The proximity effect in a close group of QSOs

Liske, J.; Williger, G. M.

doi:10.1046/j.1365-8711.2001.04908.x

Cited by 55 publications

(83 citation statements)

References 81 publications

(147 reference statements)

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“…Discussion The anisotropic clustering pattern of absorbers around quasars suggests that the transverse direction is less likely to be illuminated by ionizing photons than the line-of-sight. This suggestion gains credibility in light of the null detections of the transverse proximity effect in the Lyα forests of projected quasar pairs (Crotts 1989;Dobrzycki & Bechtold 1991;Fernandez-Soto, Barcons, Carballo, & Webb 1995;Liske & Williger 2001;Schirber, Miralda-Escudé, & McDonald 2004;Croft 2004, but see Jakobsen et al2003. Although these studies are each based only on a handful of projected pairs, they all come to similar conclusions: the amount of (optically thin) Lyα forest absorption, in the background quasar sightline near the redshift of the foreground quasar, is larger than average rather than smallerthe opposite of what is expected from the transverse proximity effect.…”

Section: Discussionmentioning

confidence: 88%

“…If Nature provides a nearby background quasar sightline, one can also study the transverse proximity effect, which is the expected decrease in absorption in a background quasar's Lyα forest, caused by the transverse ionizing flux of a foreground quasar. The transverse effect has yet to be detected, in spite of many attempts (Crotts 1989;Dobrzycki & Bechtold 1991;Fernandez-Soto, Barcons, Carballo, & Webb 1995;Liske & Williger 2001;Schirber, Miralda-Escudé, & McDonald 2004;Croft 2004, but see Jakobsen et al2003.…”

Section: Introductionmentioning

confidence: 99%

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Quasars Probing Quasars. II. The Anisotropic Clustering of Optically Thick Absorbers around Quasars

Prochaska

2007

ApJ

137

180

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With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We used a sample of 17 Lyman limit systems with column density N HI > 10 19 cm −2 selected from 149 projected quasar pair sightlines, to investigate the clustering pattern of optically thick absorbers around luminous quasars at z ∼ 2.5. Specifically, we measured the quasar-absorber correlation function in the transverse direction, and found a comoving correlation length of r 0 = 9.2 +1.5 −1.7 h −1 Mpc (comoving) assuming a power law correlation function, ξ ∝ r −γ , with γ = 1.6. Applying this transverse clustering strength to the line-of-sight, would predict that ∼ 15 − 50% of all quasars should show a N HI > 10 19 cm −2 absorber within a velocity window of ∆v < 3000 km s −1 . This overpredicts the number of absorbers along the line-of-sight by a large factor, providing compelling evidence that the clustering pattern of optically thick absorbers around quasars is highly anisotropic. The most plausible explanation for the anisotropy is that the transverse direction is less likely to be illuminated by ionizing photons than the line-of-sight, and that absorbers along the line-of-sight are being photoevaporated. A simple model for the photoevaporation of absorbers subject to the ionizing flux of a quasar is presented, and it is shown that absorbers with volume densities n H 0.1 cm −3 will be photoevaporated if they lie within ∼ 1 Mpc (proper) of a luminous quasar. Using this simple model, we illustrate how comparisons of the transverse and line-of-sight clustering around quasars can ultimately be used to constrain the distribution of gas in optically thick absorption line systems.

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Section: Discussionmentioning

confidence: 88%

Section: Introductionmentioning

confidence: 99%

Quasars Probing Quasars. II. The Anisotropic Clustering of Optically Thick Absorbers around Quasars

Prochaska

2007

ApJ

137

180

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“…An estimate of the ionizing rate can be obtained by determining the distance from the quasar at which the number of lines is equal to the background expectation. Over the years, measurements of J À21 (assuming UV $ 1:8) have taken values between $0.7 and $3 (Williger et al 1994;Bechtold 1994;Fernandez-Soto et al 1995;Cristiani et al 1995;Giallongo et al 1996;Cooke, Espey, & Carswell 1997;Scott et al 2000;Liske & Williger 2001). Since what is effectively measured is a ratio of proper distances, this analysis should be relatively independent of cosmological parameters.…”

Section: Proximity Effectmentioning

confidence: 99%

Faint Active Galactic Nuclei and the Ionizing Background

Schirber

Bullock

2003

ApJ

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We determine the evolution of the faint, high-redshift, optical luminosity function (LF) of active galactic nuclei (AGNs) implied by several observationally motivated models of the ionizing background from 3 < z < 5. Our results depend crucially on whether we use the total ionizing rate measured by the proximity effect technique or the lower determination favored by the flux decrement distribution of Ly forest lines. Assuming a faint-end LF slope of 1.58 and the SDSS estimates of the bright-end slope and normalization, we find that the LF must break at M Ã B ¼ À24:2, À22.3, À20.8 at z ¼ 3, 4, 5 if we adopt the lower ionization rate and assume no stellar contribution to the background. The breaks must occur at M Ã B ¼ À20:6, À18.7, À18.7 for the proximity effect estimate. Since stars may also contribute to the background, these values are lower limits on the break luminosity, and they brighten by as much as $2 mag if the escape fraction of ionizing photons from high-z galaxies is consistent with recent estimates: f esc ¼ 0:16. By comparing our expectations to faint AGN searches in the Hubble Deep Field and high-z galaxy fields, we find that typically quoted proximity effect estimates of the background imply an overabundance of AGNs compared to the faint counts (even with f esc ¼ 1). Even adopting the lower bound on proximity effect measurements, the stellar escape fraction must be high: f esc e0:2. Conversely, the lower flux decrement-derived background requires a smaller number of ionizing sources, and faint AGN counts are consistent with this estimate only if there is a limited stellar contribution, f esc d0:05. Our derived LFs together with the locally estimated black hole density suggest that the efficiency of converting mass to light in optically unobscured AGNs is somewhat lower than expected, d0:05 (all models). Comparison with similar estimates based on X-ray counts suggests that more than half of all AGNs are obscured in the UV/optical. We also derive lower limits on typical AGN lifetimes and obtain e10 7 yr for favored cases.

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“…Traditionally, the proximity effect has been considered as a statistical effect and large samples of up to ∼100 quasars have been compiled to measure the mean UVB at 2 < ∼ z < ∼ 4 (e.g. Bajtlik et al 1988;Lu et al 1991;Giallongo et al 1996;Cooke et al 1997;Scott et al 2000;Liske & Williger 2001). Nevertheless, the proximity effect can also be detected towards individual quasars (e.g.…”

Section: Introductionmentioning

confidence: 99%

An unbiased measurement of the UV background and its evolution via the proximity effect in quasar spectra

Dall'Aglio

Wisotzki

Worseck

2008

A&A

104

134

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We investigated a set of high-resolution (R ∼ 45 000), high signal-to-noise (S /N ∼ 70) quasar spectra to search for the signature of the so-called proximity effect in the H i Lyα forest. The sample consists of 40 bright quasars with redshifts in the range 2.1 < z < 4.7. Using the flux transmission statistic, we determined the redshift evolution of the H i effective optical depth in the Lyman forest between 2 < ∼ z < ∼ 4.5, finding good agreement with previous measurements based on smaller samples. We also see the previously reported dip in τ eff (z) around redshift z ∼ 3.3, but as the significance of that feature is only 2.6σ, we consider this detection tentative. Comparing the flux transmission near each quasar with what was expected from the overall trend of τ eff (z), we clearly detect the proximity effect not only in the combined quasar sample, but also towards each individual line of sight at high significance, albeit with varying strength. We quantify this strength using a simple prescription based on a fiducial value for the intensity of the metagalactic UV background (UVB) radiation field at 1 Ryd, multiplied by a free parameter that varies from QSO to QSO. The observed proximity effect strength distribution (PESD) is asymmetric, with an extended tail towards values corresponding to a weak effect. We demonstrate that this is not simply an effect of gravitational clustering around quasars, as the same asymmetry is already present in the PESD predicted for purely Poissonian variance in the absorption lines. We present the results of running the same analysis on simulated quasar spectra generated by a simple Monte-Carlo code. Comparing the simulated PESD with observations, we argue that the standard method of determining the UVB intensity J ν 0 by averaging over several lines of sight is heavily biased towards high values of J ν 0 because of the PESD asymmetry. Using instead the mode of the PESD provides an estimate of J ν 0 that is unbiased with respect to his effect. For our sample we get a modal value for the UVB intensity of log J ν 0 = −21.51 ± 0.15 (in units of erg cm −2 s −1 Hz −1 sr −1 ) for a median quasar redshift of 2.73. With J ν 0 fixed we then corrected τ eff near each quasar for local ionisation and estimated the amount of excess H i absorption attributed to gravitational clustering. On scales of ∼3 Mpc, only a small minority of quasars show substantial overdensities of up to a factor of a few in τ eff ; these are exactly the objects with the weakest proximity effect signatures. After removing those quasars residing in overdense regions, we redetermined the UVB intensity using a hybrid approach of sample averaging and statistical correction for the PESD asymmetry bias, arriving at log J ν 0 = −21.46 +0.14 −0.21 . This is the most accurate measurement of J ν 0 to date. We present a new diagnostic based on the shape and width of the PESD that strongly supports our conclusion that there is no systematic overdensity bias for the proximity effect. This additional diagnostic breaks the oth...

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The proximity effect in a close group of QSOs

Cited by 55 publications

References 81 publications

Quasars Probing Quasars. II. The Anisotropic Clustering of Optically Thick Absorbers around Quasars

Quasars Probing Quasars. II. The Anisotropic Clustering of Optically Thick Absorbers around Quasars

Faint Active Galactic Nuclei and the Ionizing Background

An unbiased measurement of the UV background and its evolution via the proximity effect in quasar spectra

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