A HIGH-RESOLUTIONHUBBLE SPACE TELESCOPESTUDY OF APPARENT LYMAN CONTINUUM LEAKERS ATz∼ 3

Abstract

We present U 336 V 606 J 125 H 160 follow-up Hubble Space Telescope (HST) observations of 16 z ∼ 3 candidate Lyman continuum (LyC) emitters in the HS1549+1919 field. With these data, we obtain high spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500 Å to 900 Å fluxdensity ratio of F F 4.0 2.0 UV LyC obs ( ) =  , compatible with predictions from stellar population synthesis models. Assuming minimal IGM absorption, this ratio corresponds to a relative (absolute) escape fraction of f esc,rel MD5b = 75%-100% ( f esc,abs MD5b = 14%-19%). The stellar population fit to MD5b indicates an age of 50 Myr, which is in the youngest 10% of the HST sample and the youngest third of typical z ∼ 3 Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at z = 2.85, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from z ∼ 3 star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.

The Lyman continuum escape fraction of galaxies atz= 3.3 in the VUDS-LBC/COSMOS field

“…The new upper limit by these observations is f rel esc ∼ 7−9%, consistent with the limits by Vanzella et al (2010b) and Boutsia et al (2011). In addition, Mostardi et al (2015) reported an HST follow-up analysis of 16 candidate LyC emitters found by Mostardi et al (2013). These authors obtained deep U 336 , V 606 , J 125 , and H 160 imaging with WFC3 and ACS for these galaxies: thanks to the high spatial resolution of HST, they showed that 15 emitters can be explained by contamination of low-z objects, wrong spectroscopic redshifts, or uncertain photometric redshifts.…”

“…Imposing the constraint of a physical value for the relative escape fraction of f rel esc < 100%, the LoS of MD5b must be very transparent, exp(−τ IGM ) > 0.75, which corresponds to a probability <10 −4 , according to Inoue et al (2014). This could be an indication that this galaxy is also a low-z contaminant, similar to the other cases studied by Mostardi et al (2015).…”

“…These conclusions have been strengthened by recent observations by Siana et al (2015), who found no convincing detection in their deep HST imaging with WFC3-UVIS of five LyC emitters extracted from the sample of Nestor et al (2011), or by Mostardi et al (2015), who only found one robust LyC emitter after a reanalysis of a sample of 16 galaxies by Mostardi et al (2013). More interestingly, the only robust candidate LyC emitter by Mostardi et al (2015), the galaxy MD5b, has an observed ratio F UV /F LyC = 4.0 ± 2.0, equivalent to a relative escape fraction of 75%, when assuming complete transmission of the IGM. Instead, if a mean value of exp(−τ IGM ) = 0.4 at z ∼ 3.1 is adopted, following Inoue et al (2014), the relative escape fraction of MD5b turns out to be 188%.…”

“…These authors obtained deep U 336 , V 606 , J 125 , and H 160 imaging with WFC3 and ACS for these galaxies: thanks to the high spatial resolution of HST, they showed that 15 emitters can be explained by contamination of low-z objects, wrong spectroscopic redshifts, or uncertain photometric redshifts. Only one galaxy shows a robust LyC detection, according to Mostardi et al (2015). On the basis of these results, they concluded that galaxies at z ∼ 3 provide the same contribution of QSOs to the HI photoionizing background, although with large uncertainties.…”

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.…”