896 research outputs found
A Hubble diagram for quasars
We present a new method to test the ΛCDM 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 Å 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 = 0–1.4 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 ΛCDM 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
Towards an informed quest for accretion disc winds in quasars: the intriguing case of Ton 28
We report on the detection of a blueshifted Fe K absorption feature in two consecutive XMM–Newton observations of the luminous blue quasar Ton 28, at the 4σ cumulative significance. The rest energy of 9.2 keV implies the presence of an accretion disc wind with bulk outflow velocity of ∼0.28c, while the kinetic power is most likely a few per cent of the quasar luminosity. Remarkably, Ton 28 had been specifically selected as an optimal target to reveal an ultra-fast X-ray wind based on its total luminosity (Lbol > 1046 erg s−1) and [O III] λ5007 Å equivalent width (EW < 6 Å), suggestive of high accretion rate and low inclination, respectively. Other peculiar optical/UV emission-line properties include narrow Hβ, strong Fe II, and blueshifted C IV . These are key parameters in the Eigenvector 1 formalism, and are frequently found in active galaxies with ongoing accretion disc winds, hinting at a common physical explanation. Provided that the effectiveness of our selection method is confirmed with similar sources, this result could represent the first step towards the characterization of black hole winds through multiwavelength indicators in the absence of high-quality X-ray spectra
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 LX − LUV relation for a sample of quasars in the redshift range 4 < z < 7, 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 (0.5–2 keV) and hard (2–10 keV) unabsorbed luminosities. We find that the relation shows no evidence for evolution with redshift. The intrinsic dispersion of the LX–LUV 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
Demography of obscured and unobscured AGN: prospects for a Wide Field X-ray Telescope
We discuss some of the main open issues in the evolution of Active Galactic
Nuclei which can be solved by the sensitive, wide area surveys to be performed
by the proposed Wide Field X-ray Telescope mission.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in
Bologna, Italy, Nov. 25-26 2009. To appear in Memorie della Societa'
Astronomica Italiana 2010 (arXiv:1010.5889
QSO Museum I: A sample of 61 extended Ly α-emission nebulae surrounding z ∼ 3 quasars
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
The SOUX AGN sample: Optical/UV/X-ray SEDs and the nature of the disc
We use the SOUX sample of ∼700 active galactic nucleus (AGN) to form average optical-ultraviolet (UV)-X-rays spectral energy distributions (SEDs) on a two-dimensional (2D) grid of MBH and L2500. We compare these with the predictions of a new AGN SED model, qsosed, which includes prescriptions for both hot and warm Comptonization regions as well as an outer standard disc. This predicts the overall SED fairly well for 7.5 < log(MBH/M⊙) < 9.0 over a wide range in L/LEdd, but at higher masses the outer disc spectra in the model are far too cool to match the data. We create optical-UV composites from the entire Sloan Digital Sky Survey sample and use these to show that the mismatch is due to there being no significant change in spectral shape of the optical-UV continuum across several decades of MBH at constant luminosity. We show for the first time that this cannot be matched by standard disc models with high black hole spin. These apparently fit, but are not self-consistent as they do not include the General Relativistic effects for the emission to reach the observer. At high spin, increased gravitational redshift compensates for almost all of the higher temperature emission from the smaller inner disc radii. The data do not match the predictions made by any current accretion flow model. Either the disc is completely covered by a warm Comptonization layer whose properties change systematically with L/LEdd, or the accretion flow structure is fundamentally different to that of the standard disc models
Centri ecclesiastici e dinamiche di popolamento: la fondazione dei borghi nuovi subalpini e l’eredità romanica
Le testimonianze di architettura dell’XI e del XII secolo costituiscono non solo documenti utili allo studio della cultura artistica e tecnica, ma sono indicatori significativi per lo studio della storia delle dinamiche insediative. Il processo di patrimonializzazione delle architetture romaniche ha seguito infatti percorsi strettamente legati alle sorti del popolamento nelle aree loro circostanti: nei casi in cui i villaggi si siano riallocati o ri-concentrati nel basso medioevo, si è verificato il fenomeno della ‘ruralizzazione’ delle architetture romaniche, associato a una sostanziale conservazione ‘fossile’ della loro facies originaria; ove invece l’insediamento abbia avuto fortuna demica e politica presso la chiesa, l’edificio medievale è stato conservato in maniera parziale o selettiva, subendo ordinariamente interventi di adeguamento liturgico e di gusto. Il fenomeno è stato studiato attraverso alcuni casi-studio relativi al dinamismo insediativo della fondazione di borghi nuovi tra XII e XIV secolo.
The architectural examples dating to 11th and 12th centuries are not only important to studying artistic and technical culture, but they are also meaningful indicators about history of settlement dynamics. In fact, the process of patrimonialization of Romanesque architectures followed ways closely linked to the destiny of the population in the nearby areas. When villages were re-positioned or re-focused during the late Middle Ages, Romanesque architectures experienced phenomenon of ‘ruralisation’, in association with a significant conservation of their original appearance as a ‘fossil’. Instead, when settlements had demographic and political successful outcomes beside the church, the medieval buildings have been partially or selectively preserved, routinely undergoing interventions of liturgical and style adaptation. The phenomenon is studied through case studies related to the dynamism of the foundation of new villages between 12th and 14th centuries
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