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Register a new userWeighing the Black Holes in z~2 Submillimeter-Emitting Galaxies Hosting Active Galactic Nuclei
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We place direct observational constraints on the black-hole masses of the cosmologically important z~2 submillimeter-emitting galaxy (SMG; f850>4mJy) population, and use measured host-galaxy masses to explore their evolutionary status. We employ the well-established virial black-hole mass estimator to weigh the black holes of a sample of z~2 SMGs with broad Halpha or Hbeta emission. The average black-hole mass and Eddington ratio (eta) of the lower-luminosity broad-line SMGs (L_X~10^44 erg/s} are log(M_BH/M_sol)~8.0 and eta~0.2, respectively. These lower-luminosity broad-line SMGs lie in the same location of the L_X-L_FIR plane as more typical SMGs hosting X-ray obscured AGN and may be intrinsically similar systems, but orientated so that the rest-frame optical nucleus is visible. Under this hypothesis, we conclude that SMGs host black holes with log(M_BH/M_odot)~7.8; we find supporting evidence from observations of local ULIRGs. Combining these black-hole mass constraints with measured host-galaxy masses, we find that the black holes in SMGs are >3 times smaller than those found in comparably massive normal galaxies in the local Universe, albeit with considerable uncertainty, and >10 times smaller than those predicted for z~2 luminous quasars and radio galaxies. These results imply that the growth of the black hole lags that of the host galaxy in SMGs, in stark contrast with that previously suggested for radio galaxies and luminous quasars at z~2. On the basis of current host-galaxy mass constraints, we show that SMGs and their descendants cannot lie significantly above the locally defined M_BH-M_GAL relationship. We argue that the black holes in the z~0 descendents of SMGs will have log(M_BH/M_odot)~8.6, indicating that they only need to grow by a factor of ~6 by the present day (ABRIDGED).
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We review recent evidence for a clear association between accretion onto supermassive black holes and star formation up to z~1 in the zCOSMOS survey. Star formation rates (SFRs) are determined from the [OII] emission-line strength and a correction for the AGN contribution. We find that SFRs of X-ray selected AGN span a distribution of 1-100 solar masses per year and evolve in a manner that is indistinguishable from that of massive, star-forming galaxies. The close relationship between AGN activity and star formation is further supported by an increase in the AGN fraction with bluer rest-frame colors (U-V); we further illustrate how the location of AGNs in a color-magnitude diagram can be misleading in luminosity-limited samples due to the dependence of AGN activity on the stellar mass and the low mass-to-light ratios of blue cloud galaxies. To conclude, our results support a co-evolutionary scenario up to z~1 based on the constancy with redshift of the ratio between mass accretion rate and SFR.
Reverberation mapping methods have been used to measure masses in about three dozen AGNs. The consistency of the virial masses computed from line widths and time delays, the relationship between black hole mass and host-galaxy stellar bulge velocity dispersion, and the consistency with black hole masses estimated from stellar dynamics in the two cases in which such determinations are possible all indicate that reverberation mass measurements are robust and are accurate to typically a factor of a few. The reverberation-mapped AGNs are of particular importance because they anchor the scaling relationships that allow black hole mass estimation based on single spectra. We discuss potential sources of systematic error, particularly with regard to how the emission line widths are measured.
Basing our analysis on ROGUE I, a catalog of over 32,000 radio sources associated with optical galaxies, we provide two diagnostics to select the galaxies where the radio emission is due to an active galactic nucleus (AGN). Each of these diagnostics can be applied independently. The first one, dubbed MIRAD, compares the flux $F_{W3}$ in the $W3$ mid-infrared band of the WISE telescope, with the radio flux at 1.4 GHz, $Frad$. MIRAD requires no optical spectra. The second diagnostic, dubbed DLM, relates the 4000 AA break strength, $D_{rm n}(4000)$, with the radio luminosity per unit stellar mass. The DLM diagram has already been used in the past, but not as standalone. For these two diagrams, we propose simple, empirical dividing lines that result in the same classification for the objects in common. These lines correctly classify as radio-AGN 99.5 percent of the extended radio sources in the ROGUE~I catalog, and as star-forming (SF) galaxies 98--99 percent of the galaxies identified as such by their emission line ratios. Both diagrams clearly show that radio AGNs are preferentially found among elliptical galaxies and among galaxies hosting the most massive black holes. Most of the radio sources classified as radio-AGNs in the MIRAD or DLM diagrams are either optically weak AGNs or retired galaxies.
Submillimeter-emitting galaxies (SMGs) are z~2 bolometrically luminous systems hosting energetic starburst and AGN activity. SMGs may represent a rapid growth phase that every massive galaxy undergoes before lying on the well-established black-hole-spheroid mass relationship in the local Universe. Here we briefly discuss our recent results from Alexander et al. (2008) where we estimated the masses of the black holes in SMGs using the black-hole virial mass estimator, finding M_BH~6x10^7 M_solar for typical SMGs. We show that the black-hole-spheroid mass ratio for SMGs at z~2 was suggestively below that found for massive galaxies in the local Universe and more than an order of magnitude below the black-hole-spheroid mass ratio estimated for z~2 quasars and radio galaxies. We demonstrate that SMGs and their progeny cannot lie on the elevated z~2 black-hole-spheroid mass relationship of quasars-radio galaxies without overproducing the space density of the most massive black holes (M_BH~10^9 M_solar), unless the galaxy spheroid of SMGs is an order of magnitude lower than that typically assumed (M_SPH~10^10 M_solar). We also show that the relative black-hole-spheroid growth rates of typical SMGs appear to be insufficient to significantly increase the black-hole-spheroid mass ratio, without requiring long duty cycles (~10^9 years), and argue that a more AGN-dominated phase (e.g., an optically bright quasar) is required to significantly move SMGs (and their progeny) up the black-hole-spheroid mass plane.
To investigate the role of feedback from Active Galactic Nuclei (AGN) in driving the evolution of their host galaxies, we have carried out a study of the environments and optical properties of galaxies harboring X-ray luminous AGN in the Cl1604 supercluster at z~0.9. Making use of Chandra, HST/ACS and Keck/DEIMOS observations, we examine the integrated colors, morphologies and spectral properties of nine moderate-luminosity (L_x ~ 10^43 erg s^-1) type 2 Seyferts detected in the Cl1604 complex. We find that the AGN are predominantly hosted by luminous spheroids and/or bulge dominated galaxies which have colors that place them in the valley between the blue cloud and red sequence in color-magnitude space, consistent with predictions that AGN hosts should constitute a transition population. Half of the hosts have bluer overall colors as a result of blue resolved cores in otherwise red spheroids and a majority show signs of recent or pending interactions. We also find a substantial number exhibit strong Balmer absorption features indicative of post-starburst galaxies, despite the fact that we detect narrow [OII] emission lines in all of the host spectra. If the [OII] lines are due in part to AGN emission, as we suspect, then this result implies that a significant fraction of these galaxies (44%) have experienced an enhanced level of star formation within the last ~1 Gyr which was rapidly suppressed. Overall we find that the properties of the nine host galaxies are generally consistent with a scenario in which recent interactions have triggered both increased levels of nuclear activity and an enhancement of centrally concentrated star formation, followed by a rapid truncation of the latter, possibly as a result of feedback from the AGN itself. [Abridged]
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