arXiv subjects

Jeremy Darling

Publications and source records attributed to Jeremy Darling.

At least 91 records · Page 5Linked to original sources

arXiv2016

Water Masers in the Andromeda Galaxy: II. Where Do Masers Arise?

We present a comparative multi-wavelength analysis of water maser-emitting regions and non-maser-emitting luminous 24 micron star-forming regions in the Andromeda Galaxy (M31) to identify the sites most likely to produce luminous water masers useful for astrometry and proper motion studies. Included in the analysis are Spitzer 24 micron photometry, Herschel 70 and 160 micron photometry, H$α$ emission, dust temperature, and star formation rate. We find significant differences between the maser-emitting and non-maser-emitting regions: water maser-emitting regions tend to be more IR-luminous and show higher star formation rates. The five water masers in M31 are consistent with being analogs of water masers in Galactic star-forming regions and represent the high-luminosity tail of a larger (and as yet undetected) population. Most regions likely to produce water masers bright enough for proper motion measurements using current facilities have already been surveyed, but we suggest three ways to detect additional water masers in M31: (1) Re-observe the most luminous mid- or far-IR sources with higher sensitivity than was used in the Green Bank Telescope survey; (2) Observe early-stage star-forming regions selected by mm continuum that have not already been selected by their 24 micron emission, and (3) Re-observe the most luminous mid- or far-IR sources, and rely on maser variability for new detections.

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arXiv2016

Toward gas exhaustion in the W51 high-mass protoclusters

We present new JVLA observations of the high-mass cluster-forming region W51A from 2 to 16 GHz with resolution $θ_{fwhm} \approx$ 0.3 - 0.5". The data reveal a wealth of observational results: (1) Currently-forming, very massive (proto-O) stars are traced by o-H2CO $2_{1,1}-2_{1,2}$ emission, suggesting that this line can be used efficiently as a massive protostar tracer. (2) There is a spatially distributed population of $\sim$mJy continuum sources, including hypercompact H ii regions and candidate colliding wind binaries, in and around the W51 proto-clusters. (3) There are two clearly detected protoclusters, W51e and W51 IRS2, that are gas-rich but may have most of their mass in stars within their inner $\sim$ 0.05 pc. The majority of the bolometric luminosity in W51 most likely comes from a third population of OB stars between these clusters. The presence of a substantial population of exposed O-stars coincident with a population of still-forming massive stars, along with a direct measurement of the low mass loss rate via ionized gas outflow from W51 IRS2, together imply that feedback is ineffective at halting star formation in massive protoclusters. Instead, feedback may shut off the large-scale accretion of diffuse gas onto the W51 protoclusters, implying that they are evolving towards a state of gas exhaustion rather than gas expulsion. Recent theoretical models predict gas exhaustion to be a necessary step in the formation of gravitationally bound stellar clusters, and our results provide an observational validation of this process.

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arXiv2016

Measurement of the Black Hole Mass in NGC 1332 from ALMA Observations at 0.044 Arcsecond Resolution

We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 3 observations of CO(2-1) emission from the circumnuclear disk in the E/S0 galaxy NGC 1332 at 0.044" resolution. The disk exhibits regular rotational kinematics and central high-velocity emission (+/-500 km/s) consistent with the presence of a compact central mass. We construct models for a thin, dynamically cold disk in the gravitational potential of the host galaxy and black hole, and fit the beam-smeared model line profiles directly to the ALMA data cube. Model fits successfully reproduce the disk kinematics out to r=200 pc. Fitting models just to spatial pixels within projected r=50 pc of the nucleus (two times larger than the black hole's gravitational radius of influence), we find M_BH=6.64(-0.63,+0.65)*10^8 solar masses. This observation demonstrates ALMA's powerful capability to determine the masses of supermassive black holes by resolving gas kinematics on small angular scales in galaxy nuclei.

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arXiv2016

Identifying OH Imposters in the ALFALFA Neutral Hydrogen Survey

OH megamasers (OHMs) are rare, luminous molecular masers that are typically observed in (ultra) luminous infrared galaxies and serve as markers of major galaxy mergers. In blind emission line surveys such as the Arecibo Legacy Fast Arecibo L-Band Feed Array (ALFALFA) survey for neutral hydrogen (HI), OHMs at z~0.2 can mimic z~0.05 HI lines. We present the results of optical spectroscopy of ambiguous HI detections in the ALFALFA 40% data release detected by the Wide Field Infrared Survey Explorer (WISE) but with uncertain optical counterparts. The optical redshifts, obtained from observations at the Apache Point Observatory, revealed five new OHMs and identified 129 HI optical counterparts. Sixty candidates remain ambiguous. The new OHMs are the first detected in a blind spectral line survey. The number of OHMs in ALFALFA is consistent with predictions from the OH luminosity function. Additionally, the mid-infrared magnitudes and colors of the OHM host galaxies found in a blind survey do not seem to differ from those found in previous targeted surveys. This validates the methods used in previous IR-selected OHM surveys and indicates there is no previously unknown OHM-producing population at z~0.2. We also provide a method for future surveys to separate OH megamasers from 99% of HI line emitters without optical spectroscopy by using WISE infrared colors and magnitudes. Since the fraction of OHMs found in flux-limited HI surveys is expected to increase with the survey's redshift, this selection method can be applied to future flux-limited high-redshift hydrogen surveys.

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arXiv2016

The SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Overview and Early Data

The Extended Baryon Oscillation Spectroscopic Survey (eBOSS) will conduct novel cosmological observations using the BOSS spectrograph at Apache Point Observatory. Observations will be simultaneous with the Time Domain Spectroscopic Survey (TDSS) designed for variability studies and the Spectroscopic Identification of eROSITA Sources (SPIDERS) program designed for studies of X-ray sources. eBOSS will use four different tracers to measure the distance-redshift relation with baryon acoustic oscillations (BAO). Using more than 250,000 new, spectroscopically confirmed luminous red galaxies at a median redshift z=0.72, we project that eBOSS will yield measurements of $d_A(z)$ to an accuracy of 1.2% and measurements of H(z) to 2.1% when combined with the z>0.6 sample of BOSS galaxies. With ~195,000 new emission line galaxy redshifts, we expect BAO measurements of $d_A(z)$ to an accuracy of 3.1% and H(z) to 4.7% at an effective redshift of z= 0.87. A sample of more than 500,000 spectroscopically-confirmed quasars will provide the first BAO distance measurements over the redshift range 0.9 2.1; these new data will enhance the precision of $d_A(z)$ and H(z) by a factor of 1.44 relative to BOSS. Furthermore, eBOSS will provide improved tests of General Relativity on cosmological scales through redshift-space distortion measurements, improved tests for non-Gaussianity in the primordial density field, and new constraints on the summed mass of all neutrino species. Here, we provide an overview of the cosmological goals, spectroscopic target sample, demonstration of spectral quality from early data, and projected cosmological constraints from eBOSS.

astro-ph.CO↗

arXiv2015

Invisible Active Galactic Nuclei. II Radio Morphologies & Five New HI 21 cm Absorption Line Detections

We have selected a sample of 80 candidates for obscured radio-loud active galactic nuclei and presented their basic optical/near-infrared (NIR) properties in Paper 1. In this paper, we present both high-resolution radio continuum images for all of these sources and HI 21cm absorption spectroscopy for a few selected sources in this sample. A-configuration 4.9 and 8.5 GHz VLA continuum observations find that 52 sources are compact or have substantial compact components with size <0.5" and flux density >0.1 Jy at 4.9 GHz. The most compact 36 sources were then observed with the VLBA at 1.4 GHz. One definite and 10 candidate Compact Symmetric Objects (CSOs) are newly identified, a detection rate of CSOs ~3 times higher than the detection rate previously found in purely flux-limited samples. Based on possessing compact components with high flux densities, 60 of these sources are good candidates for absorption-line searches. Twenty seven sources were observed for HI 21cm absorption at their photometric or spectroscopic redshifts with only 6 detections made (one detection is tentative). However, five of these were from a small subset of six CSOs with pure galaxy optical/NIR spectra and for which accurate spectroscopic redshifts place the redshifted 21cm line in a RFI-free spectral window. It is likely that the presence of ubiquitous RFI and the absence of accurate spectroscopic redshifts preclude HI detections in similar sources (only one detection out of the remaining 22 sources observed, 14 of which have only photometric redshifts). Future searches for highly-redshifted HI and molecular absorption can easily find more distant CSOs among bright, blank field' radio sources but will be severely hampered by an inability to determine accurate spectroscopic redshifts for them due to their lack of rest-frame UV continuum.

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arXiv2015

Next Generation Very Large Array Memo No. 9 Science Working Group 4: Time Domain, Fundamental Physics, and Cosmology

We report here on key science topics for the Next Generation Very Large Array in the areas of time domain, fundamental physics, and cosmology. Key science cases considered are pulsars in orbit around the Galactic Center massive black hole, Sagittarius A*, electromagnetic counterparts to gravitational waves, and astrometric cosmology. These areas all have the potential for ground-breaking and transformative discovery. Numerous other topics were discussed during the preparation of this report and some of those discussions are summarized here, as well. There is no doubt that further investigation of the science case will reveal rich and compelling opportunities.

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arXiv2015

Dense gas in the Galactic central molecular zone is warm and heated by turbulence

The Galactic center is the closest region in which we can study star formation under extreme physical conditions like those in high-redshift galaxies. We measure the temperature of the dense gas in the central molecular zone (CMZ) and examine what drives it. We mapped the inner 300 pc of the CMZ in the temperature-sensitive J = 3-2 para-formaldehyde (p-H$_2$CO) transitions. We used the $3_{2,1} - 2_{2,0} / 3_{0,3} - 2_{0,2}$ line ratio to determine the gas temperature in $n \sim 10^4 - 10^5 $cm$^{-3}$ gas. We have produced temperature maps and cubes with 30" and 1 km/s resolution and published all data in FITS form. Dense gas temperatures in the Galactic center range from ~60 K to > 100 K in selected regions. The highest gas temperatures T_G > 100 K are observed around the Sgr B2 cores, in the extended Sgr B2 cloud, the 20 km/s and 50 km/s clouds, and in "The Brick" (G0.253+0.016). We infer an upper limit on the cosmic ray ionization rate $ζ_{CR} < 10^{-14}$ 1/s. The dense molecular gas temperature of the region around our Galactic center is similar to values found in the central regions of other galaxies, in particular starburst systems. The gas temperature is uniformly higher than the dust temperature, confirming that dust is a coolant in the dense gas. Turbulent heating can readily explain the observed temperatures given the observed line widths. Cosmic rays cannot explain the observed variation in gas temperatures, so CMZ dense gas temperatures are not dominated by cosmic ray heating. The gas temperatures previously observed to be high in the inner ~75 pc are confirmed to be high in the entire CMZ.

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arXiv2015

The dense gas mass fraction in the W51 cloud and its protoclusters

We present new 2 cm and 6 cm maps of H2CO, radio recombination lines, and the radio continuum in the W51 star forming complex acquired with Arecibo and the Green Bank Telescope at ~50" resolution. We use H2CO absorption to determine the relative line-of-sight positions of molecular and ionized gas. We measure gas densities using the H2CO densitometer, including continuous measurements of the dense gas mass fraction (DGMF) over the range $10^4$ cm$^{-3}$ < n(H$_2$) < $10^6$ cm$^{-3}$- this is the first time a dense gas mass fraction has been measured over a range of densities with a single data set. The DGMF in W51A is high,f >~70% above $n>10^4$ cm$^{-3}$, while it is low, f<20%, in W51 B. We did not detect any H2CO emission throughout the W51 GMC; all gas dense enough to emit under normal conditions is in front of bright continuum sources and therefore is seen in absorption instead. The data set has been made public at http://dx.doi.org/10.7910/DVN/26818. Conclusions. (1) The dense gas fraction in the W51 A and B clouds shows that W51 A will continue to form stars vigorously, while star formation has mostly ended in W51 B. The lack of dense, star-forming gas around W51 C indicates that collect-and-collapse is not acting or is inefficient in W51. (2) Ongoing high-mass star formation is correlated with n ~ 1x10$^5$ cm$^{-3}$ gas. Gas with n > 10$^4$ cm$^{-3}$ is weakly correlated with low and moderate mass star formation, but does not strongly correlate with high-mass star formation. (3) The nondetection of H$_2$CO emission implies that the emission detected in other galaxies, e.g. Arp 220, comes from high-density gas that is not directly affiliated with already-formed massive stars. Either the non-star-forming ISM of these galaxies is very dense, implying the star formation density threshold is higher, or H II regions have their emission suppressed.

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arXiv2014

The Comparison of Physical Properties Derived from Gas and Dust in a Massive Star-Forming Region

We explore the relationship between gas and dust in massive star-forming regions by comparing physical properties derived from each. We compare the temperatures and column densities in a massive star-forming Infrared Dark Cloud (IRDC, G32.02+0.05), which shows a range of evolutionary states, from quiescent to active. The gas properties were derived using radiative transfer modeling of the (1,1), (2,2), and (4,4) transitions of NH3 on the Karl G. Jansky Very Large Array (VLA), while the dust temperatures and column densities were calculated using cirrus-subtracted, modified blackbody fits to Herschel data. We compare the derived column densities to calculate an NH3 abundance, 4.6 x 10^-8. In the coldest star-forming region, we find that the measured dust temperatures are lower than the measured gas temperatures (mean and standard deviations T_dust ~ 11.6 +/- 0.2 K vs. T_gas ~ 15.2 +/- 1.5 K), which may indicate that the gas and dust are not well-coupled in the youngest regions (~0.5 Myr) or that these observations probe a regime where the dust and/or gas temperature measurements are unreliable. Finally, we calculate millimeter fluxes based on the temperatures and column densities derived from NH3 which suggest that millimeter dust continuum observations of massive star-forming regions, such as the Bolocam Galactic Plane Survey or ATLASGAL, can probe hot cores, cold cores, and the dense gas lanes from which they form, and are generally not dominated by the hottest core.

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arXiv2014

The Onset of Massive Star Formation: The Evolution of Temperature and Density Structure in an Infrared Dark Cloud

We present new NH3 (1,1), (2,2), and (4,4) observations from the Karl G. Jansky Very Large Array (VLA) compiled with work in the literature to explore the range of conditions observed in young, massive star-forming regions. To sample the effects of evolution independent from those of distance/resolution, abundance, and large-scale environment, we compare clumps in different evolutionary stages within a single Infrared Dark Cloud (IRDC), G32.02+0.06. We find that the early stages of clustered star formation are characterized by dense, parsec-scale filamentary structures interspersed with complexes of dense cores (<0.1 pc cores clustered in complexes separated by ~1 pc) with masses from about 10 to 100 Msun. The most quiescent core is the most extended while the star-forming cores are denser and more compact, showing very similar column density structure before and shortly after the onset of massive star formation, with peak surface densities >~ 1 g cm^-2. Quiescent cores and filaments show smoothly varying temperatures from 10-20 K, rising to over 40 K in star-forming cores. We calculate the virial parameters for 16 cores and find that the level of support provided by turbulence is generally insufficient to support them against gravitational collapse (alpha_vir ~ 0.6). The star-forming filaments show smooth velocity fields, punctuated by discontinuities at the sites of active star formation. We discuss the Massive Molecular Filament (MMF; M > 10^5 Msun, l > 60 pc) hosting the IRDC, hypothesizing that it may have been shaped by previous generations of massive stars.

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arXiv2014

The Hubble Expansion is Isotropic in the Epoch of Dark Energy

The isotropy of the universal Hubble expansion is a fundamental tenet of physical cosmology, but it has not been precisely tested during the current epoch, when dark energy is dominant. Anisotropic expansion will produce a shearing velocity field, causing objects to stream toward directions of faster expansion and away from directions of slower expansion. This work tests the basic cosmological assumption of isotropic expansion and thus the isotropy of dark energy. The simplest anisotropy will manifest as a quadrupolar curl-free proper motion vector field. We derive this theoretical signature using a tri-axial expanding metric with a flat geometry (Bianchi I model), generalizing and correcting previous work. We then employ the best current data, the Titov & Lambert (2013) proper motion catalog of 429 objects, to measure the isotropy of universal expansion. We demonstrate that the Hubble expansion is isotropic to 7% (1 $σ$), corresponding to streaming motions of 1 microarcsecond/yr, in the best-constrained directions (-19% and +17% in the least-constrained directions) and does not significantly deviate from isotropy in any direction. The Gaia mission, which is expected to obtain proper motions for 500,000 quasars, will likely constrain the anisotropy below 1%.

astro-ph.CO↗

arXiv2013

Ammonia Thermometry of Star Forming Galaxies

With a goal toward deriving the physical conditions in external galaxies, we present a study of the ammonia (NH$_3$) emission and absorption in a sample of star forming systems. Using the unique sensitivities to kinetic temperature afforded by the excitation characteristics of several inversion transitions of NH$_3$, we have continued our characterization of the dense gas in star forming galaxies by measuring the kinetic temperature in a sample of 23 galaxies and one galaxy offset position selected for their high infrared luminosity. We derive kinetic temperatures toward 13 galaxies, 9 of which possess multiple kinetic temperature and/or velocity components. Eight of these galaxies exhibit kinetic temperatures $>100$ K, which are in many cases at least a factor of two larger than kinetic temperatures derived previously. Furthermore, the derived kinetic temperatures in our galaxy sample, which are in many cases at least a factor of two larger than derived dust temperatures, point to a problem with the common assumption that dust and gas kinetic temperatures are equivalent. As previously suggested, the use of dust emission at wavelengths greater than 160 $μ$m to derive dust temperatures, or dust heating from older stellar populations, may be skewing derived dust temperatures in these galaxies to lower values. We confirm the detection of high-excitation OH $^2Π_{3/2}$ J=9/2 absorption toward Arp220 (Ott et. al. 2011). We also report the first detections of non-metastable NH$_3$ inversion transitions toward external galaxies in the (2,1) (NGC253, NGC660, IC342, and IC860), (3,1), (3,2), (4,3), (5,4) (all in NGC660) and (10,9) (Arp220) transitions.

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arXiv2013

Objects Appear Smaller as They Recede: How Proper Motions Can Directly Reveal the Cosmic Expansion, Provide Geometric Distances, and Measure the Hubble Constant

Objects and structures gravitationally decoupled from the Hubble expansion will appear to shrink in angular size as the universe expands. Observations of extragalactic proper motions can thus directly reveal the cosmic expansion. Relatively static structures such as galaxies or galaxy clusters can potentially be used to measure the Hubble constant, and test masses in large scale structures can measure the overdensity. Since recession velocities and angular separations can be precisely measured, apparent proper motions can also provide geometric distance measurements to static structures. The apparent fractional angular compression of static objects is 15 microarcseconds per year in the local universe; this motion is modulated by the overdensity in dynamic expansion-decoupled structures. We use the Titov et al. quasar proper motion catalog to examine the pairwise proper motion of a sparse network of test masses. Small-separation pairs (< 200 Mpc comoving) are too few to measure the expected effect, yielding an inconclusive 8.3 +/- 14.9 microarcsec/yr. Large-separation pairs (200-1500 Mpc) show no net convergence or divergence for z < 1, -2.7 +/- 3.7 microarcsec/yr, consistent with pure Hubble expansion and significantly inconsistent with static structures, as expected. For all pairs a "null test" gives -0.36 +/- 0.62 microarcsec/yr, consistent with Hubble expansion, and excludes a static locus at ~5-10 sigma significance for z ~ 0.5-2.0. The observed large-separation pairs provide a reference frame for small-separation pairs that will significantly deviate from the Hubble flow. The current limitation is the number of small-separation objects with precise astrometry, but Gaia will address this and will likely detect the cosmic recession.

astro-ph.CO↗

arXiv2013

A measurement of the turbulence-driven density distribution in a non-star-forming molecular cloud

Molecular clouds are supersonically turbulent. This turbulence governs the initial mass function and the star formation rate. In order to understand the details of star formation, it is therefore essential to understand the properties of turbulence, in particular the probability distribution of density in turbulent clouds. We present formaldehyde volume density measurements of a non-star-forming cloud along the line of sight towards W49A. We use these measurements in conjunction with total mass estimates from 13CO to infer the shape of the density probability distribution function. This method is complementary to measurements of turbulence via the column density distribution and should be applicable to any molecular cloud with detected CO. We show that turbulence in this cloud is probably compressively driven, with a compressive-to-total Mach number ratio $b = \mathcal{M}_C/\mathcal{M}>0.4$. We measure the standard deviation of the density distribution, constraining it to the range $1.5 < σ_s < 1.9$ assuming that the density is lognormally distributed. This measurement represents an essential input into star formation laws. The method of averaging over different excitation conditions to produce a model of emission from a turbulent cloud is generally applicable to optically thin line observations.

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arXiv2013

Formaldehyde Densitometry of Starburst Galaxies: Density-Independent Global Star Formation

Accurate techniques which allow for the derivation of the spatial density in star formation regions are rare. A technique which has found application for the derivation of spatial densities in Galactic star formation regions utilizes the density-sensitive properties of the K-doublet transitions of formaldehyde (H2CO). In this paper, we present an extension of our survey of the formaldehyde 1(10)-1(11) (lambda = 6.2 cm) and 2(11)-2(12) (lambda = 2.1 cm) K-doublet transitions of H2CO in a sample of 56 starburst systems (Mangum etal. 2008). We have extended the number of galaxies in which both transitions have been detected from 5 to 13. We have improved our spatial density measurements by incorporating kinetic temperatures based upon NH3 measurements of 11 of the galaxies with a total of 14 velocity components in our sample (Mangum etal. 2013). Our spatial density measurements lie in a relatively narrow range of from 10^(4.5) to 10^(5.5) cm^(-3). This implies that the Schmidt-Kennicutt relation between L_(IR) and M_(dense): (1) Is an indication of the dense gas mass reservoir available to form stars, and (2) Is not directly dependent upon a higher average density driving the star formation process in the most luminous starburst galaxies. We have also used our H2CO measurements to derive two separate measures of the dense gas mass which are generally smaller, in many cases by a factor of 10^2-10^3, than those derived using HCN. This disparity suggests that H2CO traces a denser, more compact, component of the giant molecular clouds in our starburst galaxy sample. We also report measurements of the rotationally-excited lambda = 6.3 cm 2P_(1/2) J=1/2 state of OH and the H111alpha radio recombination line taken concurrently with our H2CO 1(10)-1(11) measurements.

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arXiv2012

Toward a Direct Measurement of the Cosmic Acceleration

We present precise HI 21 cm absorption line redshifts observed in multiple epochs to directly constrain the secular redshift drift dz/dt_o or the cosmic acceleration, dv/dt_o. A comparison of literature analog spectra to contemporary digital spectra shows significant acceleration likely attributable to systematic instrumental errors. However, we obtain robust constraints using primarily Green Bank Telescope digital data. Ten objects spanning z=0.09-0.69 observed over 13.5 years show dz/dt_o = (-2.3 +/- 0.8) x 10^-8 yr^-1 or dv/dt_o = -5.5 +/- 2.2 m/s/yr. The best constraint from a single object, 3C286 at = 0.692153275(85), is dz/dt_o = (1.6 +/- 4.7) x 10^-8 yr^-1 or dv/dt_o =2.8 +/- 8.4 m/s/yr. These measurements are three orders of magnitude larger than the theoretically expected acceleration at z=0.5, dz/dt_o = 2 x 10^-11 yr^-1 or dv/dt_o = 0.3 cm/s/yr, but they demonstrate the lack of peculiar acceleration in absorption line systems and the long-term frequency stability of modern radio telescopes. A comparison of UV metal absorption lines to the 21 cm line improves constraints on the cosmic variation of physical constants: Delta(alpha^2 g_p mu)/(alpha^2 g_p mu) = (-1.2 +/- 1.4) x 10^-6 in the redshift range z=0.24-2.04. The linear evolution over the last 10.4 Gyr is (-0.2 +/- 2.7) x 10^-16 yr^-1, consistent with no variation. The cosmic acceleration could be directly measured in ~125 years using current telescopes or in ~5 years using a Square Kilometer Array, but systematic effects will arise at the 1 cm/s/yr level.

astro-ph.CO↗

arXiv2012

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

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.

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