 3C 295 and E1821+643. 3C 295 is merger/interaction and the cooling flow scenarios. The two a powerful FRII radio galaxy in a cluster at z = 0.46, and scenarios make very different predictions about the dynami- E1821+643 is an optically luminous quasar at z = 0.297 cal state of AGN host clusters, and by investigating the clus- situated in a very rich cluster. ter dynamics, it may be possible to find which, if any, of these The cooling flow scenario has been discussed by e.g. two mechanisms are responsible for the AGN–environment Fabian et al. (1986), Fabian & Crawford (1990) and Bremer, link. Fabian & Crawford (1997). In this picture, cooling flows in sub-clusters at earlier epochs provide fuel for the AGN, but A cooling flow is expected to be associated with a single as sub-clusters merge toward the present epoch, the cool- deep potential well in an evolved, virialized cluster (Edge et ing flows are disrupted and the quasar activity switched off. al. 1992), and in this case we expect a strong weak lensing Alternatively, the quasar may switch off when the cooling signal and a mass distribution characterized by only a single flow gas is exhausted (Fabian & Crawford 1990). Several peak. In contrast, if the AGN activity is related to galaxy observations have been made of extended emission-line gas interactions, these will be much more disruptive if the en- around powerful quasars which seem to support the cooling counter velocities are relatively low and comparable to the flow model (e.g. Crawford & Fabian 1989; Forbes et al. 1990; internal velocity dispersions of the galaxies involved. Conse- Bremer et al. 1992). quently, a cluster that is still forming will be preferred, as However, it is not established yet whether extended in such a cluster there is an increased probability for low- emission line gas indicates the presence of a cooling flow. velocity encounters. A dynamically young cluster is expected For instance, the emission lines could be produced by ra- to have an irregular mass distribution, possibly with several dio source shocks instead of gas cooling out of the flow (e.g. sub-clumps (Schindler 2001). Tadhunter et al. 2000). The standard cooling flow model Our method of cluster selection is based on the detec- (Fabian 1994 and references therein) has received some crit- tion of weak gravitational shear and the presence of an AGN. XMM icism lately since observations of cooling flow clusters This is different from the usual methods of detecting clus- do not show emission lines at temperatures below 1 keV as ters, by optical richness or X-ray luminosity. Whereas op- predicted (Peterson et al. 2001; Tamura et al. 2001; Kaastra tical and X-ray cluster surveys are biased toward the most et al. 2001). This suggests that the cooling flow model may baryon-rich systems, clusters selected by their weak shear need to be re-evaluated, and in many cases mass deposi- signal are biased toward the most massive systems in terms tion rates may turn out to be lower than originally thought. of dark+luminous mass. If there is a large range in the Heating by AGN has been proposed as a possible cause of baryonic-to-dark matter ratio in clusters, clusters with high Chandra the apparent lack of cold gas, but new data seem mass-to-light (hereafter M/L) ratios will have been missed to indicate that the coolest gas is close to the radio lobes, in optical and X-ray surveys. Although we present a case- contrary to what is expected if there is heating by the AGN by-case study here, it is nevertheless interesting to compare (Fabian 2001). the masses and the M/L ratios of the AGN host clusters to If cooling flows were the chief mechanism for providing those of clusters detected on the basis of optical richness or fuel to the most powerful AGN in clusters, we should ob- X-ray luminosity. serve more rich clusters with central, powerful AGN than we do. In fact, most rich AGN host clusters do not contain The outline of the paper is as follows. In Section 2 we anything more powerful than an FRI source (like M87 in the describe the selection of the targets and review some of their Virgo cluster). In a sample of 260 rich-cluster radio galaxies properties. Section 3 describes the observations and the data studied by Ledlow, Owen & Eilek (2002), there are almost reduction. Before the weak lensing analysis is performed, we no FRII radio galaxies with luminosities at 1.4 GHz greater investigate the colour-magnitude diagrams of the clusters than 1025.5 W Hz−1.