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An upper limit to [C II] emission in a z~5 galaxy Marsden, Anthony Gaelen

Abstract

Low ionization state far-IR emission lines play an important role in cooling star-forming regions, as they are an unambiguous link between UV and IR photons. Star-forming galaxies have large dust masses which obscure UV radiation, so we rely on the IR to probe these star-forming regions. Such lines may be useful diagnostics of star-formation activity in young galaxies, and at high redshift may be detectable from the ground. In practice, however, very little is known about how strong this line emission might be in the early universe. We attempted to detect the 158 μm [C II] line from a lensed galaxy at z = 4.92 using the Caltech Submillimeter Observatory. This source is an ordinary galaxy, in the sense that it does not show extreme star formation, but lensing makes it visible. Our analysis includes a careful consideration of the calibrations and weighting of the individual scans. We find only modest improvement over the simpler reduction methods, however, and the final spectrum remains dominated by systematic baseline ripple effects. We obtain a 95% confidence upper limit of 33mJy for a 200 km s⁻¹ FWHM line, corresponding to an unlensed magnitude of 1 x 10⁹ L[sub ⊙] for a standard cosmological model. Combining this with a marginal detection of the continuum emission using the James Clerk Maxwell Telescope, we can derive an upper limit of 0.4% for the ratio of L[sub C II]/L[sub FIR].

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