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Star formation in half of massive galaxies was quenched by the time the Universe was 3 billion years old 1 . Very low amounts of molecular gas seem to be responsible for this, at least in some cases 2 – 7 , although morphological gas stabilization, shock heating or activity associated with accretion onto a central supermassive black hole are invoked in other cases 8 – 11 . Recent studies of quenching by gas depletion have been based on upper limits that are insufficiently sensitive to determine this robustly 2 – 7 , or stacked emission with its problems of averaging 8 , 9 . Here we report 1.3 mm observations of dust emission from 6 strongly lensed galaxies where star formation has been quenched, with magnifications of up to a factor of 30. Four of the six galaxies are undetected in dust emission, with an estimated upper limit on the dust mass of 0.0001 times the stellar mass, and by proxy (assuming a Milky Way molecular gas-to-dust ratio) 0.01 times the stellar mass in molecular gas. This is two orders of magnitude less molecular gas per unit stellar mass than seen in star forming galaxies at similar redshifts 12 – 14 . It remains difficult to extrapolate from these small samples, but these observations establish that gas depletion is responsible for a cessation of star formation in some fraction of high-redshift galaxies. The authors report 1.3 mm observations of dust emission from strongly lensed galaxies where star formation is quenched, demonstrating that gas depletion is responsible for the cessation of star formation in some high-redshift galaxies.

Dust grains absorb half of the radiation emitted by stars throughout the history of the universe, re-emitting this energy at infrared wavelengths. Polycyclic aromatic hydrocarbons (PAHs) are large organic molecules that trace millimetre-size dust grains and regulate the cooling of interstellar gas within galaxies. Observations of PAH features in very distant galaxies have been difficult owing to the limited sensitivity and wavelength coverage of previous infrared telescopes. Here we present James Webb Space Telescope observations that detect the 3.3 μm PAH feature in a galaxy observed less than 1.5 billion years after the Big Bang. The high equivalent width of the PAH feature indicates that star formation, rather than black hole accretion, dominates infrared emission throughout the galaxy. The light from PAH molecules, hot dust and large dust grains and stars are spatially distinct from one another, leading to order-of-magnitude variations in PAH equivalent width and ratio of PAH to total infrared luminosity across the galaxy. The spatial variations we observe suggest either a physical offset between PAHs and large dust grains or wide variations in the local ultraviolet radiation field. Our observations demonstrate that differences in emission from PAH molecules and large dust grains are a complex result of localized processes within early galaxies.

The completion of the Atacama Large Millimeter/submillimeter Array (ALMA) has led to the ability to make observations with unprecedented resolution at sub-millimeter wavelengths, allowing novel probes of the ISM and kinematics of high-redshift galaxies. Because they are magnified by foreground galaxies or clusters, gravitationally lensed galaxies allow the highest possible spatial resolution to be obtained, and/or a sharp reduction in the observing time required to detect faint objects or spectral lines. These benefits have made lensed galaxies useful benchmark systems for ALMA, enabling a wide variety of science cases. Here I focus in particular on spatially-resolved observations of massive galactic outflows in the very distant z > 4 universe, summarizing plausible tracers of the cold molecular phase of these outflows. The prospects of joint JWST and ALMA observations will be revolutionary, including the chance to take a full census of galactic outflows in multiple gas phases at matched spatial resolution.

The heavy element content (‘metallicity’) of the Universe is a record of the total star formation history. Gas-phase metallicity in galaxies, as well as its evolution with time, is of particular interest as a tracer of accretion and outflow processes. However, metallicities from the widely used electron temperature (Te) method are typically approximately two times lower than the values based on the recombination line method. This ‘abundance discrepancy factor’ is well known and is commonly ascribed to bias due to temperature fluctuations. We present a measurement of oxygen abundance in the nearby (3.4-Mpc) system, Markarian 71, using a combination of optical and far-infrared emission lines to measure and correct for temperature fluctuation effects. Our far-infrared result is inconsistent (>2σ significance) with the metallicity from recombination lines and, instead, indicates little to no bias in the standard Te method, ruling out the long-standing hypothesis that the abundance discrepancy factor is explained by temperature fluctuations for this object. Our results provide a framework to accurately measure metallicity across cosmic history, including with recent data reaching within the first billion years, with the James Webb Space Telescope and the Atacama Large Millimeter Array.A method for measuring oxygen abundances using optical and far-infrared emission lines provides absolute metallicities of the interstellar gas in Markarian 71 and could be applied across cosmic history.

NRC publication: Yes

Since their discovery, submillimetre-selected galaxies 1 , 2 have revolutionized the field of galaxy formation and evolution. From the hundreds of square degrees mapped at submillimetre wavelengths 3 – 5 , only a handful of sources have been confirmed to lie at z  > 5 (refs 6 – 10 ) and only two at z ≥ 6 (refs 11 , 12 ). All of these submillimetre galaxies are rare examples of extreme starburst galaxies with star formation rates of ≳1,000 M ⊙ yr −1 and therefore are not representative of the general population of dusty star-forming galaxies. Consequently, our understanding of the nature of these sources, at the earliest epochs, is still incomplete. Here, we report the spectroscopic identification of a gravitationally amplified ( μ  = 9.3 ± 1.0) dusty star-forming galaxy at z  = 6.027. After correcting for gravitational lensing, we derive an intrinsic less-extreme star formation rate of 380 ± 50 M ⊙ yr −1 for this source and find that its gas and dust properties are similar to those measured for local ultra luminous infrared galaxies, extending the local trends to a poorly explored territory in the early Universe. The star-formation efficiency of this galaxy is similar to those measured in its local analogues 13 , despite a ~12 Gyr difference in cosmic time. This paper reports the detection of a high-redshift galaxy that may be more representative of ‘normal’ star-forming galaxies formed in the first billion years of the Universe than the extreme starbursts discovered to date.

In this dissertation, I study various aspects related to the gas and star formation in dusty star-forming galaxies in the distant universe. My dissertation is heavily based on observations made by the Atacama Large Millimeter/submillimeter Array (ALMA), observing a sample of gravitationally lensed high-redshift dusty galaxies originally discovered by the South Pole Telescope (SPT). In addition to the introductions to the individual chapters, Chapter 1 provides a broader background to the study of these objects and places them in the overall context of galaxy evolution. In Chapter 2 I describe a technique designed to search for faint molecular lines in the spectrum of high-redshift dusty galaxies. The brightest molecular lines in the spectra of these objects are due to carbon monoxide, but a host of other species are present in the interstellar media. These other molecules trace gas of a wide range of temperatures and densities, but are generally ten times fainter than the brighter CO lines. I detected several other molecular lines, and used them to characterize the conditions of the interstellar gas. This work was published in Spilker et al. (2014). In Chapter 3, I describe a technique for modeling the effects of gravitational lensing which is optimized for data from interferometers such as ALMA. Using these models and data for a large sample of objects from ALMA, I studied the intrinsic properties of the sample such as the source sizes and luminosities. I used these intrinsic properties to revisit topics from the literature which benefit from the additional size information I determined. This work was published in Spilker et al. (2016). In Chapter 4, I use the modeling technique I developed to investigate the relationship between the star formation and the cold molecular gas from which stars form in two objects selected from the SPT sample. Using the models of the source, I was able to determine the mass of molecular gas in these objects using several independent methods. I found that the molecular gas reservoirs are more extended than the star formation, which has implications for the ``law'' used as a prescription for star formation in many simulations. This work was published in Spilker et al. (2015). Chapter 5 describes ongoing work to determine what will happen to the dusty galaxies after their active phase of star formation ends, and what processes dominate that change. Since their discovery, these dusty galaxies have been thought to be progenitors of early quiescent galaxies. In this chapter, I show observations of a massive molecular outflow from a single object, which may be responsible for removing the raw material for star formation. Finally, in Chapter 6, I end with a summary of this dissertation.

In Chen et al., 2023 (C23; arXiv:2304.09898), we introduced a new method to directly measure temperature fluctuations and applied it to a nearby dwarf galaxy, Mrk 71, finding a temperature fluctuation parameter \\(t^2 = 0.008\\pm 0.043\\). This result is lower by \\(\\sim 2\\sigma\\) than the value required to explain the abundance discrepancy (AD) in this object. In the Matters Arising article submitted by Mendez-Delgado et al. (arXiv:2310.01197), the authors claim that using the same data presented in C23 in a different way, it is possible to conclude that the measurements are consistent with a larger \\(t^2 \\simeq 0.1\\) inferred indirectly from recombination lines (RLs). However, this requires a higher density such that the infrared [O III] 52 \\(\\mu\\)m and [O III] 88 \\(\\mu\\)m lines -- which form the basis of the direct measurement method -- are mutually inconsistent. Moreover, to reach agreement between the direct \\(t^2\\) measurement and the larger \\(t^2\\) value inferred from RLs requires systematically varying four parameters by \\(\\sim 1\\sigma\\) from their best-determined values, which collectively amount to a \\(\\sim2\\sigma\\) difference, consistent with the significance (\\(\\sim 2 \\sigma\\)) originally reported in C23. Therefore, we conclude that the results of C23 hold, and that the combined optical and infrared [O III] data disfavour \\(t^2 \\simeq 0.1\\) at the \\(\\approx2\\sigma\\) level in Mrk 71. Future work is nonetheless warranted to better understand the AD associated with both optical and infrared emission line analysis.

SPT2349\\(-\\)56 is a protocluster discovered in the 2500 deg\\(^2\\) South Pole Telescope (SPT) survey. In this paper, we study the kinematics of the galaxies found in the core of SPT2349\\(-\\)56 using high-resolution (1.55 kpc spatial resolution at \\(z = 4.303\\)) redshifted [CII] 158-\\(\\mu\\)m data. Using the publicly available code 3D-Barolo, we analyze the seven far-infrared (FIR) brightest galaxies within the protocluster core. Based on conventional definitions for the detection of rotating discs, we classify six sources to be rotating discs in an actively star-forming protocluster environment, with weighted mean \\(V_{\\mathrm{rot}}/\\sigma_{\\mathrm{disp}} = 4.5 \\pm 1.3\\). The weighted mean rotation velocity (\\(V_{\\mathrm{rot}}\\)) and velocity dispersion (\\(\\sigma_{\\mathrm{disp}}\\)) for the sample are \\( 357.1 \\pm 114.7\\) km s\\(^{-1}\\) and \\(43.5 \\pm 23.5\\) km s\\(^{-1}\\), respectively. We also assess the disc stability of the galaxies and find a mean Toomre parameter of \\(Q_\\mathrm{T} = 0.9 \\pm 0.3\\). The galaxies show a mild positive correlation between disc stability and dynamical support. Using the position-velocity maps, we find that five sources further classify as disturbed discs, and one classifies as a strictly rotating disc. Our sample joins several observations at similar redshift with high \\(V/\\sigma\\) values, with the exception that they are morphologically disturbed, kinematically rotating and interacting galaxies in an extreme protocluster environment.