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Characterizing the physical conditions (density, temperature, ionization state, metallicity, etc) of the interstellar medium is critical to improving our understanding of the formation and evolution of galaxies. In this work, we present a multi-line study of the interstellar medium in the host galaxy of a quasar atz ≈ 6.4, that is, when the universe was 840 Myr old. This galaxy is one of the most active and massive objects emerging from the dark ages and therefore represents a benchmark for models of the early formation of massive galaxies. We used the Atacama Large Millimeter Array to target an ensemble of tracers of ionized, neutral, and molecular gas, namely the following fine-structure lines: [O III] 88 μm, [N II] 122 μm, [C II] 158 μm, and [C I] 370 μm – as well as the rotational transitions of CO(7–6), CO(15–14), CO(16–15), and CO(19–18); OH 163.1 μm and 163.4 μm; along with H₂O 3(0,3)–2(1,2), 3(3,1)–4(0,4), 3(3,1)–3(2,2), 4(0,4)–3(1,3), and 4(3,2)–4(2,3). All the targeted fine-structure lines were detected, along with half of the targeted molecular transitions. By combining the associated line luminosities with the constraints on the dust temperature from the underlying continuum emission and predictions from photoionization models of the interstellar medium, we find that the ionized phase accounts for about one-third of the total gaseous mass budget and is responsible for half of the total [C II] emission. This phase is characterized by a high density (n ∼ 180 cm⁻³) that typical of HII regions. The spectral energy distribution of the photoionizing radiation is comparable to that emitted by B-type stars. Star formation also appears to be driving the excitation of the molecular medium. We find marginal evidence for outflow-related shocks in the dense molecular phase, but not in other gas phases. This study showcases the power of multi-line investigations in unveiling the properties of the star-forming medium in galaxies at cosmic dawn.

 

We report the discovery of two companion sources to a strongly lensed galaxy SPT0418-47 (“ring”) at redshift 4.225, targeted by the JWST Early Release Science program. We confirm that these sources are at a similar redshift to the ring based on Hαdetected in the NIRSpec spectrum and [Cii]λ158μm line from the Atacama Large Millimeter/submillimeter Array (ALMA). Using multiple spectral lines detected in JWST/NIRSpec, the rest-frame optical to infrared images from NIRCam and MIRI and far-infrared dust continuum detected by ALMA, we argue that the newly discovered sources are actually lensed images of the same companion galaxy SPT0418-SE, hereafter referred to “SE,” located within 5 kpc in the source plane of the ring. The star formation rate derived using [Cii] and the dust continuum puts a lower limit of 17M_☉yr⁻¹, while the SFR_Hαis estimated to be >2 times lower, thereby confirming that SE is a dust-obscured star-forming galaxy. Analysis using optical strong line diagnostics suggests that SE has near-solar elemental abundance, while the ring appears to have supersolar metallicity O/H and N/O. We attempt to reconcile the high metallicity in this system by invoking early onset of star formation with continuous high star-forming efficiency or by suggesting that optical strong line diagnostics need revision at high redshift. We suggest that SPT0418-47 resides in a massive dark-matter halo with yet-to-be-discovered neighbors. This work highlights the importance of joint analysis of JWST and ALMA data for a deep and complete picture of the early universe.

 

We present recent developments on Cornell’s 2nd generation z (redshift) and Early Universe Spectrometer (ZEUS-2). ZEUS-2 is a long-slit echelle-grating spectrometer, originally implemented to deliver R∼1000 spectroscopy in the 350-, and 450-micron telluric windows using NIST Transition-Edge Sensed (TES) bolometer arrays. We have expanded its capabilities to also cover the 200-micron window, and present first-light data for the new array from our 2019 observing campaign on the Atacama Pathfinder EXperiment (APEX) telescope. We also discuss the various enhancements we have implemented to improve observing efficiency and noise performance, including identifying and mitigating vibrations in hardware and improving the stability and robustness of the control software for the detector temperature. Furthermore, we have implemented several software routines to interface with the telescope control systems. These improvements, demonstrated during our recent observing campaign in Nov-Dec 2021, resulted in enhanced reliability and ease of operation, as well as increased sensitivity. A data-driven software pipeline, leveraging data from all 300 detectors on the array to remove common-mode noise, was implemented, and noise performance was further improved by robustly detecting unstable detectors and disabling them during observations. 

An often unglamorous, yet critical, part of most millimeter/submillimeter astronomical instruments is cryogenic temperature monitoring and control. Depending on the operating wavelength of the instrument and detector technology, this could be stable temperatures in the Kelvin range for millimeter heterodyne systems to 100 mK temperatures at sub-micro-Kelvin stability as for many submillimeter bolometer systems. Here we describe a project of the HARDWARE.astronomy initiative to build a low-cost open-source temperature monitoring and control system. The HARDWARE.astronomy Housekeeping Box, or H.aHk Box (pronounced “hack box”) is developed primarily by undergraduates and employs existing open-source devices (e.g Arduino, Raspberry Pi) to reduce costs while also limiting the complexity of the development. The H.aHk Box features a chassis with a control computer and ten expansion slots that can be filled with a variety of expansion cards. These cards include initially an AC 4-wire temperature monitor and PID control cards. Future work will develop 2-wire temperature monitors, stepper motor controller, and high-power supply. The base-system will also be able to interface with other house-keeping systems over USB, serial port and ethernet. The first deployment of the H.aHk Box will be for the ZEUS-2 submillimeter grating spectrometer. All designs, firmware, software and parts list will be published online allowing for other projects to adopt the system and create custom expansion cards as needed. Here we describe the design (including mechanical, electrical, firmware, and software components) and initial performance of the H.aHk Box system with initial AC/DC 4-wire and PID cards. 

We investigate the molecular gas content of z  ∼ 6 quasar host galaxies using the Institut de Radioastronomie Millimétrique Northern Extended Millimeter Array. We targeted the 3 mm dust continuum, and the line emission from CO(6–5), CO(7–6), and [C  I ] 2−1 in ten infrared–luminous quasars that have been previously studied in their 1 mm dust continuum and [C  II ] line emission. We detected CO(7–6) at various degrees of significance in all the targeted sources, thus doubling the number of such detections in z  ∼ 6 quasars. The 3 mm to 1 mm flux density ratios are consistent with a modified black body spectrum with a dust temperature T dust  ∼ 47 K and an optical depth τ ν  = 0.2 at the [C  II ] frequency. Our study provides us with four independent ways to estimate the molecular gas mass, M H2 , in the targeted quasars. This allows us to set constraints on various parameters used in the derivation of molecular gas mass estimates, such as the mass per luminosity ratios α CO and α [CII] , the gas-to-dust mass ratio δ g/d , and the carbon abundance [C]/H 2 . Leveraging either on the dust, CO, [C  I ], or [C  II ] emission yields mass estimates of the entire sample in the range M H2  ∼ 10 10 –10 11 M ⊙ . We compared the observed luminosities of dust, [C  II ], [C  I ], and CO(7–6) with predictions from photo-dissociation and X-ray dominated regions. We find that the former provide better model fits to our data, assuming that the bulk of the emission arises from dense ( n H  > 10 4 cm −3 ) clouds with a column density N H  ∼ 10 23 cm −2 , exposed to a radiation field with an intensity of G 0  ∼ 10 3 (in Habing units). Our analysis reiterates the presence of massive reservoirs of molecular gas fueling star formation and nuclear accretion in z  ∼ 6 quasar host galaxies. It also highlights the power of combined 3 mm and 1 mm observations for quantitative studies of the dense gas content in massive galaxies at cosmic dawn.