Abstract
We report a reanalysis of a near-pristine absorption system, located at a redshift
toward the quasar Q1243+307, based on the combination of archival and new data obtained with the HIRES ...echelle spectrograph on the Keck telescope. This absorption system, which has an oxygen abundance O/H = −2.769 ± 0.028 (≃1/600 of the solar abundance), is among the lowest metallicity systems currently known where a precise measurement of the deuterium abundance is afforded. Our detailed analysis of this system concludes, on the basis of eight D
i
absorption lines, that the deuterium abundance of this gas cloud is
, which is in very good agreement with the results previously reported by Kirkman et al., but with an improvement on the precision of this single measurement by a factor of ∼3.5. Combining this new estimate with our previous sample of six high precision and homogeneously analyzed D/H measurements, we deduce that the primordial deuterium abundance is
or, expressed as a linear quantity,
this value corresponds to a one percent determination of the primordial deuterium abundance. Combining our result with a big bang nucleosynthesis (BBN) calculation that uses the latest nuclear physics input, we find that the baryon density derived from BBN agrees to within 2
σ
of the latest results from the
Planck
cosmic microwave background data.
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.
ABSTRACT
Observations of reionization-era analogues at z ∼ 3 are a powerful tool for constraining reionization. Rest-ultraviolet observations are particularly useful, in which both direct and ...indirect tracers of ionizing-photon production and escape can be observed. We analyse a sample of 124 z ∼ 3 galaxies from the Keck Lyman Continuum Spectroscopic Survey, with sensitive spectroscopic measurements of the Lyman continuum region. We present a method of removing foreground contamination from our sample using high-resolution, multiband Hubble Space Telescope imaging. We re-measure the global properties of the cleaned sample of 13 individually detected Lyman continuum sources and 107 individually undetected sources, including a sample-averaged absolute escape fraction of fesc, abs = 0.06 ± 0.01 and a sample-averaged ratio of ionizing to non-ionizing ultraviolet flux density of 〈f900/f1500〉out = 0.040 ± 0.006, corrected for attenuation from the intergalactic and circumgalactic media. Based on composite spectra, we also recover a strong positive correlation between 〈f900/f1500〉out and Lyα equivalent width (Wλ(Ly$\rm \alpha$)) and a negative correlation between 〈f900/f1500〉out and UV luminosity. As in previous work, we interpret the relationship between 〈f900/f1500〉out and Wλ(Ly$\rm \alpha$) in terms of the modulation of the escape of ionizing radiation from star-forming galaxies based on the covering fraction of neutral gas. We also use a Wλ(Ly$\rm \alpha$)-weighted 〈f900/f1500〉out to estimate an ionizing emissivity from star-forming galaxies at z ∼ 3 as ϵLyC ≃ 5.5 × 1024 erg s−1 Hz−1 Mpc−3. This estimate, evaluated using the uncontaminated sample of this work, affirms that the contribution of galaxies to the ionizing background at z ∼ 3 is comparable to that of active galactic nuclei.
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 present the first spectroscopic measurements of the shape of the far-ultraviolet (far-UV; lambda = 950-1500 A) dust attenuation curve at high redshift (z~ 3). Our analysis employs rest-frame UV ...spectra of 933 galaxies at z~ 3, 121 of which have very deep spectroscopic observations (> ~7 hr) at lambda = 850-1300 A, with the Low Resolution Imaging Spectrograph on the Keck Telescope. By using an iterative approach in which we calculate the ratios of composite spectra in different bins of continuum color excess, E(B- V), we derive a dust curve that implies a lower attenuation in the far-UV for a given E(B- V) than those obtained with standard attenuation curves. We demonstrate that the UV composite spectra of z~ 3 galaxies can be modeled well by assuming our new attenuation curve, a high covering fraction of H I, and absorption from the Lyman-Werner bands of H sub(2) with a small (< ~20%) covering fraction. The low covering fraction of H sub(2) relative to that of the H sub(1) and dust suggests that most of the dust in the ISM of typical galaxies at z~ 3 is unrelated to the catalysis of H sub(2), and is associated with other phases of the ISM (i.e., the ionized and neutral gas). The far-UV dust curve implies a factor of thickapproximate2 lower dust attenuation of Lyman continuum (ionizing) photons relative to those inferred from the most commonly assumed attenuation curves for L* galaxies at z~ 3. Our results may be utilized to assess the degree to which ionizing photons are attenuated in H II regions or, more generally, in the ionized or low column density (N(H I) < ~ 10 super(17,2) cm super(-2)) neutral ISM of high-redshift galaxies.
Using a large sample of spectroscopically confirmed z~ 3 galaxies, we establish an empirical relationship between reddening (E(B- V)), neutral gas covering fraction (f sub(cov)(H I)), and the escape ...of ionizing (Lyman continuum, LyC) photons. Our sample includes 933 galaxies at z~ 3, 121 of which have deep spectroscopic observations (> ~7 hr) at 850 < ~ lambda sub(rest)< ~ 1300 A with the Low Resolution Imaging Spectrograph on Keck. The high covering fraction of outflowing optically thick H i indicated by the composite spectra of these galaxies implies that photoelectric absorption, rather than dust attenuation, dominates the depletion of LyC photons. By modeling the composite spectra as the combination of an unattenuated stellar spectrum including nebular continuum emission with one that is absorbed by H 1 and reddened by a line-of-sight extinction, we derive an empirical relationship between E(B- V) and f sub(cov)(H 1). Galaxies with redder UV continua have larger covering fractions of H 1 characterized by higher line-of-sight extinctions. We develop a model which connects the ionizing escape fraction with E(B- V), and which may be used to estimate the ionizing escape fraction for an ensemble of galaxies. Alternatively, direct measurements of the escape fraction for our sample allow us to constrain the intrinsic LyC-to-UV flux density ratio to be left angle bracket S(900 A)/S(1500 A)right angle bracket sub(int)> ~ 0.20, a value that favors stellar population models that include weaker stellar winds, a flatter initial mass function, and/or binary evolution. Last, we demonstrate how the framework discussed here may be used to assess the pathways by which ionizing radiation escapes from high-redshift 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.
ABSTRACT
We present a new semi-analytical formalism for modelling metal absorption lines that emerge from a clumpy galactic environment, ALPACA. We predict the “down-the-barrel” (DTB) metal ...absorption line profiles and the equivalent width (EW) of absorption at different impact parameters (b) as a function of the clump properties, including clump kinematics, clump volume filling factor, clump number density profile, and clump ion column densities. With ALPACA, we jointly model the stacked DTB C ii λ1334 spectrum of a sample of z ∼ 3 Lyman break galaxies and the EW versus b profile of a sample of z ∼ 2 star-forming galaxy–galaxy pairs. ALPACA successfully reproduced two data sets simultaneously, and the best fit prefers a low clump volume filling factor (∼3 × 10−3). The radial velocities of the clumps are a superposition of a rapidly accelerated outflow with a maximum velocity of $\sim 400 \, {\mathrm{km}\, \mathrm{s}^{-1}}$ and a velocity dispersion of $\sigma \sim 120 \, {\mathrm{km}\, \mathrm{s}^{-1}}$. The joint modelling reveals a physical scenario where the absorption observed at a particular velocity is contributed by the clumps distributed over a fairly broad range of radii. We also find that the commonly adopted Sobolev approximation is at best only applicable within a narrow range of radii where the clumps are undergoing rapid acceleration in a non-volume-filling clumpy medium. Lastly, we find that the clump radial velocity profile may not be fully constrained by the joint modelling and spatially resolved Ly α emission modelling may help break the degeneracy.
We use a newly assembled sample of 3545 star-forming galaxies with secure spectroscopic, grism, and photometric redshifts at z = 1.5-2.5 to constrain the relationship between UV slope (β) and dust ...attenuation (LIR/LUV IRX). Our sample significantly extends the range of LUV and β probed in previous UV-selected samples, including those as faint as M1600 = −17.4 ( ) and −2.6 β 0.0. IRX is measured using stacks of deep Herschel data, and the results are compared with predictions of the IRX−β relation for different assumptions of the stellar population model and obscuration curve. We find that z = 1.5-2.5 galaxies have an IRX−β relation that is consistent with the predictions for an SMC curve if we invoke subsolar-metallicity models currently favored for high-redshift galaxies, while the commonly assumed starburst curve overpredicts the IRX at a given β by a factor of 3. IRX is roughly constant with LUV for LUV 3 × 109 L . Thus, the commonly observed trend of fainter galaxies having bluer β may simply reflect bluer intrinsic slopes for such galaxies, rather than lower obscurations. The IRX−β relation for young/low-mass galaxies at z 2 implies a dust curve that is steeper than the SMC. The lower attenuations and higher ionizing photon output for low-metallicity stellar populations point to Lyman continuum production efficiencies, ion, that may be elevated by a factor of 2 relative to the canonical value for L* galaxies, aiding in their ability to keep the universe ionized at z ∼ 2.
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