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1. CloudFlex: A Flexible Parametric Model for the Small-scale Structure of the Circumgalactic Medium

   https://doi.org/10.3847/1538-4357/ad5965  

   Hummels, Cameron_B; Rubin, Kate_H_R; Schneider, Evan_E; Fielding, Drummond_B (September 2024, The Astrophysical Journal)

   Abstract We presentCloudFlex, an open-source tool for predicting absorption-line signatures of cool gas in galaxy halos with small-scale structure. Motivated by analyses of ∼10⁴K material in hydrodynamical simulations of turbulent, multiphase media, we model cool gas structures as complexes of cloudlets sampled from a power-law distribution of mass $\propto m_{\mathrm{cl}}^{-\alpha }$with velocities drawn from a turbulent velocity field. The user may specifyα, the lower limit of the cloudlet mass distribution ( $m_{\mathrm{cl},\min }$), and several other parameters that set the mass, size, and velocity distribution of the complex. This permits investigation of the relation between these parameters and absorption-line observables. As a proof-of-concept, we calculate the Mgiiλ2796 absorption induced by the cloudlets in background quasi-stellar object (QSO) spectra. We demonstrate that, at fixed metallicity, the covering fraction of sight lines with equivalent widthsW₂₇₉₆< 0.3 Å increases significantly with decreasing $m_{\mathrm{cl},\min }$, cloudlet number density (n_cl), and complex size. We then use this framework to predict the halo-scaleW₂₇₉₆distribution around ∼L^*galaxies. We show that the observed incidences ofW₂₇₉₆> 0.3 Å sight lines with impact parameters 10 kpc <R_⊥< 50 kpc in projected QSO–galaxy studies are consistent with our model over much of parameter space. However, they are underpredicted by models with $m_{\mathrm{cl},\min }\ge 100M_{\odot }$andn_cl≥ 0.03 cm⁻³, in keeping with a picture in which the inner cool circumgalactic medium (CGM) is dominated by numerous low-mass cloudlets (m_cl≲ 100M_⊙) with a volume filling factor ≲1%. When used to model absorption-line data sets built from multi-sight line and/or spatially extended background probes,CloudFlexenables detailed constraints on the size and velocity distributions of structures comprising the photoionized CGM. 

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2. On the Metallicities and Kinematics of the Circumgalactic Media of Damped Lyα Systems at z ∼ 2.5*

   https://doi.org/10.3847/1538-4357/acd34a  

   Urbano_Stawinski, Stephanie_M; Rubin, Kate_H_R; Prochaska, J_Xavier; Hennawi, Joseph_F; Tejos, Nicolas; Fumagalli, Michele; Rafelski, Marc; Kirby, Evan_N; Lusso, Elisabeta; Hafen, Zachary (July 2023, The Astrophysical Journal)

   Abstract We use medium- and high-resolution spectroscopy of close pairs of quasars to analyze the circumgalactic medium (CGM) surrounding 32 damped Lyαabsorption systems (DLAs). The primary quasar sightline in each pair probes an intervening DLA in the redshift range 1.6 <z_abs< 3.5, such that the secondary sightline probes absorption from Lyαand a large suite of metal-line transitions (including Oi, Cii, Civ, Siii, and Siiv) in the DLA host galaxy’s CGM at transverse distances 24 kpc ≤R_⊥≤ 284 kpc. Analysis of Lyαin the CGM sightlines shows an anticorrelation betweenR_⊥and Hicolumn density (N_HI) with 99.8% confidence, similar to that observed around luminous galaxies. The incidences of Ciiand SiiiwithN> 10¹³cm⁻²within 100 kpc of DLAs are larger by 2σthan those measured in the CGM of Lyman break galaxies (C_f(N_CII) > 0.89 and ${\mathrm{C}}_f\left(N_{\mathrm{Si}\mathrm{II}}\right)={0.75}_{-0.17}^{+0.12}$). Metallicity constraints derived from ionic ratios for nine CGM systems with negligible ionization corrections andN_HI> 10^18.5cm⁻²show a significant degree of scatter (with metallicities/limits across the range $-2.06\lesssim \log Z/Z_{\odot }\lesssim -0.75$), suggesting inhomogeneity in the metal distribution in these environments. Velocity widths of Civλ1548 and low-ionization metal species in the DLA versus CGM sightlines are strongly (>2σ) correlated, suggesting that they trace the potential well of the host halo overR_⊥≲ 300 kpc scales. At the same time, velocity centroids for Civλ1548 differ in DLA versus CGM sightlines by >100 km s⁻¹for ∼50% of velocity components, but few components have velocities that would exceed the escape velocity assuming dark matter host halos of ≥10¹²M_⊙. 

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3. The Cool Circumgalactic Medium of Low-redshift Star-forming Galaxies. I. Empirical Model and Mean Properties

   https://doi.org/10.3847/1538-4357/acf217  

   Faerman, Yakov; Werk, Jessica K (October 2023, The Astrophysical Journal)

   Abstract We present an analytic model for the cool,T∼ 10⁴K, circumgalactic medium (CGM), describing the gas distribution, and thermal and ionization states. Our model assumes (total) pressure equilibrium with the ambient warm/hot CGM, photoionization by the metagalactic radiation, and allows for nonthermal pressure support, parameterized by the ratio of thermal pressures,η=P_hot,th/P_cool,th. We apply the model to the COS-Halos measurements and find that a nominal model withη= 3, gas distribution out tor≈ 0.6R_vir, andM_cool= 3 × 10⁹M_⊙, corresponding to a volume filling fraction off_V,cool≈ 1%, reproduces the Hiand low/intermediate metal ions (Cii, Ciii, Siii, Siiii, and Mgii) mean column densities. Variation of ±0.5 dex inηorM_coolencompasses ∼2/3 of the scatter between objects. Our nominal model underproduces the measured Civand Siivcolumns, and these can be reproduced with (i) a cool phase withM_cool∼ 10¹⁰M_⊙andη≈ 5, or (ii) cooling or mixing gas at intermediate temperatures, withM∼ 1.5 × 10¹⁰M_⊙and occupying ∼1/2 of the total CGM volume. For cool gas withf_V,cool≈ 1%, we estimate an upper limit on the cloud sizes,R_cl≲ 0.5 kpc. Our results suggest that for the average galaxy CGM, the mass and nonthermal support in the cool phase are lower than previously estimated, and extreme scenarios are not necessary. We estimate the rates of cool gas depletion and replenishment, and find accretion onto the galaxy can be offset, allowing ${\dot{M}}_{\mathrm{cool}}\approx 0$over long timescales. 

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4. Discovery and Spectroscopic Confirmation of Aquarius III: A Low-mass Milky Way Satellite Galaxy

   https://doi.org/10.3847/1538-4357/ad8eba  

   Cerny, W.; Chiti, A.; Geha, M.; Mutlu-Pakdil, B.; Drlica-Wagner, A.; Tan, C_Y; Adamów, M.; Pace, A_B; Simon, J_D; Sand, D_J; et al (January 2025, The Astrophysical Journal)

   Abstract We present the discovery of Aquarius III, an ultra-faint Milky Way satellite galaxy identified in the second data release of the DECam Local Volume Exploration survey. Based on deeper follow-up imaging with DECam, we find that Aquarius III is a low-luminosity ( $M_V=-{2.5}_{-0.5}^{+0.3};$ $L_V={850}_{-260}^{+380}{L}_{\odot }$), extended ( $r_{1/2}={41}_{-8}^{+9}$pc) stellar system located in the outer halo (D_⊙= 85 ± 4 kpc). From medium-resolution Keck/DEIMOS spectroscopy, we identify 11 member stars and measure a mean heliocentric radial velocity of $v_{\mathrm{sys}}=-{13.1}_{-0.9}^{+1.0}\mathrm{km}{\mathrm{s}}^{-1}$for the system and place an upper limit ofσ_v< 3.5 km s⁻¹(σ_v< 1.6 km s⁻¹) on its velocity dispersion at the 95% (68%) credible level. Based on calcium-triplet metallicities of the six brightest red giant members, we find that Aquarius III is very metal-poor ([Fe/H]= − 2.61 ± 0.21) with a statistically significant metallicity spread ( ${\sigma }_{\left[\mathrm{Fe}/\mathrm{H}\right]}={0.46}_{-0.14}^{+0.26}$dex). We interpret this metallicity spread as strong evidence that the system is a dwarf galaxy as opposed to a star cluster. Combining our velocity measurement with Gaia proper motions, we find that Aquarius III is currently situated near its orbital pericenter in the outer halo (r_peri= 78 ± 7 kpc) and that it is plausibly on first infall onto the Milky Way. This orbital history likely precludes significant tidal disruption from the Galactic disk, notably unlike other satellites with comparably low velocity dispersion limits in the literature. Thus, if further velocity measurements confirm that its velocity dispersion is truly belowσ_v≲ 2 km s⁻¹, Aquarius III may serve as a useful laboratory for probing galaxy formation physics in low-mass halos. 

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5. FLIMFLAM DR1: The First Constraints on the Cosmic Baryon Distribution from Eight Fast Radio Burst Sight Lines

   https://doi.org/10.3847/1538-4357/ad6567  

   Khrykin, Ilya_S; Ata, Metin; Lee, Khee-Gan; Simha, Sunil; Huang, Yuxin; Prochaska, J_Xavier; Tejos, Nicolas; Bannister, Keith_W; Cooke, Jeff; Day, Cherie_K; et al (September 2024, The Astrophysical Journal)

   Abstract The dispersion measure of fast radio bursts (FRBs), arising from the interactions with free electrons along the propagation path, constitutes a unique probe of the cosmic baryon distribution. Their constraining power is further enhanced in combination with observations of the foreground large-scale structure and intervening galaxies. In this work, we present the first constraints on the partition of the cosmic baryons between the intergalactic medium (IGM) and circumgalactic medium (CGM), inferred from the FLIMFLAM spectroscopic survey. In its first data release, the FLIMFLAM survey targeted galaxies in the foreground of eight localized FRBs. Using Bayesian techniques, we reconstruct the underlying ∼Mpc-scale matter density field that is traced by the IGM gas. Simultaneously, deeper spectroscopy of intervening foreground galaxies (at impact parametersb_⊥≲r₂₀₀) and the FRB host galaxies constrains the contribution from the CGM. Applying Bayesian parameter inference to our data and assuming a fiducial set of priors, we infer the IGM cosmic baryon fraction to be $f_{\mathrm{igm}}={0.59}_{-0.10}^{+0.11}$and a CGM gas fraction of $f_{\mathrm{gas}}={0.55}_{-0.29}^{+0.26}$for 10¹⁰M_⊙≲M_halo≲ 10¹³M_⊙halos. The mean FRB host dispersion measure (rest-frame) in our sample is $〈{\mathrm{DM}}_{\mathrm{host}}〉={90}_{-19}^{+29}\mathrm{pc}{\mathrm{cm}}^{-3}$, of which $〈{\mathrm{DM}}_{\mathrm{host}}^{\mathrm{unk}}〉={69}_{-19}^{+28}\mathrm{pc}{\mathrm{cm}}^{-3}$arises from the host galaxy interstellar medium (ISM) and/or the FRB progenitor environment. While our currentf_igmandf_gasuncertainties are too broad to constrain most galactic feedback models, this result marks the first measurement of the IGM and CGM baryon fractions, as well as the first systematic separation of the FRB host dispersion measure into two components: arising from the halo and from the inner ISM/FRB engine. 

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