APEX SV projects

Molecules in QSO absorption lines

Coordinator: M. Zwaan, C. Peroux

Abstract:
We propose to search for molecular absorption line systems at $z_{abs}$=0.885 in front of a known strong flat spectrum radio source, PKS1830-211 ($z_{em}$=2.507). To date only a handful of molecular detections are known in damped Ly-$\alpha$ (DLA) systems. At optical wavelengths, nine H$_2$ measurements have been possible among the many quasar lines of sight studied (Ledoux, Petitjean \& Srianand 2003, MNRAS, 346, 209). Formation of molecular hydrogen at high redshifts is believed to be directly connected to the amount of dust in the gas phase and the amount of dust in DLAs is an issue that is currently hotly debated. At low redshift, sub-mm observations have been used to attempt to detect molecular absorption line systems in front of radio sources. Again, only few successful measurements have been possible despite many hours of dedicated observations: 4 detections of CO in absorption have been made, and many other molecular species have been detected in these same systems (Drinkwater et~al. 1996, A\&A, 312, 771; Wiklind \& Combes 1994, A\&A, 288L, 41; Combes \& Wiklind 1995, A\&A, 303L, 61; Wiklind et~al. 1995, A\&A, 297, 643; Curran et~al. 2004, MNRAS, 352, 563). Only two systems in which molecules have been found are truly intervening, the other two being associated to the background radio source. CO provides direct information on the dust content of DLAs while detection of HCN is an alternative indicator of star formation. With APEX, such absorption systems can be used to study detailed chemistry at cosmological distances. The proposed programme is a pilote study to check upon APEX capabilities and to prepare for larger future programmes to search blindly for molecules in absorption at high redshifts. We propose to confirm the presence of HCO+, HCN and HNC at $z_{abs}$=0.885 towards PKS1830-211 whose low transitions (2-1) were detected with IRAM and SEST (Wiklind \& Combes 1998, ApJ, 500, 129) at higher transitions (6-7) and (7-8) and to search for new molecules such as CS (12-13) and (13-14).

Data:
Program is available and data products can be downloaded

Scientific Justification:
Molecular lines can be detected in absorption in the spectrum of a background compact source. Observing molecules in absorption is extremely powerful: the detection sensitivity is not dependent on the properties of the system under study but rather depends solely on the a priori unrelated characteristics of the background sources. Absorption line systems provide the most detailed information on the dense, pre-star forming Inter-Stellar Medium (ISM) in galaxies at cosmologically significant redshifts as well as the global cosmic chemical evolution. In particular, molecular gas is known to measure mass better than optical light. These lines are expected to be very narrow at sub-mm wavelengths. Today, only a handful of molecular detections are known in absorption. Only few successful observations have been possible despite many hours of dedicated observations: 4 detections of CO in absorption have been reported (Drinkwater et al. 1996, AA, 312, 771; Wiklind & Combes 1994, AA, 288L, 41; Combes & Wiklind 1995, AA, 303L, 61; Wiklind et al. 1995, AA, 297, 643; Curran et al. 2004, MNRAS, 352, 563). Among these 4 detections, two are found to be associated with the background source. Therefore, we know so far of only two genuine CO detections in intervening absorbers: 1) the z(abs)=0.685 system towards z(em)=0.936 BLac B0218+357 (Wiklind & Combes, 1995, A&A, 299, 382) and 2) the z(abs)=0.886 system towards z(em)=2.507 PKS 1830-211 (the same quasar has another z(abs)=0.192 double system along its line-of-sight; (Wiklind & Combes, 1996, Nature, 379, 139).

Technical Justification:
We propose to search for molecular absorption line systems in front of a known strong flat spectrum radio source, PKS 1830-211. From 30--IRAM and 15--SEST observations, many molecules 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 in this system. We choose to observe these low-excitation lines because the higher J levels of CO were not detected in previous short science verification observations, indicating that the excitation temperature is low. Note that H2O towards the other molecule rich absorber B0218+357 has already been detected, illustrating the feasability of our proposed observations (Combes & Wiklind, 1997, ApJ, 486L, 79).
The rest frequencies of the transitions we propose to observe are 556.936 GHz, 624.975 GHz and 572.498 GHz respectively (Schoier, F.L. et al. 2005, AA, 432, 369). These correspond to the following redshifted frequencies: 295.328 GHz, 331.408 GHz and 303.580 GHz. The bandwidth of 1024 MHz with 2048 channels would give a velocity resolution of 0.5 km/s. Other molecular lines in this system show velocity widths of ~ 30 km/s, implying that we can rebin the spectrum to increase the signal to noise, and still resolve the absorption profile. The quasar is known to be 0.5 Jy at these frequencies. Typical system temperatures in the 2A band are 200K, which means we will reach noise levels of ~ 3.5 mK per channel per session of 2 hours on and 2 hours off. The noise levels that we expect are very similar to those from Combes & Wiklind (1997, ApJ, 486L, 79) from their IRAM observations of the absorber against B0218+357. The flux of our background source is also comparable to that of B0218+357, which means that we will reach similar optical depth limits in PKS 1830-211. Combes & Wiklind detected the H2O line at 5 sigma.
The rms noise level in our observations is ~3.5 mK per 0.5 MHz channel, but given the expected width of the line, we can rebin the spectrum to at least 5 MHz, giving an rms noise level of 1.1 mK. The H2O line detected by Combes & Wiklind (1997, ApJ, 486L, 79) had a peak depth of 5.5 mK. We therefore estimate that 4 hrs per absorption line are needed (i.e. including position switching). In the period 19-29 October 2005, this target is expected to be visible in the early hours of the night.