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 (H 2 CO) over cosmic time. The K-doublet rotational transitions of H 2 CO become "refrigerated"-their excitation temperatures are driven below the CMB temperature-via collisional pumping by molecular hydrogen (H 2 ). "Anti-inverted" H 2 CO line ratios thus provide an accurate measurement of the H 2 density in molecular clouds. Using a radiative transfer model, we demonstrate that H 2 CO centimeter wavelength line excitation and detectability are nearly independent of redshift or gas kinetic temperature. Since the H 2 CO K-doublet lines absorb CMB light, and since the CMB lies behind every galaxy and provides an exceptionally uniform extended illumination source, H 2 CO 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 H 2 CO 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 meter-wave H 2 CO lines in galaxies during the peak of cosmic star formation may contaminate H i 21 cm tomography of the Epoch of Reionization.

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

“…By assuming a line FWHM of 200 km s −1 , we derive median 6σ limits (with no negative line candidates found above this threshold) of 0.18 and 0.55 mJy beam −1 for the COSMOS and GOODS-N fields, respectively, corresponding to ΔT Obs of −0.03 and −0.11 K at the average frequency and beam size of our survey. We note that the beam size of our observations (∼3″) is likely to be larger than the absorbing molecular regions (∼0 25-1 25 at z∼1; Darling & Zeiger 2012), implying a dilution of the expected signal strength due to the beam filling factor of ∼0.025-0.1. We use the absence of significant negative detections to infer a probability distribution for their space abundance by assuming a uniform uncorrelated distribution of sources over the cosmic volume covered by our survey and, therefore, a Poisson number count.…”

“…Although negative line features can usually be assumed to be due to noise, line absorption against the uniform CMB has been suggested to be a potential source of such negative lines. In particular, formaldehyde in dense molecular gas in galaxies has been confirmed at low-z to have the potential to produce such absorption against the CMB (Zeiger & Darling 2010;Darling & Zeiger 2012).…”

Microwave background temperature at a redshift of 6.34 from H2O absorption

“…3c ), where only a few spectra at rest-frame 538 μm with sufficient signal-to-noise ratio to detect the effect exist. This differs from molecules like H 2 CO, for which absorption against the CMB has been predicted to occur at any redshift up to the present day 12 , but for which no detections at high redshift currently exist 13 . For starbursts with dust as warm as HFLS3, its relative strength is expected to continue to increase with redshift all the way up to z ~ 7–8, and to remain observable back to the earliest epochs when such galaxies existed.…”

“…any redshift up to the present day 12 , but for which no detections at high redshift currently exist 13 . For starbursts with dust as warm as HFLS3, its relative strength is expected to continue to increase with redshift all the way up to z ~ 7-8, and to remain observable back to the earliest epochs when such galaxies existed.…”

Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices

“…It has been studied as a potential probe of planet formation in protoplanetary disks (PPDs), 11 and it is even used in cosmology as an extinction-free tracer of star formation across the epoch of Galaxy evolution. 12 Its protonated form, H 2 COH + , detected for the first time in Sgr B2 and several hot cores, 13 has been recently observed in a cold prestellar core. 14 In the coma of several comets, its abundance has been found at the percent level relative to H 2 O (e.g.…”