Abstract
The relationship between galaxy characteristics and the reionization of the universe remains elusive, mainly due to the observational difficulty in accessing the Lyman continuum (LyC) at ...these redshifts. It is thus important to identify low-redshift LyC-leaking galaxies that can be used as laboratories to investigate the physical processes that allow LyC photons to escape. The weakness of the S
ii
nebular emission lines relative to typical star-forming galaxies has been proposed as a LyC predictor. In this paper, we show that the S
ii
deficiency is an effective method to select LyC-leaking candidates using data from the Low-redshift LyC Survey, which has detected flux below the Lyman edge in 35 out of 66 star-forming galaxies with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We show that LyC leakers tend to be more S
ii
deficient and that the fraction of their detections increases as S
ii
deficiency becomes more prominent. Correlational studies suggest that S
ii
deficiency complements other LyC diagnostics (such as strong Ly
α
emission and high O
iii
/O
ii
). Our results verify an additional technique by which reionization-era galaxies could be studied.
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
The Lyman continuum (LyC) cannot be observed at the epoch of reionization (
z
≳ 6) owing to intergalactic H
i
absorption. To identify LyC emitters (LCEs) and infer the fraction of escaping ...LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Ly
α
, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the O
iii
/O
ii
flux ratio and star formation rate surface density, predict whether a galaxy is an LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H
i
column density, ionization parameter, and stellar feedback. We find that LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.
Abstract
We present new high-resolution (
R
=
λ
/Δ
λ
∼ 14,000) spectra of the two brightest He
ii
-transparent quasars in the far-UV at
z
em
> 3.5, HE2QS J2311−1417 (
z
em
= 3.70) and HE2QS ...J1630+0435 (
z
em
= 3.81), obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. In the predominantly saturated He
ii
absorption spectra, both sight lines show several isolated resolved (FWHM ≳ 50 km s
−1
) transmission spikes in He
ii
Ly
α
and He
ii
Ly
β
. The incidence of such spikes decreases with increasing redshift, but both sight lines show significant spikes at
z
≳ 3.5, signaling the presence of fully ionized regions in the
z
≳ 3.5 intergalactic medium (IGM). We employ an automated algorithm to compare the number of detected He
ii
transmission spikes to predictions from mock spectra created from the outputs of a cubic (146 cMpc)
3
optically thin
Nyx
hydrodynamical simulation, assuming a range of UV background photoionization rates Γ
He
II
. From the incidence of Ly
α
and Ly
β
transmission spikes we infer similar photoionization rates of
at 3.51 <
z
< 3.66 and Γ
He
II
= (0.9 ± 0.3) × 10
−15
s
−1
at 3.460 <
z
< 3.685, respectively. Because the transmission spikes indicate fully ionized regions at
z
≳ 3.5 along both lines of sight, our observations provide further evidence that He
ii
reionization had substantially progressed at these redshifts.
The Low-Redshift Lyman Continuum Survey Saldana-Lopez, Alberto; Schaerer, Daniel; Chisholm, John ...
Astronomy and astrophysics (Berlin),
07/2022, Volume:
663
Journal Article
Peer reviewed
Open access
Aims.
Combining 66 ultraviolet (UV) spectra and ancillary data from the recent Low-Redshift Lyman Continuum Survey (LzLCS) and 23 LyC observations by earlier studies, we form a statistical sample of ...star-forming galaxies at
z
∼ 0.2 − 0.4 with which we study the role of cold interstellar medium (ISM) gas in the leakage of ionizing radiation. We also aim to establish empirical relations between the H
I
neutral and low-ionization state (LIS) absorption lines with different galaxy properties.
Methods.
We first constrain the massive star content (stellar ages and metallicities) and UV attenuation by fitting the stellar continuum with a combination of simple stellar population models. The models, together with accurate LyC flux measurements, allow us to determine the absolute LyC photon escape fraction for each galaxy (
f
esc
abs
). We then measure the equivalent widths and residual fluxes of multiple H
I
and LIS lines, and the geometrical covering fraction of the UV emission, adopting the picket-fence model.
Results.
The LyC escape fraction spans a wide range, with a median
f
esc
abs
(0.16, 0.84 quantiles) of 0.04 (0.02, 0.20), and 50 out of the 89 galaxies detected in the LyC (1
σ
upper limits of
f
esc
abs
≲ 0.01 for non-detections, typically). The H
I
and LIS line equivalent widths scale with the UV luminosity and attenuation, and inversely with the residual flux of these lines. Additionally, Ly
α
equivalent widths scale with both the H
I
and LIS residual fluxes, but anti-correlate with the corresponding H
I
or LIS equivalent widths. The H
I
and LIS residual fluxes are correlated, indicating that the neutral gas is spatially traced by the low-ionization transitions. We find that the observed trends of the absorption lines and the UV attenuation are primarily driven by the geometric covering fraction of the gas. The observed nonuniform gas coverage also demonstrates that LyC photons escape through low-column-density channels in the ISM. The equivalent widths and residual fluxes of both the H
I
and LIS lines strongly correlate with
f
esc
abs
: strong LyC leakers (highest
f
esc
abs
) show weak absorption lines, low UV attenuation, and large Ly
α
equivalent widths. We provide several empirical calibrations to estimate
f
esc
abs
from UV absorption lines. Finally, we show that simultaneous UV absorption line and dust attenuation measurements can, in general, predict the escape fraction of galaxies. We apply our method to available measurements of UV LIS lines of 15 star-forming galaxies at
z
∼ 4 − 6 (plus 3 high-
z
galaxy composites), finding that these high-redshift, UV-bright galaxies (
M
UV
≲ −21) may have low escape fractions,
f
esc
abs
≲ 0.1.
Conclusions.
UV absorption lines trace the cold ISM gas of galaxies, which governs the physics of the LyC escape. We show that, with some assumptions, the absolute LyC escape can be statistically predicted using UV absorption lines, and the method can be applied to study galaxies across a wide redshift range, including in the epoch of cosmic reionization.
Abstract
Star-forming galaxies are considered the likeliest source of the H
i
ionizing Lyman continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above
z
≳ 6, ...the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of
indirect
indicators for LyC that can be calibrated at lower redshifts and applied in the epoch of reionization. Emission from the Mg
ii
λλ
2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for eight LyC emitter candidates, selected to have strong Mg
ii
emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2
σ
significance. This high detection rate suggests that strong Mg
ii
emitters might be more likely to leak LyC than similar galaxies without strong Mg
ii
. Using photoionization models, we constrain the escape fraction of Mg
ii
as ∼15%–60%. We confirm that the escape fraction of Mg
ii
correlates tightly with that of Ly
α
, which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg
ii
emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg
ii
emission can be adopted to estimate the escape fraction of Ly
α
and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.
Abstract
We present statistics of He
ii
Ly
α
transmission spikes and large-scale absorption troughs using archival high-resolution (
R
=
λ
/Δ
λ
≃ 12,500–18,000) far-UV spectra of eight He
ii
...-transparent quasars obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers the redshift range 2.5 ≲
z
≲ 3.8, thereby probing the rapidly evolving He
ii
absorption at the end of He
ii
reionization epoch. The measured lengths of the troughs decrease dramatically from
L
≳ 100 cMpc at
z
> 3 to
L
≃ 30 cMpc at
z
∼ 2.7, signaling a significant progression of He
ii
reionization at these redshifts. Furthermore, unexpectedly long
L
∼ 65 cMpc troughs detected at
z
≃ 2.9 suggest that the UV background fluctuates at larger scales than predicted by current models. By comparing the measured incidence of transmission spikes to predictions from forward-modeled mock spectra created from the outputs of a (146 cMpc)
3
optically thin
Nyx
hydrodynamical simulation employing different UV background models, we infer the redshift evolution of the He
ii
photoionization rate Γ
He
II
(
z
). The photoionization rate decreases with increasing redshift from ≃ 4.6 × 10
−15
s
−1
at
z
≃ 2.6 to ≃ 1.2 × 10
−15
s
−1
at
z
≃ 3.2, in agreement with previous inferences from the He
ii
effective optical depth, and following expected trends of current models of a fluctuating He
ii
-ionizing background.
We present statistics of HeII Lya transmission spikes and large-scale
absorption troughs using archival high-resolution ($R=\lambda /\Delta \lambda
\simeq 12,500$-$18,000$) far-UV spectra of eight ...HeII-transparent quasars
obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope.
The sample covers the redshift range 2.5<z<3.8, thereby probing the rapidly
evolving HeII absorption at the end of the HeII reionization epoch. The
measured lengths of the troughs decrease dramatically from L>100cMpc at z>3 to
L~30cMpc at z~2.7, signaling a significant progression of HeII reionization at
these redshifts. Furthermore, unexpectedly long L~65cMpc troughs detected at
z~2.9 suggest that the UV background fluctuates at larger scales than predicted
by current models. By comparing the measured incidence of transmission spikes
to predictions from forward-modeled mock spectra created from the outputs of a
(146cMpc)^3 optically thin Nyx hydrodynamical simulation employing different UV
background models, we infer the redshift evolution of the HeII photoionization
rate $\Gamma_\mathrm{He\,II}(z)$. The photoionization rate decreases with
increasing redshift from $\simeq 4.6\times 10^{-15}\mathrm{\,s^{-1}}$ at z~2.6
to $\simeq 1.2 \times 10^{-15}\mathrm{\,s^{-1}}$ at z~3.2, in agreement with
previous inferences from the HeII effective optical depth, and following
expected trends of current models of a fluctuating HeII-ionizing background.
We present new high-resolution (R~14,000) spectra of the two brightest HeII-transparent quasars in the far-UV (FUV) at z>3.5, HE2QSJ2311-1417 (z=3.70) and HE2QSJ1630+0435 (z=3.81), obtained with the ...Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST). In the predominantly saturated HeII absorption spectra, both sightlines show several isolated resolved (full width at half maximum FWHM>50 km/s) transmission spikes in HeII Ly\(\alpha\) and HeII Ly\(\beta\). The incidence of such spikes decreases with increasing redshift, but both sightlines show significant spikes at z>3.5, signaling the presence of fully ionized regions in the z>3.5 intergalactic medium (IGM). We employ an automated algorithm to compare the number of detected HeII transmission spikes to predictions from mock spectra created from the outputs of a cubic (100/h cMpc)^3 optically thin Nyx hydrodynamical simulation, assuming a range of UV background photoionization rates \(\Gamma_\mathrm{HeII}\). From the incidence of Ly\(\alpha\) and Ly\(\beta\) transmission spikes we infer similar photoionization rates of \(\Gamma_\mathrm{HeII}=2.0^{+0.6}_{-0.5}\times 10^{-15}\)s\(^{-1}\) at 3.51<z<3.66 and \(\Gamma_\mathrm{HeII}=0.9\pm0.3 \times 10^{-15}\)s\(^{-1}\) at 3.460<z<3.685, respectively. Because the transmission spikes indicate fully ionized regions at z>3.5 along both lines of sight, our observations provide further evidence that HeII reionization had substantially progressed at these redshifts.
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