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| Paper: |
Formaldehyde: A High-Redshift Tracer of Pre-Starburst Molecular Gas? |
| Volume: |
375, From Z-Machines to ALMA: (Sub)Millimeter Spectroscopy of Galaxies |
| Page: |
208 |
| Authors: |
Darling, J. |
| Abstract: |
Formaldehyde (H2CO) 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 H2. In an extragalactic setting, H2CO 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 H2CO 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 ΔJ = ±1 rotation transitions of H2CO to arbitrary redshift, both in emission and absorption, potentially providing redshifts for systems undetectable in CO or CII lines. Combined with cm-wave observations of H2CO ΔJ = 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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