40 research outputs found
Observational constraints on the spin of the most massive black holes from radio observations
We use recent progress in simulating the production of magnetohydrodynamic jets around black holes to derive the cosmic spin history of the most massive black holes, with masses >~10^8 Msol. Assuming the jet efficiency depends on spin a, we can approximately reproduce the observed `radio loudness' of quasars and the local radio luminosity function. Using the X-ray luminosity function and the local mass function of supermassive black holes, SMBHs we can reproduce the individual radio luminosity functions of radio sources showing high- and low-excitation narrow emission lines. The data favour spin distributions that are bimodal, with one component around spin zero and the other close to maximal spin. In the low-excitation galaxies, the two components have similar amplitudes. For the high-excitation galaxies, the amplitude of the high-spin peak is typically much smaller than that of the low-spin peak. A bimodality should be seen in the radio loudness of quasars. We predict that the low-excitation galaxies are dominated by SMBHs with masses >~10^8 Msol, down to radio luminosity densities ~10^21 W Hz-1 sr-1 at 1.4~GHz. Our model is also able to predict the radio luminosity function at z=1, and predicts it to be dominated by high-excitation galaxies above luminosity densities >~10^26 W Hz-1 sr-1, in full agreement with the observations. From our parametrisation and using the best fitting jet efficiencies there is marginal evidence for evolution in spin: the mean spin increases slightly from ~0.25 at z=1 to ~0.35 at z=0, and the fraction of SMBHs with a>=0.5 increases from 0.16+-0.03 at z=1 to 0.24+-0.09 at z=0. Our results are in excellent agreement with the mean radiative efficiency of quasars, as well as recent cosmological simulations. We discuss the implications in terms of accretion and SMBH mergers, and galactic black holes (Abridged).</a
VLT and GTC observations of SDSS J0123+00: a type 2 quasar triggered in a galaxy encounter?
We present long-slit spectroscopy, continuum and [OIII]5007 imaging data
obtained with the Very Large Telescope and the Gran Telescopio Canarias of the
type 2 quasar SDSS J0123+00 at z=0.399. The quasar lies in a complex, gas-rich
environment. It appears to be physically connected by a tidal bridge to another
galaxy at a projected distance of ~100 kpc, which suggests this is an
interacting system. Ionized gas is detected to a distance of at least ~133 kpc
from the nucleus. The nebula has a total extension of ~180 kpc. This is one of
the largest ionized nebulae ever detected associated with an active galaxy.
Based on the environmental properties, we propose that the origin of the nebula
is tidal debris from a galactic encounter, which could as well be the
triggering mechanism of the nuclear activity. SDSS J0123+00 demonstrates that
giant, luminous ionized nebulae can exist associated with type 2 quasars of low
radio luminosities, contrary to expectations based on type 1 quasar studies.Comment: 6 pages, 5 figures. Accepted for publication in MNRAS Letter
Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 III: Far-IR to Radio Properties and Optical Spectral Diagnostics
We present the far-IR, millimeter, and radio photometry as well as optical
and near-IR spectroscopy of a sample of 48 z~1-3 Spitzer-selected ULIRGs with
IRS mid-IR spectra. Our goals are to compute their bolometric emission, and to
determine both the presence and relative strength of their AGN and starburst
components. We find that strong-PAH sources tend to have higher 160um and 1.2mm
fluxes than weak-PAH sources. The depth of the 9.7um silicate feature does not
affect MAMBO detectability. We fit the far-IR SEDs of our sample and find an
average ~7x10^{12}Lsun for our z>1.5 sources. Spectral decomposition
suggests that strong-PAH sources typically have ~20-30% AGN fractions. Weak-PAH
sources by contrast tend to have >~70% AGN fractions, with a few sources having
comparable contributions of AGN and starbursts. The optical line diagnostics
support the presence of AGN in the bulk of the weak-PAH sources. With one
exception, our sources are narrow-line sources, show no obvious correspondence
between the optical extinction and the silicate feature depth, and, in two
cases, show evidence for outflows. Radio AGN are present in both strong-PAH and
weak-PAH sources. This is supported by our sample's far-IR-to-radio ratios (q)
being consistently below the average value of 2.34 for local star-forming
galaxies. We use survival analysis to include the lower-limits given by the
radio-undetected sources, arriving at =2.07+/-0.01 for our z>1.5 sample. In
total, radio and, where available, optical line diagnostics support the
presence of AGN in 57% of the z>1.5 sources, independent of IR-based
diagnostics. For higher-z sources, the AGN luminosities alone are estimated to
be >10^{12}Lsun, which, supported by the [OIII] luminosities, implies that the
bulk of our sources host obscured quasars.Comment: 22 pages, 14 figures, accepted for publication in Ap
A population of high-redshift type-2 quasars-II. Radio Properties
We present multi-frequency radio observations of a sample of z~2 obscured
(type-2) quasars in the Spitzer extragalactic First Look Survey area. We
combine the public data at 1.4 GHz, used in the selection of these sources,
with new observations at 610 MHz (GMRT) and at 4.9 GHz (VLA). We find the
sample includes sources with steep, flat and gigahertz-peaked spectra. There
are no strong correlations between the presence or absence of emission lines in
the optical spectra and the radio spectral properties of the sample. However,
there are no secure flat-spectrum type-2 quasars with narrow emission lines
which would be problematic for unified schemes. Most of the population have
straight radio spectra with spectral index alpha~1 as is expected for
developed, potentially FRI-like, jets in which continous injection of
relativistic electrons is accompanied by inverse-Compton losses against the
cosmic microwave background.Comment: 6 pages, 2 colour figures, submitted to MNRA
Chasing highly obscured QSOs in the COSMOS field
(abridged) We take advantage of the deep Chandra and Spitzer coverage of a
large area (more than 10 times the area covered by the Chandra deep fields,
CDFs in the COSMOS field, to extend the search of highly obscured,
Compton-thick active nuclei to higher luminosity. These sources have low
surface density and large samples can be provided only through large area
surveys, like the COSMOS survey. We analyze the X-ray properties of COSMOS MIPS
sources with 24m fluxes higher than 550Jy. For the MIPS sources not
directly detected in the Chandra images we produce stacked images in soft and
hard X-rays bands. To estimate the fraction of Compton-thick AGN in the MIPS
source population we compare the observed stacked count rates and hardness
ratios to those predicted by detailed Monte Carlo simulations including both
obscured AGN and star-forming galaxies. The density of lower luminosity
Compton-thick AGN (logL(2-10keV)=43.5-44) at z=0.7--1.2 is Mpc, corresponding to of that of X-ray selected
AGN. The comparison between the fraction of infrared selected, Compton thick
AGN to the X-ray selected, unobscured and moderately obscured AGN at high and
low luminosity suggests that Compton-thick AGN follow a luminosity dependence
similar to that discovered for Compton-thin AGN, becoming relatively rarer at
high luminosities. We estimate that the fraction of AGN (unobscured, moderately
obscured and Compton thick) to the total MIPS source population is ,
a value significantly higher than that previously estimated at similar 24m
fluxes. We discuss how our findings can constrain AGN feedback models.Comment: Paper submitted to ApJ. This is the revised version after all
referee's comments have been addresse
Molecular Gas in a Submillimeter Galaxy at z=4.5: Evidence for a Major Merger at 1 Billion Years after the Big Bang
We report the detection of CO molecular line emission in the z=4.5
millimeter-detected galaxy COSMOS_J100054+023436 (hereafter: J100+0234) using
the IRAM Plateau de Bure interferometer (PdBI) and NRAO's Very Large Array
(VLA). The CO(4-3) line as observed with PdBI has a full line width of ~1000
km/s, an integrated line flux of 0.66 Jy km/s, and a CO luminosity of 3.2e10
L_sun. Comparison to the 3.3sigma detection of the CO(2-1) line emission with
the VLA suggests that the molecular gas is likely thermalized to the J=4-3
transition level. The corresponding molecular gas mass is 2.6e10 M_sun assuming
an ULIRG-like conversion factor. From the spatial offset of the red- and
blue-shifted line peaks and the line width a dynamical mass of 1.1e11 M_sun is
estimated assuming a merging scenario. The molecular gas distribution coincides
with the rest-frame optical and radio position of the object while being offset
by 0.5'' from the previously detected Ly emission. J1000+0234 exhibits
very typical properties for lower redshift (z~2) sub-millimeter galaxies (SMGs)
and thus is very likely one of the long sought after high redshift (z>4)
objects of this population. The large CO(4-3) line width taken together with
its highly disturbed rest-frame UV geometry suggest an ongoing major merger
about a billion years after the Big Bang. Given its large star formation rate
(SFR) of >1000 M_sun/yr and molecular gas content this object could be the
precursor of a 'red-and-dead' elliptical observed at a redshift of z=2.Comment: 12 pages, 3 figures, accepted for publications by ApJ
Spectral Energy Distributions of type 2 QSOs: obscured star formation at high redshifts
We present new mid-infrared and submillimetre observations for a sample of
eight high redshift type-2 QSOs located in the Chandra Deep Field South. The
sources are X-ray absorbed with luminosities in excess of 10^44 erg/s. Two of
the targets have robust detections, S/N > 4, while a further three targets are
marginally detected with S/N > =2.5. All sources are detected in multiple
mid-infrared bands with the Spitzer Space Telescope. The multiwavelength
spectral energy distributions (SEDs) of the type-2 QSOs are compared to those
of two local ultraluminous galaxies (Arp220 and IR22491) in order to assess
contributions from a star-forming component in various parts of the SED. We
suggest that their submillimetre emission is possibly due to a starburst while
a large fraction of the mid-infrared energy is likely to originate in the
obscured central quasar. Using the mid-infrared and submm observations we
derive infrared luminosities which are found to be in excess of L>10^12Lsun.
The submillimetre (850micron) to X-ray (2 keV) spectral indices (alpha_SX) span
a wide range. About half of the type-2 QSOs have values typical for a
Compton-thick AGN with only 1 per cent of the nuclear emission seen through
scattering and, the remaining with values typical of submm-bright galaxies.
Combining the available observational evidence we outline a possible scenario
for the early stages of evolution of these sources.Comment: Accepted for publication in MNRA
The dusty heart of nearby active galaxies. I. High-spatial resolution mid-IR spectro-photometry of Seyfert galaxies
We present 8-13 micron imaging and spectroscopy of 9 type 1 and 10 type 2 AGN
obtained with the VLT/VISIR instrument at spatial resolution <100 pc. The
emission from the host galaxy sources is resolved out in most cases. The
silicate absorption features are moderately deep and emission features are
shallow. We compare the mid-IR luminosities to AGN luminosity tracers and found
that the mid-IR radiation is emitted quite isotropically. In two cases, IC5063
and MCG-3-34-64, we find evidence for extended dust emission in the narrow-line
region. We confirm the correlation between observed silicate feature strength
and Hydrogen column density recently found in Spitzer data. In a further step,
our 3D clumpy torus model has been used to interpret the data. We show that the
strength of the silicate feature and the mid-IR spectral index can be used to
get reasonable constraints on the dust distribution in the torus. The mid-IR
spectral index, alpha, is almost exclusively determined by the radial dust
distribution power-law index, a, and the silicate feature depth is mostly
depending on the average number of clouds, N0, along an equatorial
line-of-sight and the torus inclination. A comparison of model predictions to
our type 1 and type 2 AGN reveals typical average parameters a=-1.0+/-0.5 and
N0=5-8, which means that the radial dust distribution is rather shallow. As a
proof-of-concept of this method, we compared the model parameters derived from
alpha and the silicate feature to more detailed studies of IR SEDs and
interferometry and found that the constraints on a and N0 are consistent.
Finally, we might have found evidence that the radial structure of the torus
changes from low to high AGN luminosities towards steeper dust distributions,
and we discuss implications for the IR size-luminosity relation. (abridged)Comment: 22 pages, 13 figues, 6 tables; Accepted for publication in A&A; Note
that this is the second submitted paper from the series, but we changed paper
order. This one will be referred to as paper I, the previously submitted
arXiv:0909.4539 will become paper I
A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation
We present Hubble Space Telescope Wide Field Camera 3 slitless grism
spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The
high sensitivity of these grism observations, with 1-sigma detections of
emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the
sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than
previously studied z~2 emission-line galaxies. Despite their lower mass, the
galaxies have OIII/Hb ratios which are very similar to previously studied z~2
galaxies and much higher than the typical emission-line ratios of local
galaxies. The WFC3 grism allows for unique studies of spatial gradients in
emission lines, and we stack the two-dimensional spectra of the galaxies for
this purpose. In the stacked data the OIII emission line is more spatially
concentrated than the Hb emission line with 98.1 confidence. We additionally
stack the X-ray data (all sources are individually undetected), and find that
the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0
obscured active galaxies and star-forming galaxies. Together the compactness of
the stacked OIII spatial profile and the stacked X-ray data suggest that at
least some of these low-mass, low-metallicity galaxies harbor weak active
galactic nuclei.Comment: ApJ accepted. 8 pages, 6 figure