721 research outputs found
Star formation across cosmic time with radio surveys. The promise of the SKA
This lecture briefly reviews the major recent advances in radio astronomy
made possible by ultra-deep surveys, reaching microJansky flux density levels.
A giant step forward in many fields, including the study of the evolution of
the cosmic star formation history is expected with the advent of the Square
Kilometer Array (SKA).Comment: 28 pages, 3 figures, to be published in the Proceedings of the 3rd
Cosmology School in Cracow, July 201
A universal average spectral energy distribution for quasars from optical to extreme ultraviolet
The well-known anti-correlation between the optical/ultraviolet (UV) emission
line equivalent widths of active galactic nuclei and the continuum luminosity
(the so-called Baldwin effect) is a long-standing puzzle. One common hypothesis
is that more luminous sources have softer spectral energy distribution (SED) in
the extreme UV (EUV), as revealed by some observational studies. In this work
we revisit this issue through cross-matching SDSS quasars with GALEX
far-UV/near-UV catalogs and correcting the effect of a severe observational
bias of significant UV detection incompleteness, i.e., the more luminous in
observed-frame optical, the more likely detected in observed-frame UV. We find
that, for GALEX detected quasars at 1.8 < z < 2.2, the rest-frame mean UV SED
(~ 500 -- 3000 Angstrom) bewilderingly shows no luminosity dependence at
log(\nu L_\nu(2200 Angstrom)) > 45 (up to 47.3), contrary to the standard thin
disc model predictions and the observed Baldwin effect in this luminosity
range. Probably, the universal mean UV SED is the result of a local
atomic-originated process, and in fainter quasars stronger disk turbulence
launching more clouds is the main origin of the Baldwin effect. After
correcting for the absorption of the intergalactic medium, a rest-frame
intrinsic mean EUV SED is derived from a sub-sample of bright quasars and is
found to be much redder in the EUV than all previous quasar composite spectra,
highlighting the significance of properly accounting for the sample
incompleteness. Interestingly, the global consistence between our extremely red
mean EUV SED and the line-driven wind model again supports an origin of a local
physical process.Comment: 27 pages, 15 figures, author's initial version submitted to Nature
Astronom
EUCLIA - Exploring the UV/optical continuum lag in active galactic nuclei. I. a model without light echoing
The tight inter-band correlation and the lag-wavelength relation among
UV/optical continua of active galactic nuclei have been firmly established.
They are usually understood within the widespread reprocessing scenario,
however, the implied inter-band lags are generally too small. Furthermore, it
is challenged by new evidences, such as the X-ray reprocessing yields too much
high frequency UV/optical variations as well as it fails to reproduce the
observed timescale-dependent color variations among {\it Swift} lightcurves of
NGC 5548. In a different manner, we demonstrate that an upgraded inhomogeneous
accretion disk model, whose local {\it independent} temperature fluctuations
are subject to a speculated {\it common} large-scale temperature fluctuation,
can intrinsically generate the tight inter-band correlation and lag across
UV/optical, and be in nice agreement with several observational properties of
NGC 5548, including the timescale-dependent color variation. The emergent lag
is a result of the {\it differential regression capability} of local
temperature fluctuations when responding to the large-scale fluctuation. An
average speed of propagations as large as of the speed of light
may be required by this common fluctuation. Several potential physical
mechanisms for such propagations are discussed. Our interesting
phenomenological scenario may shed new light on comprehending the UV/optical
continuum variations of active galactic nuclei.Comment: 18 pages, 8 figures. ApJ accepted. Further comments are very welcome
An intrinsic link between long-term UV/optical variations and X-ray loudness in quasars
Observations have shown that UV/optical variation amplitude of quasars depend
on several physi- cal parameters including luminosity, Eddington ratio, and
likely also black hole mass. Identifying new factors which correlate with the
variation is essential to probe the underlying physical processes. Combining
~ten years long quasar light curves from SDSS stripe 82 and X-ray data from
Stripe 82X, we build a sample of X-ray detected quasars to investigate the
relation between UV/optical variation amplitude () and X-ray
loudness. We find that quasars with more intense X-ray radiation (com- pared to
bolometric luminosity) are more variable in UV/optical. Such correlation
remains highly significant after excluding the effect of other parameters
including luminosity, black hole mass, Ed- dington ratio, redshift, rest-frame
wavelength (i.e., through partial correlation analyses). We further find the
intrinsic link between X-ray loudness and UV/optical variation is gradually
more prominent on longer timescales (up to 10 years in the observed frame), but
tends to disappear at timescales < 100 days. This suggests a slow and long-term
underlying physical process. The X-ray reprocessing paradigm, in which
UV/optical variation is produced by a variable central X-ray emission
illuminating the accretion disk, is thus disfavored. The discovery points to an
interesting scheme that both the X-ray corona heating and UV/optical variation
is quasars are closely associated with magnetic disc turbulence, and the
innermost disc turbulence (where corona heating occurs) correlates with the
slow turbulence at larger radii (where UV/optical emission is produced).Comment: 9 pages, 4 figures, 1 table, accepted by Ap
Interpreting the statistical properties of High- z extragalactic sources detected by the South Pole Telescope Survey
Abstract: The results of the recently published spectroscopically complete survey of dusty star-forming galaxies detected by the South Pole Telescope over 2500 deg2 proved to be challenging for galaxy formation models that generally underpredict the observed abundance of high-z galaxies. In this paper we interpret these results in the light of a physically grounded model for the evolution of spheroidal galaxies. The model accurately reproduces the measured redshift distribution of galaxies without any adjustment of the parameters. The data do not support the indications of an excess of z > 4 dusty galaxies reported by some analyses of Herschel surveys
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