422 research outputs found
Deep near-infrared imaging of the HE0450-2958 system
The QSO HE0450-2958 and the companion galaxy with which it is interacting,
both ultra luminous in the infrared, have been the subject of much attention in
recent years, as the quasar host galaxy remained undetected. This led to
various interpretations on QSO and galaxy formation and co-evolution, such as
black hole ejection, jet induced star formation, dust obscured galaxy, or
normal host below the detection limit. We carried out deep observations in the
near-IR in order to solve the puzzle concerning the existence of any host. The
object was observed with the ESO VLT and HAWK-I in the near-IR J-band for 8
hours. The images have been processed with the MCS deconvolution method
(Magain, Courbin & Sohy, 1998), permitting accurate subtraction of the QSO
light from the observations. The compact emission region situated close to the
QSO, called the blob, which previously showed only gas emission lines in the
optical spectra, is now detected in our near-IR images. Its high brightness
implies that stars likely contribute to the near-IR emission. The blob might
thus be interpreted as an off-centre, bright and very compact host galaxy,
involved in a violent collision with its companion.Comment: 4 pages, 3 figures, accepted for publication in A&
Blazars in the early Universe
We investigate the relative occurrence of radio--loud and radio-quiet quasars
in the first billion years of the Universe, powered by black holes heavier than
one billion solar masses. We consider the sample of high-redshfit blazars
detected in the hard X-ray band in the 3-years all sky survey performed by the
Burst Alert Telescope (BAT) onboard the Swift satellite. All the black holes
powering these blazars exceed a billion solar mass, with accretion luminosities
close to the Eddington limit. For each blazar pointing at us, there must be
hundreds of similar sources (having black holes of similar masses) pointing
elsewhere. This puts constraints on the density of billion solar masses black
holes at high redshift (z>4), and on the relative importance of (jetted)
radio-loud vs radio-quiet sources. We compare the expected number of high
redshift radio--loud sources with the high luminosity radio-loud quasars
detected in the Sloan Digital Sky Survey (SDSS), finding agreement up to z~3,
but a serious deficit at z>3 of SDSS radio-loud quasars with respect to the
expectations. We suggest that the most likely explanations for this
disagreement are: i) the ratio of blazar to misaligned radio-sources decreases
by an order of magnitude above z=3, possibly as a result of a decrease of the
average bulk Lorentz factor; ii) the SDSS misses a large fraction of radio-loud
sources at high redshifts, iii) the SDSS misses both radio-loud and radio-quiet
quasars at high redshift, possibly because of obscuration or because of
collimation of the optical-UV continuum in systems accreting near Eddington.
These explanations imply very different number density of heavy black holes at
high redshifts, that we discuss in the framework of the current ideas about the
relations of dark matter haloes at high redshifts and the black hole they host.Comment: MNRAS, in pres
Urea Activation by an External Brønsted Acid: Breaking Self-Association and Tuning Catalytic Performance
In this work, we hypothesize that Bronsted acids can activate urea-based catalysts by diminishing its self-assembly tendency. As a proof of concept, we used the asymmetric Friedel-Crafts alkylation of indoles with nitroalkenes as a benchmark reaction. The resulting 3-substituted indole derivatives were obtained with better results due to cooperative effects of the chiral urea and a Bronsted acid additive. Such synergy has been rationalized in terms of disassembly of the supramolecular catalyst aggregates, affording a more acidic and rigid catalytic complex
Constraining stellar assembly and AGN feedback at the peak epoch of star formation
We study stellar assembly and feedback from active galactic nuclei (AGN)
around the epoch of peak star formation (1<z<2), by comparing hydrodynamic
simulations to rest-frame UV-optical galaxy colours from the Wide Field Camera
3 (WFC3) Early-Release Science (ERS) Programme. Our Adaptive Mesh Refinement
simulations include metal-dependent radiative cooling, star formation, kinetic
outflows due to supernova explosions, and feedback from supermassive black
holes. Our model assumes that when gas accretes onto black holes, a fraction of
the energy is used to form either thermal winds or sub-relativistic
momentum-imparting collimated jets, depending on the accretion rate. We find
that the predicted rest-frame UV-optical colours of galaxies in the model that
includes AGN feedback is in broad agreement with the observed colours of the
WFC3 ERS sample at 1<z<2. The predicted number of massive galaxies also matches
well with observations in this redshift range. However, the massive galaxies
are predicted to show higher levels of residual star formation activity than
the observational estimates, suggesting the need for further suppression of
star formation without significantly altering the stellar mass function. We
discuss possible improvements, involving faster stellar assembly through
enhanced star formation during galaxy mergers while star formation at the peak
epoch is still modulated by the AGN feedback.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
Clustering of Radio Galaxies and Quasars
We compute the cross-correlation between a sample of 14,000 radio-loud AGN
(RLAGN) with redshifts between 0.4 and 0.8 selected from the Sloan Digital Sky
Survey and a reference sample of 1.2 million luminous red galaxies in the same
redshift range. We quantify how the clustering of radio-loud AGN depends on
host galaxy mass and on radio luminosity. Radio-loud AGN are clustered more
strongly on all scales than control samples of radio-quiet galaxies with the
same stellar masses and redshifts, but the differences are largest on scales
less than 1 Mpc. In addition, the clustering amplitude of the RLAGN varies
significantly with radio luminosity on scales less than 1 Mpc. This proves that
the gaseous environment of a galaxy on the scale of its dark matter halo, plays
a key role in determining not only the probability that a galaxy is radio-loud
AGN, but also the total luminosity of the radio jet. Next, we compare the
clustering of radio galaxies with that of radio-loud quasars in the same
redshift range. Unified models predict that both types of active nuclei should
cluster in the same way. Our data show that most RLAGN are clustered more
strongly than radio-loud QSOs, even when the AGN and QSO samples are matched in
both black hole mass and radio luminosity. Only the most extreme RLAGN and
RLQSOs in our sample, with radio luminosities in excess of 10^26 W/Hz, have
similar clustering properties. The majority of the strongly evolving RLAGN
population at z~0.5 are found in different environments to the quasars, and
hence must be triggered by a different physical mechanism.Comment: 12 pages, 13 Figures, submitted to MNRA
An ultraviolet-optical flare from the tidal disruption of a helium-rich stellar core
The flare of radiation from the tidal disruption and accretion of a star can
be used as a marker for supermassive black holes that otherwise lie dormant and
undetected in the centres of distant galaxies. Previous candidate flares have
had declining light curves in good agreement with expectations, but with poor
constraints on the time of disruption and the type of star disrupted, because
the rising emission was not observed. Recently, two `relativistic' candidate
tidal disruption events were discovered, each of whose extreme X-ray luminosity
and synchrotron radio emission were interpreted as the onset of emission from a
relativistic jet. Here we report the discovery of a luminous
ultraviolet-optical flare from the nuclear region of an inactive galaxy at a
redshift of 0.1696. The observed continuum is cooler than expected for a simple
accreting debris disk, but the well-sampled rise and decline of its light curve
follows the predicted mass accretion rate, and can be modelled to determine the
time of disruption to an accuracy of two days. The black hole has a mass of
about 2 million solar masses, modulo a factor dependent on the mass and radius
of the star disrupted. On the basis of the spectroscopic signature of ionized
helium from the unbound debris, we determine that the disrupted star was a
helium-rich stellar core.Comment: To appear in Nature on May 10, 201
Viruses in extreme environments
The original publication is available at www.springerlink.comInternational audienceThe tolerance limits of extremophiles in term of temperature, pH, salinity, desiccation, hydrostatic pressure, radiation, anaerobiosis far exceed what can support non-extremophilic organisms. Like all other organisms, extremophiles serve as hosts for viral replication. Many lines of evidence suggest that viruses could no more be regarded as simple infectious ‘‘fragments of life'' but on the contrary as one of the major components of the biosphere. The exploration of niches with seemingly harsh life conditions as hypersaline and soda lakes, Sahara desert, polar environments or hot acid springs and deep sea hydrothermal vents, permitted to track successfully the presence of viruses. Substantial populations of double-stranded DNA virus that can reach 109 particles per milliliter were recorded. All these viral communities, with genome size ranging from 14 kb to 80 kb, seem to be genetically distinct, suggesting specific niche adaptation. Nevertheless, at this stage of the knowledge, very little is known of their origin, activity, or importance to the in situ microbial dynamics. The continuous attempts to isolate and to study viruses that thrive in extreme environments will be needed to address such questions. However, this topic appears to open a new window on an unexplored part of the viral world
Cosmic Evolution of Size and Velocity Dispersion for Early Type Galaxies
[abridged] Massive, passively evolving galaxies at redshifts z>1 exhibit on
the average physical sizes smaller by factors ~3 than local early type galaxies
(ETGs) endowed with the same stellar mass. Small sizes are in fact expected on
theoretical grounds, if dissipative collapse occurs. Recent results show that
the size evolution at z<1 is limited to less than 40%, while most of the
evolution occurs at z>1, where both compact and already extended galaxies are
observed and the scatter in size is remarkably larger than locally. The
presence at high z of a significant number of ETGs with the same size as their
local counterparts as well as of ETGs with quite small size, points to a
timescale to reach the new, expanded equilibrium configuration of less than the
Hubble time. We demonstrate that the projected mass of compact, high-z galaxies
and that of local ETGs within the *same physical radius*, the nominal
half-luminosity radius of high-z ETGs, differ substantially, in that the high-z
ETGs are on the average significantly denser. We propose that quasar activity,
which peaks at z~2, can remove large amounts of gas from central galaxy regions
on a timescale shorter than of the dynamical one, triggering a puffing up of
the stellar component at constant stellar mass; in this case the size increase
goes together with a decrease of the central mass. The size evolution is
expected to parallel that of the quasars and the inverse hierarchy, or
downsizing, seen in the quasar evolution is mirrored in the size evolution.Comment: 18 pages, 6 figures, uses RevTeX4 + emulateapj.cls and apjfonts.sty.
Accepted by Ap
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