Differential X - ray Absorption and Dust - To - Gas Ratios of the Lens

We analyzed Chandra observations of three gravitational lenses, SBS 0909+523, FBQS 0951+2635, and B 1152+199, to measure the differential X-ray absorption and the dust-to-gas ratio of the lens galaxies. We successfully detected the differential X-ray absorption in SBS 0909+523 and B 1152+199, and failed to detect it in FBQS 0951+2635 due to the dramatic drop in its flux from the ROSAT epoch. These measurements significantly increase the sample of dust-to-gas ratio measurements in cosmologically-distant, normal galaxies. Using the larger sample, we obtain an average dust-to-gas ratio of E(B− V )/NH= (1.5± 0.5)× 10 −22 mag cm atoms with an estimated intrinsic dispersion in the ratio of ≃40%. This average dust-to-gas ratio is consistent with our previous measurement, and the average Galactic value of 1.7 × 10 mag cm atoms and the estimated intrinsic dispersion is also consistent with the 30% observed in the Galaxy. A larger sample size is still needed to improve the measurements and to begin studying the evolution in the ratio with cosmic time. We also detected X-ray microlensing in SBS 0909+523 and significant X-ray variability in FBQS 0951+2635. Subject headings: galaxy: evolution – ISM: dust, extinction