NARROWBAND LYMAN-CONTINUUM IMAGING OF GALAXIES ATz∼ 2.85

Abstract

We present results from a survey for z ∼ 2.85 Lyman-continuum (LyC) emission in the HS1549+1933 field and place constraints on the amount of ionizing radiation escaping from star-forming galaxies. Using a custom narrowband filter (NB3420) tuned to wavelengths just below the Lyman limit at z 2.82, we probe the LyC spectral region of 49 Lyman break galaxies (LBGs) and 91 Lyα emitters (LAEs) spectroscopically confirmed at z 2.82. Four LBGs and seven LAEs are detected in NB3420. Using V-band data probing the rest-frame nonionizing UV, we observe that many NB3420-detected galaxies exhibit spatial offsets between their LyC and nonionizing UV emission and are characterized by extremely blue NB3420−V colors, corresponding to low ratios of nonionizing to ionizing radiation (F UV /F LyC ) that are in tension with current stellar population synthesis models. We measure average values of (F UV /F LyC ) for our LBG and LAE samples, correcting for foreground galaxy contamination and H i absorption in the intergalactic medium. We find (F UV /F LyC ) LBG corr = 82 ± 45 and (F UV /F LyC ) LAE corr = 7.4 ± 3.6. These flux density ratios correspond, respectively, to relative LyC escape fractions of f LBG esc, rel = 5%-8% and f LAE esc, rel = 18%-49%, absolute LyC escape fractions of f LBG esc = 1%-2% and f LAE esc = 5%-15%, and a comoving LyC emissivity from star-forming galaxies of 8.8-15.0 × 10 24 erg s −1 Hz −1 Mpc −3 . In order to study the differential properties of galaxies with and without LyC detections, we analyze narrowband Lyα imaging and rest-frame near-infrared imaging, finding that while LAEs with LyC detections have lower Lyα equivalent widths on average, there is no substantial difference in the rest-frame near-infrared colors of LBGs or LAEs with and without LyC detections. These preliminary results are consistent with an orientation-dependent model where LyC emission escapes through cleared paths in a patchy interstellar medium.

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

“…In this case the lack of high spatial resolution data from HST for the majority of the sample prevents any detailed analysis about possible contamination by interlopers/foregrounds. 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.…”

“…Iwata et al (2009), Nestor et al (2011, Mostardi et al (2013) found a significant number of ionizing sources in their surveys. In the SSA22 area, Iwata et al (2009) found evidence for f rel esc > 4% for the whole population of 73 LBGs analysed, but also provided evidence for a significant spatial offset between the ionizing and non-ionizing emission for some of them.…”

“…We can translate this ionizing luminosity density into a metagalactic hydrogen photoionization rate Γ −12 , (ionization rate of HI in unit of 10 −12 s −1 atom −1 ) using the formula by Meiksin (2009) and Mostardi et al (2013),…”

“…where σ HI = 6.3 × 10 −18 cm 2 is the hydrogen photoionization cross section at λ = 912 Å, α UV = −3.0 is the spectral slope of the stellar ionizing radiation (from Mostardi et al 2013), and h P is the Planck constant. The mean free path of ionizing photons, Δl is 83.5 proper Mpc at z = 3.3, according to recent estimates by Worseck et al (2014a).…”

Using Lyman-αto detect galaxies that leak Lyman continuum

“…The vast majority of known high-redshift galaxies are also opaque to their own ionising radiation, as demonstrated by the success of the well-known Lyman-break selection (as emphasised recently in Cooke et al 2014). Only a few tens of objects are detected in the LyC (Iwata et al 2009;Nestor et al 2011Nestor et al , 2013Mostardi et al 2013). A LyC-leaking galaxy has then peculiar rare properties that we describe below.…”