An Intercomparison Study of Two Proximate Damped Ly$\alpha$ Systems with Residual Flux upon the Ly$\alpha$ 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 Lya systems (PDLAs) with residual flux upon the Lya absorption trough. Though they both have residual flux as luminous as $10^{43}erg/s$, their PDLAs are quite different in, e.g., HI column density, metal line absorption strength, high-ionization absorption lines as well as residual flux strength. For J1456, the HI column density is $20.6\pm0.2$, with za=2.3138, nearly identical to the quasar redshift (z=2.3142) determined from the [OIII] 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, we measure the HI column density to be $21.5\pm0.1$ at za=3.511, slightly redshifted with respect to the quasar (z=3.490) measured from CIII]. 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 (1000 km/s) in J1456, with a significance of $8\sigma$, while also wide (1500 km/s) but with a smaller significance of $3\sigma$ in J2153. Among many explanations, we find that Lya 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 similar cases 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.