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

Abstract

We report sensitive imaging observations of the CO J = 1–0 line emission in the powerful high-redshift radio galaxy 4C 41.17 (z = 3.8) 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 J = 4–3 line. Our observations fail to detect the J = 1–0 line but yield sensitive lower limits on the R_(43) = (4 − 3)/(1 − 0) brightness temperature ratio of R43 ∼ 0.55 − ≳ 1.0 for the bulk of the H_2 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.

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