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

Abstract

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...

An X-Shooter composite of bright 1 <z< 2 quasars from UV to infrared

“…A common way to correct for this absorption is to obtain a spectrum at a sufficiently high resolution to be able to trace the continuum emission by visual inspection. At z < 2 the Lyα forest absorption is not yet heavily blended (Dall'Aglio et al 2008), and the X-Shooter resolution allows us to effectively trace the continuum. To recover the continuum we insert points blueward of Lyα where the continuum is visually estimated.…”

“…No correction has been applied in the composites by Francis et al (1991) andVanden Berk et al (2001) and a sharp decrease in continuum flux is detected. Because higher redshift objects contribute mostly at shorter wavelengths, significant Lyα absorption is expected regardless due to the Lyα opacity evolution (Moller & Jakobsen 1990;Madau 1995;Dall'Aglio et al 2008). The IGM correction of Telfer et al (2002) is a hybrid in which some manual and some statistical correction has been applied.…”

A principal component analysis of quasar UV spectra atz ~ 3

“…The eigenvectors and coefficients as derived by S05 at low redshift were used to generate mock spectra to test different analyses at high redshift (Dall'Aglio et al 2008;Kirkman et al 2005). …”

“…Numerous authors have performed this measurement using high and/or intermediate spectral resolution data (e.g. Songaila 2004;Bernardi et al 2003;Dall'Aglio et al 2008Faucher-Giguère et al 2008). The evolution is smooth except for a possible bump at z ∼ 3.2, which could be related to the He ii reionization (Schaye et al 2000), although this is not the only possible explanation (Faucher-Giguère et al 2008).…”

Fluctuations of the intergalactic ionization field at redshiftz~ 2

“…The cross section of the sphere of influence is defined by a radiusR at which the AGN radiation flux at 1 Ryd, J q ν (R), becomes equal to the mean background metagalactic flux, J bg ν (z). At redshift z = 1.9, the value of J bg ν ∼ 6 × 10 −22 erg cm −2 s −1 Hz −1 sr −1 at 1 Ryd was measured in Tytler et al (2004), Bolton et al (2005), and Dall'Aglio et al (2008). Taking the AGN comoving space density at this redshift from Wolf et al (2003) and Croom et al (2009) and choosing Δz = 0.05, we calculate the intersection probability p depending on the source absolute magnitude M B (blue curve in Fig.…”