ABSTRACT
We demonstrate how the stellar and nebular conditions in star-forming galaxies modulate the emission and spectral profile of H i Ly α emission line. We examine the net Ly α output, ...kinematics, and in particular emission of blueshifted Ly α radiation, using spectroscopy from with the Cosmic Origins Spectrograph on Hubble Space Telescope (HST), giving a sample of 87 galaxies at redshift z = 0.05−0.44. We contrast the Ly α spectral measurements with properties of the ionized gas (from optical spectra) and stars (from stellar modelling). We demonstrate correlations of unprecedented strength between the Ly α escape fraction (and equivalent width) and the ionization parameter (p ≈ 10−15). The relative contribution of blueshifted emission to the total Ly α also increases from ≈0 to ≈40 per cent over the range of O32 ratios (p ≈ 10−6). We also find particularly strong correlations with estimators of stellar age and nebular abundance, and weaker correlations regarding thermodynamic variables. Low ionization stage absorption lines suggest the Ly α emission and line profile are predominantly governed by the column of absorbing gas near zero velocity. Simultaneous multiparametric analysis over many variables shows we can predict 80 per cent of the variance on Ly α luminosity, and ∼50 per cent on the EW. We determine the most crucial predictive variables, finding that for tracers of the ionization state and H β luminosity dominate the luminosity prediction whereas the Ly α EW is best predicted by H β EW and the H α/H β ratio. We discuss our results with reference to high-redshift observations, focussing upon the use of Ly α to probe the nebular conditions in high-z galaxies and cosmic reionization.
Abstract
Extreme, young stellar populations are considered to be the primary contributor to cosmic reionization. How the Lyman continuum (LyC) escapes these galaxies remains highly elusive, and it is ...challenging to observe this process in actual LyC emitters without resolving the relevant physical scales. We investigate the Sunburst Arc, a strongly lensed LyC emitter at
z
= 2.37 that reveals an exceptionally small-scale (tens of parsecs) region of high LyC escape. The small (<100 pc) LyC-leaking region has extreme properties: a very blue UV slope (
β
= −2.9 ± 0.1), a high ionization state (O
iii
λ
5007/O
ii
λ
3727 = 11 ± 3 and O
iii
λ
5007/H
β
= 6.8 ± 0.4), strong oxygen emission (EW(O
iii
) = 1095 ± 40 Å), and a high Ly
α
escape fraction (0.3 ± 0.03), none of which are found in nonleaking regions of the galaxy. The leaking region’s UV slope is consistent with approximately “pure” stellar light that is minimally contaminated by the surrounding nebular continuum emission or extinguished by dust. These results suggest a highly anisotropic LyC escape process such that LyC is produced and escapes from a small, extreme starburst region where the stellar feedback from an ionizing star cluster creates one or more “pencil-beam” channels in the surrounding gas through which LyC can directly escape. Such anisotropic escape processes imply that random sight-line effects drive the significant scatters between measurements of galaxy properties and LyC escape fraction, and that strong lensing is a critical tool for resolving the processes that regulate the ionizing budget of galaxies for reionization.
Abstract As the nearest confirmed Lyman continuum (LyC) emitter, Haro 11 is an exceptional laboratory for studying LyC escape processes crucial to cosmic reionization. Our new Hubble Space ...Telescope/Cosmic Origins Spectrograph G130M/1055 observations of its three star-forming knots now reveal that the observed LyC originates in Knots B and C, with 903–912 Å luminosities of 1.9 ± 1.5 × 10 40 erg s −1 and 0.9 ± 0.7 × 10 40 erg s −1 , respectively. We derive local escape fractions f esc,912 = 3.4% ± 2.9% and 5.1% ± 4.3% for Knots B and C, respectively. Our Starburst99 modeling shows dominant populations on the order of ∼1–4 Myr and 1–2 × 10 7 M ⊙ in each knot, with the youngest population in Knot B. Thus, the knot with the strongest LyC detection has the highest LyC production. However, LyC escape is likely less efficient in Knot B than in Knot C due to higher neutral gas covering. Our results therefore stress the importance of the intrinsic ionizing luminosity, and not just the escape fraction, for LyC detection. Similarly, the Ly α escape fraction does not consistently correlate with LyC flux, nor do narrow Ly α red peaks. High observed Ly α luminosity and low Ly α peak velocity separation, however, do correlate with higher LyC escape. Another insight comes from the undetected Knot A, which drives the Green Pea properties of Haro 11. Its density-bounded conditions suggest highly anisotropic LyC escape. Finally, both of the LyC-leaking Knots, B and C, host ultraluminous X-ray sources (ULXs). While stars strongly dominate over the ULXs in LyC emission, this intriguing coincidence underscores the importance of unveiling the role of accretors in LyC escape and reionization.
Abstract
We analyze spectra of a gravitationally lensed galaxy, known as the Sunburst Arc, that is leaking ionizing photons, also known as the Lyman continuum (LyC). Magnification from gravitational ...lensing permits the galaxy to be spatially resolved into one region that leaks ionizing photons and several that do not. Rest-frame UV and optical spectra from Magellan target 10 different regions along the lensed Arc, including six multiple images of the LyC leaking region and four regions that do not show LyC emission. The rest-frame optical spectra of the ionizing photon emitting regions reveal a blueshifted (Δ
V
= 27 km s
−1
) broad emission component (FWHM = 327 km s
−1
), comprising 55% of the total O
iii
line flux, in addition to a narrow component (FWHM = 112 km s
−1
), suggesting the presence of strong highly ionized gas outflows. This is consistent with the high-velocity ionized outflow inferred from the rest-frame UV spectra. In contrast, the broad emission component is less prominent in the nonleaking regions, comprising ∼26% of total O
iii
line flux. The high-ionization absorption lines are prominent in both the leaker and the nonleaker, but the low-ionization absorption lines are very weak in the leaker, suggesting that the line-of-sight gas is highly ionized in the leaker. Analyses of stellar wind features reveal that the stellar population of the LyC leaking regions is considerably younger (∼3 Myr) than that of the nonleaking regions (∼12 Myr), emphasizing that stellar feedback from young stars may play an important role in ionizing photon escape.
Abstract
Star-forming galaxies are believed to be a major source of Lyman continuum (LyC) radiation responsible for reionizing the early universe. Direct observations of escaping ionizing radiation ...have however been sparse and with low escape fractions. In the local universe, only 10 emitters have been observed, with typical escape fractions of a few percent. The mechanisms regulating this escape need to be strongly evolving with redshift in order to account for the epoch of reionization. Gas content and star formation feedback are among the main suspects, known to both regulate neutral gas coverage and evolve with cosmic time. In this paper, we reanalyze
Hubble Space Telescope
(
HST
)-Cosmic Origins Spectrograph (COS) spectrocopy of the first detected local LyC leaker, Haro 11. We examine the connection between LyC leakage and Ly
α
line shape, and feedback-influenced neutral interstellar medium (ISM) properties like kinematics and gas distribution. We discuss the two extremes of an optically thin, density bounded ISM and a riddled, optically thick, ionization bounded ISM, and how Haro 11 fits into theoretical predictions. We find that the most likely ISM model is a clumpy neutral medium embedded in a highly ionized medium with a combined covering fraction of unity and a residual neutral gas column density in the ionized medium high enough to be optically thick to Ly
α
, but low enough to be at least partly transparent to LyC and undetected in Si
ii
. This suggests that star formation feedback and galaxy-scale interaction events play a major role in opening passageways for ionizing radiation through the neutral medium.
Abstract
We present Ly
α
imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The galaxies have been selected to have moderate to high star formation rates ...(SFRs) using far-ultraviolet (FUV) luminosity and H
α
equivalent width criteria, but no constraints on Ly
α
luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum-subtracted Ly
α
, H
α
, and H
β
maps. We find that Ly
α
is less concentrated than FUV and optical line emission in almost all galaxies with significant Ly
α
emission. We present global measurements of Ly
α
and other quantities measured in apertures designed to capture all of the Ly
α
emission. We then show how the escape fraction of Ly
α
relates to a number of other measured quantities (mass, metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Ly
α
escape fractions are inconsistent with common dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present an SFR calibration based on Ly
α
luminosity, where the equivalent width of Ly
α
is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Ly
α
for the galaxies that can be used to find aperture loss fractions at a given physical radius.
Abstract
The Ly
α
emission line is one of the main observables of galaxies at high redshift, but its output depends strongly on the neutral gas distribution and kinematics around the star-forming ...regions where UV photons are produced. We present observations of Ly
α
and 21 cm H
i
emission at comparable scales with the goal to qualitatively investigate how the neutral interstellar medium (ISM) properties impact Ly
α
transfer in galaxies. We have observed 21 cm H
i
at the highest possible angular resolution (≈3″ beam) with the Very Large Array in two local galaxies from the Lyman Alpha Reference Sample. We compare these data with Hubble Space Telescope Ly
α
imaging and spectroscopy, and Multi Unit Spectroscopic Explorer and Potsdam MultiAperture Spectrophotometer ionized gas observations. In LARS08, high-intensity Ly
α
emission is cospatial with high column density H
i
where the dust content is the lowest. The Ly
α
line is strongly redshifted, consistent with a velocity redistribution that allows Ly
α
escape from a high column density neutral medium with a low dust content. In eLARS01, high-intensity Ly
α
emission is located in regions of low column density H
i
, below the H
i
data sensitivity limit ( < 2 × 10
20
cm
−2
). The perturbed ISM distribution with low column density gas in front of the Ly
α
emission region plays an important role in the escape. In both galaxies, the faint Ly
α
emission (∼1×10
−16
erg s
−1
cm
−2
arcsec
−2
) traces intermediate H
α
emission regions where H
i
is found, regardless of the dust content. Dust seems to modulate, but not prevent, the formation of a faint Ly
α
halo. This study suggests the existence of scaling relations between dust, H
α
, H
i
, and Ly
α
emission in galaxies.
Abstract
We present a strong-lensing analysis of the cluster PSZ1 G311.65−18.48, based on Hubble Space Telescope imaging, archival VLT/MUSE spectroscopy, and Chandra X-ray data. This cool-core ...cluster (
z
= 0.443) lenses the brightest lensed galaxy known, dubbed the “Sunburst Arc” (
z
= 2.3703), a Lyman continuum (LyC) emitting galaxy multiply imaged 12 times. We identify in this field 14 additional strongly lensed galaxies to constrain a strong-lens model and report secure spectroscopic redshifts of four of them. We measure a projected cluster core mass of
M
(<250 kpc) =
2.93
−
0.02
+
0.01
×
10
14
M
⊙
. The two least magnified but complete images of the Sunburst Arc’s source galaxy are magnified by ∼13×, while the LyC clump is magnified by ∼4–80×. We present time delay predictions and conclusive evidence that a discrepant clump in the Sunburst Arc, previously claimed to be a transient, is not variable, thus strengthening the hypothesis that it results from an exceptionally high magnification. A source plane reconstruction and analysis of the Sunburst Arc finds its physical size to be 1 × 2 kpc and that it is resolved in three distinct directions in the source plane, 0°, 40°, and 75° (east of north). We place an upper limit of
r
≲ 50 pc on the source plane size of unresolved clumps and
r
≲ 32 pc for the LyC clump. Finally, we report that the Sunburst Arc is likely in a system of two or more galaxies separated by ≲6 kpc in projection. Their interaction may drive star formation and could play a role in the mechanism responsible for the leaking LyC radiation.
Abstract
Understanding the escape of ionizing (Lyman continuum) photons from galaxies is vital for determining how galaxies contributed to reionization in the early universe. While directly detecting ...the Lyman continuum from high-redshift galaxies is impossible due to the intergalactic medium, low-redshift galaxies in principle offer this possibility but require observations from space. The first local galaxy for which Lyman continuum escape was found is Haro 11, a luminous blue compact galaxy at
z
= 0.02, where observations with the FUSE satellite revealed an escape fraction of 3.3%. However, the FUSE aperture covers the entire galaxy, and it is not clear from where the Lyman continuum is leaking out. Here we utilize Hubble Space Telescope/Cosmic Origins Spectrograph spectroscopy in the wavelength range 1100–1700 Å of the three knots (A, B, and C) of Haro 11 to study the presence of Ly
α
emission and the properties of intervening gas. We find that all knots have bright Ly
α
emission. UV absorption lines, originating in the neutral interstellar medium, as well as lines probing the ionized medium, are seen extending to blueshifted velocities of 500 km s
−1
in all three knots, demonstrating the presence of an outflowing multiphase medium. We find that knots A and B have large covering fractions of neutral gas, making LyC escape along these sightlines improbable, while knot C has a much lower covering fraction (≲50%). Knot C also has the the highest Ly
α
escape fraction, and we conclude that it is the most likely source of the escaping Lyman continuum detected in Haro 11.
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