V ERSION ACCEPTED BY THE A STROPHYSICAL J OURNAL Preprint typeset using LATEX style emulateapj v. 10/09/06
THE CHANDRA DEEP PROTOCLUSTER SURVEY: LYα BLOBS ARE POWERED BY HEATING, NOT COOLING J. E. G EACH 1 , D. M. A LEXANDER 1 , B. D. L EHMER 1 , I AN S MAIL 2 , Y. M ATSUDA 1 , S. C. C HAPMAN 3,4 , C. A. S CHARF 5 , R. J. I VISON 6,7 , M. VOLONTERI 8 , T. YAMADA 9 , A. W. B LAIN 10 , R. G. B OWER 2 , F. E. BAUER 5 & A. BASU -Z YCH 5
arXiv:0904.0452v3 [astro-ph.CO] 23 Jun 2009
Version accepted by the Astrophysical Journal
ABSTRACT We present the results of a 400 ks Chandra survey of 29 extended Lyα emitting nebulae (Lyα Blobs, LABs) in the z = 3.09 proto-cluster in the SS A22 field. We detect luminous X-ray counterparts in five LABs, implying 44 −1 a large fraction of active galactic nuclei (AGN) in LABs, fAGN = 17+12 −7 % down to L2−32keV ∼ 10 erg s . All of the AGN appear to be heavily obscured, with spectral indices implying obscuring column densities of NH > 1023 cm−2 . The AGN fraction should be considered a lower limit, since several more LABs not detected with Chandra show AGN signatures in their mid-infrared emission. We show that the UV luminosities of the AGN are easily capable of powering the extended Lyα emission via photo-ionization alone. When combined with the UV flux from a starburst component, and energy deposited by mechanical feedback, we demonstrate that ‘heating’ by a central source, rather than gravitational cooling is the most likely power source of LABs. We argue that all LABs could be powered in this manner, but that the luminous host galaxies are often just below the sensitivity limits of current instrumentation, or are heavily obscured. No individual LABs show evidence > 9 Ms exposure of an average LAB also yields no for extended X-ray emission, and a stack equivalent to a ∼ statistical detection of a diffuse X-ray component. The resulting diffuse X-ray/Lyα luminosity limit implies > 107 K) gas component in these halos, and also rules out inverse Compton scattering of there is no hot (T ∼ cosmic microwave background photons, or local far-infrared photons, as a viable power source for LABs. Subject headings: galaxies: active – galaxies: high-redshift – galaxies: evolution 1. INTRODUCTION
It appears that feedback between galaxies and the intergalactic medium (IGM) plays a significant role in the formation and evolution of galaxies (Bower et al. 2006; Croton et al. 2006). Without it, even some of the basic properties of galaxies (such as stellar mass) cannot be re-produced in current models of galaxy formation. Gas cooling within dark matter halos is countered by outflows from starbursts and active galactic nuclei (AGN) and other heating mechanisms. These not only heat, but can also enrich the intergalactic medium (IGM), and truncate star formation within the host galaxies – preventing a glut of >L⋆ galaxies in the local Universe. Placing empirical constraints on these processes, and understanding their detailed physics, is therefore of vital importance. Recently there has been great interest in the highly extended (∼30–200 kpc in projected linear extent) Lyα lineemitting nebulae (LLyα ∼ 1043−44 erg s−1 ) identified in highredshift narrowband surveys: ‘Lyα Blobs’ (LABs) (Fynbo et 1 Department of Physics, Durham University, South Road, Durham DH1 3LE, UK. E-mail:
[email protected] 2 Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK. 3 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK 4 Department of Physics and Astronomy, University of Victoria, Victoria, B.C., V8P 1A1, Canada 5 Columbia Astrophysics Laboratory, Columbia University, Pupin Laboratories, 550 W. 120th St., Rm 1418, New York, NY 10027, USA 6 SUPA, Institute for Astronomy, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK 7 Astronomy Technology Centre, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK 8 Department of Astronomy, University of Michigan, Ann Arbor, MI, USA 9 National Astronomical Observatory of Japan, Tokyo 181-8588, Japan 10 Department of Astronomy, California Institute of Technology, MC 10524, 1200, East California Blvd, Pasadena, CA, 91125. USA.
al. 1999; Keel et al. 1999; Steidel et al. 2000; Francis et al. 2001; Palunas et al. 2004; Matsuda et al. 2004; Dey et al. 2005; Smith et al. 2008). The most important questions in LAB studies remain unanswered: how are they formed and what maintains their power? One of the main reasons that these objects have aroused curiosity is the possibility that they trace feedback events during the formation of massive galaxies (Chapman et al. 2001; Geach et al. 2005, 2007; Webb et al. 2009), but we still lack a definitive model of LAB formation. What are the possible formation mechanisms of LABs? At first glance, these objects appear to be good candidates for the Lyα ‘fuzz’ predicted to exist around primordial galaxies in simple models of galaxy formation (e.g. Rees & Ostriker 1977; Haiman et al. 2000; Haiman & Rees 2001; Birnboim & Dekel 2003). Cooling of pristine gas within a dark matter halo via Lyα emission could, in part, provide the energy required to power a LAB via the release of gravitational potential energy (e.g. Fardal et al. 2001; Nilsson et al. 2006; Smith & Jarvis 2007). However, this has to be reconciled with the fact that many LABs appear to be associated with extremely luminous galaxies (Chapman et al. 2001; Dey et al. 2005; Geach et al. 2005, 2007; Colbert et al. 2006; Beleen et al. 2008; Webb et al. 2009) with bolometric luminosities several orders of magnitude greater than that of the Lyα emission. Therefore, some models of LAB formation propose a ‘heating’ scenario, where the energy release associated with intense star formation or AGN within the LABs’ host galaxies powers the extended line emission (e.g. Ohyama et al. 2003). It has also been postulated that inverse Compton scattered cosmic microwave background (CMB) photons could go on to photoionize a neutral gas halo (e.g. Fabian et al. 2009). This mechanism is thought to give rise to extended X-ray emission around luminous radio galaxies at z > 2 (Scharf et al. 2003). Unfortunately the current limits on the soft, diffuse X-ray emission
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LAB S
ARE POWERED BY HEATING , NOT COOLING TABLE 1 X- RAY PROPERTIES OF LAB S IN SSA 22.
LAB ID
αJ2000 (h m s)
δJ2000 (◦ ′ ′′ )
f0.5−2 keV
f2−8 keV f0.5−8 keV (10−16 erg s−1 cm−2 )
L2−32 keV (1044 erg s−1 )
Γeff
Offset (′′ )
0.81±0.03 2.13±0.02 0.91±0.03 1.82±0.02 1.59±0.03
