A Unique Measurement of the Dust-to-Gas Ratio in an Absorption Line System

Computer Science

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Hst Proposal Id #8336 Quasar Absorption Lines

Scientific paper

The dust--to--gas ratio in damped Ly-alpha absorbers has been determined from ISM metal abundances and from the effect of extinction on the background quasar continuua. Both methods have significant uncertainties. A damped system lies along the line-of-sight to both bright components of the gravitationally lensed quasar, 0957+561. If we observe the complete UV to optical spectrum of each component separated in time by the known time delay in this system, we can make a differential measurement of the reddening which does not depend on the intrinsic continuum shapes. This will yield a precise measurement of the dust-to-gas ratio in an absorption system at redshift z=1.3911 and will test whether the extinction laws of the Galaxy and Magellanic clouds are the same as those in extragalactic systems. We can also test our assumptions about how abundance measurements translate into dust-to-gas ratios by comparing these results with dust-to-gas ratios derived using high-reso lution spectra of both components obtained at Keck Observatory. The UV portion of these observations is required to discriminate between different extinction laws and clearly requires HST. Because the extinction in the optical is lower, very high accuracy in the relative flux calibration of the two objects is required. This accuracy is not attainable from the ground because of cross-contamination of the two spectra and contamination by the lensing galaxy.

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