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Polarimetry as a clue towards FeLoBAL quasar geometry
D. Lawther
Dark Cosmology Centre, Juliane Maries Vej 30, Copenhagen, Denmark, [email protected]
Around 10-30% of quasars display broad (over 1000 km/s) absorption troughs superimposed on broad emission
lines such as C IV λ1549 and Mg II λ2800. These quasars are known as broad absorption line (BAL) quasars.
The broad absorption lines are typically blueshifted compared to the systemic velocity, and are therefore generally
interpreted as due to outflowing gas. All BAL quasars for which reliable spectra at 1550 Å are available display
absorption in the C IV line. Some BAL quasars additionally display absorption in low-ionization lines such as Mg II
- these are known as LoBAL quasars. A subset of LoBALs displays strong iron absorption - these are known as
FeLoBALs. The FeLoBALs are quite rare, comprising only about 1.5% of the total quasar population, and display
very reddened continuum spectra, probably due to a combination of dust absorption and overlapping iron
absorption lines.
Two scenarios have been suggested to explain the properties of BAL quasars. One suggestion is that the
presence of the BAL phenomenon in quasar spectra could be orientation-dependent: outflows carrying broad
emission line gas may be present in all quasars, but have a small covering fraction, leading us to only observe
BAL troughs at certain viewing angles to the central engine. See Figure 1 below for an example of such a model.
A second suggestion is that the BAL outflow may be an evolutionary effect: early on in a quasar's lifespan,
perhaps soon after the onset of the merger-triggered accretion, the quasar ejects a 'cocoon' of gas and dust. This
cocoon would present itself as a BAL outflow across a wider range of viewing angles. The jury is still out on
whether the BAL class as a whole can be described adequately by one of these scenarios; orientation indicators
such as polarization angles and radio-spectrum steepness point towards a range of viewing angles for BAL
quasars. This motivates a closer study of the properties of BAL subgroups: perhaps one of the above scenarios
applies to a subgroup of BALs, but not the entire BAL population?
I am currently studying FeLoBAL host galaxies in HST ACS and NICMOS imaging. I want to determine whether
FeLoBAL host galaxies have similar properties to the host galaxies of ‘ordinary’ quasars, and to the host galaxies
of the wider BAL quasar population. Ultimately, I would like to combine information on the FeLoBAL host galaxies
with other clues to the nature of FeLoBALs, and determine if they fit better into an orientation-dependent or
evolutionary model. This talk will discuss some ways that polarimetric observations could be used as valuable
tools to examine the structure of FeLoBALs.
Elvis, M 2012: Quasar structure emerges from the three forms of radiation pressure, ASP Conference Series,
arxiv: 1201.3520 v1, 17 Jan 2012
Figure 1: An example of a model quasar structure that generates an outflowing wind from the accretion disk.
Observers looking directly down the wind, as indicated by the arrow, will see this quasar as a BAL quasar. This is
Figure 1 of Elvis (2012).