Identifying the lens galaxy B 1152+199 as a ghostly damped Lyman α system by the cosmic origins spectrograph

Abstract

ABSTRACT Strong quasar-galaxy lensing provides a powerful tool to probe the interstellar medium (ISM) of the lens galaxy using radiation from the background quasar. Using the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope, we study the cold ISM properties of the lens galaxy in B 1152+199 at a redshift of z = 0.4377. Since existing optical extinction and X-ray absorption measurements of the lens have revealed a large amount of cold ISM, we expected to detect a damped Ly α absorption (DLA) system in the near ultraviolet spectrum; however, our upper limit on the H i column density is several orders of magnitude below the expectation. We also marginally detect O i and C ii absorption lines associated with the lens galaxy in the COS spectrum. Thus, the lens galaxy is identified as a ghostly DLA system, and further investigations of these ghostly DLA systems would be important to characterize the biases of using DLAs to probe the matter density of the universe. Although preliminary, the most likely explanation of the non-detection of the DLA is because of the Ly α emission of the lens galaxy that fills in the absorption trough, with a Ly α luminosity of $4\times 10^{42}\, \hbox{erg~s$^{-1}$}$.