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Published October 1, 2012 | Published
Journal Article Open

Measurements of CO Redshifts with Z-Spec for Lensed Submillimeter Galaxies Discovered in the H-ATLAS Survey


We present new observations from Z-Spec, a broadband 185-305 GHz spectrometer, of five submillimeter bright lensed sources selected from the Herschel-Astrophysical Terahertz Large Area Survey science demonstration phase catalog. We construct a redshift-finding algorithm using combinations of the signal to noise of all the lines falling in the Z-Spec bandpass to determine redshifts with high confidence, even in cases where the signal to noise in individual lines is low. We measure the dust continuum in all sources and secure CO redshifts for four out of five (z ~ 1.5-3). In one source, SDP.17, we tentatively identify two independent redshifts and a water line, confirmed at z = 2.308. Our sources have properties characteristic of dusty starburst galaxies, with magnification-corrected star formation rates of 10^(2–3) M_☉ yr^(–1). Lower limits for the dust masses (~a few 10^8 M_☉) and spatial extents (~1 kpc equivalent radius) are derived from the continuum spectral energy distributions, corresponding to dust temperatures between 54 and 69 K. In the local thermodynamic equilibrium (LTE) approximation, we derive relatively low CO excitation temperatures (≲100 K) and optical depths (τ ≲ 1). Performing a non-LTE excitation analysis using RADEX, we find that the CO lines measured by Z-Spec (from J = 4 → 3 to 10 → 9, depending on the galaxy) localize the best solutions to either a high-temperature/low-density region or a low/temperature/high-density region near the LTE solution, with the optical depth varying accordingly. Observations of additional CO lines, CO(1-0) in particular, are needed to constrain the non-LTE models.

Additional Information

© 2012 American Astronomical Society. Received 2010 September 29; accepted 2012 August 13; published 2012 September 12. We are indebted to the staff of the Caltech Submillimeter Observatory for their unflagging support. This work was supported by NSF grant AST-0807990 to J. Aguirre and by the CSO NSF Cooperative Agreement AST-0838261. Support was provided to J. Kamenetzky by an NSF Graduate Research Fellowship. Z-spec was constructed under NASA SARA grants NAGS-11911 and NAGS-12788 and an NSF Career grant (AST-0239270) and a Research Corporation Award (RI0928) to J. Glenn, in collaboration with the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We acknowledge Peter Ade and his group for their filters and Lionel Duband for the 3He/4He refrigerator in Z-Spec, and are grateful for their help in the early integration of the instrument. R.L. thanks Tom Loredo for useful discussions regarding the significance of the redshift determination, and P. Papadopoulos for help improving the gas mass discussion. R.L. also thanks the anonymous referee for the thorough read and the helpful comments for improving the paper. We appreciate the help of Robert Hanni and Jon Rodriguez with observing.

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