418 research outputs found
New observational Constraints on the Growth of the First Supermassive Black Holes
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. Studying galaxies
obtained from the deepest Hubble Space Telescope images combined with the
Chandra 4 Msec observations of the Chandra Deep Field South, we achieve the
most restrictive constraints on total black hole growth in the early Universe.
We estimate an accreted mass density <1000Mo Mpc^-3 at z~6, significantly lower
than the previous predictions from some existing models of early black hole
growth and earlier prior observations. These results place interesting
constraints on early black growth and mass assembly by accretion and imply one
or more of the following: (1) only a fraction of the luminous galaxies at this
epoch contain active black holes; (2) most black hole growth at early epochs
happens in dusty and/or less massive - as yet undetected - host galaxies; (3)
there is a significant fraction of low-z interlopers in the galaxy sample; (4)
early black hole growth is radiatively inefficient, heavily obscured and/or is
due to black hole mergers as opposed to accretion or (5) the bulk of the black
hole growth occurs at late times. All of these possibilities have important
implications for our understanding of high redshift seed formation models.Comment: ApJ Accepted, 10 pages, 7 figures, 1 table, in emulateapj forma
Improved and Quality-assessed Emission and Absorption Line measurements in Sloan Digital Sky Survey galaxies
We present a new database of absorption and emission-line measurements based
on the entire spectral atlas from the Sloan Digital Sky Survey (SDSS) 7th data
release of galaxies within a redshift of 0.2. Our work makes use of the
publicly available penalized pixel-fitting(pPXF) and gas and absorption line
fitting (gandalf) IDL codes, aiming to improve the existing measurements for
stellar kinematics, the strength of various absorption-line features, and the
flux and width of the emissions from different species of ionised gas. Our fit
to the stellar continuum uses both standard stellar population models and
empirical templates obtained by combining a large number of stellar spectra in
order to fit a subsample of high-quality SDSS spectra for quiescent galaxies.
Furthermore, our fit to the nebular spectrum includes an exhaustive list of
both recombination and forbidden lines. Foreground Galactic extinction is
implicitly treated in our models, whereas reddening in the SDSS galaxies is
included in the form of a simple dust screen component affecting the entire
spectrum that is accompanied by a second reddening component affecting only the
ionised gas emission. In order to check for systematic departures, we provide a
quality assessment for our fit to the SDSS spectra in our sample. This quality
assessment also allows the identification of objects with either problematic
data or peculiar features. For example, based on the quality assessment,
approximately 1% of the SDSS spectra classified as "galaxies" by the SDSS
pipeline do in fact require additional broad lines to be matched, even though
they do not show a strong continuum from an active nucleus, as do the SDSS
objects classified as "quasars". Finally, we provide new spectral templates for
galaxies of different Hubble types, obtained by combining the results of our
spectral fit for a subsample of 452 morphologically selected objects.Comment: Accepted for publication in ApJS. 23 pages, 14 figures, 4 tables. A
version with high-resolution figures is available at
http://gem.yonsei.ac.kr/~ksoh/ossy/arXiv/Oh_11_OSSY.pd
Major Galaxy Mergers Only Trigger the Most Luminous AGN
Using multiwavelength surveys of active galactic nuclei across a wide range
of bolometric luminosities (10^{43}<L_{bol}(erg/s<5x10^{46}) and redshifts
(0<z<3), we find a strong, redshift-independent correlation between the AGN
luminosity and the fraction of host galaxies undergoing a major merger. That
is, only the most luminous AGN phases are connected to major mergers, while
less luminous AGN appear to be driven by secular processes. Combining this
trend with AGN luminosity functions to assess the overall cosmic growth of
black holes, we find that ~50% by mass is associated with major mergers, while
only 10% of AGN by number, the most luminous, are connected to these violent
events. Our results suggest that to reach the highest AGN luminosities -where
the most massive black holes accreted the bulk of their mass - a major merger
appears to be required. The luminosity dependence of the fraction of AGN
triggered by major mergers can successfully explain why the observed scatter in
the M-\sigma relation for elliptical galaxies is significantly lower than in
spirals. The lack of a significant redshift dependence of the
L_{bol}-f_{merger} relation suggests that downsizing, i.e., the general decline
in AGN and star formation activity with decreasing redshift, is driven by a
decline in the frequency of major mergers combined with a decrease in the
availability of gas at lower redshifts.Comment: Accepted for publication by Astrophysical Journal Letters, 6 pages in
emulateapj format, 3 figure
Recent star formation in high-redshift early-type galaxies: insights from the rest-frame UV
We combine deep UBVRIzJK photometry from the MUSYC survey with redshifts from
the COMBO-17 survey to study the rest-frame ultraviolet (UV) properties of 674
high-redshift (0.5<z<1) early-type galaxies, drawn from the Extended Chandra
Deep Field South (E-CDFS). Galaxy morphologies are determined through visual
inspection of Hubble Space Telescope (HST) images taken from the GEMS survey.
We harness the sensitivity of the UV to young (<1 Gyrs old) stars to quantify
the recent star formation history of the early-type population. We find
compelling evidence that early-types of all luminosities form stars over the
lifetime of the Universe, although the bulk of their star formation is already
complete at high redshift. Luminous (-23<M(V)<-20.5) early-types form 10-15
percent of their mass after z=1, while their less luminous (M(V)>-20.5)
counterparts form 30-60 percent of their mass in the same redshift range.Comment: To appear in the proceedings of the IAU 245, eds. M. Bureau, E.
Athanassoula, and B. Barbu
The Space Density of Compton-thick AGN
We constrain the number density and evolution of Compton-thick Active
Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray
background. In the local Universe we use the wide area surveys from the Swift
and INTEGRAL satellites, while for high redshifts we explore candidate
selections based on mid-IR parameters. We present the properties of a sample of
211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South
(ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR
ratios for these sources are significantly larger than that of star-forming
galaxies and ~2 orders of magnitude smaller than for the general AGN
population, suggesting column densities of NH>5x10^24 cm^-2. The space density
of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a
strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.Comment: 4 pages, 3 figures, to appear in proceedings for 'X-ray Astronomy
2009', Bologna 09/2009, AIP Conference Series, Eds. A. Comastri, M. Cappi, L.
Angelini; author list fixe
The green valley is a red herring : Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early-and late-type galaxies
This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record [Kevin Schawinski, et al, 'The green valley is a red herring: Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies' MNRAS, Vol. 440(1): 889-907, May 2014] is available online at: https://doi.org/10.1093/mnras/stu327.We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in lowredshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of quiescent galaxies in the colour-mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue early-type galaxies move rapidly across the green valley after the morphologies are transformed from disc to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant discs, and they retain their late-type morphologies as their star formation rates decline very slowly. We summarize a range of observations that lead to these conclusions, including UV-optical colours and halo masses, which both show a striking dependence on morphological type. We interpret these results in terms of the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes. In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disc to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disc to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly active galactic nuclei feedback.Peer reviewe
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