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1. Detection of Cosmological 21 cm Emission with the Canadian Hydrogen Intensity Mapping Experiment

   https://doi.org/10.3847/1538-4357/acb13f  

   The CHIME Collaboration ; Amiri, Mandana ; Bandura, Kevin ; Chen, Tianyue ; Deng, Meiling ; Dobbs, Matt ; Fandino, Mateus ; Foreman, Simon ; Halpern, Mark ; Hill, Alex S. ; et al ( April 2023 , The Astrophysical Journal)

   We present a detection of 21 cm emission from large-scale structure (LSS) between redshift 0.78 and 1.43 made with the Canadian Hydrogen Intensity Mapping Experiment. Radio observations acquired over 102 nights are used to construct maps that are foreground filtered and stacked on the angular and spectral locations of luminous red galaxies (LRGs), emission-line galaxies (ELGs), and quasars (QSOs) from the eBOSS clustering catalogs. We find decisive evidence for a detection when stacking on all three tracers of LSS, with the logarithm of the Bayes factor equal to 18.9 (LRG), 10.8 (ELG), and 56.3 (QSO). An alternative frequentist interpretation, based on the likelihood ratio test, yields a detection significance of 7.1σ(LRG), 5.7σ(ELG), and 11.1σ(QSO). These are the first 21 cm intensity mapping measurements made with an interferometer. We constrain the effective clustering amplitude of neutral hydrogen (Hi), defined as${\mathit{}}_{\mathrm{H}\mathrm{I}}\equiv {10}^3{\mathrm{\Omega }}_{\mathrm{H}\mathrm{I}}\left(b_{\mathrm{H}\mathrm{I}}+〈f{\mu }^2〉\right)$, where Ω_Hiis the cosmic abundance of Hi,b_Hiis the linear bias of Hi, and 〈fμ²〉 = 0.552 encodes the effect of redshift-space distortions at linear order. We find${\mathit{}}_{\mathrm{H}\mathrm{I}}={1.51}_{-0.97}^{+3.60}$for LRGs (z= 0.84),${\mathit{}}_{\mathrm{H}\mathrm{I}}={6.76}_{-3.79}^{+9.04}$for ELGs (z= 0.96), and${\mathit{}}_{\mathrm{H}\mathrm{I}}={1.68}_{-0.67}^{+1.10}$for QSOs (z= 1.20), with constraints limited by modeling uncertainties at nonlinear scales. We are also sensitive to bias in the spectroscopic redshifts of each tracer, and we find a nonzero bias Δv= − 66 ± 20 km s⁻¹for the QSOs. We split the QSO catalog into three redshift bins and have a decisive detection in each, with the upper bin atz= 1.30 producing the highest-redshift 21 cm intensity mapping measurement thus far.

    

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2. The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER). III. The Mass Function of Young Stellar Clusters in M33

   https://doi.org/10.3847/1538-4357/ac51cf  

   Wainer, Tobin M. ; Johnson, L. Clifton ; Seth, Anil C. ; TorresVillanueva, Estephani E. ; Dalcanton, Julianne J. ; Durbin, Meredith J. ; Dolphin, Andrew ; Weisz, Daniel R. ; Williams, Benjamin F. ; PHATTER Collaboration ( March 2022 , The Astrophysical Journal)

   We measure the star cluster mass function (CMF) for the Local Group galaxy M33. We use the catalog of stellar clusters selected from the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region survey. We analyze 711 clusters in M33 with$7.0<\log (\mathrm{Age}/\mathrm{yr})<8.5$, and log(M/M_⊙) > 3.0 as determined from color–magnitude diagram fits to individual stars. The M33 CMF is best described by a Schechter function with power-law slopeα= −${2.06}_{-0.13}^{+0.14}$, and truncation mass log(M_c/M_⊙)$={4.24}_{-0.13}^{+0.16}$. The data show strong evidence for a high-mass truncation, thus strongly favoring a Schechter function fit over a pure power law. M33's truncation mass is consistent with the previously identified linear trend betweenM_c, and star formation rate surface density, Σ_SFR. We also explore the effect that individual cluster mass uncertainties have on derived mass function parameters, and find evidence to suggest that large cluster mass uncertainties have the potential to bias the truncation mass of fitted mass functions at the 1σlevel.

    

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3. The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon∗

   https://doi.org/10.3847/1538-4357/acb153  

   Gburek, Timothy ; Siana, Brian ; Alavi, Anahita ; Emami, Najmeh ; Richard, Johan ; Freeman, William R. ; Stark, Daniel P. ; Snapp-Kolas, Christopher ( May 2023 , The Astrophysical Journal)

   We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲z≲ 2.6 (z_mean= 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of$\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.29}_{-0.43}^{+0.51}$and a median star formation rate of${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{H}\alpha }^{\mathrm{m}\mathrm{e}\mathrm{d}}={2.25}_{-1.26}^{+2.15}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$. We measure the faint electron-temperature-sensitive [Oiii]λ4363 emission line at 2.5σ(4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of$12+\log {(\mathrm{O}/\mathrm{H})}_{\mathrm{direct}}={7.88}_{-0.22}^{+0.25}$(${0.15}_{-0.06}^{+0.12}{Z}_{\odot }$). We investigate the applicability at highzof locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixedM_*, our composite is well represented by thez∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories$(\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.92}_{-0.22}^{+0.31})$, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, at fixedM_*and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii]λ3729/[Oii]λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of$n_e=1_{-0}^{+215}{\mathrm{cm}}^{-3}$($n_e=1_{-0}^{+74}{\mathrm{cm}}^{-3}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies atz∼ 2.

    

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4. Resolved Measurements of the CO-to-H 2 Conversion Factor in 37 Nearby Galaxies

   https://doi.org/10.3847/1538-4357/ad23ed  

   Chiang 江, I-Da 宜達 ; Sandstrom, Karin M. ; Chastenet, Jérémy ; Bolatto, Alberto D. ; Koch, Eric W. ; Leroy, Adam K. ; Sun 孙, Jiayi 嘉懿 ; Teng, Yu-Hsuan ; Williams, Thomas G. ( March 2024 , The Astrophysical Journal)

   We measure the CO-to-H₂conversion factor (α_CO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements ofα_COfor CO (2–1) and (1–0), respectively. The mean values forα_{CO (2–1)}andα_{CO (1–0)}are${9.3}_{-5.4}^{+4.6}$and${4.2}_{-2.0}^{+1.9}{M}_{\odot }{\mathrm{pc}}^{-2}{(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1})}^{-1}$, respectively. The CO-intensity-weighted mean is 5.69 forα_{CO (2–1)}and 3.33 forα_{CO (1–0)}. We examine howα_COscales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ($\overline{U}$). Among them,$\overline{U}$, Σ_SFR, and the integrated CO intensity (W_CO) have the strongest anticorrelation with spatially resolvedα_CO. We provide linear regression results toα_COfor all quantities tested. At galaxy-integrated scales, we observe significant correlations betweenα_COandW_CO, metallicity,$\overline{U}$, and Σ_SFR. We also find thatα_COin each galaxy decreases with the stellar mass surface density (Σ_⋆) in high-surface-density regions (Σ_⋆≥ 100M_⊙pc⁻²), following the power-law relations${\alpha }_{\mathrm{CO}(2–1)}\propto {\mathrm{\Sigma }}_{\star }^{-0.5}$and${\alpha }_{\mathrm{CO}(1–0)}\propto {\mathrm{\Sigma }}_{\star }^{-0.2}$. The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease inα_COwith increasing Σ_⋆as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction inα_CO. The decrease inα_COat high Σ_⋆is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburst-like” conversion factors.

    

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5. Characterization of Turbulent Fluctuations in the Sub-Alfvénic Solar Wind

   https://doi.org/10.3847/1538-4357/ad34ab  

   Zank, G. P. ; Zhao, L. -L. ; Adhikari, L. ; Telloni, D. ; Baruwal, Prashant ; Baruwal, Prashrit ; Zhu, Xingyu ; Nakanotani, M. ; Pitňa, A. ; Kasper, J. C. ; et al ( April 2024 , The Astrophysical Journal)

   Parker Solar Probe (PSP) observed sub-Alfvénic solar wind intervals during encounters 8–14, and low-frequency magnetohydrodynamic (MHD) turbulence in these regions may differ from that in super-Alfvénic wind. We apply a new mode decomposition analysis to the sub-Alfvénic flow observed by PSP on 2021 April 28, identifying and characterizing entropy, magnetic islands, forward and backward Alfvén waves, including weakly/nonpropagating Alfvén vortices, forward and backward fast and slow magnetosonic (MS) modes. Density fluctuations are primarily and almost equally entropy- and backward-propagating slow MS modes. The mode decomposition provides phase information (frequency and wavenumberk) for each mode. Entropy density fluctuations have a wavenumber anisotropy ofk_∥≫k_⊥, whereas slow-mode density fluctuations havek_⊥>k_∥. Magnetic field fluctuations are primarily magnetic island modes (δBⁱ) with anO(1) smaller contribution from unidirectionally propagating Alfvén waves (δB^A+) giving a variance anisotropy of$〈{\delta B^i}^2〉/〈{\delta B^A}^2〉=4.1$. Incompressible magnetic fluctuations dominate compressible contributions from fast and slow MS modes. The magnetic island spectrum is Kolmogorov-like$k_{\perp }^{-1.6}$in perpendicular wavenumber, and the unidirectional Alfvén wave spectra are$k_{\parallel }^{-1.6}$and$k_{\perp }^{-1.5}$. Fast MS modes propagate at essentially the Alfvén speed with anticorrelated transverse velocity and magnetic field fluctuations and are almost exclusively magnetic due toβ_p≪ 1. Transverse velocity fluctuations are the dominant velocity component in fast MS modes, and longitudinal fluctuations dominate in slow modes. Mode decomposition is an effective tool in identifying the basic building blocks of MHD turbulence and provides detailed phase information about each of the modes.

    

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