An Intercomparison Study of Two Proximate Damped Lyα Systems with Residual Flux upon the Lyα Absorption Trough toward Quasars

In this paper, we present an intercomparison study of two quasars, SDSS J145618.32+340037.2 and SDSS J215331.50-025514.1, which have proximate damped Ly alpha systems (PDLAs) with residual flux upon the Ly alpha absorption trough. Though they both have residual flux as luminous as 10(43) erg s(-1), their PDLAs are quite different in, e.g., neutral hydrogen column density, metal line absorption strength, high-ionization absorption lines as well as residual flux strength. For J1456+3400, the H I column density is log(N-H I/cm(-2)) = 20.6 +/- 0.2, with z(abs) = 2.3138, nearly identical to the quasar redshift (z = 2.3142) determined from the [O III] emission line. The metallicity of this system is typical of DLAs and there is high ionization therein, suggesting that the PDLA system is multiphase, putting it in the quasar environment. For J2153-0255, we measure the H I column density to be log(N-H I/cm(-2)) = 21.5 +/- 0.1 at z(abs) = 3.511, slightly redshifted with respect to the quasar (z = 3.490) measured from C III]. The metallicity of this system is quite low and there is a lack of significant high-ionization absorption lines therein, suggesting that the system is beyond the quasar host galaxy. The residual flux is wide (similar to 1000 km s(-1)) in J1456, with a significance of similar to 8 sigma, while also wide (similar to 1500 km s(-1)) but with a smaller significance of similar to 3 sigma in J2153. Among many explanations, we find that Ly alpha fuzz or resonant scattering can be used to explain the residual flux in the two sources while partial coverage cannot be excluded for J1456. By comparing these two cases, together with a similar case reported previously, we suggest that the strength of the residual flux is related to properties such as metallicity and high-ionization absorption lines of PDLAs. The residual flux recorded upon the PDLA absorption trough opens a window for us to see the physical conditions and processes of the quasar environment, and their profile and strength further remind us of their spatial scales.