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

TL;DR

This paper proposes formaldehyde (H2CO) as a novel, distance-independent tracer of dense molecular gas in galaxies, enabling unbiased mapping of star formation history across cosmic time using CMB absorption features.

Contribution

It introduces the concept of using H2CO anti-inverted lines as a universal, extinction-free method to measure dense gas and star formation, and explores their potential for cosmology and EoR studies.

Findings

H2CO lines are nearly redshift-independent in excitation and detectability.

H2CO absorption can trace dense gas mass in galaxies across all epochs.

Potential for H2CO to serve as a galaxy ruler and impact EoR observations.

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.