Abstract
ABSTRACT We use simple models of the spatial structure of the quasar broad-line region (BLR) to investigate the properties of so-called ghostly damped Ly α (DLA) systems detected in Sloan Digital Sky Survey (SDSS) data. These absorbers are characterized by the presence of strong metal lines but no H i Ly α trough is seen in the quasar spectrum indicating that, although the region emitting the quasar continuum is covered by an absorbing cloud, the BLR is only partially covered. One of the models has a spherical geometry, another one is the combination of two wind flows, whereas the third model is a Keplerian disc. The models can reproduce the typical shape of the quasar Ly α emission and different ghostly configurations. We show that the DLA H i column density can be recovered precisely independently of the BLR model used. The size of the absorbing cloud and its distance to the centre of the AGN are correlated. However, it may be possible to disentangle the two using an independent estimate of the radius from the determination of the particle density. Comparison of the model outputs with SDSS data shows that the wind and disc models are more versatile than the spherical one and can be more easily adapted to the observations. For all the systems, we derive log N(H i)(cm−2) > 20.5. With higher quality data, it may be possible to distinguish between the models.
Highlights
One of the most challenging issues in quasar physics is to understand how black-holes are fed with infalling gas
The left-hand side panel of Fig. 8 shows the fit of the quasar template with this model. It can be seen on the figure that the broad line region (BLR) spectrum shows two peaks widely separated implying that the needed narrow line region (NLR) emission has a broader width, FW H M = 900 km s−1 in this case
The resulting spectrum depends on several characteristics of the absorbing cloud: its column density, its position, its size; but it depends on the inclination of the BLR with respect to the observer in the case of the wind and disk models
Summary
One of the most challenging issues in quasar physics is to understand how black-holes are fed with infalling gas. Since the DLA and the background quasar are located at almost the same redshift, the DLA can act as a natural coronagraph, blocking the quasar blazing radiation in Lyman-α This can allow us, depending on the dimension of these so-called eclipsing DLAs, to detect fainter emission from star-forming regions in the host galaxy and the extended quasar halo and/or to observe the narrow line region (NLR) of the AGN. The BLR is thought to be composed of approximately virialised gas in the vicinity of the black hole (Netzer 2008) From this idea, it is possible to derive the typical size of the emission by performing reverberation mapping analysis (Shen et al 2019).
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