ABSTRACT We discuss spatially resolved emission line spectroscopy secured for a total sample of 15 gravitationally lensed star-forming galaxies at a mean redshift of based on Keck laser-assisted ...adaptive optics observations undertaken with the recently improved OSIRIS integral field unit (IFU) spectrograph. By exploiting gravitationally lensed sources drawn primarily from the CASSOWARY survey, we sample these sub-L galaxies with source-plane resolutions of a few hundred parsecs ensuring well-sampled 2D velocity data and resolved variations in the gas-phase metallicity. Such high spatial resolution data offer a critical check on the structural properties of larger samples derived with coarser sampling using multiple-IFU instruments. We demonstrate how kinematic complexities essential to understanding the maturity of an early star-forming galaxy can often only be revealed with better sampled data. Although we include four sources from our earlier work, the present study provides a more representative sample unbiased with respect to emission line strength. Contrary to earlier suggestions, our data indicate a more diverse range of kinematic and metal gradient behavior inconsistent with a simple picture of well-ordered rotation developing concurrently with established steep metal gradients in all but merging systems. Comparing our observations with the predictions of hydrodynamical simulations suggests that gas and metals have been mixed by outflows or other strong feedback processes, flattening the metal gradients in early star-forming galaxies.
Abstract
Nearby dwarf galaxies provide a unique laboratory in which to test stellar population models below Z⊙/2. Such tests are particularly important for interpreting the surprising high-ionization ...ultraviolet (UV) line emission detected at z > 6 in recent years. We present HST/COS UV spectra of 10 nearby metal-poor star-forming galaxies selected to show He ii emission in SDSS optical spectra. The targets span nearly a dex in gas-phase oxygen abundance (7.8 < 12 + log O/H < 8.5) and present uniformly large specific star formation rates (sSFR ∼102 Gyr−1). The UV spectra confirm that metal-poor stellar populations can power extreme nebular emission in high-ionization UV lines, reaching C iii equivalent widths comparable to those seen in systems at z ∼ 6–7. Our data reveal a marked transition in UV spectral properties with decreasing metallicity, with systems below 12 + log O/H ≲ 8.0 (Z/Z⊙ ≲ 1/5) presenting minimal stellar wind features and prominent nebular emission in He ii and C iv. This is consistent with nearly an order of magnitude increase in ionizing photon production beyond the He+-ionizing edge relative to H-ionizing flux as metallicity decreases below a fifth solar, well in excess of standard stellar population synthesis predictions. Our results suggest that often-neglected sources of energetic radiation such as stripped binary products and very massive O-stars produce a sharper change in the ionizing spectrum with decreasing metallicity than expected. Consequently, nebular emission in C iv and He ii powered by these stars may provide useful metallicity constraints in the reionization era.
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.
Abstract
Detections and non-detections of Lyman alpha (Lyα) emission from z > 6 galaxies (<1 Gyr after the big bang) can be used to measure the timeline of cosmic reionization. Of key interest to ...measuring reionization’s mid-stages, but also increasing observational challenge, are observations at z > 7, where Lyα redshifts to near infra-red wavelengths. Here we present a search for z > 7.2 Lyα emission in 53 intrinsically faint Lyman Break Galaxy candidates, gravitationally lensed by massive galaxy clusters, in the KMOS Lens-Amplified Spectroscopic Survey (KLASS). With integration times of ∼7–10 h, we detect no Lyα emission with signal-to-noise ratio (S/N) > 5 in our sample. We determine our observations to be 80 per cent complete for 5σ spatially and spectrally unresolved emission lines with integrated line flux >5.7 × 10−18 erg s−1 cm−2. We define a photometrically selected sub-sample of 29 targets at z = 7.9 ± 0.6, with a median 5σ Lyα EW limit of 58 Å. We perform a Bayesian inference of the average intergalactic medium (IGM) neutral hydrogen fraction using their spectra. Our inference accounts for the wavelength sensitivity and incomplete redshift coverage of our observations, and the photometric redshift probability distribution of each target. These observations, combined with samples from the literature, enable us to place a lower limit on the average IGM neutral hydrogen fraction of $\gt 0.76 \,\, (68{{\ \rm per\ cent}}), \,\, \gt 0.46 \,\, (95{{\ \rm per\ cent}})$ at z ∼ 8, providing further evidence of rapid reionization at z ∼ 6–8. We show that this is consistent with reionization history models extending the galaxy luminosity function to $M_ \rm {\small UV}\lesssim -12$, with low ionizing photon escape fractions, $f_\textrm{esc} \lesssim 15{{\ \rm per\ cent}}$.
ABSTRACT The fraction of ionizing photons escaping from high-redshift star-forming galaxies is a key obstacle in evaluating whether galaxies were the primary agents of cosmic reionization. We ...previously proposed using the covering fraction of low-ionization gas, measured via deep absorption-line spectroscopy, as a proxy. We now present a significant update, sampling seven gravitationally lensed sources at 4 < z < 5. We show that the absorbing gas in our sources is spatially inhomogeneous, with a median covering fraction of 66%. Correcting for reddening according to a dust-in-cloud model, this implies an estimated absolute escape fraction of 19% 6%. With possible biases and uncertainties, collectively we find that the average escape fraction could be reduced to no less than 11%, excluding the effect of spatial variations. For one of our lensed sources, we have sufficient signal-to-noise ratio to demonstrate the presence of such spatial variations and scatter in its dependence on the Ly equivalent width, consistent with recent simulations. If this source is typical, our lower limit to the escape fraction could be reduced by a further factor 2. Across our sample, we find a modest anticorrelation between the inferred escape fraction and the local star formation rate, consistent with a time delay between a burst and leaking Lyman continuum photons. Our analysis demonstrates considerable variations in the escape fraction, consistent with being governed by the small-scale behavior of star-forming regions, whose activities fluctuate over short timescales. This supports the suggestion that the escape fraction may increase toward the reionization era when star formation becomes more energetic and burst-like.
The fraction of ionizing photons that escape from young star-forming galaxies is one of the largest uncertainties in determining the role of galaxies in cosmic reionization. Yet traditional ...techniques for measuring this fraction are inapplicable at the redshifts of interest due to foreground screening by the Ly alpha forest. In an earlier study, we demonstrated a reduction in the equivalent width of low-ionization absorption lines in composite spectra of Lyman break galaxies at z Asymptotically = to 4 compared to similar measures at z Asymptotically = to 3. This might imply a lower covering fraction of neutral gas and hence an increase with redshift in the escape fraction of ionizing photons. However, our spectral resolution was inadequate to differentiate between several alternative explanations, including changes with redshift in the outflow kinematics. Here we present higher quality spectra of three gravitationally lensed Lyman break galaxies at z Asymptotically = to 4 with a spectral resolution sufficient to break this degeneracy of interpretation. We present a method for deriving the covering fraction of low-ionization gas as a function of outflow velocity and compare the results with similar quality data taken for galaxies at lower redshift. We find an interesting but tentative trend of lower covering fractions of low-ionization gas for galaxies with strong Ly alpha emission. In combination with the demographic trends of Ly alpha emission with redshift from our earlier work, our results provide new evidence for a reduction in the average HI covering fraction, and hence an increase in the escape fraction of ionizing radiation from Lyman break galaxies, with redshift.
We report the first measurements with sub-kiloparsec spatial resolution of strongly inverted gas-phase metallicity gradients in two dwarf galaxies at z ∼ 2. The galaxies have stellar masses ∼109 , ...specific star formation rate ∼20 Gyr−1, and global metallicity (1/4 solar), assuming the strong-line calibrations of O iii/Hβ and O ii/Hβ from Maiolino et al. Their radial metallicity gradients are measured to be highly inverted, i.e., 0.122 0.008 and 0.111 0.017 dex kpc−1, which is hitherto unseen at such small masses in similar redshift ranges. From the Hubble Space Telescope observations of the source nebular emission and stellar continuum, we present two-dimensional spatial maps of star formation rate surface density, stellar population age, and gas fraction, which show that our galaxies are currently undergoing rapid mass assembly via disk inside-out growth. More importantly, using a simple chemical evolution model, we find that the gas fractions for different metallicity regions cannot be explained by pure gas accretion. Our spatially resolved analysis based on a more advanced gas regulator model results in a spatial map of net gaseous outflows, triggered by active central starbursts, that potentially play a significant role in shaping the spatial distribution of metallicity by effectively transporting stellar nucleosynthesis yields outwards. The relation between wind mass loading factors and stellar surface densities measured in different regions of our galaxies shows that a single type of wind mechanism, driven by either energy or momentum conservation, cannot explain the entire galaxy. These sources present a unique constraint on the effects of gas flows on the early phase of disk growth from the perspective of spatially resolved chemical evolution within individual systems.
ABSTRACT
Despite significant progress both observationally and theoretically, the origin of high-ionization nebular He ii emission in galaxies dominated by stellar photoionization remains unclear. ...Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing He+-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input spectral energy distributions containing the joint contribution of both stellar atmospheres and a multicolour disc model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power He ii, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of 1042 erg s−1 per unit star formation rate (SFR). To test this, we assemble a sample of 11 low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and He ii emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed He ii strengths, with typical X-ray luminosities or upper limits thereon of only 1040–1041 erg s−1 per SFR. This indicates that HMXBs are not the dominant source of He+ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.
Galactic-scale outflows regulate the stellar mass growth and chemical enrichment of galaxies, yet key outflow properties such as the chemical composition and mass-loss rate remain largely unknown. We ...address these properties with Keck/ESI echellete spectra of nine gravitationally lensed z 2-3 star-forming galaxies, probing a range of absorption transitions. Interstellar absorption in our sample is dominated by outflowing material with typical velocities of ∼−150 . Approximately 80% of the total column density is associated with a net outflow. Mass-loss rates in the low-ionization phase are comparable to or in excess of the star formation rate, with total outflow rates likely higher when accounting for ionized gas. On the order of half of the heavy element yield from star formation is ejected in the low-ionization phase, confirming that outflows play a critical role in regulating galaxy chemical evolution. Covering fractions vary and are in general non-uniform, with most galaxies having incomplete covering by the low ions across all velocities. Low-ion abundance patterns show remarkably little scatter, revealing a distinct "chemical fingerprint" of outflows. Gas-phase Si/Fe abundances are significantly supersolar (Si/Fe 0.4), indicating a combination of -enhancement and dust depletion. The derived properties are comparable to the most kinematically broad, metal-rich, and depleted intergalactic absorption systems at similar redshifts, suggesting that these extreme systems are associated with galactic outflows at impact parameters conservatively within a few tens of kiloparsecs. We discuss implications of the abundance patterns in z 2-3 galaxies and the role of outflows at this epoch.
In 1964, Refsdal hypothesized that a supernova whose light traversed multiple paths around a strong gravitational lens could be used to measure the rate of cosmic expansion. We report the discovery ...of such a system. In Hubble Space Telescope imaging, we have found four images of a single supernova forming an Einstein cross configuration around a redshift z = 0.54 elliptical galaxy in the MACS J1149.6+2223 cluster. The cluster's gravitational potential also creates multiple images of the z = 1.49 spiral supernova host galaxy, and a future appearance of the supernova elsewhere in the cluster field is expected. The magnifications and staggered arrivals of the supernova images probe the cosmic expansion rate, as well as the distribution of matter in the galaxy and cluster lenses.
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