Eight per cent leakage of Lyman continuum photons from a compact, star-forming dwarf galaxy

Abstract

One of the key questions in observational cosmology is the identification of the sources responsible for ionisation of the Universe after the cosmic Dark Ages, when the baryonic matter was neutral. The currently identified distant galaxies are insufficient to fully reionise the Universe by redshift z ∼ 6 1-3 , but low-mass star-forming galaxies are thought to be responsible for the bulk of the ionising radiation 4-6 . Since direct observations at high redshift are difficult for a variety of reasons, one solution is to identify local proxies of this galaxy population. However, starburst galaxies at low redshifts are generally opaque to their ionising radiation 7-9 . This radiation with small escape fractions of ∼1-3 % is directly detected only in three low-redshift galaxies 10, 11 . Here we present far-ultraviolet observations of a nearby low-mass star-forming galaxy, J0925+1403, selected for its compactness and high excitation. The galaxy is leaking ionising radiation, with an escape fraction of ∼ 8%. The total number of photons emitted during the starburst phase is sufficient to ionize intergalactic medium material, which is about 40 times more massive than the stellar mass of the galaxy.So-called "Green Peas" (GP), low-mass compact galaxies with very active star formation 12-15 , may be promising candidates for escaping ionising radiation. The GP galaxy J0925+1403 was selected from the Sloan Digital Sky Survey (SDSS) according to the following properties (Methods section): 1) a compact structure; 2) the presence of emission lines with high equivalent widths in its SDSS spectrum, suggesting active ongoing star formation and numerous hot O stars producing ionising Lyman continuum (LyC) radiation; 3) sufficiently bright in the far-ultraviolet (FUV) with a magnitude of 20.7 mag and redshifted enough (z = 0.301) to allow direct LyC observations with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST); and 4) a high O 32 = [OIII]λ5007/[OII]λ3727 flux ratio of 5 (see Fig. 1), which may indicate the presence of density-bounded HII regions 16 , i.e. escaping LyC radiation. We first derive some general properties of the galaxy, using the emission-line fluxes measured from the SDSS optical spectrum. After correction for the Milky Way extinction of A V,MW = 0.084 mag, we obtain an internal extinction A V,int = 0.36 mag, and a low oxygen abundance 12 + logO/H = 7.91±0.03, or less than 0.2 solar. The details of these determinations are given in the Methods 1 section. Everywhere in the paper the errors are 1σ errors.The same SDSS spectrum is used to fit a spectral energy distribution (SED) to derive the galaxy's global parameters, including the stellar mass and the age of the present burst of star formation (see Methods section). We obtain a starburst age of 2.6±0.2 Myr, a young stellar mass of (2.4±0.3)×108 M ⊙ , and a total galaxy stellar mass of (8.2±0.7)×10 8 M ⊙ . The star-formation rate is 52.2 M ⊙ yr −1 , as determined from the extinction-corrected Hβ line flux. With its low mass, low metall...

The efficiency of ionizing photon production and the radiation energy balance in compact star-forming galaxies

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The HDUV Survey: Six Lyman Continuum Emitter Candidates at z ∼ 2 Revealed by HST UV Imaging*

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