EDP Sciences
Free access
Volume 480, Number 2, March III 2008
Page(s) 359 - 368
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20077088

A&A 480, 359-368 (2008)
DOI: 10.1051/0004-6361:20077088

The line-of-sight proximity effect in individual quasar spectra

A. Dall'Aglio, L. Wisotzki, and G. Worseck

Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
    e-mail: adaglio@aip.de

(Received 11 January 2007 / Accepted 7 January 2008)

We exploit a set of high signal-to-noise (~70), low-resolution (R ~ 800) quasar spectra to search for the signature of the so-called proximity effect in the $\ion{H}{i}$ Ly$\alpha$ forest. Our sample consists of 17 bright quasars in the redshift range 2.7 < z < 4.1. Analysing the spectra with the flux transmission technique, we detect the proximity effect in the sample at high significance. We use this to estimate the average intensity of the metagalactic UV background, assuming it to be constant over this redshift range. We obtain a value of J = (9 $\pm$ 4)$\times$10-22 erg cm-2 s-1 Hz-1 sr-1, in good agreement with previous measurements at similar z. We then apply the same procedure to individual lines of sight, finding a significant deficit in the effective optical depth close to the emission redshift in every single object except one (which by a different line of evidence does nevertheless show a noticeable proximity effect). Thus, we clearly see the proximity effect as a universal phenomenon associated with individual quasars. Using extensive Monte-Carlo simulations to quantify the error budget, we assess the expected statistical scatter in the strength of the proximity effect due to shot noise (cosmic variance). The observed scatter is larger than the predicted one, so that additional sources of scatter are required. We rule out a dispersion of spectral slopes as a significant contributor. Possible effects are long time-scale variability of the quasars and/or gravitational clustering of Ly$\alpha$ forest lines. We speculate on the possibility of using the proximity effect as a tool to constrain individual quasar ages, finding that ages between ~106 and ~108 yrs might produce a characteristic signature in the optical depth profile towards the QSO. We identify one possible candidate for this effect in our sample.

Key words: cosmology: diffuse radiation -- galaxies: intergalactic medium -- galaxies: quasars: absorption lines

© ESO 2008