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.
ABSTRACT
Finding reliable indicators of Lyman continuum (Ly C) photon leakage from galaxies is essential in order to infer their escape fraction in the epoch of reionization, where direct ...measurements of Ly C flux are impossible. To this end, here we investigate whether strong C iv λλ1548, 1550 emission in the rest-frame UV spectra of galaxies traces conditions ripe for ample production and escape of Ly C photons. We compile a sample of 19 star-forming galaxies in the redshift range $z$ = 3.1–4.6 from the VANDELS survey that exhibit strong C iv emission, producing a stacked spectrum where all major rest-UV emission lines are clearly detected. Best-fitting spectral energy distribution models containing both stellar and nebular emission suggest the need for low stellar metallicities ($Z=0.1--0.2\, Z_\odot$), young stellar ages ($\log (\rm {age\,yr^{-1}}) = 6.1--6.5$), a high ionization parameter (log U = −2) and little to no dust attenuation (E(B − V) = 0.00–0.01). However, these models are unable to fully reproduce the observed C iv and He ii line strengths. We find that the Ly α line in the stacked spectrum is strong and peaks close to the systemic velocity, features that are indicative of significant Ly C photon leakage along the line of sight. The covering fractions of low-ionization interstellar absorption lines are also low, implying Ly C escape fraction in the range ≈0.05–0.30, with signatures of outflowing gas. Finally, C iv/C iii ratios of >0.75 for a subset of individual galaxies with reliable detections of both lines are also consistent with physical conditions that enable significant Ly C leakage. Overall, we report that multiple spectroscopic indicators of Ly C leakage are present in the stacked spectrum of strong C iv emitting galaxies, potentially making C iv an important tracer of Ly C photon escape at $z$ > 6.
ABSTRACT
We report the discovery of J0121+0025, an extremely luminous and young star-forming galaxy (MUV = −24.11, log$L_{\rm Ly \alpha } / \rm erg~s^{-1} = 43.8$) at z = 3.244 showing copious Lyman ...continuum (LyC) leakage ($f_{\rm esc, abs} \approx 40{{\ \rm per\ cent}}$). High signal-to-noise ratio rest-frame UV spectroscopy with the Gran Telescopio Canarias reveals a high significance (7.9σ) emission below the Lyman limit (<912 Å), with a flux density level f900 = 0.78 ± 0.10μJy, and strong P-Cygni in wind lines of O vi 1033 Å, N v 1240 Å, and C iv 1550 Å that are indicative of a young age of the starburst (<10 Myr). The spectrum is rich in stellar photospheric features, for which a significant contribution of an AGN at these wavelengths is ruled out. Low-ionization interstellar medium (ISM) absorption lines are also detected, but are weak ($EW_{0} \rm \simeq 1$ Å) and show large residual intensities, suggesting a clumpy geometry of the gas with a non-unity covering fraction or a highly ionized ISM. The contribution of a foreground and AGN contamination to the LyC signal is unlikely. Deep optical to Spitzer/IRAC 4.5 μm imaging show that the spectral energy distribution of J0121+0025 is dominated by the emission of the young starburst, with log($M_{\star }^{\rm burst}/M_{\odot }) = 9.9\pm 0.1$ and $\rm SFR = 981\pm 232$ M⊙ yr−1. J0121+0025 is the most powerful LyC emitter known among the star-forming galaxy population. The discovery of such luminous and young starburst leaking LyC radiation suggests that a significant fraction of LyC photons can escape in sources with a wide range of UV luminosities and are not restricted to the faintest ones as previously thought. These findings might shed further light on the role of luminous starbursts to the cosmic reionization.
ABSTRACT
We present a study designed to measure the average Lyman-continuum escape fraction (〈fesc〉) of star-forming galaxies at z ≃ 3.5. We assemble a sample of 148 galaxies from the VANDELS ...spectroscopic survey at 3.35 ≤ zspec ≤ 3.95, selected to minimize line-of-sight contamination of their photometry. For this sample, we use ultra-deep, ground-based, U-band imaging and Hubble Space Telescope V-band imaging to robustly measure the distribution of $\mathcal {R_{\rm obs}}\, =(L_{\rm LyC}/L_{\rm UV})_{\rm obs}$. We then model the $\mathcal {R_{\rm obs}}$ distribution as a function of 〈fesc〉, carefully accounting for attenuation by dust, the intergalactic medium and the circumgalactic medium. A maximum likelihood fit to the $\mathcal {R_{\rm obs}}$ distribution returns a best-fitting value of $\langle f_{\rm esc}\rangle =0.07^{+0.02}_{-0.02}$, a result confirmed using an alternative Bayesian inference technique (both techniques exclude 〈fesc〉 = 0.0 at >3σ). By splitting our sample in two, we find evidence that 〈fesc〉 is positively correlated with Ly α equivalent width (Wλ(Ly α)), with high and low Wλ(Lyα) subsamples returning values of $\langle f_{\rm esc}\rangle =0.12^{+0.06}_{-0.04}$ and $\langle f_{\rm esc} \rangle =0.02^{+0.02}_{-0.01}$, respectively. In contrast, we find evidence that 〈fesc〉 is anticorrelated with intrinsic UV luminosity and UV dust attenuation; with low UV luminosity and dust attenuation subsamples both returning best fits in the range 0.10 ≤ 〈fesc〉 ≤ 0.22. We do not find a clear correlation between fesc and galaxy stellar mass, suggesting stellar mass is not a primary indicator of fesc. Although larger samples are needed to further explore these trends, our results suggest that it is entirely plausible that the low dust, low-metallicity galaxies found at z ≥ 6 will display the 〈fesc〉 ≥ 0.1 required to drive reionization.
ABSTRACT
We present the discovery and analysis of J1316+2614 at z = 3.6130, a UV-bright star-forming galaxy (MUV ≃ −24.7) with large escape of Lyman continuum (LyC) radiation. J1316+2614 is a young ...(≃10 Myr) star-forming galaxy with SFR ≃ 500 M⊙ yr−1 and a starburst mass of log(M⋆/M⊙) ≃ 9.7. It shows a very steep UV continuum, βUV = −2.59 ± 0.05, consistent with residual dust obscuration, E(B − V) ≃ 0. LyC emission is detected with high significance (≃17σ) down to 830 Å, for which a very high relative (absolute) LyC escape fraction $f_{\rm esc} \rm (LyC) \simeq 0.92$ (≃0.87) is inferred. The contribution of a foreground or active galactic nucleus contamination to the LyC signal is discussed, but is unlikely. J1316+2614 is the most powerful ionizing source known among the star-forming galaxy population, both in terms of production (QH ≈ 1056 s−1) and escape of ionizing photons ($f_{\rm esc} \rm (LyC) \approx 0.9$). Nebular emission in Ly α, H β, and other rest-frame optical lines are detected, but these are weak ($EW_{0} \rm H\beta \simeq 35$ Å), with their strengths reduced roughly by $\simeq 90{{\ \rm per\ cent}}$. J1316+2614 is the first case known where the effect of large escape of ionizing photons on the strength of nebular lines and continuum emission is clearly observed. Gas inflows are detected in J1316+2614 from the blue-dominated peak Ly α emission (with a blue-to-red peak line ratio Iblue/Ired ≃ 3.7) and redshifted interstellar medium absorption (≃100 km s−1). Our results suggest that J1316+2614 is undergoing a gas compaction event, possibly representing a short-lived phase in the evolution of massive and compact galaxies, where strong gas inflows have triggered an extreme star formation episode and nearly 100 per cent LyC photons are escaping.
ABSTRACT
We present Lyman continuum (LyC) radiation escape fraction (fesc) measurements for 183 spectroscopically confirmed star-forming galaxies in the redshift range 3.11 < z < 3.53 in the Chandra ...Deep Field South. We use ground-based imaging to measure fesc, and use ground- and space-based photometry to derive galaxy physical properties using spectral energy distribution (SED) fitting. We additionally derive O iii + H β equivalent widths (that fall in the observed K band) by including nebular emission in SED fitting. After removing foreground contaminants, we report the discovery of 11 new candidate LyC leakers at ≳ 2σ level, with fesc in the range 0.14−0.85. From non-detections, we place 1σ upper limits of fesc <0.12, where the Lyman-break selected galaxies have fesc <0.11 and ‘blindly’ discovered galaxies with no prior photometric selection have fesc <0.13. We find a slightly higher 1σ limit of fesc <0.20 from extreme emission line galaxies with rest-frame O iii + H β equivalent widths >300 Å. For candidate LyC leakers, we find a weak negative correlation between fesc and galaxy stellar masses, no correlation between fesc and specific star-formation rates (sSFRs) and a positive correlation between fesc and EW0(O iii + H β). The weak/no correlations between stellar mass and sSFRs may be explained by misaligned viewing angles and/or non-coincident time-scales of starburst activity and periods of high fesc. Alternatively, escaping radiation may predominantly occur in highly localized star-forming regions, or fesc measurements may be impacted by stochasticity of the intervening neutral medium, obscuring any global trends with galaxy properties. These hypotheses have important consequences for models of reionization.
ABSTRACT
We present γ-ray optical-depth calculations from a recently published extragalactic background light (EBL) model built from multiwavelength galaxy data from the Hubble Space Telescope Cosmic ...Assembly Near-Infrared Deep Extragalactic Legacy Survey (HST/CANDELS). CANDELS gathers one of the deepest and most complete observations of stellar and dust emissions in galaxies. This model resulted in a robust derivation of the evolving EBL spectral energy distribution up to z ∼ 6, including the far-infrared peak. Therefore, the optical depths derived from this model will be useful for determining the attenuation of γ-ray photons coming from high-redshift sources, such as those detected by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope, and for multi-TeV photons that will be detected from nearby sources by the future Cherenkov Telescope Array. From these newly calculated optical depths, we derive the cosmic γ-ray horizon and also measure the expansion rate and matter content of the Universe including an assessment of the impact of the EBL uncertainties. We find H0 = 62.4 $^{+4.1}_{-3.9}$ km s−1 Mpc−1 when fixing Ωm = 0.32, and H0 = 65.1 $^{+6.0}_{-4.9}$ km s−1 Mpc−1 and Ωm = 0.19 ± 0.08, when exploring these two parameters simultaneously.
ABSTRACT
The physical properties of Epoch of Reionization (EoR) galaxies are still poorly constrained by observations. To better understand the ionizing properties of galaxies in the EoR, we ...investigate deep, rest-frame ultraviolet (UV) spectra of ≃500 star-forming galaxies at 3 ≤ z ≤ 5 selected from the public ESO-VANDELS spectroscopic survey. The absolute ionizing photon escape fraction ($f_{\rm esc}^{\rm abs}$, i.e. the ratio of leaking against produced ionizing photons) is derived by combining absorption line measurements with estimates of the UV attenuation. The ionizing production efficiency (ξion, i.e. the number of ionizing photons produced per non-ionizing UV luminosity) is calculated by fitting the far-UV (FUV) stellar continuum of the VANDELS galaxies. We find that the $f_{\rm esc}^{\rm abs}$ and ξion parameters increase towards low-mass, blue UV-continuum slopes and strong Ly α emitting galaxies, and both are slightly higher-than-average for the UV-faintest galaxies in the sample. Potential Lyman Continuum Emitters (LCEs, $f_{\rm esc}^{\rm abs} \ge 5{{\ \rm \, per\ cent}}$) and selected Lyman Alpha Emitters (LAEs, WLyα ≤ −20 Å) show systematically higher ξion (log ξion(Hz erg−1) ≈ 25.38, 25.41) than non-LCEs and non-LAEs (log ξion(Hz erg−1) ≈ 25.18, 25.14) at similar UV magnitudes. This indicates very young underlying stellar populations (≈10 Myr) at relatively low metallicities (≈0.2 Z⊙). The FUV non-ionizing spectra of potential LCEs is characterized by blue UV slopes (≤−2), enhanced Ly α emission (≤−25 Å), strong UV nebular lines (e.g. high ${\rm C\, \small {IV}}$1550/${\rm C\, \small {III}}$1908 ≥0.75 ratios), and weak absorption lines (≤1 Å). The latter suggests the existence of low gas-column-density channels in the interstellar medium, which enables the escape of ionizing photons. By comparing our VANDELS results against other surveys in the literature, our findings imply that the ionizing budget in the EoR was likely dominated by UV-faint, low-mass, and dustless galaxies.
The properties that govern the production and escape of hydrogen-ionizing photons (Lyman continuum, LyC; with energies > 13.6 eV) in star-forming galaxies are still poorly understood, but they are ...key to identifying and characterizing the sources that reionized the Universe. Here we empirically explore the relation between the hardness of ionizing radiation and the LyC leakage in a large sample of low-
z
star-forming galaxies from the recent
Hubble
Space Telescope Low-
z
Lyman Continuum Survey. Using Sloan Digital Sky Survey stacks and deep X-shooter observations, we investigate the hardness of the ionizing spectra (
Q
He
+
/
Q
H
) between 54.4 eV (He
+
) and 13.6 eV (H) from the optical recombination lines He
II
4686 Å and H
β
4861 Å for galaxies with LyC escape fractions spanning a wide range,
f
esc
(LyC) ≃ 0−90%. We find that the observed intensity of He
II
/H
β
is primarily driven by variations in the metallicity, but is not correlated with LyC leakage. Both very strong (<
f
esc
(LyC)> ≃ 0.5) and nonleakers (<
f
esc
(LyC)> ≃ 0) present similar observed intensities of He
II
and H
β
at comparable metallicity, between ≃0.01 and ≃0.02 for 12 + log(O/H)> 8.0 and < 8.0, respectively. Our results demonstrate that
Q
He
+
/
Q
H
does not correlate with
f
esc
(LyC), which implies that strong LyC emitters do not show harder ionizing spectra than nonleakers at similar metallicity.
Aims.
We investigate the production efficiency of ionizing photons (
ξ
ion
∗
) of 1174 galaxies with secure redshift at
z
= 2 − 5 from the VANDELS survey to determine the relation between ionizing ...emission and physical properties of bright and massive sources.
Methods.
We constrained
ξ
ion
∗
and galaxy physical parameters by means of spectrophotometric fits performed with the
BEAGLE
code. The analysis exploits the multi-band photometry in the VANDELS fields and the measurement of UV rest-frame emission lines (CIII
λ
1909, HeII
λ
1640, and OIII
λ
1666) from deep VIMOS spectra.
Results.
We find no clear evolution of
ξ
ion
∗
with redshift within the probed range. The ionizing efficiency slightly increases at fainter
M
UV
and bluer UV slopes, but these trends are less evident when the analysis is restricted to a complete subsample at log(
M
star
/
M
⊙
) > 9.5. We find a significant trend of increasing
ξ
ion
∗
with increasing EW(Ly
α
), with an average log(
ξ
ion
∗
/Hz erg
−1
) > 25 at EW > 50 Å and a higher ionizing efficiency for high-EW CIII
λ
1909 and OIII
λ
1666 emitters. The most significant correlations are found with respect to stellar mass, specific star formation rate (sSFR), and SFR surface density (Σ
SFR
). The relation between
ξ
ion
∗
and sSFR increases monotonically from log(
ξ
ion
∗
/Hz erg
−1
)∼24.5 at log(sSFR) ∼ −9.5 yr
−1
to ∼25.5 at log(sSFR) ∼ −7.5 yr
−1
. This relation has a low scatter and only a weak dependence on mass. The objects above the main sequence of star formation consistently have higher than average
ξ
ion
∗
. A clear increase in
ξ
ion
∗
with Σ
SFR
is also found, with log(
ξ
ion
∗
/Hz erg
−1
) > 25 for objects at Σ
SFR
> 10
M
⊙
yr
−1
kpc
−2
.
Conclusions.
Bright (
M
UV
≲ 20) and massive (log(
M
star
/
M
⊙
)≳9.5) galaxies at
z
= 2 − 5 have a moderate ionizing efficiency. However, the correlation between
ξ
ion
∗
and sSFR, together with the known increase in the average sSFR with redshift at fixed stellar mass, suggests that similar galaxies in the epoch of reionization can be efficient sources of ionizing photons. The availability of sSFR and Σ
SFR
as proxies for
ξ
ion
∗
can be fundamentally important in determining the role of galaxy populations at
z
≳ 10 that were recently discovered by the
James Webb
Space Telescope in the onset of reionization.
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