Abstract
The chemical abundance patterns of gas and stars in galaxies are powerful probes of galaxies’ star formation histories and the astrophysics of galaxy assembly but are challenging to measure ...with confidence in distant galaxies. In this paper, we report the first measurements of the correlation between stellar mass (
M
*
) and multiple tracers of chemical enrichment (including O, N, and Fe) in individual
z
∼ 2–3 galaxies, using a sample of 195 star-forming galaxies from the Keck Baryonic Structure Survey. The galaxies’ chemical abundances are inferred using photoionization models capable of reconciling high-redshift galaxies’ observed extreme rest-UV and rest-optical spectroscopic properties. We find that the
M
*
–O/H relation for our sample is relatively shallow, with moderately large scatter, and is offset ∼0.35 dex higher than the corresponding
M
*
–Fe/H relation. The two relations have very similar slopes, indicating a high level of
α
-enhancement—O/Fe ≈ 2.2 × (O/Fe)
⊙
—across two decades in
M
*
. The
M
*
–N/H relation has the steepest slope and largest intrinsic scatter, which likely results from the fact that many
z
∼ 2 galaxies are observed near or past the transition from “primary” to “secondary” N production, and may reflect uncertainties in the astrophysical origin of N. Together, these results suggest that
z
∼ 2 galaxies are old enough to have seen substantial enrichment from intermediate-mass stars, but are still young enough that Type Ia supernovae have not had time to contribute significantly to their enrichment.
We present the first comprehensive evolutionary analysis of the rest-frame UV spectroscopic properties of star-forming galaxies at z ∼ 2-4. We match samples at different redshifts in UV luminosity ...and stellar mass, and perform systematic measurements of spectral features and stellar population modeling. By creating composite spectra grouped according to Ly equivalent width (EW) and various galaxy properties, we study the evolutionary trends among Ly , low- and high-ionization interstellar (LIS and HIS) absorption features, and integrated galaxy properties. We also examine the redshift evolution of Ly and LIS absorption kinematics, and fine-structure emission EWs. The connections among the strengths of Ly , LIS lines, and dust extinction are redshift independent, as is the decoupling of the Ly and HIS line strengths, and the bulk outflow kinematics as traced by the LIS lines. Stronger Ly emission is observed at higher redshift at fixed UV luminosity, stellar mass, SFR, and age. Much of this variation in the average Ly strength with redshift, and the variation in Ly strength at fixed redshift, can be explained in terms of variations in the neutral gas covering fraction and/or dust content in the ISM and CGM. However, based on the connection between Ly and C iii emission strengths, we additionally find evidence for variations in the intrinsic production rate of Ly photons at the highest Ly EWs. The challenge now is to understand the observed evolution of the neutral gas covering fraction and dust extinction within a coherent model for galaxy formation, and make robust predictions for the escape of ionizing radiation at z > 6.
We present initial results of a deep near-IR spectroscopic survey covering the 15 fields of the Keck Baryonic Structure Survey using the recently commissioned MOSFIRE spectrometer on the Keck 1 ...telescope. Using photoionization models, we argue that the offset of the z ~ 2.3 BPT locus relative to that at z ~ 0 is caused by a combination of harder stellar ionizing radiation field, higher ionization parameter, and higher N/O at a given O/H compared to most local galaxies, and that the position of a galaxy along the z ~ 23 star-forming BPT locus is surprisingly insensitive to gas-phase oxygen abundance. We critically assess the applicability at high redshift of commonly used strong line indices for estimating gas-phase metallicity, and consider the implications of the small intrinsic scatter of the empirical relationship between excitation-sensitive line indices and M sub(*) (i.e., the "mass-metallicity" relation) at z Asymptotically = to 23.
In the redshift interval of 2 < z < 3, the physical conditions of the interstellar medium (ISM) in star-forming galaxies are likely to be different from those in the local Universe
because of lower ...gaseous metallicities, higher gas fractions, and higher star formation activities. In fact, observations suggest that higher electron densities, higher ionization parameters, and harder UV radiation fields are common. In this paper, based on the spectra of H α-selected star-forming galaxies at z = 2.5 taken with Multi-Object Spectrometer for InfraRed Exploration on Keck-1 telescope, we measure electron densities (n
e) using the oxygen line ratio (O ii λλ3726,3729), and investigate the relationships between the electron density of ionized gas and other physical properties. As a result, we find that the specific star formation rate (sSFR) and the surface density of SFR (ΣSFR) are correlated with the electron density at z = 2.5 for the first time. The ΣSFR–n
e relation is likely to be linked to the star formation law in H ii regions (where star formation activity is regulated by interstellar pressure). Moreover, we discuss the mode of star formation in those galaxies. The correlation between sSFR and ΣSFR suggests that highly star-forming galaxies (with high sSFR) tend to be characterized by higher surface densities of star formation (ΣSFR) and thus higher n
e values as well.
We use quasar absorption lines to study the physical conditions in the circumgalactic medium of redshift z ≈ 2.3 star-forming galaxies taken from the Keck Baryonic Structure Survey. In Turner et al. ...we used the pixel optical depth technique to show that absorption by H i and the metal ions O vi, N v, C iv, C iii, and Si iv is strongly enhanced within |Δv| ≲ 170 km s−1 and projected distances |d| ≲ 180 proper kpc from sightlines to the background quasars. Here we demonstrate that the O vi absorption is also strongly enhanced at fixed H i, C iv, and Si iv optical depths, and that this enhancement extends out to ∼350 km s−1. At fixed H i the increase in the median O vi optical depth near galaxies is 0.3–0.7 dex and is detected at 2–3σ confidence for all seven H i bins that have
$\log _{10}\tau _{\rm H\, {\small I}} \ge -1.5$
. We use ionization models to show that the observed strength of O vi as a function of H i is consistent with enriched, photoionized gas for pixels with
$\tau _{\rm H\, {\small I}} \gtrsim 10$
. However, for pixels with
$\tau _{\rm H\, {\small I}} \lesssim 1$
this would lead to implausibly high metallicities at low densities if the gas were photoionized by the background radiation. This indicates that the galaxies are surrounded by gas that is sufficiently hot to be collisionally ionized (T > 105 K) and that a substantial fraction of the hot gas has a metallicity ≳10−1 of solar. Given the high metallicity and large velocity extent (out to ∼1.5 v
circ) of this gas, we conclude that we have detected hot, metal-enriched outflows arising from star-forming galaxies.
We present results from the Keck Baryonic Structure Survey (KBSS) including the first detailed measurements of the column densities, kinematics, and internal energy of metal-bearing gas within the ...virial radius (35-100 physical kpc) of eight ∼L* galaxies at z ∼ 2. From our full sample of 130 metal-bearing absorbers, we infer that halo gas is kinematically complex when viewed in singly, doubly, and triply ionized species. Broad O vi and C iv absorbers are detected at velocities similar to the lower-ionization gas but with a very different kinematic structure, indicating that the circumgalactic medium (CGM) is multiphase. There is a high covering fraction of metal-bearing gas within 100 kpc, including highly ionized gas such as O vi; however, observations of a single galaxy probed by a lensed background QSO suggest the size of metal-bearing clouds is small (<400 pc for all but the O vi-bearing gas). The mass in metals found within the halo is substantial, equivalent to 25% of the metal mass within the interstellar medium. The gas kinematics unambiguously show that 70% of galaxies with detected metal absorption have some unbound metal-enriched gas, suggesting galactic winds may commonly eject gas from halos at z ∼ 2. When modeled assuming that ions with different ionization potentials can originate within a single gaseous structure, significant thermal broadening is detected in CGM absorbers that dominates the internal energy of the gas. Some 40% of the detected gas has temperatures in the range 104.5-5.5 K where cooling times are short, suggesting the CGM is dynamic, with constant heating or cooling to produce this short-lived thermal phase.
ABSTRACT
We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using ...the spectra of ≃3000 galaxies with redshifts 〈z〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $\rm{H\,{\small I}}$$\rm Ly\,\alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $\rm{H\,{\small I}}$ gas as a function of projected distance Dtran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $\rm{H\,{\small I}}$. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s$^{-1}\,$ dominate the kinematics of circumgalactic $\rm{H\,{\small I}}$ out to Dtran ≃ 50 kpc, while $\rm{H\,{\small I}}$ at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ circular velocity. Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the $\rm{H\,{\small I}}$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $\rm Ly\,\alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.
Abstract
Ly
α
emission is widely used to detect and confirm high-redshift galaxies and characterize the evolution of the intergalactic medium (IGM). However, many galaxies do not display Ly
α
...emission in typical spectroscopic observations, and intrinsic Ly
α
emitters represent a potentially biased set of high-redshift galaxies. In this work, we analyze a set of 703 galaxies at 2 ≲
z
≲ 3 with both Ly
α
spectroscopy and measurements of other rest-frame ultraviolet and optical properties in order to develop an empirical model for Ly
α
emission from galaxies and understand how the probability of Ly
α
emission depends on other observables. We consider several empirical proxies for the efficiency of Ly
α
photon production, as well as the subsequent escape of these photons through their local interstellar medium. We find that the equivalent width of metal-line absorption and the O3 ratio of rest-frame optical nebular lines are advantageous empirical proxies for Ly
α
escape and production, respectively. We develop a new quantity,
, that combines these two properties into a single predictor of net Ly
α
emission, which we find describes ∼90% of the observed variance in Ly
α
equivalent width when accounting for our observational uncertainties. We also construct conditional probability distributions demonstrating that galaxy selection based on measurements of galaxy properties yield samples of galaxies with widely varying probabilities of net Ly
α
emission. The application of the empirical models and probability distributions described here may be used to infer the selection biases of current galaxy surveys and evaluate the significance of high-redshift Ly
α
(non)detections in studies of reionization and the IGM.
Abstract
Nyx is a nearby, prograde, and high-eccentricity stellar stream physically contained in the thick disk, but its origin is unknown. Nyx could be the remnant of a disrupted dwarf galaxy, in ...which case the associated dark matter substructure could affect terrestrial dark matter direct-detection experiments. Alternatively, Nyx could be a signature of the Milky Way’s disk formation and evolution. To determine the origin of Nyx, we obtained high-resolution spectroscopy of 34 Nyx stars using Keck/HIRES and Magellan/MIKE. A differential chemical abundance analysis shows that most Nyx stars reside in a metal-rich (Fe/H > −1) high-
α
component that is chemically indistinguishable from the thick disk. This rules out the originally suggested scenario that Nyx is the remnant of a single massive dwarf galaxy merger. However, we also identify 5 substantially more metal-poor stars (Fe/H ∼ −2.0) whose chemical abundances are similar to those of the metal-weak thick disk. It remains unclear how stars that are chemically identical to the thick disk can be on such prograde, high-eccentricity orbits. We suggest two most likely scenarios: that Nyx is the result of an early minor dwarf galaxy merger, or that it is a record of the early spin-up of the Milky Way disk—although neither perfectly reproduces the chemodynamic observations. The most likely formation scenarios suggest that future spectroscopic surveys should find Nyx-like structures outside of the solar neighborhood.
Abstract Measurements of chemical abundances in high- z star-forming (SF) galaxies place important constraints on the enrichment histories of galaxies and the physical conditions in the early ...Universe. The James Webb Space Telescope (JWST) is beginning to enable direct chemical abundance measurements in galaxies at z > 2 via the detection of the faint T e -sensitive auroral line O iii λ 4364. However, abundances of other elements (e.g., S and Ar) in high- z galaxies remain unconstrained owing to a lack of T e data and wavelength coverage. Here we present multiple direct abundances in Q2343-D40, a galaxy at z = 2.9628 ± 0.0001 observed with JWST/NIRSpec as part of the CECILIA program. We report the first simultaneous measurement of T e O iii and T e S iii in a high- z galaxy, finding good agreement with the temperature trends in local SF systems. We measure a gas-phase metallicity of 12+log(O/H) = 8.07 ± 0.06, and the N/O abundance, log(N/O) = −1.37 ± 0.21, is indicative of primary nucleosynthesis. The S/O and Ar/O relative abundances, log(S/O) = −1.88 ± 0.10 and log(Ar/O) = −2.80 ± 0.12, are both >0.3 dex lower than the solar ratios. However, the relative Ar 2+ /S 2+ abundance is consistent with the solar ratio, suggesting that the relative S-to-Ar abundance does not evolve significantly with redshift. Recent nucleosynthesis models find that significant amounts of S and Ar are produced in Type Ia supernovae, such that the S/O and Ar/O abundances in Q2343-D40 could be the result of predominantly core-collapse supernova enrichment. Future JWST observations of high- z galaxies will uncover whether S/O and Ar/O are sensitive to the timescales of these different enrichment mechanisms.
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