148 research outputs found
Sample variance and Lyman α forest transmission statistics
We compare the observed probability distribution function (PDF) of the transmission in the H I Lyman α forest, measured from the Ultraviolet and Visual Echelle Spectrograph (UVES) âLarge Programmeâ sample at redshifts z = [2, 2.5, 3], to results from the GIMIC cosmological simulations. Our measured values for the mean transmission and its PDF are in good agreement with published results. Errors on statistics measured from high-resolution data are typically estimated using bootstrap or jackknife resampling techniques after splitting the spectra into chunks. We demonstrate that these methods tend to underestimate the sample variance unless the chunk size is much larger than is commonly the case. We therefore estimate the sample variance from the simulations. We conclude that observed and simulated transmission statistics are in good agreement; in particular, we do not require the temperatureâdensity relation to be âinvertedâ
The Extended Baryon Oscillation Spectroscopic Survey: Variability Selection and Quasar Luminosity Function
The SDSS-IV/eBOSS has an extensive quasar program that combines several
selection methods. Among these, the photometric variability technique provides
highly uniform samples, unaffected by the redshift bias of traditional
optical-color selections, when quasars cross the stellar locus
or when host galaxy light affects quasar colors at . Here, we present
the variability selection of quasars in eBOSS, focusing on a specific program
that led to a sample of 13,876 quasars to over a 94.5
deg region in Stripe 82, an areal density 1.5 times higher than over the
rest of the eBOSS footprint. We use these variability-selected data to provide
a new measurement of the quasar luminosity function (QLF) in the redshift range
. Our sample is denser, reaches deeper than those used in previous
studies of the QLF, and is among the largest ones. At the faint end, our QLF
extends to at low redshift and to
at . We fit the QLF using two independent double-power-law models with
ten free parameters each. The first model is a pure luminosity-function
evolution (PLE) with bright-end and faint-end slopes allowed to be different on
either side of . The other is a simple PLE at , combined with a
model that comprises both luminosity and density evolution (LEDE) at .
Both models are constrained to be continuous at . They present a
flattening of the bright-end slope at large redshift. The LEDE model indicates
a reduction of the break density with increasing redshift, but the evolution of
the break magnitude depends on the parameterization. The models are in
excellent accord, predicting quasar counts that agree within 0.3\% (resp.,
1.1\%) to (resp., ). The models are also in good agreement over
the entire redshift range with models from previous studies.Comment: 15 pages, 12 figures, accepted for publication in A&
Discovery of a Perseus-like cloud in the early Universe: HI-to-H2 transition, carbon monoxide and small dust grains at zabs=2.53 towards the quasar J0000+0048
We present the discovery of a molecular cloud at zabs=2.5255 along the line
of sight to the quasar J0000+0048. We perform a detailed analysis of the
absorption lines from ionic, neutral atomic and molecular species in different
excitation levels, as well as the broad-band dust extinction. We find that the
absorber classifies as a Damped Lyman-alpha system (DLA) with
logN(HI)(cm^-2)=20.8+/-0.1. The DLA has super-Solar metallicity with a
depletion pattern typical of cold gas and an overall molecular fraction ~50%.
This is the highest f-value observed to date in a high-z intervening system.
Most of the molecular hydrogen arises from a clearly identified narrow (b~0.7
km/s), cold component in which CO molecules are also found, with logN(CO)~15.
We study the chemical and physical conditions in the cold gas. We find that the
line of sight probes the gas deep after the HI-to-H2 transition in a ~4-5
pc-size cloud with volumic density nH~80 cm^-3 and temperature of only 50 K.
Our model suggests that the presence of small dust grains (down to about 0.001
{\mu}m) and high cosmic ray ionisation rate (zeta_H a few times 10^-15 s^-1)
are needed to explain the observed atomic and molecular abundances. The
presence of small grains is also in agreement with the observed steep
extinction curve that also features a 2175 A bump. The properties of this cloud
are very similar to what is seen in diffuse molecular regions of the nearby
Perseus complex. The high excitation temperature of CO rotational levels
towards J0000+0048 betrays however the higher temperature of the cosmic
microwave background. Using the derived physical conditions, we correct for a
small contribution (0.3 K) of collisional excitation and obtain TCMB(z =
2.53)~9.6 K, in perfect agreement with the predicted adiabatic cooling of the
Universe. [abridged]Comment: 24 pages, 24 figures, accepted for publication in A&
Nature and statistical properties of quasar associated absorption systems in the XQ-100 Legacy Survey
We statistically study the physical properties of a sample of narrow
absorption line (NAL) systems looking for empirical evidences to distinguish
between intrinsic and intervening NALs without taking into account any a priori
definition or velocity cut-off. We analyze the spectra of 100 quasars with 3.5
< z < 4.5, observed with X-shooter/VLT in the context of the XQ-100
Legacy Survey. We detect a 8 excess in the number density of
absorbers within 10,000 km/s of the quasar emission redshift with respect to
the random occurrence of NALs. This excess does not show a dependence on the
quasar bolometric luminosity and it is not due to the redshift evolution of
NALs. It extends far beyond the standard 5000 km/s cut-off traditionally
defined for associated absorption lines. We propose to modify this definition,
extending the threshold to 10,000 km/s when also weak absorbers (equivalent
width < 0.2 \AA) are considered. We infer NV is the ion that better traces the
effects of the quasar ionization field, offering the best statistical tool to
identify intrinsic systems. Following this criterion we estimate that the
fraction of quasars in our sample hosting an intrinsic NAL system is 33
percent. Lastly, we compare the properties of the material along the quasar
line of sight, derived from our sample, with results based on close quasar
pairs investigating the transverse direction. We find a deficiency of cool gas
(traced by CII) along the line of sight associated with the quasar host galaxy,
in contrast with what is observed in the transverse direction.Comment: 18 pages, 13 figures, 5 table
An 800-million-solar-mass black hole in a significantly neutral Universe at redshift 7.5
Quasars are the most luminous non-transient objects known and as a result
they enable studies of the Universe at the earliest cosmic epochs. Despite
extensive efforts, however, the quasar ULAS J1120+0641 at z=7.09 has remained
the only one known at z>7 for more than half a decade. Here we report
observations of the quasar ULAS J134208.10+092838.61 (hereafter J1342+0928) at
redshift z=7.54. This quasar has a bolometric luminosity of 4e13 times the
luminosity of the Sun and a black hole mass of 8e8 solar masses. The existence
of this supermassive black hole when the Universe was only 690 million years
old---just five percent of its current age---reinforces models of early
black-hole growth that allow black holes with initial masses of more than about
1e4 solar masses or episodic hyper-Eddington accretion. We see strong evidence
of absorption of the spectrum of the quasar redwards of the Lyman alpha
emission line (the Gunn-Peterson damping wing), as would be expected if a
significant amount (more than 10 per cent) of the hydrogen in the intergalactic
medium surrounding J1342+0928 is neutral. We derive a significant fraction of
neutral hydrogen, although the exact fraction depends on the modelling.
However, even in our most conservative analysis we find a fraction of more than
0.33 (0.11) at 68 per cent (95 per cent) probability, indicating that we are
probing well within the reionization epoch of the Universe.Comment: Updated to match the final journal versio
Variability selected high-redshift quasars on SDSS Stripe 82
The SDSS-III BOSS Quasar survey will attempt to observe z>2.15 quasars at a
density of at least 15 per square degree to yield the first measurement of the
Baryon Acoustic Oscillations in the Ly-alpha forest. To help reaching this
goal, we have developed a method to identify quasars based on their variability
in the u g r i z optical bands. The method has been applied to the selection of
quasar targets in the SDSS region known as Stripe 82 (the Southern equatorial
stripe), where numerous photometric observations are available over a 10-year
baseline. This area was observed by BOSS during September and October 2010.
Only 8% of the objects selected via variability are not quasars, while 90% of
the previously identified high-redshift quasar population is recovered. The
method allows for a significant increase in the z>2.15 quasar density over
previous strategies based on optical (ugriz) colors, achieving a density of
24.0 deg^{-2} on average down to g~22 over the 220 deg^2 area of Stripe 82. We
applied this method to simulated data from the Palomar Transient Factory and
from Pan-STARRS, and showed that even with data that have sparser time sampling
than what is available in Stripe 82, including variability in future quasar
selection strategies would lead to increased target selection efficiency in the
z>2.15 redshift range. We also found that Broad Absorption Line quasars are
preferentially present in a variability than in a color selection.Comment: 14 pages, 21 figures, accepted for publication in A&
Discovery of extreme [O III] λ5007 à outflows in high-redshift red quasars
International audienceBlack hole feedback is now a standard component of galaxy formation models. These models predict that the impact of black hole activity on its host galaxy likely peaked at z = 2-3, the epoch of strongest star formation activity and black hole accretion activity in the Universe. We used XSHOOTER on the Very Large Telescope to measure rest-frame optical spectra of four z similar to 2.5 extremely red quasars with infrared luminosities similar to 10(47) erg s(-1). We present the discovery of very broad (full width at half max = 2600-5000 km s(-1)), strongly blueshifted (by up to 1500 km s(-1)) [O III] lambda 5007 angstrom emission lines in these objects. In a large sample of type 2 and red quasars, [O III] kinematics are positively correlated with infrared luminosity, and the four objects in our sample are on the extreme end in both [O III] kinematics and infrared luminosity. We estimate that at least 3 per cent of the bolometric luminosity in these objects is being converted into the kinetic power of the observed wind. Photo-ionization estimates suggest that the [O III] emission might be extended on a few kpc scales, which would suggest that the extreme outflow is affecting the entire host galaxy of the quasar. These sources may be the signposts of the most extreme form of quasar feedback at the peak epoch of galaxy formation, and may represent an active `blow-out' phase of quasar evolution
The Sloan Digital Sky Survey Reverberation Mapping Project: Rapid CIV Broad Absorption Line Variability
We report the discovery of rapid variations of a high-velocity CIV broad
absorption line trough in the quasar SDSS J141007.74+541203.3. This object was
intensively observed in 2014 as a part of the Sloan Digital Sky Survey
Reverberation Mapping Project, during which 32 epochs of spectroscopy were
obtained with the Baryon Oscillation Spectroscopic Survey spectrograph. We
observe significant (>4sigma) variability in the equivalent width of the broad
(~4000 km/s wide) CIV trough on rest-frame timescales as short as 1.20 days
(~29 hours), the shortest broad absorption line variability timescale yet
reported. The equivalent width varied by ~10% on these short timescales, and by
about a factor of two over the duration of the campaign. We evaluate several
potential causes of the variability, concluding that the most likely cause is a
rapid response to changes in the incident ionizing continuum. If the outflow is
at a radius where the recombination rate is higher than the ionization rate,
the timescale of variability places a lower limit on the density of the
absorbing gas of n_e > 3.9 x 10^5 cm^-3. The broad absorption line variability
characteristics of this quasar are consistent with those observed in previous
studies of quasars, indicating that such short-term variability may in fact be
common and thus can be used to learn about outflow characteristics and
contributions to quasar/host-galaxy feedback scenarios.Comment: 15 pages, 14 figures. Accepted for publication in the Astrophysical
Journa
XQ-100: A legacy survey of one hundred 3.5 72 z 72 4.5 quasars observed with VLT/X-shooter
We describe the execution and data reduction of the European Southern Observatory Large Programme "Quasars and their absorption lines: a legacy survey of the high-redshift Universe with VLT/X-shooter" (hereafter "XQ-100"). XQ-100 has produced and made publicly available a homogeneous and high-quality sample of echelle spectra of 100 quasars (QSOs) at redshifts z 43 3.5-4.5 observed with full spectral coverage from 315 to 2500 nm at a resolving power ranging from R \u303. 4000 to 7000, depending on wavelength. The median signal-to-noise ratios are 33, 25 and 43, as measured at rest-frame wavelengths 1700, 3000 and 3600 \uc5, respectively. This paper provides future users of XQ-100 data with the basic statistics of the survey, along with details of target selection, data acquisition and data reduction. The paper accompanies the public release of all data products, including 100 reduced spectra. XQ-100 is the largest spectroscopic survey to date of high-redshift QSOs with simultaneous rest-frame UV/optical coverage, and as such enables a wide range of extragalactic research, from cosmology and galaxy evolution to AGN astrophysics. \ua9 2016 ESO
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