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1. Infrared Spectral Energy Distributions and Dust Masses of Sub-solar Metallicity Galaxies at z ∼ 2.3

   https://doi.org/10.3847/1538-4357/ac54a9  

   Shivaei, Irene ; Popping, Gergö ; Rieke, George ; Reddy, Naveen ; Pope, Alexandra ; Kennicutt, Robert ; Mobasher, Bahram ; Coil, Alison ; Fudamoto, Yoshinobu ; Kriek, Mariska ; et al ( March 2022 , The Astrophysical Journal)

   Abstract We present results from Atacama Large Millimeter/submillimeter Array (ALMA) 1.2 mm continuum observations of a sample of 27 star-forming galaxies at z = 2.1–2.5 from the MOSFIRE Deep Evolution Field survey with metallicity and star formation rate measurements from optical emission lines. Using stacks of Spitzer, Herschel, and ALMA photometry (rest frame ∼8–400 μ m), we examine the infrared (IR) spectral energy distributions (SED) of z ∼ 2.3 subsolar-metallicity (∼0.5 Z ⊙ ) luminous infrared galaxies (LIRGs). We find that the data agree well with an average template of higher-luminosity local low-metallicity dwarf galaxies (reduced χ 2 = 1.8). When compared with the commonly used templates for solar-metallicity local galaxies or high-redshift LIRGs and ultraluminous IR galaxies, even in the most favorable case (with reduced χ 2 = 2.8), the templates are rejected at >98% confidence. The broader and hotter IR SED of both the local dwarfs and high-redshift subsolar-metallicity galaxies may result from different grain properties or a harder/more intense ionizing radiation field that increases the dust temperature. The obscured star formation rate (SFR) indicated by the far-IR emission of the subsolar-metallicity galaxies is only ∼60% of the total SFR, considerably lower than that of the local LIRGs with ∼96%–97% obscured fractions. Due to the evolving IR SED shape, the local LIRG templates fit to mid-IR data overestimate the Rayleigh–Jeans tail measurements by a factor of 2–20. These templates underestimate IR luminosities if fit to the observed ALMA fluxes by >0.4 dex. At a given stellar mass or metallicity, dust masses at z ∼ 2.3 are an order of magnitude higher than z ∼ 0. Given the predicted molecular gas fractions, the observed z ∼ 2.3 dust-to-stellar mass ratios suggest lower dust-to-molecular gas masses than in local galaxies with similar metallicities. 

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2. CO Emission, Molecular Gas, and Metallicity in Main-sequence Star-forming Galaxies at z ∼ 2.3*

   Sanders, Ryan L. ; Shapley, Alice E. ; Jones, Tucker ; Shivaei, Irene ; Popping, Gergö ; Reddy, Naveen A. ; Davé, Romeel ; Price, Sedona H. ; Mobasher, Bahram ; Kriek, Mariska ; et al ( January 2023 , The Astrophysical Journal)

   Abstract We present observations of CO(3−2) in 13 main-sequence z = 2.0–2.5 star-forming galaxies at log ( M * / M ⊙ ) = 10.2 – 10.6 that span a wide range in metallicity (O/H) based on rest-optical spectroscopy. We find that L CO ( 3 − 2 ) ′ /SFR decreases with decreasing metallicity, implying that the CO luminosity per unit gas mass is lower in low-metallicity galaxies at z ∼ 2. We constrain the CO-to-H 2 conversion factor ( α CO ) and find that α CO inversely correlates with metallicity at z ∼ 2. We derive molecular gas masses ( M mol ) and characterize the relations among M * , SFR, M mol , and metallicity. At z ∼ 2, M mol increases and the molecular gas fraction ( M mol / M * ) decreases with increasing M * , with a significant secondary dependence on SFR. Galaxies at z ∼ 2 lie on a near-linear molecular KS law that is well-described by a constant depletion time of 700 Myr. We find that the scatter about the mean SFR− M * , O/H− M * , and M mol − M * relations is correlated such that, at fixed M * , z ∼ 2 galaxies with larger M mol have higher SFR and lower O/H. We thus confirm the existence of a fundamental metallicity relation at z ∼ 2, where O/H is inversely correlated with both SFR and M mol at fixed M * . These results suggest that the scatter of the z ∼ 2 star-forming main sequence, mass–metallicity relation, and M mol – M * relation are primarily driven by stochastic variations in gas inflow rates. We place constraints on the mass loading of galactic outflows and perform a metal budget analysis, finding that massive z ∼ 2 star-forming galaxies retain only 30% of metals produced, implying that a large mass of metals resides in the circumgalactic medium. 

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3. The MOSDEF survey: the mass–metallicity relationship and the existence of the FMR at z ∼ 1.5

   https://doi.org/10.1093/mnras/stab1793  

   Topping, Michael W ; Shapley, Alice E ; Sanders, Ryan L ; Kriek, Mariska ; Reddy, Naveen A ; Coil, Alison L ; Mobasher, Bahram ; Siana, Brian ; Freeman, William R ; Shivaei, Irene ; et al ( July 2021 , Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We analyse the rest-optical emission-line ratios of z ∼ 1.5 galaxies drawn from the Multi-Object Spectrometer for Infra-Red Exploration Deep Evolution Field (MOSDEF) survey. Using composite spectra, we investigate the mass–metallicity relation (MZR) at z ∼ 1.5 and measure its evolution to z = 0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z ∼ 1.5 to z = 0 depends on stellar mass, evolving by $\Delta \rm log(\rm O/H) \sim 0.25$ dex at M*< $10^{9.75}\, \mathrm{M}_{\odot }$ down to $\Delta \rm log(\rm O/H) \sim 0.05$ at M* ≳ $10^{10.5}\, \mathrm{M}_{\odot }$. In contrast, the O3N2-based MZR shows a constant offset of $\Delta \rm log(\rm O/H) \sim 0.30$ across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z ∼ 1.5 sample. We investigated the secondary dependence of the MZR on star formation rate (SFR) by measuring correlated scatter about the mean M*-specific SFR and M*−$\log (\rm O3N2)$ relations. We find an anticorrelation between $\log (\rm O/H)$ and sSFR offsets, indicating the presence of a M*−SFR−Z relation, though with limited significance. Additionally, we find that our z ∼ 1.5 stacks lie along the z = 0 metallicity sequence at fixed μ = log (M*/M⊙) − 0.6 × $\log (\rm SFR / M_{\odot } \, yr^{-1})$ suggesting that the z ∼ 1.5 stacks can be described by the z = 0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [N ii]/H α scales with galaxy properties is crucial to accurately describe the effects of blended [N ii] and H α on redshift and H α fiux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope. 

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4. Jansky Very Large Array Detections of CO(1–0) Emission in H i-absorption-selected Galaxies at z ≳ 2

   https://doi.org/10.3847/2041-8213/ac7bdd  

   Kaur, B. ; Kanekar, N. ; Rafelski, M. ; Neeleman, M. ; Prochaska, J. X. ; Revalski, M. ( July 2022 , The Astrophysical Journal Letters)

   We report a Karl G. Jansky Very Large Array search for redshifted CO(1–0) emission from three Hi-absorption-selected galaxies atz≈ 2, identified earlier in their CO(3–2) or CO(4–3) emission. We detect CO(1–0) emission from DLA B1228-113 atz≈ 2.1933 and DLA J0918+1636 atz≈ 2.5848; these are the first detections of CO(1–0) emission in high-zHi-selected galaxies. We obtain high molecular gas masses,M_mol≈ 10¹¹× (α_CO/4.36)M_⊙, for the two objects with CO(1–0) detections, which are a factor of ≈1.5–2 lower than earlier estimates. We determine the excitation of the mid-JCO rotational levels relative to theJ= 1 level,r_J1, in Hi-selected galaxies for the first time, obtainingr₃₁= 1.00 ± 0.20 andr₄₁= 1.03 ± 0.23 for DLA J0918+1636, andr₃₁= 0.86 ± 0.21 for DLA B1228-113. These values are consistent with thermal excitation of theJ= 3 andJ= 4 levels. The excitation of theJ= 3 level in the Hi-selected galaxies is similar to that seen in massive main-sequence and submillimeter galaxies atz≳2, but higher than that in main-sequence galaxies atz≈ 1.5; the higher excitation of the galaxies atz≳ 2 is likely to be due to their higher star formation rate (SFR) surface density. We use Hubble Space Telescope Wide Field Camera 3 imaging to detect the rest-frame near-ultraviolet (NUV) emission of DLA B1228-113, obtaining an NUV SFR of 4.44 ± 0.47M_⊙yr⁻¹, significantly lower than that obtained from the total infrared luminosity, indicating significant dust extinction in thez≈ 2.1933 galaxy.

    

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5. Unveiling the warm and dense ISM in z  > 6 quasar host galaxies via water vapor emission

   https://doi.org/10.1051/0004-6361/202243406  

   Pensabene, A. ; van der Werf, P. ; Decarli, R. ; Bañados, E. ; Meyer, R. A. ; Riechers, D. ; Venemans, B. ; Walter, F. ; Weiß, A. ; Brusa, M. ; et al ( November 2022 , Astronomy & Astrophysics)

   Water vapor (H₂O) is one of the brightest molecular emitters after carbon monoxide (CO) in galaxies with high infrared (IR) luminosity, allowing us to investigate the warm and dense phase of the interstellar medium (ISM) where star formation occurs. However, due to the complexity of its radiative spectrum, H₂O is not frequently exploited as an ISM tracer in distant galaxies. Therefore, H₂O studies of the warm and dense gas at high-zremain largely unexplored. In this work, we present observations conducted with the Northern Extended Millimeter Array (NOEMA) toward threez > 6 IR-bright quasarsJ2310+1855,J1148+5251, andJ0439+1634targeted in their multiple para- and ortho-H₂O transitions (3₁₂ − 3₀₃, 1₁₁ − 0₀₀, 2₂₀ − 2₁₁, and 4₂₂ − 4₁₃), as well as their far-IR (FIR) dust continuum. By combining our data with previous measurements from the literature, we estimated the dust masses and temperatures, continuum optical depths, IR luminosities, and star formation rates (SFR) from the FIR continuum. We modeled the H₂O lines using the MOLPOP-CEP radiative transfer code, finding that water vapor lines in our quasar host galaxies are primarily excited in the warm, dense (with a gas kinetic temperature and density ofT_kin = 50 K,n_H2 ∼ 10^4.5 − 10⁵ cm⁻³) molecular medium with a water vapor column density ofN_H2O ∼ 2 × 10¹⁷ − 3 × 10¹⁸ cm⁻³. High-JH₂O lines are mainly radiatively pumped by the intense optically-thin far-IR radiation field associated with a warm dust component at temperatures ofT_dust ∼ 80 − 190 K that account for < 5 − 10% of the total dust mass. In the case of J2310+1855, our analysis points to a relatively high value of the continuum optical depth at 100 μm (τ₁₀₀ ∼ 1). Our results are in agreement with expectations based on the H₂O spectral line energy distribution of local and high-zultra-luminous IR galaxies and active galactic nuclei (AGN). The analysis of the Boltzmann diagrams highlights the interplay between collisions and IR pumping in populating the high H₂O energy levels and it allows us to directly compare the excitation conditions in the targeted quasar host galaxies. In addition, the observations enable us to sample the high-luminosity part of the H₂O–total-IR (TIR) luminosity relations (L_H2O − L_TIR). Overall, our results point to supralinear trends that suggest H₂O–TIR relations are likely driven by IR pumping, rather than the mere co-spatiality between the FIR continuum- and line-emitting regions. The observedL_H2O/L_TIRratios in ourz > 6 quasars do not show any strong deviations with respect to those measured in star-forming galaxies and AGN at lower redshifts. This supports the notion that H₂O can be likely used to trace the star formation activity buried deep within the dense molecular clouds.

    

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