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Published November 1, 2009 | Published
Journal Article Open

The Warm Molecular Gas around the Cloverleaf Quasar


We present the first broadband λ = 1 mm spectrum toward the z = 2.56 Cloverleaf quasar, obtained with Z-Spec, a grating spectrograph on the 10.4 m Caltech Submillimeter Observatory. The 190-305 GHz observation band corresponds to the rest frame 272-444 μm, and we measure the dust continuum as well as all four transitions of carbon monoxide (CO) lying in this range. The power-law dust emission, F_ν = 14 mJy(ν/240 GHz)^(3.9) is consistent with the published continuum measurements. The CO J = 6 → 5, J = 8 → 7, and J = 9 → 8 measurements are the first, and now provide the highest-J CO information in this source. Our measured CO intensities are very close to the previously published interferometric measurements of J = 7 → 6, and we use all available transitions and our ^(13)CO upper limits to constrain the physical conditions in the Cloverleaf molecular gas disk. We find a large mass (2-50 × 10^9 M_⊙) of highly excited gas with thermal pressure nT > 10^6 K cm^(–3). The ratio of the total CO cooling to the far-IR dust emission exceeds that in the local dusty galaxies, and we investigate the potential heating sources for this bulk of warm molecular gas. We conclude that both UV photons and X-rays likely contribute, and discuss implications for a top-heavy stellar initial mass function arising in the X-ray-irradiated starburst. Finally, we present tentative identifications of other species in the spectrum, including a possible detection of the H_2O 2_(0,2) → 1_(1,1) transition at λ_(rest) = 303 μm.

Additional Information

© 2009 The American Astronomical Society. Received 2009 May 29; accepted 2009 August 18; published 2009 October 8. We are indebted to the staff of the Caltech Submillimeter Observatory for their help in Z-Spec's commissioning and observing. We acknowledge Peter Ade and his group for some of the filters and Lionel Duband for the 3He/4He refrigerator in ZSpec, and are grateful for their help in the early integration of the instrument. We benefitted from conversations with Tom Phillips, Paul Goldsmith, Simon Radford, and Andy Harris, as well as helpful comments from Xinyu Dai and an anonymous referee. Finally, we acknowledge the following grants and fellowships: NASA SARA grants NAGS-11911 and NAGS-12788, an NSF Career grant (AST-0239270) and a Research Corporation Award (RI0928) to J. Glenn, a Caltech Millikan and JPL Director's fellowships to C.M.B., a NRAO Jansky fellowship to J. Aguirre, and NASA GSRP fellowship to L. Earle.

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