A great diversity of spectral shapes in the ionising spectra of z ~ 0.6-1 galaxies revealed by HST/COS and possible detection of nebular LyC emission

TL;DR

This study extends direct LyC measurements with HST/COS to galaxies at z ~ 0.6–1, covering rest-frame LyC wavelengths down to ~600 Å and revealing a surprising diversity of LyC spectral shapes. By combining COS data with SDSS, STIS, and Xshooter spectra, the authors derive intrinsic LyC fluxes and quantify the escape fraction $f_ extrm{esc}$, finding 7 of 11 galaxies with $f_ extrm{esc}$ in the ~2%–60% range and uncovering nebular LyC emission bumps in two targets. The detection of nebular LyC near the Lyman limit implies that f_esc estimates can be biased if the nebular continuum is not accounted for, especially in near-LyC measurements around z ~ 0.3–0.4; this underscores the importance of broad UV coverage and careful modeling of stellar plus nebular LyC. The results demonstrate a wide diversity in LyC spectral shapes among low-mass star-forming galaxies and provide crucial constraints for interpreting the role of such galaxies in cosmic reionization, as well as for developing indirect LyC indicators such as the $O_{32}$ ratio.

Abstract

We present observations of eleven compact star-forming galaxies in the redshift range z = 0.6145 - 1.0053, with the Cosmic Origins Spectrograph (COS) on board the Hubble Space Telescope (HST). We aim to spectroscopically measure for the first time the Lyman continuum (LyC) over a wider rest-frame wavelength range of ~ 600 - 900A compared to ~ 850 - 900A in previous studies of galaxies at z ~ 0.3 - 0.4. The HST data are supplemented by SDSS spectra of all galaxies and by a VLT/Xshooter spectrum of one galaxy, J0232+0025. These data are used to derive the spectral energy distribution in the entire UV and optical range, the stellar mass, and the chemical composition from the nebular emission lines. We detect stellar LyC emission in seven out of eleven galaxies with escape fractions, f_esc(LyC), in the range of ~ 2 - 60%, and establish upper limits for f_esc(LyC) in the remaining galaxies. We discover for the first time nebular LyC emission as a bump just bluewards of the LyC limit at 912A in two galaxies, J0232+0025 and J1021+0436. We find a similar bump among our earlier studies in a less distant galaxy J1243+4646 with z = 0.4317. We conclude that the use of the LyC continuum in the wavelength range close to the LyC limit, which contains both the stellar and nebular continua, requires special consideration to not overestimate the observed f_esc(LyC).

Figures (8)

• Figure 1: HST COS/NUV acquisition images of the candidate LyC leaking galaxies on a surface brightness scale. The COS spectroscopic aperture with a diameter of 25 is indicated in all panels by a circle. The linear scale in each panel is derived by adopting an angular size distance (Table \ref{['tab1']}).
• Figure 2: (a) Extinction-corrected rest-frame Xshooter spectrum of J0232+0025 (grey line) overlaid with modelled SEDs. Nebular, stellar, and total (nebular+stellar) SEDs are represented by blue, red, and black lines, respectively. (b) Observed rest-frame SDSS (black line) and Xshooter (grey line) spectra of J0232+0025 overlaid with their corresponding attenuated modelled SEDs (the SDSS spectrum shown in red and the Xshooter spectrum shown in blue). (c) Observed rest-frame COS, STIS, and Xshooter spectra of J0232+0025 (grey lines) overlaid with modelled SEDs attenuated by adopting either $R(V)$ = 3.1 (black line) and $R(V)$ = 2.7 (red line), assuming the C89 reddening law. Blue-filled circles denote GALEX FUV and NUV, and SDSS $u,g,r,i,z$ photometric data. The red cross indicates the NUV flux calculated from the SED by adding the Ly$\alpha$ emission with an EW(Ly$\alpha$) = 108Å from the STIS spectrum. Dotted black lines indicate GALEX FUV and NUV response curves reduced to the J0232+0025 rest-frame. (d) Observed COS spectrum of J0232+0025 (grey line) overlaid with the intrinsic modelled SED (dash-dotted blue line) and the SED attenuated with $R(V)$ = 2.7. The short horizontal red line represents the level of the LyC stellar continuum, and the vertical dotted line indicates the location of the LyC limit at 912Å.
• Figure 3: Isophotes of the J0232+0025 COS/NUV acquisition image in arbitrary units but with equal steps. The diameter of the outer isophote is larger than the slit width of 05 used in the STIS observations.
• Figure 4: Fragments of smoothed spectra in the LyC region for 11 galaxies at $z$$\sim$ 0.6 -- 1.0 (a) -- (k) (this paper), and for J1243$+$4646 ( l) from I18b. Modelled intrinsic SEDs, attenuated by Milky Way extinction with $R(V)_\textrm{MW}$ = 3.1 and internal extinction with $R(V)_\textrm{int}$ = 2.7, assuming the C89 reddening law, and the unreddened SEDs are shown by solid red and dash-dotted blue lines, respectively. Emission features at the observed wavelengths $\lambda$1216Å and $\lambda$1301Å are geocoronal Ly$\alpha$ and O i emission lines, respectively. The parts of the observed spectra containing the LyC bumps for a) J0232+0025, h) J1021+0436, and l) J1243+4646 are highlighted by magenta lines. Dotted black lines in all panels indicate the GALEX FUV response curve reduced to the galaxy rest-frame. The vertical dotted line in all panels indicates the location of the LyC limit at 912Å. Fluxes are in 10$^{-16}$ erg s$^{-1}$ cm$^{-2}$Å$^{-1}$, and wavelengths are in angstrom.
• Figure 5: Relations between LyC escape fraction $f_\textrm{esc}$(LyC) in low-redshift LyC leaking galaxies derived from SED fits and a) the O$_{32}$ = [O iii]$\lambda$5007/[O ii]$\lambda$3727 emission-line flux ratio, and b) stellar mass $M_\star$. LyC leakers from I16aI16bI18aI18bI21a, B14, and C17 are shown by blue-filled circles. The galaxies from Fl22aFl22b and this paper with detected LyC emission are represented by green- and red-filled circles, respectively. The blue, green, and red arrows denote upper limits in galaxies with non-detected LyC emission. Dotted red lines connect the two different values of the LyC escape fraction from Table \ref{['tab6']} for J0232+0025 and J1021+0436, corresponding to higher and lower values obtained from the fluxes within the wavelength ranges with and without the LyC bump, respectively.