NEUTRAL HYDROGEN 21 cm ABSORPTION AT REDSHIFT 2.6365 TOWARD THE GRAVITATIONAL LENS MG J0414+0534

摘要

We report the detection of the ZI cm line of neutral hydrogen in absorption at a redshift of 2.6365 toward the gravitationally lensed radio source MG J0414+0534. The absorption shows two velocity components, each with a width of ～150 km s~(-1), separated by about 200 km s~(-1). We determine a total H I column density of 7.5 ± 1.3 x 10~18 (T_s) cm~(-2) assuming an H I covering factor of unity. Assuming a Galactic dust-to-gas ratio and that all the reddening toward the source is due to dust associated with the observed H I 21 cm absorption line, we derive an upper limit to the H I spin temperature T_s of 1300 K. We do not detect absorption in the HCN (J = 1-0) line toward MG J0414+0534 with a 3 σ optical depth limit of 0.047 at 19.2 km s~(-1) resolution. The implied upper limit on the HCN column density is 1 x 10~13 cm~(-2). The redshift of the H I absorption centroid differs by 200 km s~(-1) from the centroid of the recently discovered CO J = 3-2 emission from MG J0414+0534. We present two alternative scenarios that explain both the observed velocity offset between the CO emission and the H I absorption and the lack of molecular absorption toward MG J0414+0534. Either (1) the H I 21 cm line is absorbing the lensed active galactic nucleus and the regions giving rise to CO emission have a different spatial distribution than the radio continuum-emitting regions and are affected differently by gravitational lensing or (2) the H I 21 cm absorption is toward an extended, steep-spectrum radio component, which contributes sub- stantially to the radio continuum emission at low fr