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| Paper IPM / Astronomy / 16275 |
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| Abstract: | |||||||||||||||
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We present results from a search for strong H2 absorption systems proximate to quasars (zabs â zem) in the Sloan Digital Sky Survey
(SDSS) Data Release 14. The search is based on the Lyman-Werner band signature of damped H2 absorption lines without any prior
on the associated metal or neutral hydrogen content. This has resulted in the detection of 81 systems with N(H2) â¼ 1019 â 1020 cmâ2
located within a few thousand km sâ1
from the quasar. Compared to a control sample of intervening systems, this implies an excess
of proximate H2 systems by about a factor of 4 to 5. The incidence of H2 systems increases steeply with decreasing relative velocity,
reaching an order of magnitude higher than expected from intervening statistics at âv < 1000 km sâ1
. The most striking feature of
the proximate systems compared to the intervening ones is the presence of Ly-α emission in the core of the associated damped H i
absorption line in about half of the sample. This puts constraints on the relative projected sizes of the absorbing clouds to those of
the quasar line emitting regions. Using the SDSS spectra, we estimate the H i, metal and dust content of the systems, which are found
to have typical metallicities of one tenth Solar, albeit with a large spread among individual systems. We observe trends between the
fraction of leaking Ly-α emission and the relative absorber-quasar velocity as well as with the excitation of several metal species,
similar to what has been seen in metal-selected proximate DLAs. With the help of theoretical H i-H2 transition relations, we show
that the presence of H2 helps to break the degeneracy between density and strength of the UV field as main sources of excitation and
hence provides unique constraints on the possible origin and location of the absorbing clouds. We suggest that most of these systems
originate from galaxies in the quasar group, although a small fraction of them could be located in the quasar host as well. We conclude
that follow-up observations are still required to investigate the chemical and physical conditions in individual clouds and to assess the
importance of AGN feedback for the formation and survival of H2 clouds
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