# 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).

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