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.
Galaxy metallicity scaling relations provide a powerful tool for understanding galaxy evolution, but obtaining unbiased global galaxy gas-phase oxygen abundances requires proper treatment of the ...various line-emitting sources within spectroscopic apertures. We present a model framework that treats galaxies as ensembles of H ii and diffuse ionized gas (DIG) regions of varying metallicities. These models are based upon empirical relations between line ratios and electron temperature for H ii regions, and DIG strong-line ratio relations from SDSS-IV MaNGA IFU data. Flux-weighting effects and DIG contamination can significantly affect properties inferred from global galaxy spectra, biasing metallicity estimates by more than 0.3 dex in some cases. We use observationally motivated inputs to construct a model matched to typical local star-forming galaxies, and quantify the biases in strong-line ratios, electron temperatures, and direct-method metallicities as inferred from global galaxy spectra relative to the median values of the H ii region distributions in each galaxy. We also provide a generalized set of models that can be applied to individual galaxies or galaxy samples in atypical regions of parameter space. We use these models to correct for the effects of flux-weighting and DIG contamination in the local direct-method mass-metallicity and fundamental metallicity relations, and in the mass-metallicity relation based on strong-line metallicities. Future photoionization models of galaxy line emission need to include DIG emission and represent galaxies as ensembles of emitting regions with varying metallicity, instead of as single H ii regions with effective properties, in order to obtain unbiased estimates of key underlying physical properties.
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.
Abstract
We investigate the evolution of galaxy gas-phase metallicity (O/H) over the range
z
= 0–3.3 using samples of ∼300 galaxies at
z
∼ 2.3 and ∼150 galaxies at
z
∼ 3.3 from the MOSDEF survey. ...This analysis crucially utilizes different metallicity calibrations at
z
∼ 0 and
z
> 1 to account for evolving interstellar medium (ISM) conditions. We find significant correlations between O/H and stellar mass (
M
*
) at
z
∼ 2.3 and
z
∼ 3.3. The low-mass power-law slope of the mass–metallicity relation (MZR) is remarkably invariant over
z
= 0–3.3, such that O/H ∝
at all redshifts in this range. At fixed
M
*
, O/H decreases with increasing redshift as
d
log(O/H)/
dz
= −0.11 ± 0.02. We find no evidence that the fundamental metallicity relation between
M
*
, O/H, and star formation rate evolves out to
z
∼ 3.3. We employ analytic chemical evolution models to place constraints on the mass and metal loading factors of galactic outflows. The efficiency of metal removal increases toward lower
M
*
at fixed redshift and toward higher redshift at fixed
M
*
. These models suggest that the slope of the MZR is primarily set by the scaling of the outflow metal loading factor with
M
*
, not by the change in gas fraction as a function of
M
*
. The evolution toward lower O/H at fixed
M
*
with increasing redshift is driven by both higher gas fraction (leading to stronger dilution of ISM metals) and higher metal removal efficiency. These results suggest that the processes governing the smooth baryonic growth of galaxies via gas flows and star formation hold in the same form over at least the past 12 Gyr.
The epoch of galaxy assembly from 2
z
4 marks a critical stage during the evolution of today's galaxy population. During this period, the star-formation activity in the Universe was at its peak ...level, and the structural patterns observed among galaxies in the local Universe were not yet in place. A variety of novel techniques have been employed over the past decade to assemble multiwavelength observations of galaxies during this important epoch. In this primarily observational review, I present a census of the methods used to find distant galaxies and the empirical constraints on their multiwavelength luminosities and colors. I then discuss what is known about the stellar content and past histories of star formation in high-redshift galaxies; their interstellar contents including dust, gas, and heavy elements; and their structural and dynamical properties. I conclude by considering some of the most pressing and open questions regarding the physics of high-redshift galaxies, which are to be addressed with future facilities.
Using observations from the MOSFIRE Deep Evolution Field survey, we investigate the physical conditions of star-forming regions in z ~ 2.3 galaxies, specifically the electron density and ionization ...state. From measurements of the OIIlambdalambda3726,3729 and SIIlambdalambda6716,6731 doublets, we find a median electron density of ~250 cm super(-3) at z ~ 2.3, an increase of an order of magnitude compared to measurements of galaxies at z ~ 0. While z ~ 2.3 galaxies are offset toward significantly higher O sub(32) values relative to local galaxies at fixed stellar mass, we find that the high-redshift sample follows a similar distribution to the low-metallicity tail of the local distribution in the O sub(32) versus R sub(23) and O3N2 diagrams. Based on these results, we propose that z ~ 2.3 star-forming galaxies have the same ionization parameter as local galaxies at fixed metallicity. In combination with simple photoionization models, the position of local and z ~ 2.3 galaxies in excitation diagrams suggests that there is no significant change in the hardness of the ionizing spectrum at fixed metallicity from z ~ 0 to z ~ 2.3. We find that z ~ 2.3 galaxies show no offset compared to low-metallicity local galaxies in emission line ratio diagrams involving only lines of hydrogen, oxygen, and sulfur, but show a systematic offset in diagrams involving NIIlambda6584. We conclude that the offset of z ~ 2.3 galaxies from the local star-forming sequence in the NII BPT diagram is primarily driven by elevated N/O at fixed O/H compared to local galaxies. These results suggest that the local gas-phase and stellar metallicity sets the ionization state of star-forming regions at z ~ 0 and z ~ 2.
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 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.
ABSTRACT
We present constraints on the massive star and ionized gas properties for a sample of 62 star-forming galaxies at z ∼ 2.3. Using BPASS stellar population models, we fit the rest-UV spectra ...of galaxies in our sample to estimate age and stellar metallicity which, in turn, determine the ionizing spectrum. In addition to the median properties of well-defined subsets of our sample, we derive the ages and stellar metallicities for 30 high-SNR individual galaxies – the largest sample of individual galaxies at high redshift with such measurements. Most galaxies in this high-SNR subsample have stellar metallicities of 0.001 < Z* < 0.004. We then use Cloudy + BPASS photoionization models to match observed rest-optical line ratios and infer nebular properties. Our high-SNR subsample is characterized by a median ionization parameter and oxygen abundance, respectively, of log (U)med = −2.98 ± 0.25 and 12 + log (O/H)med = 8.48 ± 0.11. Accordingly, we find that all galaxies in our sample show evidence for α-enhancement. In addition, based on inferred log (U) and 12 + log (O/H) values, we find that the local relationship between ionization parameter and metallicity applies at z ∼ 2. Finally, we find that the high-redshift galaxies most offset from the local excitation sequence in the BPT diagram are the most α-enhanced. This trend suggests that α-enhancement resulting in a harder ionizing spectrum at fixed oxygen abundance is a significant driver of the high-redshift galaxy offset on the BPT diagram relative to local systems. The ubiquity of α-enhancement among z ∼ 2.3 star-forming galaxies indicates important differences between high-redshift and local galaxies that must be accounted for in order to derive physical properties at high redshift.
Abstract
We investigate the nature of the relation among stellar mass, star formation rate, and gas-phase metallicity (the
–SFR–
Z
relation) at high redshifts using a sample of 260 star-forming ...galaxies at
z
∼ 2.3 from the MOSDEF survey. We present an analysis of the high-redshift
–SFR–
Z
relation based on several emission-line ratios for the first time. We show that a
–SFR–
Z
relation clearly exists at
z
∼ 2.3. The strength of this relation is similar to predictions from cosmological hydrodynamical simulations. By performing a direct comparison of stacks of
z
∼ 0 and
z
∼ 2.3 galaxies, we find that
z
∼ 2.3 galaxies have ∼0.1 dex lower metallicity at fixed
and SFR. In the context of chemical evolution models, this evolution of the
–SFR–
Z
relation suggests an increase with redshift of the mass-loading factor at fixed
, as well as a decrease in the metallicity of infalling gas that is likely due to a lower importance of gas recycling relative to accretion from the intergalactic medium at high redshifts. Performing this analysis simultaneously with multiple metallicity-sensitive line ratios allows us to rule out the evolution in physical conditions (e.g., N/O ratio, ionization parameter, and hardness of the ionizing spectrum) at fixed metallicity as the source of the observed trends with redshift and with SFR at fixed
at
z
∼ 2.3. While this study highlights the promise of performing high-order tests of chemical evolution models at high redshifts, detailed quantitative comparisons ultimately await a full understanding of the evolution of metallicity calibrations with redshift.
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