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1. The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters

   https://doi.org/10.3847/2041-8213/acefb5  

   Lee, Khee-Gan; Khrykin, Ilya S.; Simha, Sunil; Ata, Metin; Huang, Yuxin; Prochaska, J. Xavier; Tejos, Nicolas; Cooke, Jeff; Nagamine, Kentaro; Zhang, Jielai (August 2023, The Astrophysical Journal Letters)

   Abstract The repeating fast radio burst FRB 20190520B is an anomaly of the FRB population thanks to its high dispersion measure (DM = 1205 pc cm⁻³) despite its low redshift ofz_frb= 0.241. This excess has been attributed to a large host contribution of DM_host≈ 900 pc cm⁻³, far larger than any other known FRB. In this paper, we describe spectroscopic observations of the FRB 20190520B field obtained as part of the FLIMFLAM survey, which yielded 701 galaxy redshifts in the field. We find multiple foreground galaxy groups and clusters, for which we then estimated halo masses by comparing their richness with numerical simulations. We discover two separateM_halo> 10¹⁴M_⊙galaxy clusters atz= 0.1867 and 0.2170 that are directly intersected by the FRB sight line within their characteristic halo radiusr₂₀₀. Subtracting off their estimated DM contributions, as well that of the diffuse intergalactic medium, we estimate a host contribution of ${\mathrm{D}\mathrm{M}}_{\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}={430}_{-220}^{+140}$or ${280}_{-170}^{+140}\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-3}$(observed frame), depending on whether we assume that the halo gas extends tor₂₀₀or 2 ×r₂₀₀. This significantly smaller DM_host—no longer the largest known value—is now consistent with Hαemission measures of the host galaxy without invoking unusually high gas temperatures. Combined with the observed FRB scattering timescale, we estimate the turbulent fluctuation and geometric amplification factor of the scattering layer to be $\tilde{F}G\approx 4.5–11{\left({\mathrm{pc}}^2\mathrm{km}\right)}^{-1/3}$, suggesting that most of the gas is close to the FRB host. This result illustrates the importance of incorporating foreground data for FRB analyses both for understanding the nature of FRBs and to realize their potential as a cosmological probe. 

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2. 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)

   Abstract 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 ${4.2}_{-2.0}^{+1.9}{M}_{\odot }{\mathrm{pc}}^{-2}{\left(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1}\right)}^{-1}$, 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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3. HSTPROMO Internal Proper-motion Kinematics of Dwarf Spheroidal Galaxies. I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

   https://doi.org/10.3847/1538-4357/ad571c  

   Vitral, Eduardo; van_der_Marel, Roeland_P; Sohn, Sangmo_Tony; Libralato, Mattia; del_Pino, Andrés; Watkins, Laura_L; Bellini, Andrea; Walker, Matthew_G; Besla, Gurtina; Pawlowski, Marcel_S; et al (July 2024, The Astrophysical Journal)

   Abstract We analyze four epochs of Hubble Space Telescope imaging over 18 yr for the Draco dwarf spheroidal galaxy. We measure precise proper motions for hundreds of stars and combine these with existing line-of-sight (LOS) velocities. This provides the first radially resolved 3D velocity dispersion profiles for any dwarf galaxy. These constrain the intrinsic velocity anisotropy and resolve the mass–anisotropy degeneracy. We solve the Jeans equations in oblate axisymmetric geometry to infer the mass profile. We find the velocity dispersion to be radially anisotropic along the symmetry axis and tangentially anisotropic in the equatorial plane, with a globally averaged value $\overline{{\beta }_{\mathrm{B}}}=-{0.20}_{-0.53}^{+0.28}$, (where 1 – ${\beta }_{\mathrm{B}}\equiv 〈v_{\tan }^2〉/〈v_{\mathrm{rad}}^2〉$in 3D). The logarithmic dark matter (DM) density slope over the observed radial range, Γ_dark, is $-{0.83}_{-0.37}^{+0.32}$, consistent with the inner cusp predicted in ΛCDM cosmology. As expected given Draco’s low mass and ancient star formation history, it does not appear to have been dissolved by baryonic processes. We rule out cores larger than 487, 717, and 942 pc at 1σ, 2σ, and 3σconfidence, respectively, thus imposing important constraints on the self-interacting DM cross section. Spherical models yield biased estimates for both the velocity anisotropy and the inferred slope. The circular velocity at our outermost data point (900 pc) is ${24.19}_{-2.97}^{+6.31}\mathrm{km}{\mathrm{s}}^{-1}$. We infer a dynamical distance of ${75.37}_{-4.00}^{+4.73}$kpc and show that Draco has a modest LOS rotation, with $\left(v/\sigma \right)=0.22\pm 0.09$. Our results provide a new stringent test of the so-called “cusp–core” problem that can be readily extended to other dwarfs. 

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4. Strong-lensing Analysis of SPT-CL J2325−4111 and SPT-CL J0049−2440, Two Powerful Cosmic Telescopes ( R _E > 40″) from the SPT Clusters Sample

   https://doi.org/10.3847/1538-4357/ad9aa5  

   Mahler, Guillaume; Sharon, Keren; Bayliss, Matthew; Bleem, Lindsey E; Brodwin, Mark; Floyd, Benjamin; Gassis, Raven; Gladders, Michael D; Khullar, Gourav; Remolina_González, Juan D; et al (February 2025, The Astrophysical Journal)

   Abstract We report the results from a study of two massive (M_500c> 6.0 × 10¹⁴M_⊙) strong-lensing clusters selected from the South Pole Telescope cluster survey for their large Einstein radius (R_E> 40″), SPT-CL J2325−4111 and SPT-CL J0049−2440. Ground-based and shallow Hubble Space Telescope (HST) imaging indicated extensive strong-lensing evidence in these fields, with giant arcs spanning 18″ and 31″, respectively, motivating further space-based imaging follow-up. Here, we present multiband HST imaging and ground-based Magellan spectroscopy of the fields, from which we compile detailed strong-lensing models. The lens models of SPT-CL J2325−4111 and SPT-CL J0049−2440 were optimized using nine and eight secure multiply imaged systems with a final image-plane rms of 0 $\stackrel{″}{.}$63 and 0 $\stackrel{″}{.}$73, respectively. From the lensing analysis, we measure a projected mass density within 500 kpc ofM(<500 kpc) = (7.30 ± 0.07) × 10¹⁴M_⊙and $M(<500\mathrm{kpc})=7.12_{-0.19}^{+0.16}\times 10^{14}$M_⊙for these two clusters, and subhalo mass ratios of 0.12 ± 0.01 and $0.21_{-0.05}^{+0.07}$, respectively. Both clusters produce a large area with high magnification (μ≥ 3) for a source atz= 9, ${\mathcal{A}}_{\left|\mu \right|\ge 3}^{\mathrm{lens}}=4.93_{-0.04}^{+0.03}$arcmin²and ${\mathcal{A}}_{\left|\mu \right|\ge 3}^{\mathrm{lens}}=3.64_{-0.10}^{+0.14}$arcmin², respectively, placing them in the top tier of strong-lensing clusters. We conclude that these clusters are spectacular sightlines for further observations that will reduce the systematic uncertainties due to cosmic variance. This paper provides the community with two additional well-calibrated cosmic telescopes, as strong as the Frontier Fields and suitable for studies of the highly magnified background Universe. 

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5. The Ionization and Dynamics of the Makani Galactic Wind

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

   Rupke, David S.; Coil, Alison L.; Perrotta, Serena; Davis, Julie D.; Diamond-Stanic, Aleksandar M.; Geach, James E.; Hickox, Ryan C.; Moustakas, John; Petter, Grayson C.; Rudnick, Gregory H.; et al (April 2023, The Astrophysical Journal)

   Abstract The Makani galaxy hosts the poster child of a galactic wind on scales of the circumgalactic medium. It consists of a two-episode wind in which the slow, outer wind originated 400 Myr ago (Episode I;R_I= 20 − 50 kpc) and the fast, inner wind is 7 Myr old (Episode II;R_II= 0 − 20 kpc). While this wind contains ionized, neutral, and molecular gas, the physical state and mass of the most extended phase—the warm, ionized gas—are unknown. Here we present Keck optical spectra of the Makani outflow. These allow us to detect hydrogen lines out tor= 30–40 kpc and thus constrain the mass, momentum, and energy in the wind. Many collisionally excited lines are detected throughout the wind, and their line ratios are consistent with 200–400 km s⁻¹shocks that power the ionized gas, withv_shock=σ_wind. Combining shock models, density-sensitive line ratios, and mass and velocity measurements, we estimate that the ionized mass and outflow rate in the Episode II wind could be as high as those of the molecular gas: $M_{\mathrm{II}}^{\mathrm{H}\mathrm{II}}\sim M_{\mathrm{II}}^{{\mathrm{H}}_2}=(1-2)\times {10}^9{M}_{\odot }$and $\mathit{dM}/{\mathit{dt}}_{\mathrm{II}}^{\mathrm{H}\mathrm{II}}\sim \mathit{dM}/{\mathit{dt}}_{\mathrm{II}}^{{\mathrm{H}}_2}=170-250M_{\odot }$yr⁻¹. The outer wind has slowed, so that $\mathit{dM}/{\mathit{dt}}_{\mathrm{I}}^{\mathrm{H}\mathrm{II}}\sim 10M_{\odot }$yr⁻¹, but it contains more ionized gas, $M_{\mathrm{I}}^{\mathrm{H}\mathrm{II}}=5\times {10}^9$M_⊙. The momentum and energy in the recent Episode II wind imply a momentum-driven flow (p“boost” ∼7) driven by the hot ejecta and radiation pressure from the Eddington-limited, compact starburst. Much of the energy and momentum in the older Episode I wind may reside in a hotter phase, or lie further into the circumgalactic medium. 

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