Search for: All records

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. 

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. 

Abstract We study the kinematics of the interstellar medium (ISM) viewed “down the barrel” in 20 gravitationally lensed galaxies during cosmic noon (z= 1.5–3.5). We use moderate-resolution spectra (R∼ 4000) from Keck’s Echellette Spectrograph and Imager and Magellan/MagE to spectrally resolve the ISM absorption in these galaxies into ∼10 independent elements and use double Gaussian fits to quantify the velocity structure of the gas. We find that the bulk motion of gas in this galaxy sample is outflowing, with average velocity centroid $\left(v_{\mathrm{cent}}\right)=-141$km s⁻¹(±111 km s⁻¹scatter) measured with respect to the systemic redshift. A total of 16 out of the 20 galaxies exhibit a clear positive skewness, with a blueshifted tail extending to ∼ −500 km s⁻¹. We examine scaling relations in outflow velocities with galaxy stellar mass and star formation rate, finding correlations consistent with a momentum-driven wind scenario. Our measured outflow velocities are also comparable to those reported for FIRE-2 and TNG50 cosmological simulations at similar redshift and galaxy properties. We also consider implications for interpreting results from lower-resolution spectra. We demonstrate that while velocity centroids are accurately recovered, the skewness, velocity width, and probes of high-velocity gas (e.g.,v₉₅) are subject to large scatter and biases at lower resolution. We find thatR≳ 1700 is required for accurate results for the gas kinematics of our sample. This work represents the largest available sample of well-resolved outflow velocity structure atz> 2 and highlights the need for good spectral resolution to recover accurate properties. 

ABSTRACT We present new observations of 16 bright (r = 19–21) gravitationally lensed galaxies at z ≃ 1–3 selected from the CASSOWARY survey. Included in our sample is the z = 1.42 galaxy CSWA-141, one of the brightest known reionization-era analogues at high redshift (g = 20.5), with a large specific star formation rate (31.2 Gyr−1) and an [O iii]+H β equivalent width (EW[O iii] + H β = 730 Å) that is nearly identical to the average value expected at z ≃ 7–8. In this paper, we investigate the rest-frame UV nebular line emission in our sample with the goal of understanding the factors that regulate strong C iii] emission. Although most of the sources in our sample show weak UV line emission, we find elevated C iii] in the spectrum of CSWA-141 (EWC iii] = 4.6 ± 1.9 Å) together with detections of other prominent emission lines (O iii], Si iii], Fe ii⋆, Mg ii). We compare the rest-optical line properties of high-redshift galaxies with strong and weak C iii] emission, and find that systems with the strongest UV line emission tend to have young stellar populations and nebular gas that is moderately metal-poor and highly ionized, consistent with trends seen at low and high redshift. The brightness of CSWA-141 enables detailed investigation of the extreme emission line galaxies which become common at z > 6. We find that gas traced by the C iii] doublet likely probes higher densities than that traced by [O ii] and [S ii]. Characterization of the spectrally resolved Mg ii emission line and several low-ionization absorption lines suggests neutral gas around the young stars is likely optically thin, potentially facilitating the escape of ionizing radiation. 

Physical properties of galaxies at z>7 are of interest for understanding both the early phases of star formation and the process of cosmic reionization. Chemical abundance measurements offer valuable information on the integrated star formation history, and hence ionizing photon production, as well as the rapid gas accretion expected at such high redshifts. We use reported measurements of [O III] 88μm emission and star formation rate to estimate gas-phase oxygen abundances in five galaxies at z=7.1-9.1 using the direct T_e method. We find typical abundances 12+log(O/H) = 7.9 (∼0.2 times the solar value) and an evolution of 0.9±0.5 dex in oxygen abundance at fixed stellar mass from z≃8 to 0. These results are compatible with theoretical predictions, albeit with large (conservative) uncertainties in both mass and metallicity. We assess both statistical and systematic uncertainties to identify promising means of improvement with the Atacama Large Millimeter Array (ALMA) and the James Webb Space Telescope (JWST). In particular we highlight [O III] 52μm as a valuable feature for robust metallicity measurements. Precision of 0.1-0.2 dex in T_e-based O/H abundance can be reasonably achieved for galaxies at z≈5-8 by combining [O III] 52μm with rest-frame optical strong lines. It will also be possible to probe gas mixing and mergers via resolved T_e-based abundances on kpc scales. With ALMA and JWST, direct metallicity measurements will thus be remarkably accessible in the reionization epoch. 

ABSTRACT We present improved results of the measurement of the correlation between galaxies and the intergalactic medium transmission at the end of reionization. We have gathered a sample of 13 spectroscopically confirmed Lyman-break galaxies (LBGs) and 21 Lyman-α emitters (LAEs) at angular separations 20 arcsec ≲ θ ≲ 10 arcmin (∼0.1–4 pMpc at z ∼ 6) from the sightlines to eight background z ≳ 6 quasars. We report for the first time the detection of an excess of Lyman-α transmission spikes at ∼10–60 cMpc from LAEs (3.2σ) and LBGs (1.9σ). We interpret the data with an improved model of the galaxy–Lyman-α transmission and two-point cross-correlations, which includes the enhanced photoionization due to clustered faint sources, enhanced gas densities around the central bright objects and spatial variations of the mean free path. The observed LAE(LBG)–Lyman-α transmission spike two-point cross-correlation function (2PCCF) constrains the luminosity-averaged escape fraction of all galaxies contributing to reionization to $$\langle f_{\rm esc} \rangle _{M_{\rm UV}\lt -12} = 0.14_{-0.05}^{+0.28}\, (0.23_{-0.12}^{+0.46})$$. We investigate if the 2PCCF measurement can determine whether bright or faint galaxies are the dominant contributors to reionization. Our results show that a contribution from faint galaxies ($$M_{\rm UV} \gt -20 \, (2\sigma)$$) is necessary to reproduce the observed 2PCCF and that reionization might be driven by different sub-populations around LBGs and LAEs at z ∼ 6. 

PFS (Prime Focus Spectrograph), a next generation facility instrument on the Subaru telescope, is now being tested on the telescope. The instrument is equipped with very wide (1.3 degrees in diameter) field of view on the Subaru's prime focus, high multiplexity by 2394 reconfigurable fibers, and wide waveband spectrograph that covers from 380nm to 1260nm simultaneously in one exposure. Currently engineering observations are ongoing with Prime Focus Instrument (PFI), Metrology Camera System (MCS), the first spectrpgraph module (SM1) with visible cameras and the first fiber cable providing optical link between PFI and SM1. Among the rest of the hardware, the second fiber cable has been already installed on the telescope and in the dome building since April 2022, and the two others were also delivered in June 2022. The integration and test of next SMs including near-infrared cameras are ongoing for timely deliveries. The progress in the software development is also worth noting. The instrument control software delivered with the subsystems is being well integrated with its system-level layer, the telescope system, observation planning software and associated databases. The data reduction pipelines are also rapidly progressing especially since sky spectra started being taken in early 2021 using Subaru Nigh Sky Spectrograph (SuNSS), and more recently using PFI during the engineering observations. In parallel to these instrumentation activities, the PFS science team in the collaboration is timely formulating a plan of large-sky survey observation to be proposed and conducted as a Subaru Strategic Program (SSP) from 2024. In this article, we report these recent progresses, ongoing developments and future perspectives of the PFS instrumentation.