Weighing Black Holes in High - Redshift SCUBA Galaxies

Deep SCUBA surveys have uncovered a population of dust-enshrouded star-forming galaxies at z ≈ 2. Using the ultra-deep 2 Ms Chandra Deep Field-North survey we recently showed that a large fraction of these systems are also undergoing intense black-hole growth. Here we provide further constraints on the properties of the black holes in SCUBA galaxies using the virial black-hole mass estimator. We show that typical SCUBA galaxies are likely to host black holes with M BH ≈ 10 7 –10 8 M ⊙ which are accreting at, or close to, the Eddington limit. These results provide qualitative support for our earlier conclusion that the growth of the black hole lags that of the host galaxy in these massive ultra-luminous galaxies. Deep surveys over the past decade have revolutionised our understanding of the high-z Universe and mapped the history of star formation out to z ≈ 6 (e.g., Madau, Pozzetti, & Dickinson 1998). Over the same timescale, the finding that every massive galaxy hosts a central super-massive black hole (SMBH) with a mass proportional to that of its spheroid (e.g., Tremaine et al. 2002) has raised the surprising possibility that the growth of galaxies and their SMBHs are concurrent, despite nine orders of magnitude difference in size scale. The most active sites of star formation at high redshift are identified with (sub-)millimetre emitting galaxies (SMGs), such as those detected in deep SCUBA surveys (e.g., Smail et al. 2002). The estimated star-formation rates of these systems are large enough to form a massive galaxy of ≈ 10 11 M ⊙ in just 10 8 yrs (e.g., Chapman et al. 2005; Greve et al. 2005). After intense multi-wavelength follow-up observations it is clear that these SMGs are already massive at their average redshift of z ≈ 2 and are sufficiently gas rich to maintain a vigorous level of star formation for a further ≈ 5 × 10 7 yrs (e.g., Swinbank et al. 2004; Tecza et al. 2004; Tacconi et al. 2006). These results suggest that SMGs pinpoint the formation of local > M * early-type galaxies. Given the apparent connection between the growth of galaxies and their black holes, what can be understood about the growth of black holes in SMGs? 2. The Growth of Black Holes in SMGs Using ultra-deep Chandra observations (the 2 Ms Chandra Deep Field-North survey) we recently showed that ≈ 40% (perhaps all) of the bright (f …