Search for H2O at z=0.89

Coordinator: T. Wiklind, F. Combes

Program is available and data products can be downloaded

The abundance of H2 O in the interstellar medium is not well understood. Recent resultsfrom satellite borne submm telescopes (SWAS and Odin), which has made it possible toobserve the ground transition of H2 O in different Galactic environments, indicates thatthe abundance of water vapor is lower than expected from chemical models, and that itvaries from one molecular cloud to another - and even over the extent of a single cloud(cf. Wilson et al. 2003, A&A 402, L59, and references therein).
The first detection of the ground state of water vapor was, however, done with agroundbased instrument (IRAM's 30m telescope) using an intermediate redshifted molec-ular absorption line system seen towards B0218-357 (Combes & Wiklind 1997, A&A 325,923). The redshift of the absorption line system was z=0.68 and the groundstate line of556.936 GHz was shifted to 331 GHz. Due to a low continuum strength of the backgroundsource, the H2 O line was rather faint but appeared to be saturated.
Molecular absorption lines constitute a sensitive probe of molecular transitions whichmay be too weak to be studied using emission lines. This has been shown for diffusemolecular gas in our Milky Way galaxy (cf. Lucas & Liszt in a series of papers from 1993- 2003), as well as for galaxies at intermediate redshifts (cf. Wiklind & Combes, Combes& Wiklind, in a series of papers from 1994-2000). In the latter case, 18 different molecularspecies have been detected at redshifts z= 0.25 - 0.89, often in two or more transitions.Among the detected molecules are rare species (N2 H+ , HNC, HOC+ ,...) as well as isotopicvariants (13 CO, C17 O, H13 CO+ , etc). This has shown that molecular absorption lines areexcellent probes for molecular rotational lines, with a sensitivity which to a large extentis only limited by the strength of the background continuum source.
The molecular gas causing the absorption towards PKS1830-211 is different than thatin B0218-357 and a detection towards PKS1830-211 would thus probe the water vaporabundance in different types of clouds.

Proposed observations
We propose to use APEX with the APEX-2a receiver to search for the ortho-H2 O ground-state line in the intervening molecular absorption line system at z=0.889 towards PKS1830-211. In addition we propose to observe the CO(5-4) line at restfrequency 576.268 GHz,redshifted to 305.065 GHz.
The groundstate H2 O line has a restfrequency of 556.936 GHz and the observed fre-quency will thus be 294.831 GHz. The width of the line is expected to be about 20-40km s-1 , depending on whether the line is saturated or not. The background source islensed into two components by the intervening galaxy where the absorption occurs. Onlyone of the lens components contribute to the molecular absorption. Thus, even for a fullysaturated line, we do not expect the absorption to reach zero. The continuum strengthof PKS1830-211 at 3mm is typically 1.5-2 Jy, with occasional stronger levels. At 1mmwe expect the continuum to be at least 1 Jy, which should allow detection of a saturatedline in a relatively short time period. However, should the line be unsaturated, we requirelonger integration times.
PKS1830-211 is a night time object during July/August. Pointing can be done on thecontinuum source itself or nearby SiO masers (the former is preferred but depends on theavailable pointing modes).
The configuration of the spectrometer should allow a bandwidth of 0.2-0.5 GHz, witha velocity resolution less than 5 km/s. The ideal configuration would be with a velocityresolution of 1 km s-1 and a bandwidth of 0.5-1.0 GHz. The final configuration can beleft to the science verification team. ON-OFF observation mode is suggested.
The interesting observable is in this case the line-to-continuum ratio. However, due tochanges in the atmospheric and instrument conditions, as well as the fact that only oneof the lensed components contribute to the opacity, it is desirable to also determine thestrength of the continuum source. How this is done with the present set-up is left to thescience verification team.

Time estimate

It is impossible to make an exact time estimate since the background source is variable andwe do not know whether the line is saturated or not. However, with a typical continuumstrength of 1 Jy, and assuming a (binned) velocity resolution of 4 km s-1 , a slightlyunsaturated line should be detectable in 2-3 hours. This applies to both the H2 O andCO(5-4) line. Adding time for pointing (on the continuum source itself) and considerableoverhead, the total time for this project will be 8 hours. We emphasize that the main partof this science verification proposal rests on the detection of H2 O absorption.