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The kinematics of star-forming galaxy populations at high redshifts are integral to our understanding of disk properties, merger rates, and other defining characteristics. Nebular gas emission is a common tracer of galaxies’ gravitational potential and angular momenta, but is sensitive to nongravitational forces as well as galactic outflows, and thus might not accurately trace the host galaxy dynamics. We present kinematic maps of young stars from rest-ultraviolet photospheric absorption in the star-forming galaxy CASSOWARY 13 (a.k.a. SDSS J1237+5533) atz= 1.87 using the Keck Cosmic Web Imager, alongside nebular emission measurements from the same observations. Gravitational lensing magnification of the galaxy enables good spatial sampling of multiple independent lensed images. We find close agreement between the stellar and nebular velocity fields. We measure a mean local velocity dispersion ofσ = 64 ± 12 km s⁻¹for the young stars, consistent with that of the Hiiregions traced by nebular Ciii] emission (52  ±  9 km s⁻¹). The ∼20 km s⁻¹average difference in line-of-sight velocity is much smaller than the local velocity width and the velocity gradient (≳100 km s⁻¹). We find no evidence of asymmetric drift nor evidence that outflows bias the nebular kinematics, and thus we conclude that nebular emission appears to be a reasonable dynamical tracer of young stars in the galaxy. These results support the picture of star formation in thick disks with high velocity dispersion atz ∼ 2, and they represent an important step toward establishing robust kinematics of early galaxies using collisionless tracers. 

Strong gravitational lenses with two background sources at widely separated redshifts are a promising independent probe of cosmological parameters. We can use these systems, known as double-source-plane lenses (DSPLs), to measure the ratio (β) of angular-diameter distances of the sources, which is sensitive to the matter density (Ω_m) and the equation-of-state parameter for dark-energy (w). However, DSPLs are rare and require high-resolution imaging and spectroscopy for detection, lens modeling, and measuringβ. Here, we report only the second DSPL ever used to measure cosmological parameters. We model the DSPLAGEL150745+052256 from the ASTRO 3D Galaxy Evolution with Lenses (AGEL) survey using Hubble Space Telescope/Wide-Field Camera 3 imaging and Keck Cosmic Web Imager spectroscopy. The spectroscopic redshifts for the deflector and two sources inAGEL1507 arez_defl= 0.594,z_S1 =  2.163, andz_S2= 2.591. We measure a stellar velocity dispersion ofσ_obs = 109 ± 27 km s⁻¹for the nearer source (S1). Usingσ_obsfor the main deflector (from literature) and S1, we test the robustness of our DSPL model. We measure $\beta =0.953_{-0.010}^{+0.008}$forAGEL1507 and infer Ω_m $=0.33_{-0.23}^{+0.38}$for ΛCDM cosmology. CombiningAGEL1507 with the published model of the Jackpot lens improves the precision on Ω_m(ΛCDM) andw(wCDM) by ∼10%. The inclusion of DSPLs significantly improves the constraints when combined with Planck’s cosmic microwave background observations, enhancing the precision onwby 30%. This paper demonstrates the potential constraining power of DSPLs and their complementarity to other standard cosmological probes. Tighter future constraints from larger DSPL samples discovered from ongoing and forthcoming large-area sky surveys would provide insights into the nature of dark energy. 

Abstract The baryon cycle is crucial for understanding galaxy formation, as gas inflows and outflows vary throughout a galaxy’s lifetime and affect its star formation rate. Despite the necessity of accretion for galaxy growth at high redshifts, direct observations of inflowing gas have proven elusive, especially atz ≳ 2. We present a spectroscopic analysis of a galaxy at redshiftz= 2.45, which exhibits signs of inflow in several ultraviolet interstellar absorption lines, with no clear outflow signatures. The absorption lines are redshifted by ∼250 km s⁻¹with respect to the systemic redshift, and Civshows a prominent inverse P-Cygni profile. Simple stellar population models suggest that this galaxy has a low metallicity (∼5% solar), with a very young starburst of age ∼4 Myr dominating the ultraviolet luminosity. The gas inflow velocity and nebular velocity dispersion suggest an approximate halo mass of order ∼10¹¹M_⊙, a regime in which simulations predict that bursty star formation is common at this redshift. We conclude that this system is likely in the beginning of a cycle of bursty star formation, where inflow and star formation rates are high, but where supernovae and other feedback processes have not yet launched strong outflows. In this scenario, we expect the inflow-dominated phase to be observable (e.g., with net redshifted interstellar medium absorption) for only a short timescale after a starburst onset. This result represents a promising avenue for probing the full baryon cycle, including inflows, during the formative phases of low-mass galaxies at high redshifts. 

We study the spatially resolved outflow properties of CSWA13, an intermediate-mass (M_* = 10⁹M_⊙), gravitationally lensed star-forming galaxy atz= 1.87. We use Keck/KCWI to map outflows in multiple rest-frame UV interstellar medium (ISM) absorption lines, along with fluorescent Siii* emission, and nebular emission from Ciii] tracing the local systemic velocity. The spatial structure of the outflow velocity mirrors that of the nebular kinematics, which we interpret to be a signature of a young galactic wind that is pressurizing the ISM of the galaxy but is yet to burst out. From the radial extent of Siii* emission, we estimate that the outflow is largely encapsulated within 3.5 kpc. We explore the geometry (e.g., patchiness) of the outflow by measuring the covering fraction at different velocities, finding that the maximum covering fraction is at velocitiesv ≃ −150 km s⁻¹. Using the outflow velocity (v_out), radius (R), column density (N), and solid angle (Ω) based on the covering fraction, we measure the mass-loss rate $\log {\dot{m}}_{\mathrm{out}}/\left(M_{\odot }{\text{yr}}^{-1}\right)=1.73\pm 0.23$and mass loading factor $\log \eta =0.04\pm 0.34$for the low-ionization outflowing gas in this galaxy. These values are relatively large and the bulk of the outflowing gas is moving with speeds less than the escape velocity of the galaxy halo, suggesting that the majority of the outflowing mass will remain in the circumgalactic medium and/or recycle back into the galaxy. The results support a picture of high outflow rates transporting mass and metals into the inner circumgalactic medium, providing the gas reservoir for future star formation.