Does the X-ray outflow quasar PDS 456 have a UV outflow at 0.3c?
Date Issued
2018
Author(s)
Abstract
The quasar PDS 456 (at redshift ~0.184) has a prototype ultra-fast outflow
(UFO) measured in X-rays. This outflow is highly ionized with relativistic
speeds, large total column densities log N_H(cm^-2) > 23, and large kinetic
energies that could be important for feedback to the host galaxy. A UV spectrum
of PDS 456 obtained with the Hubble Space Telescope in 2000 contains one
well-measured broad absorption line (BAL) at ~1346A (observed) that might be
Ly-alpha at v ~ 0.06c or NV 1240 at v ~ 0.08c. However, we use photoionisation
models and comparisons to other outflow quasars to show that these BAL
identifications are problematic because other lines that should accompany them
are not detected. We argue that the UV BAL is probably CIV 1549 at v ~ 0.30c.
This would be the fastest UV outflow ever reported, but its speed is similar to
the X-ray outflow and its appearance overall is similar to relativistic UV BALs
observed in other quasars. The CIV BAL identification is also supported
indirectly by the tentative detection of another broad CIV line at v ~ 0.19c.
The high speeds suggest that the UV outflow originates with the X-ray UFO
crudely 20 to 30 r_g from the central black hole. We speculate that the CIV BAL
might form in dense clumps embedded in the X-ray UFO, requiring density
enhancements of only >0.4 dex compared clumpy structures already inferred for
the soft X-ray absorber in PDS 456. The CIV BAL might therefore be the first
detection of low-ionisation clumps proposed previously to boost the opacities
in UFOs for radiative driving.
Volume
476
Issue
1
Start page
943
Issn Identifier
0035-8711
Ads BibCode
2018MNRAS.476..943H
Rights
open.access
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