We use photoionization models that are designed to reconcile the joint rest-UV-optical spectra of high-z star-forming galaxies to self-consistently infer the gas chemistry and nebular ionization and ...excitation conditions for ∼150 galaxies from the Keck Baryonic Structure Survey (KBSS), using only observations of their rest-optical nebular spectra. We find that the majority of z ∼ 2-3 KBSS galaxies are moderately O-rich, with an interquartile range in 12 + log(O/H) = 8.29-8.56, and have significantly sub-solar Fe enrichment, with an interquartile range of Fe/H = −0.79, −0.53, which contributes additional evidence in favor of super-solar O/Fe in high-z galaxies. The model-inferred ionization parameters and N/O are strongly correlated with common strong-line indices (such as O32 and N2O2), with the latter exhibiting similar behavior to local extragalactic H ii regions. In contrast, diagnostics commonly used for measuring gas-phase O/H (such as N2 and O3N2) show relatively large scatter with the overall amount of oxygen present in the gas and behave differently than observed at z ∼ 0. We provide a new calibration for using R23 to measure O/H in typical high-z galaxies, although it is most useful for relatively O-rich galaxies; combining O32 and R23 does not yield a more effective calibration. Finally, we consider the implications for the intrinsic correlations between physical conditions across the galaxy sample and find that N/O varies with O/H in high-z galaxies in a manner that is almost identical to local H ii regions. However, we do not find a strong anti-correlation between ionization parameter and metallicity (O/H or Fe/H) in high-z galaxies, which is one of the principal bases for using strong-line ratios to infer oxygen abundance.
Abstract
We present results of a deep spectroscopic survey quantifying the statistics of the escape of ionizing radiation from star-forming galaxies at
z
∼ 3. We measure the ratio of ionizing to ...non-ionizing UV flux density
, where
f
900
is the mean flux density evaluated over the range 880, 910 Å. We quantify the emergent ratio of ionizing to non-ionizing UV flux density by analyzing high signal-to-noise ratio composite spectra formed from subsamples with common observed properties and numbers sufficient to reduce the statistical uncertainty in the modeled IGM+CGM correction to obtain precise values of
, including a full-sample average
= 0.057 ± 0.006. We show that
increases monotonically with
, inducing an inverse correlation with UV luminosity as a by-product. We fit the composite spectra using stellar spectral synthesis together with models of the ISM in which a fraction
f
c
of the stellar continuum is covered by gas with column density
. We show that the composite spectra simultaneously constrain the intrinsic properties of the stars (
L
900
/
L
1500
)
int
along with
f
c
,
,
, and
f
esc,abs
, the absolute escape fraction of ionizing photons. We find a sample-averaged
f
esc,abs
= 0.09 ± 0.01, with subsamples falling along a linear relation
. Using the far-UV luminosity function, the distribution function
n
(
W
(Ly
α
)), and the relationship between
and
, we estimate the total ionizing emissivity of
z
∼ 3 star-forming galaxies with
M
uv
≤ −19.5, which exceeds the contribution of quasi-stellar objects by a factor of ∼3, and accounts for ∼50% of the total
ϵ
LyC
at
z
∼ 3 estimated using indirect methods.
We present a combined analysis of rest-frame far-UV (FUV; 1000-2000 A) and rest-frame optical (3600-7000 A) composite spectra formed from very deep Keck/LRIS and Keck/MOSFIRE observations of a sample ...of 30 star-forming galaxies with z= 2.40 + or - 0.11, selected to be broadly representative of the full KBSS-MOSFIRE spectroscopic survey. Since the same massive stars are responsible for the observed FUV continuum and for the excitation of the observed nebular emission, a self-consistent stellar population synthesis model should simultaneously match the details of the FUV stellar+nebular continuum and-when inserted as the excitation source in photoionization models-predict all observed nebular emission line ratios. We find that only models including massive star binaries, having low stellar metallicity ( Zlow */Z sub(middot in circle)Asymptotically = to 0.1) but relatively high nebular (ionized gas-phase) abundances ( Z sub(nch)/Zmid dot in circle sub(middot in circle)Asymptotically = to 0.5), can successfully match all of the observational constraints. We show that this apparent discrepancy is naturally explained by highly super-solar O/Fe (Asymptotically = to4-5 (O/Fe)Asymptotically = to), expected for a gas whose enrichment is dominated by the products of core-collapse supernovae. While O dominates the physics of the ionized gas (and thus the nebular emission lines), Fe dominates the extreme-UV (EUV) and FUV opacity and controls the mass-loss rate from massive stars, resulting in particularly dramatic effects for massive stars in binary systems. This high nebular excitation-caused by the hard EUV spectra of Fe-poor massive stars-is much more common at high redshift (z> ~ 2) than low redshift due to systematic differences in the star formation history of typical galaxies.
We study metal absorption around 854 z ≈ 2.4 star-forming galaxies taken from the Keck Baryonic Structure Survey. The galaxies examined in this work lie in the fields of 15 hyperluminous background ...quasi-stellar objects, with galaxy impact parameters ranging from 35 proper kpc (pkpc) to 2 proper Mpc (pMpc). Using the pixel optical depth technique, we present the first galaxy-centred 2D maps of the median absorption by O vi, N v, C iv, C iii, and Si iv, as well as updated results for H i. At small galactocentric radii we detect a strong enhancement of the absorption relative to randomly located regions that extend out to at least 180 pkpc in the transverse direction, and ±240 km s−1 along the line of sight (LOS, ∼1 pMpc in the case of pure Hubble flow) for all ions except N v. For C iv (and H i) we detect a significant enhancement of the absorption signal out to 2 pMpc in the transverse direction, corresponding to the maximum impact parameter in our sample. After normalizing the median absorption profiles to account for variations in line strengths and detection limits, in the transverse direction we find no evidence for a sharp drop-off in metals distinct from that of H i. We argue instead that non-detection of some metal-line species in the extended circumgalactic medium is consistent with differences in the detection sensitivity. Along the LOS, the normalized profiles reveal that the enhancement in the absorption is more extended for O vi, C iv, and Si iv than for H i. We also present measurements of the scatter in the pixel optical depths, covering fractions, and equivalent widths as a function of projected galaxy distance. Limiting the sample to the 340 galaxies with redshifts measured from nebular emission lines does not decrease the extent of the enhancement along the LOS compared to that in the transverse direction. This rules out redshift errors as the source of the observed redshift-space anisotropy and thus implies that we have detected the signature of gas peculiar velocities from infall, outflows, or virial motions for H i, O vi, C iv, C iii, and Si iv.
We present a detailed analysis of 317 2.0 ≤ z ≤ 2.7 star-forming galaxies from the Keck Baryonic Structure Survey. Using complementary spectroscopic observations with Keck/LRIS and Keck/MOSFIRE, as ...well as spectral energy distribution (SED) fits to broadband photometry, we examine the joint rest-UV and rest-optical properties of the same galaxies, including stellar and nebular dust attenuation, metallicity, and star formation rate (SFR). The inferred parameters of the stellar population (reddening, age, SFR, and stellar mass) are strongly dependent on the details of the assumed stellar population model and the shape of the attenuation curve. Nebular reddening is generally larger than continuum reddening, but with large scatter. Compared to local galaxies, high-redshift galaxies have lower gas-phase metallicities (and/or higher nebular excitation) at fixed nebular reddening, and higher nebular reddening at fixed stellar mass, consistent with gas fractions that increase with redshift. We find that continuum reddening is correlated with 12 + log(O/H)O3N2 at 3.0 significance, whereas nebular reddening is correlated with only 1.1 significance. This may reflect the dependence of both continuum reddening and O3N2 on the shape of the ionizing radiation field produced by the massive stars. Finally, we show that H -based and SED-based estimates of SFR exhibit significant scatter relative to one another, and on average agree only for particular combinations of spectral synthesis models and attenuation curves. We find that the SMC extinction curve predicts consistent SFRs if we assume the subsolar (0.14 Z ) binary star models that are favored for high-redshift galaxies.
We present observations of Q1549-C25, an ~L* star-forming galaxy at z= 3.15 for which Lyman-continuum (LyC) radiation is significantly detected in deep Keck/LRIS spectroscopy. We find no evidence of ...contamination from a lower-redshift interloper close to the line of sight in the high signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25 in V sub(606), J sub(125), and H sub(160)Hubble Space Telescope(HST) imaging reveals that the object consists of a single, isolated component within 1''. In combination, these data indicate Q1549-C25 as a clean spectroscopic detection of LyC radiation, only the second such object discovered to date at z~ 3. We model the spectral energy distribution of Q1549-C25, finding evidence of negligible dust extinction, an age (assuming continuous star formation) of ~1 Gyr, and a stellar mass of Mlow * = 7.9 x 10 super(9)Mmiddot in circle. Although it is not possible to derive strong constraints on the absolute escape fraction of LyC emission, f sub(esc)(LyC), from a single object, we use simulations of intergalactic and circumgalactic absorption to infer f sub(esc)(LyC) > or =, slanted 0.51 at 95% confidence. The combination of deep Keck/LRIS spectroscopy and Hubble Space Telescope imaging is required to assemble a larger sample of objects like Q1549-C25, and obtain robust constraints on the average f sub(esc)(LyC) at z~ 3 and beyond.
Abstract
We present a detailed study of the rest-optical (3600–7000 Å) nebular spectra of ∼380 star-forming galaxies at
, obtained with Keck/Multi-object Spectrometer for Infrared Exploration ...(MOSFIRE) as part of the Keck Baryonic Structure Survey (KBSS). The KBSS-MOSFIRE sample is representative of star-forming galaxies at these redshifts, with stellar masses
and star formation rates SFR = 3–1000
yr
−1
. We focus on robust measurements of many strong diagnostic emission lines for individual galaxies: O
ii
λλ
3727, 3729, Ne
iii
λ
3869, H
β
, O
iii
4960, 5008, N
ii
λλ
6549, 6585, H
α
, and S
ii
λλ
6718, 6732. Comparisons with observations of typical local galaxies from the Sloan Digital Sky Survey and between subsamples of KBSS-MOSFIRE show that high-redshift galaxies exhibit a number of significant differences in addition to the well-known offset in log(O
iii
/H
β
) and log(N
ii
/H
α
). We argue that the primary difference between H
ii
regions in
galaxies and those at
is an enhancement in the degree of nebular excitation, as measured by O
iii
/H
β
and
(O
iii
+O
ii
)/H
β
. At the same time, KBSS-MOSFIRE galaxies are ∼10 times more massive than
galaxies with similar ionizing spectra and have higher N/O (likely accompanied by higher O/H) at fixed excitation. These results indicate the presence of harder ionizing radiation fields at fixed N/O and O/H relative to typical
galaxies, consistent with Fe-poor stellar population models that include massive binaries, and highlight a population of massive, high-specific star formation rate galaxies at high redshift with systematically different star formation histories than galaxies of similar stellar mass today.
We present a spectroscopic survey of 318 faint (R ~ 27, L ~ 0.1Llow *), Ly alpha -emission-selected galaxies (LAEs) in regions centered on the positions of hyperluminous QSOs (HLQSOs) at 2.5 < z < 3. ...A sample of 32 LAEs with rest-frame optical emission line spectra from Keck/Multi-Object Spectrometer For InfraRed Exploration (MOSFIRE) are used to interpret the LAE spectra in the context of their systemic redshifts. The fields are part of the Keck Baryonic Structure Survey, which includes substantial ancillary multi-wavelength imaging from both the ground and space. From a quantitative analysis of the diverse Ly alpha spectral morphologies, including line widths, asymmetries, and multi-peaked profiles, we find that peak widths and separations are typically smaller than among samples of more luminous continuum-selected galaxies (Lyman-break galaxies and their analogs; LBGs) at similar redshifts. We find tentative evidence for an association between Ly alpha spectral morphology and external illumination by the nearby HLQSO. Using the MOSFIRE subsample, we find that the peak of the resolved (R approx = 1300) Ly alpha line is shifted by +200 km s super(-1) with respect to systemic across a diverse set of galaxies including both LAEs and LBGs. We also find a small number of objects with significantly blueshifted Ly alpha emission, a potential indicator of accreting gas. The Ly alpha -to-H alpha line ratios measured for the MOSFIRE subset suggest that the LAEs in this sample have Ly alpha escape fractions functionof sub(esc,Ly alpha ) approx = 30%, significantly higher than typical LBG samples. Using redshifts calibrated by our MOSFIRE sample, we construct composite LAE spectra, finding the first evidence for metal-enriched outflows in such intrinsically faint high-redshift galaxies. These outflows have smaller continuum covering fractions (functionof sub(c) approx = 0.3) and velocities (v sub(ave) approx = 100-200 km s super(-1), v sub(max) approx = 500 km s super(-1)) than those associated with typical LBGs, suggesting that the gas covering fraction is a likely driver of the high Ly alpha and Ly-continuum escape fractions of LAEs with respect to LBGs. Our results suggest a similar scaling of outflow velocity with star formation rate (SFR) as is observed at lower redshifts (v sub(outflow) ~ SFR super(0.25)) and indicate that a substantial fraction of gas is ejected with v > v sub(esc). Further observations, including deep spectroscopy in the observed near-IR, will further probe the evolution and enrichment of these galaxies in the context of their gaseous environments.
ABSTRACT
We present the rest-frame optical spectroscopic properties of 60 faint (
R
AB
∼ 27;
L
∼ 0.1
L
*
) Ly
α
-selected galaxies (LAEs) at
z
≈ 2.56. These LAEs also have rest-UV spectra of their ...Ly
α
emission line morphologies, which trace the effects of interstellar and circumgalactic gas on the escape of Ly
α
photons. We find that the LAEs have diverse rest-optical spectra, but their average spectroscopic properties are broadly consistent with the extreme low-metallicity end of the populations of continuum-selected galaxies selected at
z
≈ 2–3. In particular, the LAEs have extremely high O
iii
λ
5008/H
β
ratios (log(O
iii
/H
β
) ∼ 0.8) and low N
ii
λ
6585/H
α
ratios (log(N
ii
/H
α
) < 1.15). Coupled with a detection of the O
iii
λ
4364 auroral line, these measurements indicate that the star-forming regions in faint LAEs are characterized by high electron temperatures (
T
e
≈ 1.8 × 10
4
K), low oxygen abundances (12 + log(O/H) ≈ 8.04,
Z
neb
≈ 0.22
Z
⊙
), and high excitations with respect to their more luminous continuum-selected analogs. Several of our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) ≈ 6.9–7.4 (
Z
neb
≈ 0.02–0.05
Z
⊙
). We interpret these observations in light of new models of stellar evolution (including binary interactions) that have been shown to produce long-lived populations of hot, massive stars at low metallicities. We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionization-era galaxies. Furthermore, we investigate the physical trends accompanying Ly
α
emission across the largest current sample of combined Ly
α
and rest-optical galaxy spectroscopy, including both the 60 KBSS-Ly
α
LAEs and 368 more luminous galaxies at similar redshifts. We find that the net Ly
α
emissivity (parameterized by the Ly
α
equivalent width) is strongly correlated with nebular excitation and ionization properties and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content.
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.
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