Although Lyman-continuum (LyC) radiation from star-forming galaxies likely drove the re-ionization of the universe, observations of star-forming galaxies at low redshift generally indicate low LyC ...escape fractions. To analyze the LyC optical depths and ionizing sources of these rare, compact star-bursts, we compare nebular photo-ionization and stellar population models with observed emission lines in the Peas' Sloan Digital Sky Survey (SDSS) spectra. We focus on the six most extreme Green Peas, the galaxies with the highest OIII/OII ratios and the best candidates for escaping ionizing radiation. Although stacked spectra reveal no Wolf-Rayet (WR) features, we tentatively detect WR features in the SDSS spectra of three extreme Peas. The Peas' ages likely optimize the escape of LyC radiation; they are old enough for supernovae and stellar winds to reshape the interstellar medium, but young enough to possess large numbers of UV-luminous O or WR stars.
The aim of this paper is to explore and map the age and abundance structure of the stars in the nearby Galactic disk. We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and ...subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes, the metal-rich stellar halo, sub-structures in velocity space such as the Hercules stream and the Arcturus moving group, as well as stars that cannot (kinematically) be associated with either the thin disk or the thick disk. We present stellar parameters, stellar ages, kinematical parameters, orbital parameters, and detailed elemental abundances for O, Na, Mg, Al, Si, C a, Ti, C r, Fe, Ni, Zn, Y, and Ba for 714 nearby F and G dwarf stars. Our data show that there is an old and alpha -enhanced disk population, and a younger and less alpha -enhanced disk population. As the exact cause for this effect is unclear we chose to apply an empirical correction. Turn-off stars and more evolved stars appear to be unaffected.
ABSTRACT We report on the detection of Lyman continuum radiation in two nearby starburst galaxies. Tol 0440-381, Tol 1247-232, and Mrk 54 were observed with the Cosmic Origins Spectrograph on board ...the Hubble Space Telescope. The three galaxies have radial velocities of ∼13,000 km s−1, permitting a ∼35 window on the restframe Lyman continuum shortward of the Milky Way Lyman edge at 912 . The chosen instrument configuration using the G140L grating covers the spectral range from 912 to 2000 . We developed a dedicated background subtraction method to account for the temporal and spatial background variations of the detector, which is crucial at the low flux levels around 912 . This modified pipeline allowed us to significantly improve the statistical and systematic detector noise and will be made available to the community. We detect Lyman continuum in all three galaxies. However, we conservatively interpret the emission in Tol 0440-381 as an upper limit due to possible contamination by geocoronal Lyman series lines. We determined the current star formation properties from the far-ultraviolet continuum and spectral lines and used synthesis models to predict the Lyman continuum radiation emitted by the current population of hot stars. We discuss various model uncertainties such as, among others, atmospheres and evolution models. Lyman continuum escape fractions were derived from a comparison between the observed and predicted Lyman continuum fluxes. Tol 1247-232, Mrk 54, and Tol 0440-381 have absolute escape fractions of (4.5 1.2)%, (2.5 0.72)%, and <(7.1 1.1)%, respectively.
Abstract
As some of the only Lyman continuum (LyC) emitters at
z
∼ 0, Green Pea (GP) galaxies are possible analogs of the sources that reionized the universe. We present
HST
COS spectra of 13 of the ...most highly ionized GPs, with O
iii
/O
ii
= 6–35, and investigate correlations between Ly
α
, galaxy properties, and low-ionization UV lines. Galaxies with high O
iii
/O
ii
have higher H
α
equivalent widths (EWs), and high intrinsic Ly
α
production may explain the prevalence of high Ly
α
EWs among GPs. While the Ly
α
escape fraction is closely linked to low gas covering fractions, implying a clumpy gas geometry, narrow Ly
α
velocity peak separation (
) correlates with the ionization state, suggesting a density-bounded geometry. We therefore suggest that
may trace the residual transparency of low-column-density pathways. Metallicity is associated with both O
iii
/O
ii
and
. This trend may result from catastrophic cooling around low-metallicity star clusters, which generates a compact geometry of dense clouds within a low-density inter-clump medium. We find that the relative strength of low-ionization UV emission to absorption correlates with Ly
α
emission strength and is related to Ly
α
profile shape. However, as expected for optically thin objects, the GPs with the lowest
show both weak low-ionization emission and weak absorption. The strengths of the low-ionization absorption and emission lines in a stacked spectrum do not correspond to any individual spectrum. Galaxies with high O
iii
/O
ii
contain a high fraction of LyC emitter candidates, but O
iii
/O
ii
alone is an insufficient diagnostic of LyC escape.
Abstract
By clearing neutral gas away from a young starburst, superwinds may regulate the escape of Lyman continuum (LyC) photons from star-forming galaxies. However, models predict that superwinds ...may not launch in the most extreme, compact starbursts. We explore the role of outflows in generating low optical depths in the Green Peas (GPs), the only known star-forming population with several confirmed and candidate LyC-leaking galaxies. With
Hubble Space Telescope
UV spectra of 25 low-redshift GPs, including new observations of 13 of the most highly ionized GPs, we compare the kinematics of UV absorption lines with indirect H
i
optical depth diagnostics: Ly
α
escape fraction, Ly
α
peak separation, or low-ionization absorption line equivalent width. The data suggest that high-ionization kinematics tracing superwind activity may correlate with low optical depth in some objects. However, the most extreme GPs, including many of the best candidate LyC emitters with weak low-ionization absorption and strong, narrow Ly
α
profiles, show the
lowest
velocities. These results are consistent with models for suppressed superwinds, which suggests that outflows may not be the only cause of LyC escape from galaxies.
Abstract
The classical model of massive-star mechanical feedback is based on effects at solar metallicity (
Z
⊙
), yet feedback parameters are very different at low metallicity. Metal-poor stellar ...winds are much weaker, and more massive supernova progenitors likely collapse directly to black holes without exploding. Thus, for ∼0.4
Z
⊙
we find reductions in the total integrated mechanical energy and momentum of ∼40% and 75%, respectively, compared to values classically expected at solar metallicity. But in particular, these changes effectively delay the onset of mechanical feedback until ages of ∼10 Myr. Feedback from high-mass X-ray binaries could slightly increase mechanical luminosity between ages 5 and 10 Myr, but it is stochastic and unlikely to be significant on this timescale. Stellar dynamical mechanisms remove most massive stars from clusters well before 10 Myr, which would further promote this effect; this process is exacerbated by gas retention implied by weak feedback. Delayed mechanical feedback implies that radiation feedback therefore dominates at early ages, which is consistent with the observed absence of superwinds in some extreme starbursts. This scenario may lead to higher star formation efficiencies, multiple stellar populations in clusters, and higher Lyman continuum escape. This could explain the giant star-forming complexes in metal-poor galaxies and the small sizes of OB superbubble shells relative to their inferred ages. It could also drive modest effects on galactic chemical evolution, including on oxygen abundances. Thus, delayed low-metallicity mechanical feedback may have broad implications, including for early cosmic epochs.
Abstract
The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization ...is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a
z
= 0.2–0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.
ABSTRACT
Most of the hydrogen in the intergalactic medium (IGM) was rapidly ionized at high redshifts. While observations have established that reionization occurred, observational constraints on the ...high-redshift ionizing emissivity remain elusive. Here, we present a new analysis of the Low-redshift Lyman Continuum Survey (LzLCS) and literature observations, a combined sample of 89 star-forming galaxies at redshifts near 0.3 with Hubble Space Telescope observations of their ionizing continua (or Lyman Continuum, LyC). We find a strong (6σ significant) inverse correlation between the continuum slope at 1550 Å (defined as F$_\lambda \propto \lambda ^{\beta ^{1550}_{\rm obs}}$) and both the LyC escape fraction (fesc, LyC) and fesc, LyC times the ionizing photon production efficiency (ξion). On average, galaxies with redder continuum slopes have smaller fesc, LyC than galaxies with bluer slopes mainly due to higher dust attenuation. More than 5 per cent (20 per cent) of the LyC emission escapes galaxies with $\beta _{\rm obs}^{1550}$ <−2.1 (−2.6). We find strong correlations between $\beta _{\rm obs}^{1550}$ and the O iii/O ii flux ratio (at 7.5σ significance), galaxy stellar mass (at 5.9σ), the gas-phase metallicity (at 4.6σ), and the observed far-ultraviolet absolute magnitude (at 3.4σ). Using previous observations of $\beta _{\rm obs}^{1550}$ at high redshift, we estimate the evolution of fesc, LyC with both redshift and galaxy magnitude. The LzLCS observations suggest that fainter and lower mass galaxies dominate the ionizing photon budget at higher redshift, possibly due to their rapidly evolving metal and dust content. Finally, we use our correlation between $\beta _{\rm obs}^{1550}$ and fesc, LyC × ξion to predict the ionizing emissivity of galaxies during the epoch of reionization. Our estimated emissivities match IGM observations, and suggest that star-forming galaxies emit sufficient LyC photons into the IGM to exceed recombinations near redshifts of 7–8.
Outflows are a pervasive feature of mechanical feedback from super star clusters (SSCs) in starburst galaxies, playing a fundamental role in galaxy evolution. Observations are now starting to confirm ...that outflows can undergo catastrophic cooling, suppressing adiabatic superwinds. Here we present a suite of one-dimensional, hydrodynamic simulations that study the ionization structure of these outflows and the resulting line emission generated by the cooling gas. We use the non-equilibrium atomic chemistry package within MAIHEM, our modified version of FLASH, which evolves the ionization state of the gas and computes the total cooling rate on an ion-by-ion basis. We find that catastrophically cooling models produce strong nebular line emission compared to adiabatic outflows. We also show that such models exhibit non-equilibrium conditions, thereby generating more highly ionized states than equivalent equilibrium models. When including photoionization from the parent SSC, catastrophically cooling models show strong C iv λ1549 and O vi λ1037 emission. For density-bounded photoionization, He ii λ1640, λ4686, C iii λ1908, Si iv λ1206, and Si iii λ1400 are also strongly enhanced. These lines are seen in extreme starbursts where catastrophic cooling is likely to occur, suggesting that they may serve as diagnostics of such conditions. The higher ionization generated by these flows may help to explain line emission that cannot be attributed to SSC photoionization alone.
Context. The Lyman-alpha (Lyα) line of hydrogen is of prime importance for detecting galaxies at high redshift. For a correct data interpretation, numerical radiative transfer models are necessary ...due to Lyα resonant scattering off neutral hydrogen atoms. Aims. Recent observations have discovered an escape of ionizing Lyman-continuum radiation from a population of compact, actively star-forming galaxies at redshift z ~ 0.2−0.3, also known as “green peas”. For the potential similarities with high-redshift galaxies and impact on the reionization of the universe, we study the green pea Lyα spectra, which are mostly double-peaked, unlike in any other galaxy sample. If the double peaks are a result of radiative transfer, they can be a useful source of information on the green pea interstellar medium and ionizing radiation escape. Methods. We select a sample of twelve archival green peas and we apply numerical radiative transfer models to reproduce the observed Lyα spectral profiles, using the geometry of expanding, homogeneous spherical shells. We use ancillary optical and ultraviolet data to constrain the model parameters, and we evaluate the match between the models and the observed Lyα spectra. As a second step, we allow all the fitting parameters to be free, and examine the agreement between the interstellar medium parameters derived from the models and those from ancillary data. Results. The peculiar green pea double-peaked Lyα line profiles are not correctly reproduced by the constrained shell models. Conversely, unconstrained models fit the spectra, but parameters derived from the best-fitting models are not in agreement with the ancillary data. In particular: 1) the best-fit systemic redshifts are larger by 10–250 km s−1 than those derived from optical emission lines; 2) the double-peaked Lyα profiles are best reproduced with low-velocity (≲150 km s−1) outflows that contradict the observed ultraviolet absorption lines of low-ionization-state elements with characteristic velocities as large as 300 km s−1; and 3) the models need to consider intrinsic Lyα profiles that are on average three times broader than the observed Balmer lines. Conclusions. Differences between the modelled and observed velocities are larger for targets with prominent Lyα blue peaks. The blue peak position and flux appear to be connected to low column densities of neutral hydrogen, leading to Lyα and Lyman-continuum escape. This is at odds with the kinematic origin of the blue peak in the homogeneous shell models. Additional modelling is needed to explore alternative geometries such as clumpy media and non-recombination Lyα sources to further constrain the role and significance of the Lyα double peaks.
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