Lyα/Hα Ratio of Singly Ionized Helium in Quasars

Abstract

He II Lyα λ304/Hα λ1640 emission lines are mainly produced by recombination, and their canonical ratio of ~10 may be a sensitive reddening indicator. We obtain the high signal-to-noise ratio optical spectra of two quasars and combine them with the far-ultraviolet spectra that show the He II λ304 emission. For HS 1700+64, the He II λ1640 emission is not detected, and an upper limit to it sets the ratio greater than 20. This may not be inconsistent with the theoretical value when all observational uncertainties are taken into consideration. For Q0302-003, the ratio is very low and on the order of unity. The most plausible cause for such a low ratio is extinction in the EUV band by very fine grains of dust. Q0302-003 has a prominent narrow component of FWHM ~ 2000 km s^-1 in its major emission lines, and it appears that reddening is associated only with the line-emitting region. We suggest that the geometry of the line-emitting region in high-z quasars resembles that in low-luminosity active galaxies, with the presence of dust mostly in the outer part.

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities of Research in Astronomy, Inc., under NASA contract NAS5-26555.