The origin of nebular He II emission, which is frequently observed in low-metallicity (O/H) star-forming galaxies, remains largely an unsolved question. Using the observed anticorrelation of the ...integrated X-ray luminosity per unit of star formation rate (LX/SFR) of an X-ray binary population with metallicity and other empirical data from the well-studied galaxy I Zw 18, we show that the observed He II λ4686 intensity and its trend with metallicity is naturally reproduced if the bulk of He+ ionizing photons are emitted by the X-ray sources. We also show that a combination of X-ray binary population models with normal single and/or binary stellar models reproduces the observed I(4686)/I(Hβ) intensities and its dependency on metallicity and age. We conclude that both empirical data and theoretical models suggest that high-mass X-ray binaries are the main source of nebular He II emission in low-metallicity star-forming galaxies.
ABSTRACT
Early star-forming galaxies produced copious ionizing photons. A fraction of these photons escaped gas within galaxies to reionize the entire Universe. This escape fraction is crucial for ...determining how the Universe became reionized, but the neutral intergalactic medium precludes direct measurement of the escape fraction at high redshifts. Indirect estimates of the escape fraction must describe how the Universe was reionized. Here, we present new Keck Cosmic Web Imager spatially resolved spectroscopy of the resonant Mg ii 2800 Å doublet from a redshift 0.36 galaxy, J1503+3644, with a previously observed escape fraction of 6 per cent. The Mg ii emission has a similar spatial extent as the stellar continuum, and each of the Mg ii doublet lines are well fitted by single Gaussians. The Mg ii is optically thin. The intrinsic flux ratio of the red and blue Mg ii emission line doublet, $R=F_{2796}/F_{2803}$, is set by atomic physics to be two, but Mg$^+$ gas along the line of sight decreases R proportional to the Mg ii optical depth. Combined with the metallicity, R estimates the neutral gas column density. The observed R ranges across the galaxy from 0.8 to 2.7, implying a factor of 2 spatial variation of the relative escape fraction. All of the ionizing photons that escape J1503+3644 pass through regions of high R. We combine the Mg ii emission and dust attenuation to accurately estimate the absolute escape fractions for 10 local Lyman Continuum emitting galaxies and suggest that Mg ii can predict escape fraction within the epoch of reionization.
Following our first detection reported in Izotov et al., we present the detection of Lyman continuum (LyC) radiation of four other compact star-forming galaxies observed with the Cosmic Origins ...Spectrograph (COS) onboard the Hubble Space Telescope. These galaxies, at redshifts of z ∼ 0.3, are characterized by high emission-line flux ratios O iii λ5007/O ii λ3727 ≳ 5. The escape fractions of the LyC radiation fesc(LyC) in these galaxies are in the range of ∼6–13 per cent, the highest values found so far in low-redshift star-forming galaxies. Narrow double-peaked Ly α emission lines are detected in the spectra of all four galaxies, compatible with predictions for LyC leakers. We find escape fractions of Ly α, fesc(Ly α) ∼ 20–40 per cent, among the highest known for Ly α emitting galaxies. Surface brightness profiles produced from the COS acquisition images reveal bright star-forming regions in the centre and exponential discs in the outskirts with disc scalelengths α in the range ∼0.6–1.4 kpc. Our galaxies are characterized by low metallicity, ∼1/8–1/5 solar, low stellar mass ∼(0.2–4) × 109 M⊙, high star formation rates, SFR ∼ 14–36 M⊙ yr−1, and high SFR densities, Σ ∼ 2–35 M⊙ yr−1 kpc−2. These properties are comparable to those of high-redshift star-forming galaxies. Finally, our observations, combined with our first detection reported in Izotov et al., reveal that a selection for compact star-forming galaxies showing high O iii λ5007/O ii λ3727 ratios appears to pick up very efficiently sources with escaping LyC radiation: all five of our selected galaxies are LyC leakers.
The fraction of ionizing photons that escape high-redshift galaxies sensitively determines whether galaxies reionized the early Universe. However, this escape fraction cannot be measured from ...high-redshift galaxies because the opacity of the intergalactic medium is large at high redshifts. Without methods to measure the escape fraction of high-redshift galaxies indirectly, it is unlikely that we will know what reionized the Universe. Here, we analyze the far-ultraviolet (UV) H I (Lyman series) and low-ionization metal absorption lines of nine low-redshift, confirmed Lyman continuum emitting galaxies. We use the H I covering fractions, column densities, and dust attenuations measured in a companion paper to predict the escape fraction of ionizing photons. We find good agreement between the predicted and observed Lyman continuum escape fractions (within 1.4σ) using both the H I and ISM absorption lines. The ionizing photons escape through holes in the H I, but we show that dust attenuation reduces the fraction of photons that escape galaxies. This means that the average high-redshift galaxy likely emits more ionizing photons than low-redshift galaxies. Two other indirect methods accurately predict the escape fractions: the Lyα escape fraction and the optical O III/O II flux ratio. We use these indirect methods to predict the escape fraction of a sample of 21 galaxies with rest-frame UV spectra but without Lyman continuum observations. Many of these galaxies have low escape fractions (fesc ≤ 1%), but 11 have escape fractions >1%. Future studies will use these methods to measure the escape fractions of high-redshift galaxies, enabling upcoming telescopes to determine whether star-forming galaxies reionized the early Universe.
Context.
Identifying the physical mechanisms driving the escape of Lyman continuum (LyC) photons is crucial for the search of Lyman continuum emitter (LCE) candidates.
Aims.
To understand the ...physical properties involved in the leakage of LyC photons, we investigate the connection between the H
I
covering fraction, H
I
velocity width, the Lyman
α
(Ly
α
) properties, and the escape of LyC photons in a sample of 22 star-forming galaxies, which includes 13 confirmed LCEs.
Methods.
We fit the stellar continuum, dust attenuation, and absorption lines between 920 Å and 1300 Å to extract the H
I
covering fractions and dust attenuation. Additionally, we measure the H
I
velocity widths of the optically thick Lyman series and derive the Ly
α
equivalent widths (EW), escape fractions (
f
esc
), peak velocities, and fluxes at the minimum of the observed Ly
α
profiles.
Results.
Overall, we highlight strong observational correlations between the presence of low H
I
covering fractions and the observation of (1) low Ly
α
peak velocities; (2) more flux at the profile minimum; and (3) larger EW(Ly
α
),
f
esc
(Ly
α
), and
f
esc
obs
(LyC). Hence, low column density channels are crucial ISM ingredients for the leakage of Ly
α
and LyC photons. Additionally, galaxies with narrower H
I
absorption velocity widths have higher Ly
α
equivalent widths, larger Ly
α
escape fractions, and lower Ly
α
peak velocity separations. This may suggest that these galaxies have low H
I
column density. Finally, we find that dust also regulates the amount of Ly
α
and LyC radiation that actually escapes the ISM.
Conclusions.
The ISM porosity is one of the origins of strong Ly
α
emission, enabling the escape of ionizing photons in low-
z
leakers. However, this is not sufficient to explain the largest
f
esc
obs
(LyC), which indicates that the most extreme LCEs are likely to be density-bounded along all lines of sight to the observer. Overall, the neutral gas porosity provides a constraint for a lower limit to the escape fraction of LyC and Ly
α
photons, which offers a key estimator for assessing the leakage of ionizing photons.
Context. The processes allowing the escape of ionizing photons from galaxies into the intergalactic medium are poorly known. Aims. To understand how Lyman continuum (LyC) photons escape galaxies, we ...constrain the H I covering fractions and column densities using ultraviolet (UV) H I and metal absorption lines of 18 star-forming galaxies that have Lyman series observations. Nine of these galaxies are confirmed LyC emitters. Methods. We fit the stellar continuum, dust attenuation, metal, and H I properties to consistently determine the UV attenuation, as well as the column densities and covering factors of neutral hydrogen and metals. We used synthetic interstellar absorption lines to explore the systematics of our measurements. Then we applied our method to the observed UV spectra of low-redshift and z ~ 3 galaxies. Results. The observed H I lines are found to be saturated in all galaxies. An indirect approach using O I column densities and the observed O/H abundances yields H I column densities of log(NH I) ~ 18.6−20 cm−2. These columns are too high to allow the escape of ionizing photons. We find that the known LyC leakers have H I covering fractions less than unity. Ionizing photons escape through optically thin channels in a clumpy interstellar medium. Our simulations confirm that the H I covering fractions are accurately recovered. The Si II and H I covering fractions scale linearly, in agreement with observations from stacked Lyman break galaxy spectra at z ~ 3. Thus, with an empirical correction, the Si II absorption lines can also be used to determine the H I coverage. Finally, we show that a consistent fitting of dust attenuation, continuum, and absorption lines is required to properly infer the covering fraction of neutral gas and subsequently to infer the escape fraction of ionizing radiation. Conclusions. These measurements can estimate the LyC escape fraction, as we demonstrate in a companion paper.
We present new deep ALMA and Hubble Space Telescope (HST)/WFC3 observations of MASOSA and VR7, two luminous Ly emitters (LAEs) at z = 6.5, for which the UV continuum levels differ by a factor of ...four. No IR dust continuum emission is detected in either, indicating little amounts of obscured star formation and/or high dust temperatures. MASOSA, with a UV luminosity M1500 = −20.9, compact size, and very high Ly , is undetected in C ii to a limit of LC ii < 2.2 × 107 L , implying a metallicity Z 0.07 Z . Intriguingly, our HST data indicate a red UV slope β = −1.1 0.7, at odds with the low dust content. VR7, which is a bright (M1500 = −22.4) galaxy with moderate color (β = −1.4 0.3) and Ly EW0 = 34 , is clearly detected in C ii emission (S/N = 15). VR7's rest-frame UV morphology can be described by two components separated by 1.5 kpc and is globally more compact than the C ii emission. The global C ii/UV ratio indicates Z 0.2 Z , but there are large variations in the UV/C ii ratio on kiloparsec scales. We also identify diffuse, possibly outflowing, C ii-emitting gas at 100 km s−1 with respect to the peak. VR7 appears to be assembling its components at a slightly more evolved stage than other luminous LAEs, with outflows already shaping its direct environment at z ∼ 7. Our results further indicate that the global C ii−UV relation steepens at SFR < 30 M yr−1, naturally explaining why the C ii/UV ratio is anticorrelated with Ly EW in many, but not all, observed LAEs.
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![Resolved UV and [C ii] Stru...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-637X/small "Resolved UV and [C ii] Structures of Luminous Galaxies within the Epoch of Reionization")