Catálogo de publicaciones - revistas

<jats:title>ABSTRACT</jats:title> <jats:p>We perform a semi-automated survey for τ<jats:sub>912</jats:sub> ⩾ 2 Lyman limit systems (LLSs) in quasar spectra from the Sloan Digital Sky Survey, Data Release 7. From a starting sample of 2473 quasars with <jats:italic>z</jats:italic> <jats:sub>em</jats:sub> = 3.6–5.0, we analyze 429 spectra meeting strict selection criteria for a total redshift path Δ<jats:italic>z</jats:italic> = 93.8 and identify 190 intervening systems at <jats:italic>z</jats:italic> <jats:sub>LLS</jats:sub> ⩾ 3.3. The incidence of τ<jats:sub>912</jats:sub> ⩾ 2 LLSs per unit redshift, ℓ<jats:sub>τ⩾2</jats:sub>(<jats:italic>z</jats:italic>), is well described by a single power law at these redshifts: <jats:inline-formula> <jats:tex-math><?CDATA $\ell _{\rm {\tau \ge 2}}(z)= C_{\rm LLS} [(1+z)/(1+z_*)]^{\gamma _{\rm LLS}}$?></jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj339002ieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, with <jats:italic>z</jats:italic> <jats:sub>*</jats:sub> ≡ 3.7, <jats:italic>C</jats:italic> <jats:sub>LLS</jats:sub> = 1.9 ± 0.2, and γ<jats:sub>LLS</jats:sub> = 5.1 ± 1.5 (68% c.l.). These values are systematically lower than previous estimates (especially at <jats:italic>z</jats:italic> &lt; 4) but are consistent with recent measurements of the mean free path to ionizing radiation. Extrapolations of this power law to <jats:italic>z</jats:italic> = 0 are inconsistent with previous estimations of ℓ(<jats:italic>z</jats:italic>) at <jats:italic>z</jats:italic> &lt; 1 and may indicate a break at <jats:italic>z</jats:italic> ≈ 2, similar to that observed for the Lyα forest. Our results also indicate that the systems giving rise to LLS absorption decrease by ≈50% in comoving number density and/or physical size from <jats:italic>z</jats:italic> = 4 to 3.3, perhaps due to an enhanced extragalactic ultraviolet background. The observations place an integral constraint on the H <jats:sc>i</jats:sc> frequency distribution <jats:italic>f</jats:italic>(<jats:italic>N</jats:italic> <jats:sub>H <jats:sc>i</jats:sc> </jats:sub>, <jats:italic>X</jats:italic>) and indicate that the power-law slope <jats:inline-formula> <jats:tex-math><?CDATA $\beta \equiv d\ln f(N_{\rm H\,\mathsc{i}},X)/d\ln N_{\rm H\,\mathsc{i}}$?></jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj339002ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> is likely shallower than β = −1 at <jats:inline-formula> <jats:tex-math><?CDATA $N_{\rm H\,\mathsc{i}}\approx 10^{18} \;{\rm cm^{-2}}$?></jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj339002ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. Including other constraints on <jats:italic>f</jats:italic>(<jats:italic>N</jats:italic> <jats:sub>H <jats:sc>i</jats:sc> </jats:sub>, <jats:italic>X</jats:italic>) from the literature, we infer that β is steeper than β = −1.7 at <jats:inline-formula> <jats:tex-math><?CDATA $N_{\rm H\,\mathsc{i}}\approx 10^{15}\;{\rm cm^{-2}}$?></jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj339002ieqn4.gif" xlink:type="simple" /> </jats:inline-formula>, implying at least two inflections in <jats:italic>f</jats:italic>(<jats:italic>N</jats:italic> <jats:sub>H <jats:sc>i</jats:sc> </jats:sub>, <jats:italic>X</jats:italic>). We also perform a survey for proximate LLSs (PLLSs) and find that ℓ<jats:sub>PLLS</jats:sub>(<jats:italic>z</jats:italic>) is systematically lower (≈25%) than intervening systems. Finally, we estimate that systematic effects impose an uncertainty of 10%–20% in the ℓ(<jats:italic>z</jats:italic>) measurements; these effects may limit the precision of all future surveys.</jats:p>