Highly-luminous cool core clusters of galaxies: mechanically-driven or radiatively-driven AGN?

arXiv:1101.4026v1 [astro-ph.CO] 20 Jan 2011 Tracing the Ancestry of Galaxies (on the Land of our Ancestors) Proceedings IAU Symposium No. xxx, 2011 A.C. Editor, B.D. Editor & C.E. Editor, eds. c⃝2011 International Astronomical Union DOI: 00.0000/X000000000000000X Highly-luminous cool core clusters of galaxies: mechanically-driven or radiatively-driven AGN? Julie Hlavacek-Larrondo1⋆and Andy Fabian1 1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA email: juliehl@ast.cam.ac.uk Abstract. Cool core clusters of galaxies require strong feedback from their central AGN to offset cooling. We present a study of strong cool core, highly-luminous (most with LX ⩾1045 erg s−1 ), clusters of galaxies in which the mean central AGN jet power must be very high yet no central point X-ray source is detected. Using the unique spatial resolution of Chandra, a sample of 13 clusters is analysed, including A1835, A2204, and one of the most massive cool core clusters, RXCJ1504.1-0248. All of the central galaxies host a radio source, indicating an active nucleus, and no obvious X-ray point source. For all clusters in the sample, the nucleus has an X-ray bolometric luminosity below 2 per cent of that of the entire cluster. We investigate how these clusters can have such strong X-ray luminosities, short radiative cooling-times of the inner intracluster gas requiring strong energy feedback to counterbalance that cooling, and yet have such radiatively-inefficient cores with, on average, Lkin/ Lnuc exceeding 200. Explanations of this puzzle carry significant implications for the origin and operation of jets, as well as on establishing the importance of kinetic feedback for the evolution of galaxies and their surrounding medium. Keywords. X-rays: galaxies: clusters, (galaxies:) cooling flows, galaxies: jets 1. Introduction Clusters of galaxies with steeply rising X-ray surface brightness profiles are known as cool core clusters, and require strong feedback from their central AGN to offset cooling of the intracluster medium (ICM). For highly-luminous cool core clusters (with LX ⩾1045 erg s−1 ), the central AGN must be injecting on average 1045 erg s−1 into the surrounding medium. We do not know the black hole (BH) mass for most of these AGN but we expect that they lie between 109 and 1010 M⊙, based on the few reliable measurements in the literature (e. g. M87, Macchetto et al. 1997). Given that these objects are in an exceptional environment and are active continuously, it is not clear that the standard MBH −σ or MBH −MK relations are relevant for them. For MBH ∼109M⊙, this means that these black holes must be operating at high enough Eddington rates that they should be radiatively efficient (see Fig. 1). We would therefore expect to see an X-ray point source. We present a sample of strong cool core, highly-luminous clusters, for which there is no evidence of a nuclear X-ray point source in the Chandra images. Using these images, we derive upper limits of the nuclear luminosities with the web interface of pimms (Mukai 1993), which converts a count rate into an expected flux. We also investigate whether there is a hidden power law in the X-ray spectra, but find no such evidence in any of our objects. Finally, we calculate the energy that must be injected by the AGN in order to offset the cooling ( Loutflow) of the ICM, and compare it with the nuclear luminosity of the AGN. 119 120 Julie Hlavacek-Larrondo & Andy Fabian Figure 1. Sketch of black hole energy release as a function of mass accretion (Churazov et al. 2005). The energy in radiation dominates at high accretion rates. If a black hole is re- leasing 1045 erg s−1 , then for MBH ∼109M⊙the power exceeds 10−2 LEdd, indicating that Pradiation ⩾Poutflow, i.e. we should see a nuclear point source. For MBH ⩾1010M⊙, the power exceeds 10−3 LEdd. Here, Poutflow could dominate over Pradiation, if the black hole is in a low accretion state. 2. Results Our results are shown in Fig. 2 and Fig. 3, and reveal a significant population of objects requiring high kinetic input from an AGN to offset cooling and/or high jet power, yet are without a detected X-ray nucleus. These objects appear to be radiatively inefficient with on average, Lkin/ Lnuc ⩾200. We examine 7 possible explanations as to why these objects appear to be so radiatively-inefficient. • First, they could simply be strongly Doppler-suppressed. However, all jets would need to be aligned with the plane of the sky, and it is unlikely that all of our objects have jets with a preferred geometry. • Second, we could be dealing with Advection Dominated Accretion flows (Narayan & McClintock 2008), but it would be difficult to obtain jet powers of 1045 erg s−1 . • Next, our objects could have magnetically-dominated black holes, but the mecha- nism responsible for creating jets in magnetically-dominated accretion discs still remains poorly understood, which makes it difficult to give any definitive conclusion. • They could also be highly-abs

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