Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...510l..87m&link_type=abstract
The Astrophysical Journal, Volume 510, Issue 2, pp. L87-L90.
Astronomy and Astrophysics
Astronomy
13
Cosmology: Gravitational Lensing, Galaxies: Quasars: Absorption Lines, Radio Lines: Galaxies
Scientific paper
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+/-1.3x10^18 (T_s) m^-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 sigma optical depth limit of 0.047 at 19.2 km s^-1 resolution. The implied upper limit on the HCN column density is 1x10^13 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 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.
Carilli Chris L.
Menten Karl. M.
Moore Christopher B.
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