Dust in QSO Absorption-Line Systems

Details Dust in QSO Absorption-Line Systems Dust in QSO Absorption-Line Systems

Dec 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990apj...364..405o&link_type=abstract

Astrophysical Journal v.364, p.405

Astronomy and Astrophysics

Astronomy

17

Galaxies: Intergalactic Medium, Quasars

Scientific paper

We attempt to estimate the dust obscuration of QSOs using an approximate calibration derived from absorption line strengths in intervening QSO absorption line systems. Adopting, as calibration, a fixed ratio of dust opacity to C IV absorption line strengths, for the tenuous outer parts of high z galaxies, we do not require any further assumptions concerning dust-to-gas ratio or sky covering factor. The number of systems covering a QSO to given z, known empirically, is then used to estimate the statistical effect of dust on the visibility of high z objects given the calibration. At z = 3.0, we expect > 1.2 mag of extinction in the observer's B band, a value that roughly doubles for each added unit in z, if extinction continues to rise below 1000 Å, and if absorbers exist at even higher redshift. At this level, number counts in magnitude-limited samples may underestimate true numbers of objects by many factors of 10. However, formation of grains, of carbon, and of star-forming regions must all begin at some point as one looks back in time, and cautionary remarks are made concerning extrapolations beyond z = 3.
A synthetic spectrum calculated for Q0805 +046 including dust obscuration, due to an intervening, heavy element, absorption line system, appears to be consistent with the observed spectrum. Direct evidence for dust in the QSO absorption line systems should be sought, and some suggestions are made for further observational tests. Due to selection bias (i.e., we preferentially detect brighter, less obscured, QSOs), the average extinction to a given redshift is larger (by approximately a factor of less than or equal to 2) than the average extinction to detected objects, thereby increasing the quasar number depletion for given observed extinction.

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