A sensitive search for CO J = 1–0 emission in 4C 41.17: high-excitation molecular gas at z = 3.8

¹ Institut für Astronomie, ETH, 8093 Zürich, Switzerland e-mail: papadop@phys.ethz.ch
² California Institute of Technology, Pasadena, CA 91125, USA
³ Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁴ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ European Southern Observatory, Karl Schwarzschild Straße 2, 85748 Garching, Germany

Received: 10 June 2005
Accepted: 26 August 2005

Abstract

We report sensitive imaging observations of the CO –0 line emission in the powerful high-redshift radio galaxy 4C 41.17 () with the NRAO Very Large Array (VLA), conducted in order to detect the large concomitant H₂ gas reservoir recently unveiled in this system by De Breuck et al.(2005) via the emission of the high excitation –3 line. Our observations fail to detect the –0 line but yield sensitive lower limits on the brightness temperature ratio of for the bulk of the H₂ gas mass. Such high ratios are typical of the high-excitation molecular gas phase “fueling” the star formation in local starbursts, but quite unlike these objects, much of the molecular gas in 4C 41.17 seems to be in such a state, and thus participating in the observed starburst episode. The widely observed and unique association of highly excited molecular gas with star forming sites allows CO line emission with large (high-J)/(low-J) intensity ratios to serve as an excellent “marker” of the spatial distribution of star formation in distant dust-obscured starbursts, unaffected by extinction.