116 research outputs found
Moderate-Luminosity Growing Black Holes From 1.25 < z < 2.7: Varied Accretion In Disk-Dominated Hosts
We compute black hole masses and bolometric luminosities for 57 active
galactic nuclei (AGN) in the redshift range 1.25 < z < 2.67, selected from the
GOODS-South deep multi-wavelength survey field via their X-ray emission. We
determine host galaxy morphological parameters by separating the galaxies from
their central point sources in deep HST images, and host stellar masses and
colors by multi-wavelength SED fitting. 90% of GOODS AGN at these redshifts
have detected rest-frame optical nuclear point sources; bolometric luminosities
range from 2e43 - 2e46 erg/s. The black holes are growing at a range of
accretion rates, with at least 50% of the sample having L/L_Edd < 0.1. 70% of
host galaxies have stellar masses M* > 1e10 M_sun, with a range of colors
suggesting a complex star formation history. We find no evolution of AGN
bolometric luminosity within the sample, and no correlation between AGN
bolometric luminosity and host stellar mass, color or morphology. Fully half
the sample of host galaxies is disk-dominated, with another 25% having strong
disk components. Fewer than 15% of the systems appear to be at some stage of a
major merger. These moderate-luminosity AGN hosts are therefore inconsistent
with a dynamical history dominated by mergers strong enough to destroy disks,
indicating minor mergers or secular processes dominate the co-evolution of
galaxies and their central black holes at z ~ 2.Comment: 11 pages, 6 figures, accepted to ApJ. Sersic indices, AGN/galaxy
luminosity ratios, stellar masses etc. provided in Table
The Multiwavelength AGN Population and the X-ray Background
In order to fully understand galaxy formation we need to know when in the cosmic history are supermassive black holes (SMBHs) growing more intensively, in what type of galaxies this growth is happening and what fraction of these sources are invisible at most wavelengths due to obscuration. Active Galactic Nuclei (AGN) population synthesis models that can explain the spectral shape and intensity of the cosmic X-ray background (CXRB) indicate that most of the SMBH growth occurs in moderate-luminosity (L X ~ 1044 erg/s) sources (Seyfert-type AGN), at z~ 0.5−1 and in heavily obscured but Compton-thin, NH ~ 1023cm−2, systems. However, this is not the complete history, as a large fraction of black hole growth does not emit significantly in X-rays either due to obscuration, intrinsic low luminosities or large distances. The integrated intensity at high energies indicates that a significant fraction of the total black hole growth, 22%, occurs in heavily-obscured systems that are not individually detected in even the deepest X-ray observations. We further investigate the AGN triggering mechanism as a function of bolometric luminosity, finding evidence for a strong connection between significant black hole growth events and major galaxy mergers from z~ 0 to z~ 3, while less spectacular but longer accretion episodes are most likely due to other (stochastic) processes. AGN activity triggered by major galaxies is responsible for ~60% of the total black hole growth. Finally, we constrain the total accreted mass density in supermassive black holes at z > 6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. We estimate an accreted mass density <1000 M⊙Mpc−3 at z~ 6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observation
Heavily Obscured Quasar Host Galaxies at z~2 are Disks, Not Major Mergers
We explore the nature of heavily obscured quasar host galaxies at z~2 using
deep Hubble Space Telescope WFC3/IR imaging of 28 Dust Obscured Galaxies (DOGs)
to investigate the role of major mergers in driving black hole growth. The high
levels of obscuration of the quasars selected for this study act as a natural
coronagraph, blocking the quasar light and allowing a clear view of the
underlying host galaxy. The sample of heavily obscured quasars represents a
significant fraction of the cosmic mass accretion on supermassive black holes
as the quasars have inferred bolometric luminosities around the break of the
quasar luminosity function. We find that only a small fraction (4%, at most
11-25%) of the quasar host galaxies are major mergers. Fits to their surface
brightness profiles indicate that 90% of the host galaxies are either disk
dominated, or have a significant disk. This disk-like host morphology, and the
corresponding weakness of bulges, is evidence against major mergers and
suggests that secular processes are the predominant driver of massive black
hole growth. Finally, we suggest that the co-incidence of mergers and AGN
activity is luminosity dependent, with only the most luminous quasars being
triggered mostly by major mergers.Comment: 5 pages, 4 figures, 1 table. To appear as a Letter in MNRA
Searching for the Role of Mergers in Fast and Early SMBH Growth: Morphological Decomposition of Quasars and Their Hosts at
We present rest-frame ultraviolet (UV) images of six luminous quasars at obtained with the Hubble Space Telescope (HST). These quasars exhibit
a wide range of star formation rates (SFRs) and lie in a wide range of
environments. We carefully model and subtract the point-like quasar emission
and investigate the morphology of the underlying host galaxies at kpc scales.
The residual images allowed identification of potential companion sources,
which enabled us to explore the role of galaxy merger scenarios in the
co-evolution of the quasars and their hosts. We also search for the mechanism
driving extreme SFRs in three of the quasars. We find that the rate of
detection of potential companions to the host galaxies does not follow trends
between high- and low-SFR sources; i.e., the HST imaging suggests that both
high- and low-SFR sources are found in both dense and sparse galactic
environments. The suggested role of major mergers driving extreme SFRs cannot
be supported by the multiwavelength data in hand. Three of four companion
sources, previously revealed by sub-millimeter observations, are not detected
in the HST images of three of our quasars. An adapted high-resolution imaging
strategy focused on high-SFR sources and extended to a larger quasar sample is
required to determine the role of mergers in the processes of star formation
and supermassive black hole growth at high redshift.Comment: 22 pages, 10 figures; accepted for publication in Ap
Galaxy Zoo: constraining the origin of spiral arms
Since the discovery that the majority of low-redshift galaxies exhibit some level of spiral structure, a number of theories have been proposed as to why these patterns exist. A popular explanation is a process known as swing amplification, yet there is no observational evidence to prove that such a mechanism is at play. By using a number of measured properties of galaxies, and scaling relations where there are no direct measurements, we model samples of SDSS and S4G spiral galaxies in terms of their relative halo, bulge and disc mass and size. Using these models, we test predictions of swing amplification theory with respect to directly measured spiral arm numbers from Galaxy Zoo 2. We find that neither a universal cored or cuspy inner dark matter profile can correctly predict observed numbers of arms in galaxies. However, by invoking a halo contraction/expansion model, a clear bimodality in the spiral galaxy population emerges. Approximately 40 per cent of unbarred spiral galaxies at z 10^10 Msolar have spiral arms that can be modelled by swing amplification. This population display a significant correlation between predicted and observed spiral arm numbers, evidence that they are swing amplified modes. The remainder are dominated by two-arm systems for which the model predicts significantly higher arm numbers. These are likely driven by tidal interactions or other mechanisms
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