1999ApJ...510L..87M

We report the detection of the 21 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±1.3x10¹⁸{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 1x10¹³ 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 substantially to the radio continuum emission at low frequencies but has a negligible flux density at higher frequencies. This latter model requires free-free absorption toward the steep-spectrum component in order to result in the observed Gigahertz-peaked spectrum of MG J0414+0534's integrated radio emission.