The Formation and Growth of the First Black HolesA Whitepaper for the 2010 Decadal SurveyScience Frontier Panel: Galaxies across Cosmic Time (GCT)Authors: Bret Lehmer (Durham University), Niel Brandt (Penn State University), Dave Alexander(Durham University), Roger Brissenden (Smithsonian Astrophysical Obsevatory), Martin Elvis(Smithsonian Astrophysical Obsevatory), Ann Hornschemeier (Goddard Space Flight Center),Dan Schwartz (Smithsonian Astrophysical Obsevatory), & the Generation-X TeamContact Person: Bret LehmerDepartment of Physics, Durham University, South Road, Durham DH1 3LE (UK)Email: [email protected]: +44 191 334–377311 IntroductionThe path of observational cosmology over the next decade will be focused on the exploration of theearly Universe. To determine how the phenomena (i.e., galaxies, stars, planets, etc.) of our localUniverse evolved to their present state, we must determine how and when the first stars, galaxies,and black holes formed. In the next decade, observatories such as JWST, ALMA, LSST, TMT,GMT, and E-ELT will provide a phenomenal new perspective on this theme, allowing for the firsttime in history a direct view of the starlight that originates from the first galaxies; a regime thatis only now understood by theoretical inferences. However, it is now clear that galaxy and blackhole growth are intimately tied, and therefore knowledge of black hole formation and growth isrequired to piece together a complete picture of galaxy formation. Specifically, we will need ob-servational facilities designed to address the following questions: How and when did the first blackholes in the Universe form and grow? What influence did the energetic process of accretion ontothe first black holes have on the formation and evolution of their host galaxies and in reionizingthe Universe? Addressing these questions requires sensitive X-ray observations, which effectivelyprobe the nearby vicinity of the accreting black holes even through heavily obscured environments.The next great X-ray observatory, the International X-ray Observatory (IXO), will provide the firstkey discoveries of luminous accreting supermassive black holes (SMBHs) out to z ≈ 7–8 and con-strain the physical processes responsible for powering these sources. To extend the findings fromIXO and study for the first time the evolution of representative populations of seed black holesfrom their birth around z ≈ 8–15 requires an X-ray observatory capable of detecting extremelyfaint sources with 0.1–10 keV fluxes of ≈ 10−20ergs cm−2s−1, while remaining free from sourceconfusion. To achieve these aims requires an observatory with an effective light collecting area of≈ 50–100 m2and a resolution of ≈ 0.′′1; these specifications can provide imaging that is ≈ 1000times deeper and ≈ 5 times higher resolution than Chandra. The Generation-X mission specifica-tions have been designed with these requirements in mind. Here we illustrate how Generation-Xwill optimally address these questions.2 The First Black HolesHow and when did the first black holes in the Universe form?Theoretical studies of star formation in the early Universe require that the first populations of blackholes were formed at z ≈ 15–20 as a result of the first massive stars (>∼260 M⊙) ending their shortlives (e.g., Bromm et al. 1999; Gao etal. 2007). The initial masses of such black holes will bemore than half that of the progenitor star (e.g., Heger & Woosley 2002) and it is expected thatthese stars will produce a population of black holes with masses of order 100 M⊙. Though thedetails are not yet clear, these black holes are expected to lie in nebulous overdense halos, and soshortly after their formation, they will merge with neighboring halos to form increasingly moremassive overdensities. Through this process, the black holes can grow both via mass accretion andblack hole–black hole mergers (Volonteri etal. 2003). As the gas in these overdense regions cools,new generations of stars will form and these first galaxies and black holes will continue to grow2through accretion.X-ray observations provide a very effective means for probing the conditions in the immediatevicinity of accreting black holes and are capable of penetrating through thick columns of obscur-ing material. We now know from deep Chandra and XMM-Newton data that the X-ray band is,at present, the most effective way of identifying and studying large populations of accreting su-permassive black holes (SMBHs), seen as active galactic nuclei (AGNs), over a wide range ofluminosities and over the entire history of the Universe (e.g., Brandt & Hasinger 2005). The deep-est X-ray surveys conducted with Chandra (Alexander et al. 2003; Luo etal. 2008) find about tentimes more AGNs per square degree than the deepest optical surveys (e.g., Bauer et al. 2004), andmany of which cannot be identified via optical spectroscopy (e.g., Barger et al. 2005). Beyondthe local universe, the best way to find SMBHs will be to search for nuclear activity probed byX-ray emission. The X-ray signatures of an accreting black hole are very clear even out to z ≈ 6: aluminous X-ray source with a hard powerlaw spectrum extending to high energies (e.g., Shemmeret al. 2006). At even higher redshifts, the 0.1–10 keV bandpass probes hard rest-frame X-ray en-ergies, which are capable of penetrating extremely thick obscuring environments reaching opticalextinctions of 100s to 1000s of magnitudes.It is known from observational studies of local galaxies and their nuclear environments that by thepresent day nearly all nucleated galaxies host SMBHs in their centers (e.g., Kormendy & Richstone1995; Magorrian et al. 1998). It has also been recently discovered that the masses of these SMBHsare strongly correlated with the masses of the giant stellar bulges residing in the host galaxies(e.g., Gebhardt etal. 2000). A key inference from these observations is that SMBHs and their hostgalaxies grew in tandem, with the growth of each component influencing that of the other despitethe host galaxy being ≈ 109times larger in physical size and ≈ 1000 times more massive than theSMBH. Therefore, a more complete understanding of how galaxies and their SMBHs formed andevolved together requires observations constraining the formation of both the first black holes andgalaxies.While there are several paths to the formation of SMBHs (e.g., Rees 1984), the fact that theSMBH–galaxy bulge mass