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.
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 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.
We use extensive spectroscopy from the MOSFIRE Deep Evolution Field survey to investigate the relationships between rest-frame optical emission line equivalent widths (W) and a number of galaxy and ...interstellar medium (ISM) characteristics for a sample of 1134 star-forming galaxies at redshifts 1.4 z 3.8. We examine how the equivalent widths of , , λλ4960, 5008, + Hβ, , and , depend on stellar mass, UV slope, age, star formation rate (SFR) and specific SFR (sSFR), ionization parameter and excitation conditions (O32 and /Hβ), gas-phase metallicity, and ionizing photon production efficiency ( ion). The trend of increasing W with decreasing stellar mass is strongest for (and +Hβ). More generally, the equivalent widths of all the lines increase with redshift at a fixed stellar mass or fixed gas-phase metallicity, suggesting that high equivalent width galaxies are common at high redshift. This redshift evolution in equivalent widths can be explained by the increase in SFR and decrease in metallicity with redshift at a fixed stellar mass. Consequently, the dependence of W on sSFR is largely invariant with redshift, particularly when examined for galaxies of a given metallicity. Our results show that high equivalent width galaxies, specifically those with high , have low stellar masses, blue UV slopes, young ages, high sSFRs, ISM line ratios indicative of high ionization parameters, high ion, and low metallicities. As these characteristics are often attributed to galaxies with high ionizing escape fractions, galaxies with high W are likely candidates for the population that dominates cosmic reionization.
ABSTRACT
We present a robust measurement of the rest-frame UV luminosity function (LF) and its evolution during the peak epoch of cosmic star formation at
. We use our deep near-ultraviolet imaging ...from WFC3/UVIS on the
Hubble Space Telescope
and existing Advanced Camera for Surveys (ACS)/WFC and WFC3/IR imaging of three lensing galaxy clusters, Abell 2744 and MACS J0717 from the Hubble Frontier Field survey and Abell 1689. Combining deep UV imaging and high magnification from strong gravitational lensing, we use photometric redshifts to identify 780 ultra-faint galaxies with
AB mag at
. From these samples, we identified five new, faint, multiply imaged systems in A1689. We run a Monte Carlo simulation to estimate the completeness correction and effective volume for each cluster using the latest published lensing models. We compute the rest-frame UV LF and find the best-fit faint-end slopes of
,
, and
at
,
, and
, respectively. Our results demonstrate that the UV LF becomes steeper from
to
with no sign of a turnover down to
AB mag. We further derive the UV LFs using the Lyman break “dropout” selection and confirm the robustness of our conclusions against different selection methodologies. Because the sample sizes are so large and extend to such faint luminosities, the statistical uncertainties are quite small, and systematic uncertainties (due to the assumed size distribution, for example) likely dominate. If we restrict our analysis to galaxies and volumes above
completeness in order to minimize these systematics, we still find that the faint-end slope is steep and getting steeper with redshift, though with slightly shallower (less negative) values (
, −1.69 ± 0.07, and −1.79 ± 0.08 for
, 1.9, and 2.6, respectively). Finally, we conclude that the faint star-forming galaxies with UV magnitudes of
covered in this study produce the majority (55%–60%) of the unobscured UV luminosity density at
.
Abstract
We measure the ionizing photon production efficiency (
ξ
ion
) of low-mass galaxies (10
7.8
–10
9.8
M
⊙
) at 1.4 <
z
< 2.7 to better understand the contribution of dwarf galaxies to the ...ionizing background and reionization. We target galaxies that are magnified by strong-lensing galaxy clusters and use Keck/MOSFIRE to measure nebular emission-line fluxes and Hubble Space Telescope to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log of H
α
-to-UV luminosity ratios (
L
H
α
/
L
UV
) of galaxies to determine the standard log(
ξ
ion
). Second, we take the logarithm of the total
L
H
α
over the total
L
UV
. We prefer the latter, as it provides the total ionizing UV luminosity density of galaxies when multiplied by the nonionizing UV luminosity density. log(
ξ
ion
) calculated from the second method is ∼0.2 dex higher than the first method. We do not find any strong dependence between log(
ξ
ion
) and stellar mass, far-UV magnitude (
M
UV
), or UV spectral slope (
β
). We report a value of log(
ξ
ion
) ∼ 25.47 ± 0.09 for our UV-complete sample (
) and ∼25.37 ± 0.11 for our mass-complete sample (7.8 < log(
M
*
) < 9.8). These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log(
ξ
ion
) is 0.2–0.3 dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity or age. We also find a correlation between log(
ξ
ion
) and the equivalent widths of H
α
and O
iii
λ
5007 fluxes, confirming that these equivalent widths can be used to estimate
ξ
ion
.
Using data from the MOSFIRE Deep Evolution Field (MOSDEF) survey, we present a census of active galactic nucleus (AGN)-driven ionized outflows in a sample of 159 AGNs at 1.4 ≤ z ≤ 3.8. The sample ...spans AGN bolometric luminosities of 1044-47 erg s−1 and includes both quiescent and star-forming galaxies extending across 3 orders of magnitude in stellar mass. We identify and characterize outflows from the Hβ, O iii, H , and N ii emission line spectra. We detect outflows in 17% of the AGNs, seven times more often than in a mass-matched sample of inactive galaxies in MOSDEF. The outflows are fast and galaxy-wide, with velocities of ∼400-3500 km s and spatial extents of 0.3-11.0 kpc. The incidence of outflows among AGNs is independent of the stellar mass of the host galaxy, with outflows detected in both star-forming and quiescent galaxies. This suggests that outflows exist across different phases in galaxy evolution. We investigate relations between outflow kinematic, spatial, and energetic properties and both AGN and host galaxy properties. Our results show that AGN-driven outflows are widespread in galaxies along the star-forming main sequence. The mass-loading factors of the outflows are typically 0.1-1 and increase with AGN luminosity, capable of exceeding unity at . In these more luminous sources, the ionized outflow alone is likely sufficient to regulate star formation and, when combined with outflowing neutral and molecular gas, may be able to quench star formation in their host galaxies.
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.
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