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Register a new userHighly Accreting Quasars: Sample Definition and Possible Cosmological Implications
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We propose a method to identify quasars radiating closest to the Eddington limit, defining primary and secondary selection criteria in the optical, UV and X-ray spectral range based on the 4D eigenvector 1 formalism. We then show that it is possible to derive a redshift-independent estimate of luminosity for extreme Eddington ratio sources. Using preliminary samples of these sources in three redshift intervals (as well as two mock samples), we test a range of cosmological models. Results are consistent with concordance cosmology but the data are insufficient for deriving strong constraints. Mock samples indicate that application of the method proposed in this paper using dedicated observations would allow to set stringent limits on Omega_M and significant constraints on Omega_Lambda.
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We present an analysis of UV spectra of 13 quasars believed to belong to extreme Population A (xA) quasars, aimed at the estimation of the chemical abundances of the broad line emitting gas. Metallicity estimates for the broad line emitting gas of quasars are subject to a number of caveats, although present data suggest the possibility of an increase along the quasar main sequence along with prominence of optical Fe II emission. Extreme Population A sources with the strongest Fe II emission offer several advantages with respect to the quasar general population, as their optical and UV emission lines can be interpreted as the sum of a low-ionization component roughly at quasar rest frame (from virialized gas), plus a blueshifted excess (a disk wind), in different physical conditions. Specifically, in terms of ionization parameter, cloud density, metallicity and column density. Capitalizing on these results, we analyze the component at rest frame and the blueshifted one, exploiting the dependence (of several intensity line ratios on metallicity $Z$). We find that the validity of intensity line ratios as metallicity indicators depends on the physical conditions. We apply the measured diagnostic ratios to estimate the physical properties of sources such as density, ionization, and metallicity of the gas. Our results confirm that the two regions (the low-ionization component and the blue-shifted excess) of different dynamical conditions also show different physical conditions and suggest metallicity values that are high, and probably the highest along the quasar main sequence, with $Z gtrsim 10 Z_{odot}$. We found some evidence of an overabundance of Aluminium with respect to Carbon, possibly due to selective enrichment of the broad line emitting gas by supernova ejecta.
We present a new catalogue of ~2,400 optically selected quasars with spectroscopic redshifts and X-ray observations from either Chandra or XMM-Newton. The sample can be used to investigate the non-linear relation between the UV and X-ray luminosity of quasars, and to build a Hubble diagram up to redshift z~7.5. We selected sources that are neither reddened by dust in the optical/UV nor obscured by gas in the X-rays, and whose X-ray fluxes are free from flux-limit related biases. After checking for any possible systematics, we confirm, in agreement with our previous works, that (i) the X-ray to UV relation provides distance estimates matching those from supernovae up to z~1.5, and (ii) its slope shows no redshift evolution up to z~5. We provide a full description of the methodology for testing cosmological models, further supporting a trend whereby the Hubble diagram of quasars is well reproduced by the standard flat $Lambda$CDM model up to z~1.5-2, but strong deviations emerge at higher redshifts. Since we have minimized all non-negligible systematic effects, and proven the stability of the $L_{rm X}-L_{rm UV}$ relation at high redshifts, we conclude that an evolution of the expansion rate of the Universe should be considered as a possible explanation for the observed deviation, rather than some systematic (redshift-dependent) effect associated with high-redshift quasars.
We present new Gemini/GMOS optical spectroscopy of 16 extreme variability quasars (EVQs) that dimmed by more than 1.5 mag in the $g$ band between the Sloan Digital Sky Survey (SDSS) and the Dark Energy Survey (DES) epochs (separated by a few years in the quasar rest frame). The quasar sample covers a redshift range of $0.5 < z < 2.1$. Nearly half of these EVQs brightened significantly (by more than 0.5 mag in the $g$ band) in a few years after reaching their previous faintest state, and some EVQs showed rapid (non-blazar) variations of greater than 1-2 mag on timescales of only months. Leveraging on the large dynamic range in continuum variability between the earlier SDSS and the new GMOS spectra, we explore the associated variations in the broad Mg II,$lambda2798$ line, whose variability properties have not been well studied before. The broad Mg II flux varies in the same direction as the continuum flux, albeit with a smaller amplitude, which indicates at least some portion of Mg II is reverberating to continuum changes. However, the width (FWHM) of Mg II does not vary accordingly as continuum changes for most objects in the sample, in contrast to the case of the broad Balmer lines. Using the width of broad Mg II to estimate the black hole mass therefore introduces a luminosity-dependent bias.
Highly accreting quasars are characterized by distinguishing properties in the 4D eigenvector 1 parameter space that make them easily recognizable over a broad range range of redshift and luminosity. The 4D eigenvector 1 approach allows us to define selection criteria that go beyond the restriction to Narrow Line Seyfert 1s identified at low redshift. These criteria are probably able to isolate sources with a defined physical structure i.e., a geometrically thick, optically thick advection-dominated accretion disk (a slim disk). We stress that the importance of highly accreting quasars goes beyond the understanding of the details of their physics: their Eddington ratio is expected to saturate toward values of order unity, making them possible cosmological probes.
We report on the diversity in quasar spectra from the Baryon Oscillation Spectroscopic Survey. After filtering the spectra to mitigate selection effects and Malmquist bias associated with a nearly flux-limited sample, we create high signal-to-noise ratio composite spectra from 58,656 quasars (2.1 le z le 3.5), binned by luminosity, spectral index, and redshift. With these composite spectra, we confirm the traditional Baldwin effect (BE, i.e., the anticorrelation of C IV equivalent width (EW) and luminosity) that follows the relation W_lambda propto L^{beta_w} with slope beta_w = -0.35 pm 0.004, -0.35 pm 0.005, and -0.41 pm 0.005 for z = 2.25, 2.46, and 2.84, respectively. In addition to the redshift evolution in the slope of the BE, we find redshift evolution in average quasar spectral features at fixed luminosity. The spectroscopic signature of the redshift evolution is correlated at 98% with the signature of varying luminosity, indicating that they arise from the same physical mechanism. At a fixed luminosity, the average C IV FWHM decreases with increasing redshift and is anti-correlated with C IV EW. The spectroscopic signature associated with C IV FWHM suggests that the trends in luminosity and redshift are likely caused by a superposition of effects that are related to black hole mass and Eddington ratio. The redshift evolution is the consequence of a changing balance between these two quantities as quasars evolve toward a population with lower typical accretion rates at a given black hole mass.
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