Formaldehyde Silhouettes Against the Cosmic Microwave Background: A Mass-Limited, Distance-Independent, Extinction-Free Tracer of Star Formation Across the Epoch of Galaxy Evolution

Abstract

We examine the absorption of cosmic microwave background (CMB) photons by formaldehyde (H2CO) over cosmic time. The K-doublet rotational transitions of H2CO become "refrigerated" - their excitation temperatures are driven below the CMB temperature - via collisional pumping by molecular hydrogen (H2). "Anti-inverted" H2CO line ratios thus provide an accurate measurement of the H2 density in molecular clouds. Using a radiative transfer model, we demonstrate that H2CO centimeter wavelength line excitation and detectability are nearly independent of redshift or gas kinetic temperature. Since the H2CO K-doublet lines absorb CMB light, and since the CMB lies behind every galaxy and provides an exceptionally uniform extended illumination source, H2CO is a distance-independent, extinction-free molecular gas mass-limited tracer of dense gas in galaxies. A Formaldehyde Deep Field could map the history of cosmic star formation in a uniquely unbiased fashion and may be possible with large bandwidth wide-field radio interferometers whereby the silhouettes of star-forming galaxies would be detected across the epoch of galaxy evolution. We also examine the possibility that H2CO lines may provide a standardizable galaxy ruler for cosmology similar to the Sunyaev-Zel'dovich effect in galaxy clusters but applicable to much higher redshifts and larger samples. Finally, we explore how anti-inverted meterwave H2CO lines in galaxies during the peak of cosmic star formation may contaminate HI 21 cm tomography of the Epoch of Reionization.

Accepted by ApJ Letters. 5 pages, 5 figures

References in corpus (4)

• Formaldehyde Densitometry of Starburst Galaxies
• The Ionization Fraction of Barnard 68: Implications for Star and Planet Formation
• Galactic H2CO Densitometry I: Pilot survey of Ultracompact HII regions and methodology
• Formaldehyde Anti-Inversion at z=0.68 in the Gravitational Lens B0218+357

Cited by in corpus (10)

• Rotational quenching of H2CO by molecular hydrogen: cross-sections, rates, pressure broadening
• The CO Luminosity Density at High-z (COLDz) Survey: A Sensitive, Large Area Blind Search for Low-J CO Emission from Cold Gas in the Early Universe with the Karl G. Jansky Very Large Array
• Microwave Background Temperature at a Redshift of 6.34 from H2O Absorption
• The dense gas mass fraction in the W51 cloud and its protoclusters
• A measurement of the turbulence-driven density distribution in a non-star-forming molecular cloud
• The inhomogeneous ISM toward PKS1830-211 SW - A detailed view on molecular gas at a look-back time of 7.5 Gyr
• Vacuum ultraviolet photodesorption and photofragmentation of formaldehyde-containing ices
• H2CO and H110α observations towards NH3 sources
• Rethinking Habitability using Biogenic Precursors: Formaldehyde in Millimeter Molecular Clouds of the Inner Galaxy
• The 107 GHz methanol transition is a dasar in G0.253+0.016