We study the Ly alpha profiles of 36 spectroscopically detected Ly alpha -emitters (LAEs) at z ~ 2-3, using Keck MOSFIRE to measure systemic redshifts and velocity dispersions from rest-frame optical ...nebular emission lines. Thirty of the 36 Ly alpha emission lines are redshifted with respect to the systemic velocity with at least lsigma significance, and the velocity offset with respect to systemic Delta v sub(Ly) alpha is correlated with the R-band magnitude, M sub(UV), and the velocity dispersion measured from nebular emission lines with >3sigma significance: brighter galaxies with larger velocity dispersions tend to have larger values of Delta v sub(Ly) alpha . Our results are consistent with a scenario in which the Ly alpha profile is determined primarily by the properties of the gas near the systemic redshift; in such a scenario, the opacity to Ly alpha photons in lower mass galaxies may be reduced if large gaseous disks have not yet developed and if the gas is ionized by the harder spectrum of young, low metallicity stars.
To study the chemical evolution across cosmic epochs, we investigate Ne, S, Cl, and Ar abundance patterns in the Cosmic Origins Spectrograph Legacy Archive Spectroscopic SurveY (CLASSY). CLASSY ...comprises local star-forming galaxies (SFGs; 0.02 < z < 0.18) with enhanced star formation rates, making them strong analogues to high-z SFGs. With direct measurements of electron temperature, we derive accurate ionic abundances for all elements and assess ionization correction factors (ICFs) to account for unseen ions and derive total abundances. We find Ne/O, S/O, Cl/O, and Ar/O exhibit constant trends with gas-phase metallicity for 12+log(O/H) < 8.5 but significant correlation for Ne/O and Ar/O with metallicity for 12+log(O/H) > 8.5, likely due to ICFs. Thus, the applicability of the ICFs to integrated spectra of galaxies could bias results, underestimating true abundance ratios. Using CLASSY as a local reference, we assess the evolution of Ne/O, S/O, and Ar/O in galaxies at z > 3, finding no cosmic evolution of Ne/O, while the lack of direct abundance determinations for S/O and Ar/O can bias the interpretation of the evolution of these elements. We determine the fundamental metallicity relationship (FMR) for CLASSY and compare to the high-redshift FMR, finding no evolution. Finally, we perform the first mass–neon relationship analysis across cosmic epochs, finding a slight evolution to high Ne at later epochs. The robust abundance patterns of CLASSY galaxies and their broad range of physical properties provide essential benchmarks for interpreting the chemical enrichment of the early galaxies observed with the JWST.
ABSTRACT
The connection between the escape fraction of ionizing radiation (fesc) and the properties of galaxies, such as stellar mass ($\rm M_{\rm *}$), age, star-formation rate (SFR), and dust ...content, are key inputs for reionization models, but many of these relationships remain untested at high redshift. We present an analysis of a sample of 96 $z$ ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey (KLCS). These galaxies have both sensitive Keck/LRIS spectroscopic measurements of the Lyman continuum (LyC) region, and multiband photometry that places constraints on stellar population parameters. We construct composite spectra from subsamples binned as a function of galaxy property and quantify the ionizing-photon escape for each composite. We find a significant anti-correlation between fesc and $\rm M_{\rm *}$, consistent with predictions from cosmological zoom-in simulations. We also find significant anti-correlation between fesc and E(B−V), encoding the underlying physics of LyC escape in our sample. We also find no significant correlation between fesc and either stellar age or specific SFR (= SFR/$\rm M_{\rm *}$), challenging interpretations that synchronize recent star formation and favorable conditions for ionizing escape. The galaxy properties now shown to correlate with fesc in the KLCS are Lyα equivalent width, UV Luminosity, $\rm M_{\rm *}$, SFR, and E(B−V), but not age or sSFR. This comprehensive analysis of galaxy properties and LyC escape at high redshift will be used to guide future models and observations of the reionization epoch.
We present Keck/OSIRIS laser guide-star assisted adaptive optics (LGSAO) integral-field spectroscopy of O iii λ5007 nebular emission from 12 galaxies hosting optically faint ( = 20-25; erg s−1) ...active galactic nuclei (AGNs) at redshift z ∼ 2-3. In combination with deep Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) rest-frame optical imaging, Keck/MOSFIRE rest-optical spectroscopy, and Keck/KCWI rest-UV integral-field spectroscopy, we demonstrate that both the continuum and emission-line structures of these sources exhibit a wide range of morphologies, from compact, isolated point sources to double-AGN merging systems with extensive ∼50 kpc tidal tails. One of the 12 galaxies previously known to exhibit a proximate damped Ly system coincident in redshift with the galaxy shows evidence for both an extended O iii narrow-line emission region and spatially offset Ly emission (with morphologically distinct blueshifted and redshifted components) indicative of large-scale gas flows photoionized by the central AGN. We do not find widespread evidence of star formation in the host galaxies surrounding these AGNs; the O iii velocity dispersions tend to be high ( = 100-500 ), the continuum morphologies are much more compact than a mass-matched star-forming comparison sample, and the diagnostic nebular emission-line ratios are dominated by an AGN-like ionizing spectrum. The sample is most consistent with a population of AGNs that radiate at approximately their Eddington limit and photoionize extended O iii nebulae whose characteristic sizes scale approximately as the square root of the AGN luminosity.
ABSTRACT
Star-forming galaxies form a sequence in the O
iii
λ
5007/
versus N
ii
λ
6584/
diagnostic diagram, with low-metallicity, highly ionized galaxies falling in the upper left corner. Drawing ...from a large sample of UV-selected star-forming galaxies at
with rest-frame optical nebular emission line measurements from Keck-MOSFIRE, we select the extreme ∼5% of the galaxies lying in this upper left corner, requiring log(N
ii
/
)
and log(O
iii
/
)
. These cuts identify galaxies with
, when oxygen abundances are measured via the O3N2 diagnostic. We study the
properties of the resulting sample of 14 galaxies. The mean (median) rest-frame
equivalent width is 39 (36) Å, and 11 of the 14 objects (79%) are
emitters (LAEs) with
. We compare the equivalent width distribution of a sample of 522 UV-selected galaxies at
identified without regard to their optical line ratios; this sample has mean (median)
equivalent width −1 (−4) Å, and only 9% of these galaxies qualify as LAEs. The extreme galaxies typically have lower attenuation at
than those in the comparison sample and have ∼50% lower median oxygen abundances. Both factors are likely to facilitate the escape of
: in less dusty galaxies
photons are less likely to be absorbed during multiple scatterings, while the harder ionizing spectrum and higher ionization parameter associated with strong, low-metallicity star formation may reduce the covering fraction or column density of neutral hydrogen, further easing
escape. The use of nebular emission line ratios may prove useful in the identification of galaxies with low opacity to
photons across a range of redshifts.
ABSTRACT
We present the first statistical analysis of kinematically resolved, spatially extended $\rm Ly\alpha$ emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the ...Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse $\rm Ly\alpha$ emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged $\rm Ly\alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of $\rm Ly\alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended $\rm Ly\alpha$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $\rm Ly\alpha$ emission. The symmetry implies that the $\rm Ly\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $\rm Ly\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $\rm Ly\alpha$ emission. We speculate that this weak directional dependence of $\rm Ly\alpha$ emission becomes discernible only when the $\rm Ly\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $\rm Ly\alpha$ radiative transfer modelling.
Abstract We present the first results from Chemical Evolution Constrained Using Ionized Lines in Interstellar Aurorae (CECILIA), a Cycle 1 JWST NIRSpec/MSA program that uses ultra-deep ∼30 hr ...G235M/F170LP observations to target multiple electron temperature-sensitive auroral lines in the spectra of 33 galaxies at z ∼ 1–3. Using a subset of 23 galaxies, we construct two ∼600 object-hour composite spectra, both with and without the stellar continuum, and use these to investigate the characteristic rest-optical ( λ rest ≈ 5700–8500 Å) spectrum of star-forming galaxies at the peak epoch of cosmic star formation. Emission lines of eight different elements (H, He, N, O, Si, S, Ar, and Ni) are detected, with most of these features observed to be ≲3% the strength of H α . We report the characteristic strength of three auroral features (N ii λ 5756, S iii λ 6313, and O ii λ λ 7322, 7332), as well as other semi-strong and faint emission lines, including forbidden Ni ii λ λ 7380, 7414 and permitted O i λ 8449, some of which have never before been observed outside of the local Universe. Using these measurements, we find T e N ii = 13,630 ± 2540 K, representing the first measurement of electron temperature using N ii in the high-redshift Universe. We also see evidence for broad line emission with a FWHM of 536 − 167 + 45 km s −1 ; the broad component of H α is 6.01%–28.31% the strength of the narrow component and likely arises from star-formation-driven outflows. Finally, we briefly comment on the feasibility of obtaining large samples of faint emission lines using JWST in the future.
Abstract
We present a detailed study of the partial rest-optical (
λ
obs
≈ 3600–5600 Å) spectra of
N
= 767 star-forming galaxies at 0.6 <
z
< 1.0 from the Large Early Galaxy Astrophysics Census ...(LEGA-C). We compare this sample with low-redshift (
z
∼ 0) galaxies from the Sloan Digital Sky Survey (SDSS), intermediate-redshift (
z
∼ 1.6) galaxies from the Fiber Multi-Object Spectrograph (FMOS)-COSMOS Survey, and high-redshift (
z
∼ 2) galaxies from the Keck Baryonic Structure Survey (KBSS). At a look-back time of 6–8 Gyr, galaxies with stellar masses
log
(
M
*
/
M
⊙
)
>
10.50
appear remarkably similar to
z
∼ 0 galaxies in terms of their nebular excitation, as measured using O
iii
λ
5008/H
β
. There is some evidence that 0.6 <
z
< 1.0 galaxies with lower
M
*
have higher O
iii
λ
5008/H
β
than
z
∼ 0 galaxies and are more similar to less evolved
z
∼ 1.6 and
z
∼ 2 galaxies, which are offset from the
z
∼ 0 locus at all
M
*
. We explore the impact of selection effects, contributions from active galactic nuclei, and variations in physical conditions (ionization parameter and gas-phase oxygen abundance) on the apparent distribution of O
iii
λ
5008/H
β
and find somewhat higher ionization in 0.6 <
z
< 1.0 galaxies with lower
M
*
relative to
z
∼ 0 galaxies. We use new near-infrared spectroscopic observations of a subsample of LEGA-C galaxies to investigate other probes of enrichment and excitation. Our analysis demonstrates the importance of obtaining complete rest-optical spectra of galaxies in order to disentangle these effects.
Abstract
The Baldwin, Philips, & Terlevich diagram of O
iii
/H
β
versus N
ii
/H
α
(hereafter N2-BPT) has long been used as a tool for classifying galaxies based on the dominant source of ionizing ...radiation. Recent observations have demonstrated that galaxies at
z
∼ 2 reside offset from local galaxies in the N2-BPT space. In this paper, we conduct a series of controlled numerical experiments to understand the potential physical processes driving this offset. We model nebular line emission in a large sample of galaxies, taken from the
simba
cosmological hydrodynamic galaxy formation simulation, using the
cloudy
photoionization code to compute the nebular line luminosities from H
ii
regions. We find that the observed shift toward higher O
iii
/H
β
and N
ii
/H
α
values at high redshift arises from sample selection: when we consider only the most massive galaxies
M
*
∼ 10
10–11
M
⊙
, the offset naturally appears, due to their high metallicities. We predict that deeper observations that probe lower-mass galaxies will reveal galaxies that lie on a locus comparable to
z
∼ 0 observations. Even when accounting for samples-selection effects, we find that there is a subtle mismatch between simulations and observations. To resolve this discrepancy, we investigate the impact of varying ionization parameters, H
ii
region densities, gas-phase abundance patterns, and increasing radiation field hardness on N2-BPT diagrams. We find that either decreasing the ionization parameter or increasing the N/O ratio of galaxies at fixed O/H can move galaxies along a self-similar arc in N2-BPT space that is occupied by high-redshift galaxies.
ABSTRACT
We examine the azimuthal variations in gas-phase metallicity profiles in simulated Milky Way-mass disc galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in ...simulation suite, which includes a sub-grid turbulent metal mixing model. We produce spatially resolved maps of the discs at z ≈ 0 with pixel sizes ranging from 250 to 750 pc, analogous to modern integral field unit galaxy surveys, mapping the gas-phase metallicities in both the cold and dense gas and the ionized gas correlated with H ii regions. We report that the spiral arms alternate in a pattern of metal rich and metal poor relative to the median metallicity of the order of ≲0.1 dex, appearing generally in this sample of flocculent spirals. The pattern persists even in a simulation with different strengths of metal mixing, indicating that the pattern emerges from physics above the sub-grid scale. Local enrichment does not appear to be the dominant source of the azimuthal metallicity variations at z ≈ 0: there is no correlation with local star formation on these spatial scales. Rather, the arms are moving radially inwards and outwards relative to each other, carrying their local metallicity gradients with them radially before mixing into the larger-scale interstellar medium. We propose that the arms act as freeways channeling relatively metal poor gas radially inwards, and relatively enriched gas radially outwards.
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