SDSS-IV MaNGA: Observational Evidence of a Density-bounded Region in a Lyα Emitter

Using integral field unit spectroscopy, we present here the spatially resolved morphologies of [S ii]λ6717,6731/Hα and [S ii]λ6717,6731/[O iii]λ5007 emission line ratios for the first time in a blueberry Lyα emitter (BBLAE) at z ∼ 0.047. Our derived morphologies show that the extreme starburst region of the BBLAE, populated by young (≤10 Myr), massive Wolf–Rayet stars, is [S ii] deficient, while the rest of the galaxy is [S ii] enhanced. We infer that the extreme starburst region is density-bounded (i.e., optically thin to ionizing photons), and the rest of the galaxy is ionization-bounded, indicating a Blister-type morphology. We find that the previously reported small escape fraction (10%) of Lyα photons is from our identified density-bounded H ii region of the BBLAE. This escape fraction is likely constrained by a porous dust distribution. We further report a moderate correlation between [S ii] deficiency and inferred Lyman continuum (LyC) escape fraction using a sample of confirmed LyC leakers studied in the literature, including the BBLAE studied here. The observed correlation also reveals its dependency on the stellar mass and gas-phase metallicity of the leaky galaxies. Finally, the future scope and implications of our work are discussed in detail.

Lyman Continuum Galaxy Candidates in COSMOS

Star-forming galaxies are the sources likely to have reionized the universe. As we cannot observe them directly due to the opacity of the intergalactic medium at z ≳ 5, we study z ∼ 3–5 galaxies as proxies to place observational constraints on cosmic reionization. Using new deep Hubble Space Telescope rest-frame UV F336W and F435W imaging (30 orbits, ∼40 arcmin2, ∼29–30 mag depth at 5σ), we attempt to identify a sample of Lyman continuum galaxies (LCGs). These are individual sources that emit ionizing flux below the Lyman break (<912 Å). This population would allow us to constrain cosmic reionization parameters such as the number density and escape fraction (f esc) of ionizing sources. We compile a comprehensive parent sample that does not rely on the Lyman-break technique for redshifts. We present three new spectroscopic candidates at z ∼ 3.7–4.4 and 32 new photometric candidates. The high-resolution multiband HST imaging and new Keck/Low Resolution Imaging Spectrometer (LRIS) redshifts make these promising spectroscopic LCG candidates. Using both a traditional and a probabilistic approach, we find that the most likely f esc values for the three spectroscopic LCG candidates are >100% and therefore not physical. We are unable to confirm the true nature of these sources with the best available imaging and direct blue Keck/LRIS spectroscopy. More spectra, especially from the new class of 30 m telescopes, will be required to build a statistical sample of LCGs to place firm observational constraints on cosmic reionization.

CDFS-6664: A Candidate of Lyman-continuum Emission at z ∼ 3.8 Detected by the Hubble Deep UV Legacy Survey

We report the detection of Lyman continuum (LyC) emission from the galaxy, CDFS-6664, at z = 3.797 in a sample of Lyman break galaxies with detected [O iii] emission lines. The LyC emission is detected with a significance ∼5σ in the F336W band of the Hubble Deep UV Legacy Survey, corresponding to the 650–770 Å rest frame. The light centroid of the LyC emission is offset from the galaxy center by about 0.″2 (1.4 pkpc). The Hubble deep images at longer wavelengths show that the emission is unlikely provided by low-redshift interlopers. The photometric and spectroscopic data show that the possible contribution of an active galactic nucleus is quite low. Fitting the spectral energy distribution of this source to stellar population synthesis models, we find that the galaxy is young (∼50 Myr) and actively forming stars with a rate of 52.1 ± 4.9 M ⊙ yr−1. The significant star formation and the spatially offset LyC emission support a scenario where the ionizing photons escape from the low-density cavities in the ISM excavated by massive young stars. From the nebular model, we estimate the escape fraction of LyC photons to be 38% ± 7% and the corresponding intergalactic medium (IGM) transmission to be 60%, which deviates more than 3σ from the average transmission. The unusually high IGM transmission of LyC photons in CDFS-6664 may be related to a foreground type-2 quasar, CDF-202, at z = 3.7, with a projected separation of 1.′2 only. The quasar may have photoevaporated optically thick absorbers and enhance the transmission on the sightline of CDFS-6664.

Modeling the Hα Emission Surrounding Spica Using the Lyman Continuum from a Gravity-darkened Central Star

The large, faint Hα emission surrounding the early B-star binary Spica has been used to constrain the total hydrogen recombination rate of the nebula and indirectly probe the Lyman continuum luminosity of the primary star. Early analysis suggested that a stellar atmosphere model, consistent with Spica A’s spectral type, has a Lyman continuum luminosity about two times lower than required to account for the measured Hα surface brightness within the nebula. To more consistently model both the stellar and nebular emission, we have used a model atmosphere for Spica A that includes the effects of gravity darkening as input to photoionization models to produce synthetic Hα surface brightness distributions for comparison to data from the Southern Hα Sky Survey Atlas. This paper presents a method for the computation of projected surface brightness profiles from 1D volume emissivity models and constrains both stellar and nebular parameters. A mean effective temperature for Spica A of ≃24,800 K is sufficient to match both the observed absolute spectrophotometry, from the far-UV to the near-IR, and radial Hα surface brightness distributions. Model hydrogen densities increase with the distance from the star, more steeply and linearly toward the southeast. The northwest matter-bounded portion of the nebula is predicted to leak ∼17% of Lyman continuum photons. Model H ii region column densities are consistent with archival observations along the line of sight.

Ionizing radiation from AGNs at z > 3.3 with the Subaru Hyper Suprime-Cam Survey and the CFHT Large Area U-band Deep Survey (CLAUDS)

We use deep and wide imaging data from the CFHT Large Area U-band Deep Survey (CLAUDS) and the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to constrain the ionizing radiation (Lyman Continuum; LyC) escape fraction from AGNs at z ∼ 3–4. For 94 AGNs with spectroscopic redshifts at 3.3 &lt; z &lt; 4.0, we use their U-band / i-band flux ratios to estimate LyC transmission of individual AGNs. The distribution of their LyC transmission shows values lower than the range of LyC transmission values for IGM of the same redshift range, which suggests that LyC escape fraction of AGNs at z &gt; 3.3 is considerably lower than unity in most cases. We do not find any trend in LyC transmission values depending on their UV luminosities. Based on the photometry of stacked images we find the average flux ratio of LyC and non-ionizing UV photons escaping from the objects (fLyC/fUV)out = 0.182 ± 0.043 for AGNs at 3.3 &lt; z &lt; 3.6, which corresponds to LyC escape fraction fesc = 0.303 ± 0.072 if we assume a fiducial intrinsic SED of AGN. Based on the estimated LyC escape fraction and the UV luminosity function of AGNs, we argue that UV-selected AGNs’ contribution to the LyC emissivity at the epoch is minor, although the size of their contribution largely depends on the shape of the UV luminosity function.

Upper limits on the escape fraction of ionizing radiation from galaxies at 2 ≲ z < 6

In this work, we investigate upper limits on the global escape fraction of ionizing photons ($f_{\rm esc/global}^{\rm abs}$) from a sample of galaxies probed for Lyman-continuum (LyC) emission characterized as non-LyC and LyC leakers. We present a sample of 9 clean non-contaminated (by low redshift interlopers, CCD problems and internal reflections of the instrument) galaxies which do not show significant (&gt; 3σ) LyC flux between 880Å &lt;λrest &lt; 910Å. The 9 galaxy stacked spectrum reveals no significant LyC flux with an upper limit of $f_{\rm esc}^{\rm abs} \le 0.06$. In the next step of our analysis, we join all estimates of $f_{\rm esc}^{\rm abs}$ upper limits derived from different samples of 2 ≲ z &lt; 6 galaxies from the literature reported in last ∼20 years and include the sample presented in this work. We find the $f_{\rm esc}^{\rm abs}$ upper limit ≤ 0.084 for the galaxies recognized as non-LyC leakers. After including all known detections from literature $f_{\rm esc/global}^{\rm abs}$ upper limit ≤ 0.088 for all galaxies examined for LyC flux. Furthermore, $f_{\rm esc}^{\rm abs}$ upper limits for different groups of galaxies indicate that the strongest LyC emitters could be galaxies classified as Lyman alpha emitters. We also discuss the possible existence of a correlation among the observed flux density ratio $(F_{\nu }^{LyC}/F_{\nu }^{UV})_{\rm obs}$ and Lyman alpha equivalent width EW(Lyα), where we confirm the existence of moderately significant correlation among galaxies classified as non-LyC leakers.

Assessing the sources of reionisation: a spectroscopic case study of a 30× lensed galaxy at z ∼ 5 with Lyα, C iv, Mg ii, and [Ne iii]

We present a detailed spectroscopic analysis of a galaxy at z ≃ 4.88 that is, by chance, magnified ∼30 × by gravitational lensing. Only three sources at z ≳ 5 are known with such high magnification. This particular source has been shown to exhibit widespread, high equivalent width ${\rm C\, {\small IV}}\, \lambda \, 1549$ emission, implying it is a unique example of a metal-poor galaxy with a hard radiation field, likely representing the galaxy population responsible for cosmic reionisation. Using UV nebular line ratio diagnostics, VLT/X-shooter observations rule out strong AGN activity, indicating a stellar origin of the hard radiation field instead. We present a new detection of ${[\rm Ne\, {\small III}]}\, \lambda \, 3870$ and use the [Ne iii]/[O ii] line ratio to constrain the ionisation parameter and gas-phase metallicity. Closely related to the commonly used [O iii]/[O ii] ratio, our [Ne iii]/[O ii] measurement shows this source is similar to local “Green Pea” galaxies and Lyman-continuum leakers. It furthermore suggests this galaxy is more metal poor than expected from the Fundamental Metallicity Relation, possibly as a consequence of excess gas accretion diluting the metallicity. Finally, we present the highest redshift detection of ${\rm Mg\, {\small II}}\, \lambda \, 2796$, observed at high equivalent width in emission, in contrast to more evolved systems predominantly exhibiting Mg ii absorption. Strong Mg ii emission has been observed in most z ∼ 0 Lyman-continuum leakers known and has recently been proposed as an indirect tracer of escaping ionising radiation. In conclusion, this strongly lensed galaxy, observed just 300 Myr after reionisation ends, enables testing of observational diagnostics proposed to constrain the physical properties of distant galaxies in the JWST/ELT era.