S21A0005

We aim to find out the reason why there exists an anisotropic HI absorption around quasars; i.e., the environments around quasars are highly biased toward producing strong HI absorption in transverse direction while there exists a significant deficit of HI absorption within a few Mpc of quasars along line-of-sight (i.e., proximity effect). The most plausible explanation for this opposite trend is that the transverse direction is shadowed from the quasar UV radiation due to dusty torus, a frequently invoked structure in AGN unification models. However, a critical weakness of this idea is that we have no information on inclination angle of our sightline relative to the torus. In this program, we examine environments of quasars with broad absorption troughs in their spectra (i.e., BAL quasars) because it is widely believed that BAL troughs are observed only if the central continuum is viewed from the side. With closely separated (θ < 120^′′) 15 quasar pairs at different redshift, we examine HI absorption at foreground BAL quasars in spectra of background quasars, to confirm if the anisotropic HI absorption is also observed around BAL quasars. This is a continuation of our programs in past semesters, but those in S19A and S20A were canceled due to natural disasters.

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