Spectral variability of QSOs in the optical band

Memorie Della Societa Astronomica Italiana, 2001

We present progress results of a new survey for bright QSOs (V < 14.5, R < 15.4, B J < 15.2) covering the whole sky at |b| > 30. The surface density of QSOs brighter than B J = 14.8 turns out to be 2.9±0.8•10 −3 deg −2. The optical Luminosity Function at 0.04 < z ≤ 0.3 shows significant departures from the standard pure luminosity evolution, providing new insights in the modelling of the QSO phenomenon.

Journal of Student Research

The following study discusses the relationship of high redshift(z) Quasi Stellar Objects’(QSOs) flux, luminosity, and color indices. The paper analyzes observable trends among these 3 variables and uses them to understand the distribution of QSOs across space-time. While many such studies have been conducted in redshift ranges of 1

2008

We present the first mid-IR spectra of X-ray selected Type 2 QSOs ever taken. The objects have been selected according to their high intrinsic luminosities and column densities in the X-rays. We characterize their spectral properties, quantify the contribution of star formation in the hosts and of AGN to the combined light of these objects, and discuss the implications of

Symposium - International Astronomical Union, 1996

We find that the spectra of QSOs evolve: high redshift QSOs (z…

The Astronomical Journal, 1997

An Homogeneous Bright QSO Survey (HBQS) has been carried out in the framework of an ESO Key programme. 327 QSOs (with M B < −23 and 0.3 < z < 2.2) have been selected over an area of 555 deg 2 in the magnitude range 15 < B < 18.75. For magnitudes brighter than B = 16.4 the QSO surface density turns out to be a factor 2.2 higher than what measured by the PG survey, corresponding to a surface density of 0.013 +.007 −.005 deg −2. If the data from the Edinburgh QSO Survey are included, an overdensity of a factor 2.7 is observed, corresponding to a surface density of 0.016 ± 0.005 deg −2. In order to derive the QSO optical luminosity function we used Monte Carlo simulations that take into account of the selection criteria, photometric errors and QSO spectral slope distribution. We have combined our data with the Edinburgh QSO Survey, the Large Bright QSO Survey, Durham/AAT survey, ESO/AAT faint survey and the (ZM) 2 B survey. The luminosity function can be represented with a Pure Luminosity Evolution, L(z) ∝ (1 + z) k , of a two power-law both for q 0 = 0.5 and q 0 = 0.1. For q 0 = 0.5 the k evolution parameter is k = 3.26 ± 0.07, slower than the previous Boyle's (1992) estimate k = 3.45. A flatter slope β = −3.72 ± 0.13 of the bright part of the luminosity function is also required. If a spread in the QSO spectral slope of σ γ = 0.3 and 0.5 is taken into account, the k parameter drops to 3.2 and 3.0 respectively. The observed overdensity of bright QSOs is concentrated at redshifts lower than 0.6. It results that in the range 0.3 < z < 0.6 the luminosity function is flatter than observed at higher redshifts.

Eprint Arxiv 1109 5621, 2011

A new method of analysis of QSO spectra, usually referred to as the "Thong method", has been recently presented and made use of in a number of publications. Several of these have been withdrawn because the authors have been convicted of plagiarism. However, there exists no publication showing that the method itself, which is an original contribution of the authors, is wrong. The purpose of the present note is to show that it is and that the results obtained when using it, including limits on the time variation of the fine structure constant many times smaller than published by other authors, must therefore be ignored and discarded.

2006

We measure the luminosity function of QSOs in the redshift range 3.5 &lt; z &lt; 5.2 for the absolute magnitude interval -21 &lt; M_{145} &lt; -28. Suitable criteria are defined to select faint QSOs in the GOODS fields, checking their effectiveness and completeness in detail. The confirmed sample of faint QSOs is compared with a brighter one derived from the SDSS. Using a Monte-Carlo technique we estimate the properties of the luminosity function. Our results show that models based on pure density evolution show better agreement with observation than models based on pure luminosity evolution, even if a different break magnitude with respect to z ~ 2.1 is required at 3.5 &lt; z &lt; 5.2. According to our modeling a faint-end slope steeper than low-redshift observations is required to reproduce the data, moreover models with a steep bright-end slope score a higher probability than models with a bright-end flattening. Determining the faint-end of the luminosity function at these redshi...

2006

Aims: In this work the luminosity function of QSOs is measured in the redshift range 3.5<z<5.2 for the absolute magnitude interval -21<M_{145}<-28. The determination of the faint end of the luminosity function at these redshifts provides important constraints on models of joint evolution of galaxies and AGNs. Methods: We have defined suitable criteria to select faint QSOs in the GOODS fields, checking in detail their effectiveness and completeness. Spectroscopic follow-up of the resulting QSO candidates has been carried out. The confirmed sample of faint QSOs is compared with a brighter one derived from the SDSS. We have used a Monte-Carlo technique to estimate the properties of the luminosity function, checking various parameterizations for its shape and evolution. Results: Models based on Pure Density Evolution show better agreement with observation than models based on Pure Luminosity Evolution. However a different break magnitude with respect to z~2.1 is required at 3.5<z<5.2. Models with a steeper faint end score a higher probability. We do not find any evidence for a flattening of the bright end at redshift z>3.5. Conclusions: The estimated space density evolution of QSOs indicates a suppression of the formation and/or feeding of Supermassive Black Holes at these redshifts. The QSO contribution to the UV background is insufficient to ionize the IGM at 3.5<z<5.2.

Monthly Notices of the Royal Astronomical Society, 1996

We have compiled, from the literature, a sample of Ly α forest lines in the spectra of 69 QSOs, all observed with a resolution between 60 to 100 km s −1 .

The Astrophysical Journal, 1994

A re ned sample of 64 variable objects with stellar image structure has been identi ed in SA 57 to B 22:5, over a time baseline of 15 years, sampled at 11 distinct epochs. The photometric data typically have a root-mean-square error at B = 22 of only 0.05 mag. Thirty-ve quasars in this eld have already been spectroscopically con rmed, 34 of which are among the sample of variables. Of the other variables, 6 are known spectroscopically to be stars, 10 additional objects are stars based on reliable detection of proper motion, and 1 is spectroscopically a narrow-emission-line galaxy. Of the 13 remaining variables, it is argued that they are a mixture of distant halo subdwarfs and quasars with star-like colors. We compute the ensemble average structure function and autocorrelation function from the light curves in the respective quasar rest-frames, which are used to investigate the general dependences on apparent magnitude, absolute magnitude, and redshift.

2011

We have embarked upon a project to model the UV spectra of BALQSOs using a Monte Carlo radiative transfer code previously validated through modelling of the winds of cataclysmic variable stars (e.g. Noebauer et al. 2010). We intend to use the simulations to investigate the plausibility of geometric unification (e.g. Elvis 2000) of the different classes of QSO. Here we introduce the code, and present some initial results. These demonstrate that for reasonable geometries and mass loss rates we are able to produce synthetic spectra which reproduce the important features of observed BALQSO spectra.

Astrophysics and Space Science, 1979

The investigations on the Plasma Laser Star Model of QSOs reported in our previous paper are continued. Here we assume that QSOs are early type stars with temperatures in the range 104-10 s K. It is pointed out that the spectral lines of such stars may have asymmetric shapes and large broadening leading to errors in measurement up to 20 &. The conventional red-shift theory, however, allows fitting errors much more than this amount for many QSO emission lines. By taking the abundances of elements in QSO atmsopheres identical with the average cosmic abundance we analyze and compare the interpretations of the emission lines of 330 QSOs (263 QSOs are from Burbidge et al.'s list and the rest are more recently discovered QSOs) according to the new and the conventional theories.

The Astrophysical Journal, 1999

Two sets of relationships relate QSO UV to soft X-ray continua with the broad-line region. These are (i) the Baldwin relationships, which are inverse relationships between the broad-line equivalent width and the continuum luminosity, and (ii) Boroson & Green's optical "Principal Component 1" relationships, linking steeper soft Xray spectra with narrower Hb emission, stronger Hb blue wings, stronger optical Fe ii emission, and weaker [O iii] l5007 lines. In order to understand these relationships, we extended the spectra into the UV for 22 QSOs with high-quality soft X-ray spectra. These are from the complete sample of QSOs from the Bright Quasar Survey for which Laor et al. demonstrated strong luminosity and X-ray-optical Principal Component 1 relationships. We show that these extend to a whole new set of UV relationships: Principal Component 1 (in the sense of steeper X-ray spectra) is related to narrower C iii] l1909 lines, larger Si iii] l1892/C iii] l1909 ratios (a highdensity indicator), stronger low-ionization lines, and weaker C iv l1549 but stronger N v l1240 lines. We speculate that high accretion rates are linked to high columns of dense (∼10 11 cm ), nitrogen-enhanced, low-Ϫ3 ionization gas from nuclear starbursts. Line width, inverse Fe ii-[O iii] and inverse Fe ii-C iv relationships hint at the geometrical arrangement of this gas. These Principal Component 1 relationships appear to be independent of luminosity and therefore of the Baldwin relationships.

International Journal of Astronomy and Astrophysics, 2014

We have determined the Optical Luminosity Function (OLF) of a sample of 80946 Quasi Stellar Objects (QSOs) taken from the Sloan Digital Sky Survey Data Release Seven (SDSS DR7) with redshift range 0.3 − − = ⋅ ⋅ Ho is used. From the optical luminosity function a clear evidence of AGN downsizing is observed, i.e. the number density of the less luminous AGNs peaks at lower redshift than the number density of the more luminous AGNs.

1995

This is the rst paper in a series aimed at de ning a statistically signi cant sample of QSOs in the range 15 < B < 18:75 and 0:3 < z < 2:2. The selection is carried out using direct plates obtained at the ESO and UK Schmidt Telescopes, scanned with the COSMOS facility and searched for objects with an ultraviolet excess. Follow-up spectroscopy, carried out at ESO La Silla, is used to classify each candidate. In this initial paper, we describe the scienti c objectives of the survey; the selection and observing techniques used. We present the rst sample of 285 QSOs (M B < 23) in a 153 deg 2 area, covered by the six \deep" elds, intended to obtain signi cant statistics down B ' 18:75 with unprecedented photometric accuracy. From this database, QSO counts are determined in the magnitude range 17 < B < 18:75.