Molecular Absorption Lines at High-Redshift with APEX

Coordinator: M. Zwaan, C. Peroux

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Scientific Justification:
Molecular lines can be detected in absorptionin the spectrum of a background compact source. Observing molecules inabsorption is extremely powerful: the detection sensitivity is notdependent on the properties of the system under study but ratherdepends solely on the a priori unrelated characteristics of thebackground sources. Absorption line systems provide the most detailedinformation on the dense, pre-star forming Inter-Stellar Medium (ISM)in galaxies at cosmologically significant redshifts as well as theglobal cosmic chemical evolution. In particular, molecular gas isknown to measure mass better than optical light. These lines areexpected to be very narrow at sub-mm wavelengths. Today, only ahandful of molecular detections are known in absorption. Only fewsuccessful observations have been possible despite many hours ofdedicated observations: 4 detections of CO in absorption have beenreported (Drinkwater et al. 1996, AA, 312, 771; Wiklind & Combes 1994,AA, 288L, 41; Combes & Wiklind 1995, AA, 303L, 61; Wiklind etal. 1995, AA, 297, 643; Curran et al. 2004, MNRAS, 352, 563). Amongthese 4 detections, two are found to be associated with the backgroundsource. Therefore, we know so far of only two genuine CO detectionsin intervening absorbers: 1) the z(abs)=0.685 system towardsz(em)=0.936 BLac B0218+357 (Wiklind & Combes, 1995, A&A, 299, 382) and2) the z(abs)=0.886 system towards z(em)=2.507 PKS 1830-211 (the samequasar has another z(abs)=0.192 double system along its line-of-sight;(Wiklind & Combes, 1996, Nature, 379, 139).Technical Justification: Here, we propose to search for molecularabsorption line systems in front of a known strong flat spectrum radiosource, PKS 1830-211. From 30--IRAM and 15--SEST observations, manymolecules have already been observed in this absorber, including HCO,HCN, O2 etc. We propose here to observe o-H2O, HCl(1<-0) and o-NH3 inthis system. We choose to observe these low-excitation lines becausethe higher J levels of CO were not detected in previous short scienceverification observations, indicating that the excitation temperatureis low. Note that H2O towards the other molecule rich absorberB0218+357 has already been detected, illustrating the feasability ofour proposed observations (Combes & Wiklind, 1997, ApJ, 486L, 79).The rest frequencies of the transitions we propose to observe are556.936 GHz, 624.975 GHz and 572.498 GHz respectively (Schoier,F.L. et al. 2005, AA, 432, 369). These correspond to the followingredshifted frequencies: 295.328 GHz, 331.408 GHz and 303.580 GHz. Thebandwidth of 1024 MHz with 2048 channels would give a velocityresolution of 0.5 km/s. Other molecular lines in this system showvelocity widths of ~ 30 km/s, implying that we can rebin the spectrumto increase the signal to noise, and still resolve the absorptionprofile. The quasar is known to be 0.5 Jy at thesefrequencies. Typical system temperatures in the 2A band are 200K,which means we will reach noise levels of ~ 3.5 mK per channel persession of 2 hours on and 2 hours off. The noise levels that we expectare very similar to those from Combes & Wiklind (1997, ApJ, 486L, 79)from their IRAM observations of the absorber against B0218+357. Theflux of our background source is also comparable to that of B0218+357,which means that we will reach similar optical depth limits in PKS1830-211. Combes & Wiklind detected the H2O line at 5 sigma.The rms noise level in our observations is ~3.5 mK per 0.5 MHzchannel, but given the expected width of the line, we can rebin thespectrum to at least 5 MHz, giving an rms noise level of 1.1 mK. TheH2O line detected by Combes & Wiklind (1997, ApJ, 486L, 79) had apeak depth of 5.5 mK. We therefore estimate that 4 hrs per absorptionline are needed (i.e. including position switching). In the period 19-29October 2005, this target is expected to be visible in the early hoursof the night.