The First HeI* Broad Absorption Line Quasar

Details The First HeI* Broad Absorption Line Quasar The First HeI* Broad Absorption Line Quasar

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21541132l&link_type=abstract

American Astronomical Society, AAS Meeting #215, #411.32; Bulletin of the American Astronomical Society, Vol. 42, p.245

Astronomy and Astrophysics

Astronomy

Scientific paper

We report the discovery of the first HeI* broad absorption line quasar using SpeX on IRTF. The blue-shifted absorption profile extends in velocity space from about -11,000 to -1,000 km/s, and it shows considerable velocity structure on the order of 1,000 km/s. The maximum apparent optical depth is 0.6. Integration over the 10830Å apparent optical depth profile yields a lower limit of the log HeI* column density of 15.3. Absorption is also seen for the HeI* line in optical spectra from SDSS and the MDM Hiltner telescope. These two transitions have the same lower level; thus, the covering fraction and optical depth can be determined. A pure partial covering model yields log HeI* columns of 16.5-16.7, while a power-law absorption distribution yields 17.1. These column densities are significantly larger than the lower limit because of the high ratio of the product of the wavelength and the oscillator strength (23.3). This property, plus the relatively low densities of HeI* in ionized gas, makes HeI* absorption a valuable probe of high column densities.
Cloudy simulations were performed to investigate the nature of the HeI* absorption. The HeI* 2s level lies 19.8eV above the ground state and is therefore populated by recombination; thus HeI* is coincident with He+ in the ionized gas slab. Models integrated through the HeI* region consistently produce values of the HeI* column density that are too low by 0.5 dex or more. The HeI* column increases with ionization parameter at a slightly smaller rate than the intensity in the HeI continuum. There is no significant dependence on spectral energy distribution. There is little dependence on density; some enhancement is found at very high densities approaching 1012cm-3. High densities are excluded, however, by the lack of observed Hα absorption.
This work is funded by NSF AST-0707703.

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