Identifying the Lens Galaxy B1152+199 as a Ghostly Damped Lyman Alpha System by the Cosmic Origins Spectrograph

TL;DR

This study uses the Hubble Space Telescope's Cosmic Origins Spectrograph to identify the lens galaxy in B1152+199 as a ghostly damped Lyman alpha system, revealing complexities in using DLAs to probe cosmic matter density.

Contribution

First detection of a ghostly DLA system in a lens galaxy using UV spectroscopy, highlighting potential biases in DLA-based cosmic measurements.

Findings

Upper limit on HI column density is much lower than expected.

Detected OI and CII absorption lines associated with the lens galaxy.

Lya emission likely fills in the absorption trough, explaining the non-detection.

Abstract

Strong quasar-galaxy lensing provides a powerful tool to probe the inter-stellar 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 B1152+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 Lya absorption (DLA) system in the near ultraviolet spectrum; however, our upper limit on the HI column density is several orders of magnitude below the expectation. We also marginally detect OI and CII 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 Lya emission of the lens galaxy that fills in the absorption trough, with a Lya luminosity of 4e42 erg/s.