1,721,093 research outputs found
The evolution of the QSO luminosity function up to z = 4: The role of K-correction uncertainties
No full textUncertainties in the spectral index of the optical spectrum of QSOs induce systematic biases in the estimate of the cosmological evolution of the luminosity function. In a framework of pure luminosity evolution, we obtain a slower intrinsic evolution of the luminosity function for z < 2.2 after correction for uncertainties in the adopted K-correction. The slower evolution is consistent with the apparent density decline of high-redshift QSOs (z > 2.5) observed in the red band. Indeed, we find that a scenario of uniform luminosity evolution up to z congruent-to 4 is suggested. The apparent decline in the redshift distribution is explained in terms of the sliding down along the steep side of the luminosity function for increasing redshift
Optical variability of quasars: Statistics and cosmological properties
No full textThe long-term optical variability of a composite, complete sample of quasars is investigated. While no correlation with luminosity is detected, a positive correlation with redshift is found (rho = 0.25). These results are reconciled with previous opposite findings of other authors, arguing that the analysis of variability can be biased by the combined effect of the structure function of light curves and of cosmological time dilation. Such bias becomes negligible if variability is measured by a magnitude difference at a fixed and large time lag (4 yr). Interpretation of this cosmological trend is given in terms of spectral variability. Statistical biases on the estimate of the evolution rate for the quasar population are discussed
The optical-ultraviolet continuum of a sample of QSOs
No full textThe average optical-UV continuum shape of QSOs is investigated, using spectra of 62 QSOs that have good relative photometric calibrations. The QSO spectra were extracted from two complete color-selected samples in the magnitude interval m(B) approximate to 15-20. The analysis is performed by fitting power-law continua f(v) proportional to v(alpha) in well-defined rest-frame wavelength intervals after removing regions of the spectrum affected by strong emission lines or weak emission bumps. The average slope in the rest-frame optical-UV region 1200-5500 Angstrom, shows a rapid change around the 3000 Angstrom emission bump, with alpha similar or equal to 0.15 longward of it and alpha similar or equal to -0.65 at shorter wavelengths. Although these average slopes were obtained using spectra of QSOs with different luminosities and redshifts, there are no significant correlations of the average spectral index with these quantities. For a few QSOs in the sample, we were able to measure the same softening of the spectral shape within the individual spectrum. These results have significant consequences on the estimate of the cosmological evolution of the optically selected QSOs, as they affect, for instance, the k-corrections. New k-corrections in the BVR and Gunn I bands are computed. The derived average spectral shape in the optical-UV band places interesting constraints on the expected emission mechanisms
A new (2D+1) cluster finding algorithm for photometric redshift surveys .
No full textEditors: Gaetano Belvedere, Giuseppe Leto and Francesca Zuccarell
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