DETECTIONS OF LYMAN CONTINUUM FROM STAR-FORMING GALAXIES ATz∼ 3 THROUGH SUBARU/SUPRIME-CAM NARROW-BAND IMAGING

Abstract

Knowing the amount of ionizing photons from young star-forming galaxies is of particular importance to understanding the reionization process. Here we report initial results of Subaru/Suprime-Cam deep imaging observation of the SSA22 proto-cluster region at z = 3.09, using a special narrow-band filter to optimally trace ionizing radiation from galaxies at z ∼ 3. The unique wide field-of-view of Suprime-Cam enabled us to search for ionizing photons from 198 galaxies (73 Lyman break galaxies (LBGs) and 125 Ly-α emitters (LAEs)) with spectroscopically measured redshifts z ≃ 3.1. We detected ionizing radiation from 7 LBGs, as well as from 10 LAE candidates. Some of the detected galaxies show significant spatial offsets of ionizing radiation from non-ionizing UV emission. For some LBGs the observed non-ionizing UV to Lyman continuum flux density ratios are smaller than values expected from population synthesis models with a standard Salpeter initial mass function (IMF) with moderate dust attenuation (which is suggested from the observed UV slopes), even if we assume very transparent IGM along the sightlines of these objects. This implies an intrinsically bluer spectral energy distribution, e.g, that produced by a top-heavy IMF, for these LBGs. The observed flux desity ratios of non-ionizing UV to ionizing radiation of 7 detected LBGs range from 2.4 to 23.8 and the median is 6.6. The observed flux density ratios of the detected LAEs are even smaller than LBGs, if they are truly at z ≃ 3.1. We find that the median value of the flux density ratio for the deteced LBGs suggest that their escape fractions is likely to be higher than 4%, if the Lyman continuum escape is isotropic. The results imply that some of the LBGs in the proto-cluster at z ∼ 3 have the escape fraction significantly higher than that of galaxies (in a general field) at z ∼ 1 studied previously.

ALMA observations of az≈ 3.1 protocluster: star formation from active galactic nuclei and Lyman-alpha blobs in an overdense environment

“…Despite these caveats we note that our calculated escape fractions are broadly consistent with those estimated for star-forming galaxies at z ≈ 3 (e.g. Iwata et al 2009;Siana et al 2015).…”

Analyzing low signal-to-noise FUSE spectra

“…In previous articles where z ∼ 3 galaxies were imaged in the LyC, it was shown that emitted ionizing photons can be found at some distance from the center of the galaxies (Iwata et al 2009;Nestor et al 2011). While there is no possible way of acquiring LyC images of Haro 11, we do know that this local galaxy has star-forming regions out to, and beyond, the 30 × 30 LWRS aperture of FUSE.…”

“…Iwata et al (2009) and Nestor et al (2011) demonstrated that LyC leakage can be found spatially off-set from the main body of the non-ionizing UV continuum (although some of these detections statistically are likely to be lower redshift contaminants). To estimate the distance from the center at which we would expect most of the ionizing radiation to slip through a picket fence structure in Haro 11, we adopt a simple model with two components -the ionizing stars and the neutral gas.…”

“…To measure escape fractions and investigate the physical properties that allow leakage, we need to study more nearby objects. This can and has been done with groundbased telescopes at z ∼ 3 (e.g., Steidel et al 2001;Shapley et al 2006;Iwata et al 2009;Vanzella et al 2010;Nestor et al 2011), or locally by using space telescopes such as HUT (Leitherer et al 1995), FUSE (Deharveng et al 2001;Bergvall et al 2006;Grimes et al 2007Grimes et al , 2009, GALEX (Cowie et al 2009), and HST (Malkan et al 2003;Siana et al 2007Siana et al , 2010Bridge et al 2010). Of the four only HST and GALEX are still in operation, and until now only HUT and FUSE have been providing wavelength coverage down to 900 Å, such that local galaxies can be studied.…”

Constraints on the star formation efficiency of galaxies during the epoch of reionization

“…We allow fesc to vary between 0.01 and 1. Observations of star-forming galaxies at z ∼ 1-3 suggest a small average ionizing escape fraction from a few percent to smaller than 15%, which is likely to increase towards fainter galaxies (or equivalently towards higher redshifts, e.g., Iwata et al 2009;Siana et al 2010;Nestor et al 2013;Mostardi et al 2015). In recognition of the redshift dependence of Mmin, we model its variation by rescaling Mmin(z) as given by Okamoto et al (2008) by a factor of 0.1-10.…”