832 research outputs found
A Hubble diagram for quasars
We present a new method to test the ÎCDM cosmological model and to estimate cosmological parameters based on the nonlinear relation between the ultraviolet and X-ray luminosities of quasars. We built a data set of 1138 quasars by merging several samples from the literature with X-ray measurements at 2 keV and SDSS photometry, which was used to estimate the extinction-corrected 2500 Ă
flux. We obtained three main results: (1) we checked the nonlinear relation between X-ray and UV luminosities in small redshift bins up to z ~ 6, confirming that the relation holds at all redshifts with the same slope; (2) we built a Hubble diagram for quasars up to z ~ 6, which is well matched to that of supernovae in the common z = 0â1.4 redshift interval and extends the test of the cosmological model up to z ~ 6; and (3) we showed that this nonlinear relation is a powerful tool for estimating cosmological parameters. Using the present data and assuming a ÎCDM model, we obtain WM = 0.22 0.08 0.10 - + and WL = 0.92 0.30 0.18 - + (WM = 0.28 ± 0.04 and WL = 0.73 0.08 from a joint quasar-SNe fit). Much more precise measurements will be achieved with future surveys. A few thousand SDSS quasars already have serendipitous X-ray observations from Chandra or XMM-Newton, and at least 100,000 quasars with UV and X-ray data will be made available by the extended ROentgen Survey with an Imaging Telescope Array all-sky survey in a few years. The Euclid, Large Synoptic Survey Telescope, and Advanced Telescope for High ENergy Astrophysics surveys will further increase the sample size to at least several hundred thousand. Our simulations show that these samples will provide tight constraints on the cosmological parameters and will allow us to test for possible deviations from the standard model with higher precision than is possible today
Is there any evidence that ionised outflows quench star formation in type 1 quasars at z<1?
The aim of this paper is to test the basic model of negative AGN feedback.
According to this model, once the central black hole accretes at the Eddington
limit and reaches a certain critical mass, AGN driven outflows blow out gas,
suppressing star formation in the host galaxy and self-regulating black hole
growth. We consider a sample of 224 quasars selected from the SDSS at z<1
observed in the infrared band by Herschel. We evaluate the star formation rate
in relation to several outflow signatures traced by the [OIII]4959,5007 and
[OII]3726,3729 emission lines in about half of the sample with high quality
spectra. Most of the quasars show asymmetric and broad wings in [OIII], which
we interpret as outflow signatures. We separate the quasars in two groups,
``weakly'' and ``strongly'' outflowing, using three different criteria. When we
compare the mean star formation rate in five redshift bins in the two groups,
we find that the SFRs are comparable or slightly larger in the strongly
outflowing quasars. We estimate the stellar mass from SED fitting and the
quasars are distributed along the star formation main sequence, although with a
large scatter. The scatter from this relation is uncorrelated with respect to
the kinematic properties of the outflow. Moreover, for quasars dominated in the
infrared by starburst or by AGN emission, we do not find any correlation
between the star formation rate and the velocity of the outflow, a trend
previously reported in the literature for pure starburst galaxies. We conclude
that the basic AGN negative feedback scenario seems not to agree with our
results. Although we use a large sample of quasars, we did not find any
evidence that the star formation rate is suppressed in the presence of AGN
driven outflows on large scale. A possibility is that feedback is effective
over much longer timescales than those of single episodes of quasar activity.Comment: 18 pages, new version that implements the suggestions of the referee
and matches the AA published versio
Quasars as standard candles II
A tight non-linear relation between the X-ray and the optical-ultraviolet (UV) emission has been observed in active galactic nuclei (AGN) over a wide range of redshift and several orders of magnitude in luminosity, suggesting the existence of an ubiquitous physical mechanism regulating the energy transfer between the accretion disc and the X-ray emitting corona. Recently, our group developed a method to use this relation in observational cosmology, turning quasars into standardizable candles. This work mainly seeks to investigate the potential evolution of this correction at high redshifts. We thus studied the LXâ
ââ
LUV relation for a sample of quasars in the redshift range 4â< âzâ< â7, adopting the selection criteria proposed in our previous work regarding their spectral properties. The resulting sample consists of 53 type 1 (unobscured) quasars, observed either with Chandra or XMM-Newton, for which we performed a full spectral analysis, determining the rest-frame 2 keV flux density, as well as more general X-ray properties such as the estimate of photon index, and the soft (0.5â2 keV) and hard (2â10 keV) unabsorbed luminosities. We find that the relation shows no evidence for evolution with redshift. The intrinsic dispersion of the LXâLUV for a sample free of systematics/contaminants is of the order of 0.22 dex, which is consistent with previous estimates from our group on quasars at lower redshift
A Sensitive Quantification of HHV-6B by Real-time PCR
Human herpesvirus (HHV)-6B is a pathogen causing latent infection in virtually all humans. Nevertheless, the interaction of HHV-6B with its host cells is poorly understood. Although HHV-6B is approximately 90% homologous to HHV-6A, it expresses certain B-specific genes. In order to quantify the amount of expressed viral mRNA we have developed a method using real-time PCR on a LightCycler instrument. Here we describe an assay for the detection of the HHV-6B B6 mRNA, but our approach can easily be extended to involve other mRNAs. This method is useful during the study of HHV-6B biology and offers reliable and reproducible, quantitative detection of viral mRNA below the attomol range
The Obscured Fraction of AGN in the XMM-COSMOS Survey: A Spectral Energy Distribution Perspective
The fraction of AGN luminosity obscured by dust and re-emitted in the mid-IR
is critical for understanding AGN evolution, unification, and parsec-scale AGN
physics. For unobscured (Type-1) AGN, where we have a direct view of the
accretion disk, the dust covering factor can be measured by computing the ratio
of re-processed mid-IR emission to intrinsic nuclear bolometric luminosity. We
use this technique to estimate the obscured AGN fraction as a function of
luminosity and redshift for 513 Type-1 AGN from the XMM-COSMOS survey. The
re-processed and intrinsic luminosities are computed by fitting the 18-band
COSMOS photometry with a custom SED-fitting code, which jointly models emission
from: hot-dust in the AGN torus, the accretion disk, and the host-galaxy. We
find a relatively shallow decrease of the luminosity ratio as a function of
Lbol, which we interpret as a corresponding decrease in the obscured fraction.
In the context of the receding torus model, where dust sublimation reduces the
covering factor of more luminous AGN, our measurements require a torus height
which increases with luminosity as h ~ Lbol^{0.3-0.4}. Our obscured
fraction-luminosity relation agrees with determinations from SDSS censuses of
Type-1 and Type-2 quasars, and favors a torus optically thin to mid-IR
radiation. We find a much weaker dependence of obscured fraction on 2-10 keV
luminosity than previous determinations from X-ray surveys, and argue that
X-ray surveys miss a significant population of highly obscured Compton-thick
AGN. Our analysis shows no clear evidence for evolution of obscured fraction
with redshift.Comment: 33 pages, 24 figures, ApJ accepte
Human Herpesvirus 6 (HHV-6) Causes Severe Thymocyte Depletion in SCID-hu Thy/Liv Mice
Human herpesvirus 6 (HHV-6) is a potentially immunosuppressive agent that may act as a cofactor in the progression of AIDS. Here, we describe the first small animal model of HHV-6 infection. HHV-6 subgroup A, strain GS, efficiently infected the human thymic tissue implanted in SCID-hu Thy/Liv mice, leading to the destruction of the graft. Viral DNA was detected in Thy/Liv implants by quantitative polymerase chain reaction (PCR) as early as 4 d after inoculation and peaked at day 14. The productive nature of the infection was confirmed by electron microscopy and immunohistochemical staining. Atypical thymocytes with prominent nuclear inclusions were detected by histopathology. HHV-6 replication was associated with severe, progressive thymocyte depletion involving all major cellular subsets. However, intrathymic T progenitor cells (ITTPs) appeared to be more severely depleted than the other subpopulations, and a preferred tropism of HHV-6 for ITTPs was demonstrated by quantitative PCR on purified thymocyte subsets. These findings suggest that thymocyte depletion by HHV-6 may be due to infection and destruction of these immature T cell precursors. Similar results were obtained with strain PL-1, a primary isolate belonging to subgroup B. The severity of the lesions observed in this animal model underscores the possibility that HHV-6 may indeed be immunosuppressive in humans
AGN X-ray variability in the XMM-COSMOS survey
We took advantage of the observations carried out by XMM in the COSMOS field
during 3.5 years, to study the long term variability of a large sample of AGN
(638 sources), in a wide range of redshift (0.1<z<3.5) and X-ray luminosity
(L(2-10)). Both a simple statistical method to asses the
significance of variability, and the Normalized Excess Variance
() parameter, where used to obtain a quantitative measurement
of the variability. Variability is found to be prevalent in most AGN, whenever
we have good statistic to measure it, and no significant differences between
type-1 and type-2 AGN were found. A flat (slope -0.23+/-0.03) anti-correlation
between and X-ray luminosity is found, when significantly
variable sources are considered all together. When divided in three redshift
bins, the anti-correlation becomes stronger and evolving with z, with higher
redshift AGN being more variable. We prove however that this effect is due to
the pre-selection of variable sources: considering all the sources with
available measurement, the evolution in redshift disappears.
For the first time we were also able to study the long term X-ray variability
as a function of and Eddington ratio, for a large sample of AGN
spanning a wide range of redshift. An anti-correlation between
and is found, with the same slope of the
anti-correlation between and X-ray luminosity, suggesting
that the latter can be a byproduct of the former one. No clear correlation is
found between and the Eddington ratio in our sample.
Finally, no correlation is found between the X-ray and the
optical variability.Comment: 14 Pages, 13 figures. Accepted to the Astrophysical Journal on
December 6, 201
The first ultraviolet quasar-stacked spectrum at z ≃ 2.4 from WFC3
The ionising continuum from active galactic nuclei (AGN) is fundamental for
interpreting their broad emission lines and understanding their impact on the
surrounding gas. Furthermore, it provides hints on how matter accretes onto
supermassive black holes. Using HST's Wide Field Camera 3 we have constructed
the first stacked ultraviolet (rest-frame wavelengths 600-2500\AA) spectrum of
53 luminous quasars at z=2.4, with a state-of-the-art correction for the
intervening Lyman forest and Lyman continuum absorption. The continuum slope
() of the full sample shows a break at ~912\AA\
with spectral index at \AA\ and a
softening at shorter wavelengths ( at \AA). Our analysis proves that a proper intergalactic medium absorption
correction is required to establish the intrinsic continuum emission of
quasars. We interpret our average ultraviolet spectrum in the context of
photoionisation, accretion disk models, and quasar contribution to the
ultraviolet background. We find that observed broad line ratios are consistent
with those predicted assuming an ionising slope of -2.0,
similar to the observed ionising spectrum in the same wavelength range. The
continuum break and softening are consistent with accretion disk plus X-ray
corona models when black hole spin is taken into account. Our spectral energy
distribution yields a 30% increase to previous estimates of the specific quasar
emissivity, such that quasars may contribute significantly to the total
specific Lyman limit emissivity estimated from the Ly forest at z<3.2.Comment: 19 pagine, 13 figure, replaced with version that matches the one
published in MNRA
Reactive oxygen species, antioxidant mechanisms and serum cytokine levels in cancer patients: impact of an antioxidant treatment
Objective. So far, it is not well established whether oxidative stress found in cancer patients results from an increased production of oxidants in the body or from a failure of physiological antioxidant systems. To further investigate this question we have assessed the blood levels of reactive oxygen species as a marker of free radicals producing oxidative stress and the most relevant of the physiological body enzymes counteracting reactive oxygen species, namely glutathione peroxidase and superoxide dismutase. Serum levels of proinflammatory cytokines and IL-2 were also investigated. All these parameters were studied in relation to the clinically most important index of disease progression, namely Performance Status (ECOG PS). We also tested the reducing ability of different antioxidant agents on reactive oxygen species levels by measuring the increase in glutathione peroxidase activity, and the reduction of serum levels of IL-6 and TNF. Design, setting and subjects. We carried out an open non randomized study on 28 advanced stage cancer patients (stage III, 10.7%, and stage IV, 89.3%) with tumours at different (8) sites: all were hospitalized in the Medical Oncology Dept, University of Cagliari Interventions. The patients were divided into 5 groups and a different antioxidant treatment was administered to each group. The selected antioxidants were: alpha lipoic acid 200 mg/day orally, N-acetylcysteine 1800 mg/day i.v. or carboxycysteine-lysine salt 2.7 g/day orally, amifostine 375 mg/day i.v., reduced glutathione 600 mg/day i.v., vitamin A 30000 IU/day orally plus vitamin E 70 mg/day orally plus Vitamin C 500 mg/day orally. The antioxidant treatment was administered for 10 consecutive days. Results. Our results show that all but one of the antioxidants tested were effective in reducing reactive oxygen species levels and 2 of them (cysteine-containing compounds and amifostine) had the additional effect of increasing glutathione peroxidase activity. Comprehensively, the âantioxidant treatmentâ was found to have an effect both on reactive oxygen species levels and glutathione peroxidase activity. The antioxidant treatment also reduced serum levels of IL-6 and TNF. Patients in both ECOG PS 0-1 and ECOG PS 2-3 responded to antioxidant treatment
Orientation effects on quasars SED: the torus IR emission
We performed a spectroscopic analysis of a SDSS DR7 sample of >12000 quasars as a function of their orientation with respect to the line of sight, as indicated by the equivalent width (EW) of the [OIII] line. This confirmed the presence of orientation effects in both the narrow and the broad lines, thus providing information on the geometry and kinematics of the Narrow Line Region and the Broad Line Region.We now use the EW[OIII] indicator to shed light on the geometry of the obscuring structure, the torus in the Unified Model. Through an analysis of the photometry available for the same sample, we study the Spectral Energy Distribution (SED) from the UV to the IR as a function of the EW[OIII]. From the IR bands of the SED we confirm that the torus must be clumpy and co-planar with the accretion disk and BLR, as predicted by many theoretical models in the literature
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