Abstract
We examine the factors responsible for the variation in the ionization parameter (
U
) of high-redshift star-forming galaxies based on medium-resolution JWST/NIRSpec observations obtained by ...the Cosmic Evolution Early Release Science survey. The sample consists of 48 galaxies with redshifts
z
spec
= 2.7−6.3, which are largely representative of typical galaxies at these redshifts. The S
ii
λ
λ
6718, 6733 doublet is used to estimate electron densities (
n
e
), and dust-corrected H
α
luminosities are used to compute ionizing photon rates (
Q
). Using composite spectra of galaxies in bins of O
iii
λ
λ
4960, 5008/O
ii
λ
λ
3727, 3730 (O32) as a proxy for
U
, we determine that galaxies with higher O32 have 〈
n
e
〉 ≃ 500 cm
−3
that are ≳5 × larger than that of lower-O32 galaxies. We do not find a significant difference in 〈
Q
〉 between low- and high-O32 galaxies. Photoionization modeling indicates a large spread in
log
U
of ≈1.5 dex at a fixed
Z
neb
. On the other hand, the data indicate a highly significant correlation between
U
and star-formation-rate surface density (Σ
SFR
), which appears to be redshift invariant at
z
∼ 1.6−6.3, and possibly up to
z
∼ 9.5. We consider several avenues through which metallicity and Σ
SFR
(or gas density) may influence
U
, including variations in
n
e
and
Q
, internal dust extinction of ionizing photons, and the effects of gas density on the volume filling fraction. Based on these considerations, we conclude that gas density may play a more central role than metallicity in modulating
U
at these redshifts.
We present Keck/LRIS spectra of over 200 galaxies with well-determined redshifts between 0.4 and 1.4. We combine new measurements of near-ultraviolet, low-ionization absorption lines with previously ...measured masses, luminosities, colors, and star formation rates to describe the demographics and properties of galactic flows. Among star-forming galaxies with blue colors, we find a net blueshift of the Fe II absorption greater than 200 km s super(-1) (100 km s super(-1)) toward 2.5% (20%) of the galaxies. The fraction of blueshifted spectra does not vary significantly with stellar mass, color, or luminosity but does decline at specific star formation rates less than roughly 0.8 Gyr super(-1). The insensitivity of the blueshifted fraction to galaxy properties requires collimated outflows at these redshifts, while the decline in outflow fraction with increasing blueshift might reflect the angular dependence of the outflow velocity. The low detection rate of infalling gas, 3%-6% of the spectra, suggests an origin in (enriched) streams favorably aligned with our sightline. We find that four of these nine infalling streams have projected velocities commensurate with the kinematics of an extended disk or satellite galaxy. The strength of the Mg II absorption increases with stellar mass, B-band luminosity, and U - B color, trends arising from a combination of more interstellar absorption at the systemic velocity and less emission filling in more massive galaxies. Our results provide a new quantitative understanding of gas flows between galaxies and the circumgalactic medium over a critical period in galaxy evolution.
ABSTRACT
We present detections of O iii λ4363 and direct-method metallicities for star-forming galaxies at z = 1.7–3.6. We combine new measurements from the MOSFIRE Deep Evolution Field (MOSDEF) ...survey with literature sources to construct a sample of 18 galaxies with direct-method metallicities at z > 1, spanning 7.5 < 12+log(O/H) < 8.2 and log(M*/M⊙) = 7–10. We find that strong-line calibrations based on local analogues of high-redshift galaxies reliably reproduce the metallicity of the z > 1 sample on average. We construct the first mass–metallicity relation at z > 1 based purely on direct-method O/H, finding a slope that is consistent with strong-line results. Direct-method O/H evolves by ≲0.1 dex at fixed M* and star formation rate from z ∼ 0 to 2.2. We employ photoionization models to constrain the ionization parameter and ionizing spectrum in the high-redshift sample. Stellar models with supersolar O/Fe and binary evolution of massive stars are required to reproduce the observed strong-line ratios. We find that the z > 1 sample falls on the z ∼ 0 relation between ionization parameter and O/H, suggesting no evolution of this relation from z ∼ 0 to z ∼ 2. These results suggest that the offset of the strong-line ratios of this sample from local excitation sequences is driven primarily by a harder ionizing spectrum at fixed nebular metallicity compared to what is typical at z ∼ 0, naturally explained by supersolar O/Fe at high redshift caused by rapid formation time-scales. Given the extreme nature of our z > 1 sample, the implications for representative z ∼ 2 galaxy samples at ∼1010 M⊙ are unclear, but similarities to z > 6 galaxies suggest that these conclusions can be extended to galaxies in the epoch of reionization.
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 results from the first robust investigation of galaxy morphology as a function of environment at z > 1.5. Our study is motivated by the fact that star-forming galaxies contained within a ...protocluster at z = 2.3 in the HS 1700+64 field have significantly older ages and larger stellar masses on average than those at similar redshifts but more typical environmental densities. In the analysis of HST ACS images, we apply nonparametric statistics to characterize the rest-frame UV morphologies of a sample of 85 UV-selected star-forming galaxies at z = 1.7 2.9, 22 of which are contained in the protocluster. The remaining 63 control sample galaxies are not in the protocluster but have a similar mean redshift of z similar to 2.3. We find no environmental dependence for the distributions of morphological properties. Combining the measured morphologies with the results of population synthesis modeling, we find only weak correlations, if any, between morphological and steilar population properties such as stellar mass, age, extinction, and star formation rate. Given the incomplete census of the protocluster galaxy population, and the lack of correlation between rest-frame UV morphology and star formation history at z similar to 2 within our sample, the absence of environmental trends in the distribution of morphological properties is not surprising. In addition, using a larger sample of photometric candidates, we compare morphological distributions for 282 UV-selected and 43 near-IR-selected galaxies. While the difference in the degree of nebulosity between the two samples appears to be a by-product of the fainter average rest-frame UV surface brightness of the near-IR-selected galaxies, we find that, among the lowest surface brightness galaxies, the near-IR-selected objects have significantly smaller angular sizes.
ABSTRACT
High-redshift star-forming galaxies are likely responsible for the reionization of the universe, yet direct detection of their escaping ionizing (Lyman continuum LyC) photons has proven to ...be extremely challenging. In this study, we search for escaping LyC of the Cosmic Horseshoe, a gravitationally lensed, star-forming galaxy at
z
= 2.38 with a large magnification of ∼24. Transmission at wavelengths of low-ionization interstellar absorption lines in the rest-frame ultraviolet suggests a patchy, partially transparent interstellar medium. This makes it an ideal candidate for direct detection of the LyC. We obtained a 10-orbit Hubble near-UV image using the Wide Field Camera 3 (WFC3)/UVIS F275W filter that probes wavelengths just below the Lyman limit at the redshift of the Horseshoe in an attempt to detect escaping LyC radiation. After fully accounting for the uncertainties in the opacity of the intergalactic medium (IGM) and accounting for the charge transfer inefficiency in the WFC3 CCDs, we find a
upper limit for the relative escape fraction of
. This value is a factor of five lower than the value (0.4) predicted by the 40% transmission in the low-ion absorption lines. Though possible, it is unlikely that the nondetection is due to a high-opacity line of sight through the IGM (
% chance). We discuss several possible causes for the discrepancy between the escape fraction and the covering fraction and consider the implications for future attempts at both direct LyC detection and indirect estimates of the escape fraction.
Abstract
We present an analysis of the star formation rates (SFRs) and dust attenuation properties of star-forming galaxies at 2.7 ≤
z
< 6.5 drawn from the Cosmic Evolution Early Release Science ...Survey. Our analysis is based on JWST/NIRSpec Micro-Shutter Assembly
R
∼ 1000 spectroscopic observations covering approximately 1–5
μ
m. Our primary rest-frame optical spectroscopic measurements are H
α
/H
β
Balmer decrements, which we use as an indicator of nebular dust attenuation. In turn, we use Balmer decrements to obtain dust-corrected H
α
-based SFRs (i.e., SFR(H
α
)). We construct the relationship between SFR(H
α
) and stellar mass (
M
*
) in three bins of redshift (2.7 ≤
z
< 4.0, 4.0 ≤
z
< 5.0, and 5.0 ≤
z
< 6.5), which represents the first time the star-forming main sequence has been traced at these redshifts using direct spectroscopic measurements of Balmer emission as a proxy for SFR. In tracing the relationship between SFR(H
α
) and
M
*
back to such early times (
z
> 3), it is essential to use a conversion factor between H
α
and SFR that accounts for the subsolar metallicity prevalent among distant galaxies. We also use measured Balmer decrements to investigate the relationship between dust attenuation and stellar mass out to
z
∼ 6. The lack of significant redshift evolution in attenuation at fixed stellar mass, previously confirmed using Balmer decrements out to
z
∼ 2.3, appears to hold out to
z
∼ 6.5. Given the rapidly evolving gas, dust, and metal content of star-forming galaxies at fixed mass, this lack of significant evolution in attenuation provides an ongoing challenge to explain.
Increasingly large samples of galaxies are now being discovered at redshifts z {approx} 5-6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but ...detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper, we shed light on the physical conditions in a plausibly similar low-mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z = 2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (<100 Myr), low mass (M {sub *} {approx} 10{sup 9} M{sub sun}), low in metallicity (Z {approx} 1/6 Z{sub sun}), and unreddened (E(B - V) {approx_equal} 0.02, UV continuum slope {beta} = -2.1). We infer a metallicity 12 + log(O/H) = 7.9 {+-} 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio O III {lambda}{lambda}1661, 1666/O III {lambda}5007 to determine the electron temperature and finding 12 + log(O/H) = 7.8 {+-} 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum, which represents {approx}12 hr of integration with the Keck telescope, contains strong emission from Ly{alpha} (with rest-frame equivalent width 54 A), He II {lambda}1640 (both stellar and nebular), C III {lambda}{lambda}1907, 1909 and O III {lambda}{lambda}1661, 1666. The C IV/C III ratio indicates that the source of ionization is unlikely to be an active galactic nucleus. Analysis of the He II, O III, and C III line strengths indicates a very high ionization parameter log U {approx} -1, while Ly{alpha} and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418's unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.
Abstract
We present ultradeep Keck/MOSFIRE rest-optical spectra of two star-forming galaxies at
z
= 2.18 in the COSMOS field with bright emission lines, representing more than 20 hr of total ...integration. The fidelity of these spectra enabled the detection of more than 20 unique emission lines for each galaxy, including the first detection of the auroral O
ii
λλ
7322, 7332 lines at high redshift. We use these measurements to calculate the electron temperature in the low-ionization O
+
zone of the ionized interstellar medium and derive abundance ratios of O/H, N/H, and N/O using the direct method. The N/O and
α
/Fe abundance patterns of these galaxies are consistent with rapid formation timescales and ongoing strong starbursts, in accord with their high specific star formation rates. These results demonstrate the feasibility of using auroral O
ii
measurements for accurate metallicity studies at high redshift in a higher-metallicity and lower-excitation regime previously unexplored with the direct method in distant galaxies. These results also highlight the difficulty in obtaining the measurements required for direct-method metallicities from the ground. We emphasize the advantages that the JWST/NIRSpec instrument will bring to high-redshift metallicity studies, where the combination of increased sensitivity and uninterrupted wavelength coverage will yield more than an order of magnitude increase in efficiency for multiplexed auroral-line surveys relative to current ground-based facilities. Consequently, the advent of JWST promises to be the beginning of a new era of precision chemical abundance studies of the early universe at a level of detail rivaling that of local galaxy studies.
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