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Title:
Formaldehyde: A High-Redshift Tracer of Pre-Starburst Molecular Gas?
Authors:
Darling, J.
Publication:
From Z-Machines to ALMA: (Sub)Millimeter Spectroscopy of Galaxies ASP Conference Series, Vol. 375, proceedings of the conference held 12-14 January, 2006 at the North American ALMA Science Center, National Radio Astronomy Observatory, Charlottesville, Virginia, United States. Edited by Andrew J. Baker, Jason Glenn, Andrew I. Harris, Jeffrey G. Mangum and Min S. Yun., p.208
Publication Date:
10/2007
Origin:
ASP
Bibliographic Code:
2007ASPC..375..208D

Abstract

Formaldehyde (H_2CO) has been observed in absorption against the cosmic microwave background (CMB) in dark clouds in the Galaxy, indicating an anti-inversion of energy level populations most likely induced by collisions with H_2. In an extragalactic setting, H_2CO may provide a unique tracer of the cold molecular ISM in pre-star-forming molecular clouds. Because the CMB scales as TCMB=2.73~K (1+z) and lies behind every galaxy, the detectability of H_2CO likely increases with redshift as (1+z) and does not rely on the fortuitous alignment of flat- or inverted-spectrum continuum sources with molecular clouds that has traditionally frustrated cold molecular ISM studies. Since the ``cooling'' of formaldehyde is collisional, line ratios can indicate molecular hydrogen density as well as excitation temperatures, providing new insight into the cold molecular gas input to star formation. ALMA can detect the Δ J=±1 rotation transitions of H_2CO to arbitrary redshift, both in emission and absorption, potentially providing redshifts for systems undetectable in CO or C II lines. Combined with cm-wave observations of H_2CO Δ J=0 rotation transitions, ALMA can characterize the physical state of the cold molecular ISM in galaxies from the present to the early universe.
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