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Hubble imaging of the ionizing radiation from a star-forming galaxy at z=3.2 with fesc>50%

Star-forming galaxies are considered to be the leading candidate sources that dominate the cosmic reionization at z>7, and the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently a active line of research. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890A rest-frame, and detect LyC emission. This galaxy is very compact and also has large Oxygen ratio [OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiation spans the range 50-100\%, depending on the IGM attenuation. The LyC emission is detected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved, with effective radius R_e<200pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible with optically thin media. Irrespective to the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization, when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([OIII]+Hbeta) observed recently at z~7.5-8.5 is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at z>7 allowing a direct comparison with lower redshift LyC emitters, as reported here.


INTRODUCTION
Cosmic reionization is a major episode in the history of the Universe and the search for ionizing sources is one of the main goals of modern observational cosmology (Robertson et al. 2010). Star-forming galaxies and AGN have been proposed to be the dominant sources of ionizing radiation, possibly acting at different cosmic epochs (Haardt & Madau 2012). While the redshift evolution of the ultraviolet luminosity function of star-forming galaxies is relatively well measured up to z 7 − 8, showing that the bulk of the ultraviolet luminosity density is dominated by the faint galaxy population (L < L , e.g, Bouwens et al. 2015), the number density of high redshift and faint AGN is still highly uncertain (e.g., Giallongo et al. 2015;Georgakakis et al. 2015). In addition, understanding the mechanisms of reionization (and post-reionization z < 6) hinges on understanding how the escape fraction of ionizing radiation, f esc (LyC), changes as a function of luminosity and redshift. Because of the IGM opacity, direct observation of ionizing radiation during reionization is not feasible (Prochaska et al. 2010). A strategy to make progress is to identify LyC sources at lower redshift, e.g. 3-3.5, and study which of their properties can be used as predictors of a LyC leakage. Recent advances have been made by looking at starburst galaxies in the local universe (Borthakur et al. 2014;Izotov et al. 2016), about 10 Gyrs after reionization ended (z ∼ 6). However, it is more useful to identify LyC leakers at the highest redshifts possible (z ∼ 3, one billion years after the end of reionization), because the properties of these galaxies are better analogs to those that reionized the universe.
Escaping LyC radiation from galaxies has been searched in the recent years (Vanzella et al. 2010a;Siana et al. 2010;Nestor et al. 2013;Mostardi et al. 2013Mostardi et al. , 2015 and, to date, no convincing LyC emission has been reported at high redshift (Siana et al. 2015). Moreover, nothing is known about the spatial distribution of the emerging LyC radiation. Here we report on a LyC emission arising from a distant galaxy (z = 3.212) unambiguously confirmed with HST observations. Throughout the paper, the AB system (AB = 31.4 − 2.5log( f ν /nJy) and cosmology Ω tot , Ω M , Ω Λ = 1.0, 0.3, 0.7 with H 0 = 70kms −1 Mpc −1 are used.

THE CHALLENGING DETECTION OF ESCAPING LYC PHOTONS
The difficulty of directly and unambiguously identifing the LyC radiation in distant galaxies are due to a combination of several effects: (1) superposition of foreground sources can produce false LyC detections (Vanzella et al. 2010a(Vanzella et al. , 2012, and in this respect HST observations are crucial (Siana et al. 2015); (2)  distribution of the neutral gas in galaxies and the relatively short duty cycle of f esc (LyC) over cosmic time adds further stochasticity in the LyC visibility (Wise et al. 2014;Cen & Kimm 2015); (4) the accessible luminosity range in the LyC achievable by current large telescopes limits the analysis to bright galaxies (L > 0.5L ) for which the average escaping ionizing radiation has now been shown to be intrinsically low (< 5 − 10%, Grazian et al. 2016;Vanzella et al. 2010b;Mostardi et al. 2015;Siana et al. 2015). These aspects make the search for escaping ionizing radiation challenging and can explain the current low detection rate at high redshift. Nevertheless, the identification of examples with escaping LyC radiation lying in the tail of the large f esc (LyC) values, though rare (Vanzella et al. 2010b), represents the only empirical way we have to increase our physical insight into the mechanisms that allow ionizing photons to escape and thus providing unique reference for studies at z > 6.
2.1. The selected candidate LyC emitter A LyC candidate at z = 3.212 in the GOODS-Southern field, named Ion2 (GDS-ID 033203.24-274518.8), was first identified by Vanzella et al. (2015). The compact star-forming region, showing strong [O III]λλ4959, 5007 emission lines (with a rest-frame equivalent width of 1500Å) and large Oxygen ratio [OIII]λ5007/[OII]λ3727 (≥ 10) makes Ion2 the highest redshift "Green Pea" galaxy currently known and, accordingly to the photoionization models (Nakajima & Ouchi 2014;Jaskot & Oey 2013), an ideal candidate LyC emitter. A plausible spectroscopic LyC detection was subsequently discussed in de Barros et al. (2016). However, the presence of a close companion not resolved with ground-based spectroscopy and imaging (0.2 , see Fig. 1) casted some doubts about the association with Ion2 of the observed flux and thus on the reliability of the LyC leakage.

HST OBSERVATIONS
To confirm the LyC emission, we obtained HST WFC3/UVIS image in the F336W filter, corresponding to rest-frame 730−890 Å. Seventeen 2800s (1-orbit) dithered exposures were taken for a total integration time of 47.6 ks. The charge transfer efficiency (CTE) of the WFC3/UVIS CCDs has degraded significantly due to radiation damage and is particularly problematic in images with low background. To mitigate the effects of poor CTE, we increased the background to ∼ 12e − pix −1 with a post-flash LED (Biretta & Baggett 2013). Furthermore, we placed the target near the read-out edge of the CCD so that electrons need only transfer across ∼ 300 of the 2048 pixels. Finally, we applied a pixel-based CTE correction 12 . As the dark current is nearly half of the total background, proper dark subtraction is critical. Unfortunately, the STScI darks are only a single value and do not capture the gradient and blotchy pattern in the dark current (Teplitz et al. 2013). Also, because the darks are not CTE corrected, more than half of hot and warm pixels are not properly masked (Rafelski et al. 2015). To mitigate these issues we adopted the dark processing method explained in detail in Rafelski et al. (2015). First, we CTE-correct all raw dark images in the anneal cycle of our visits and remove the cosmic rays. We then find and mask the hot pixels that appear throughout the anneal cycle and make a mean super dark from all darks in the anneal cycle. We then dark subtract the science images while masking the hot pixels that were found to exist at the time of the observation.
We processed all of our CTE-corrected raw data using STS-DAS task CALWF3, including subtraction of our new super darks. These calibrated images were then combined using Astrodrizzle (Gonzaga & et al. 2012), which performs background subtraction, cosmic ray rejection and geometric distortion correction. The final combined F336W image has a pixel scale of 0.03 and is astrometrically aligned with the 3D-HST F606W image (Skelton et al. 2014) with a precision of 48 mas. As a by-product, Astrodrizzle produces an inverse variance image used to derive the uncertainties in the photometry. We performed aperture photometry using a growth-curve methodology. The estimated uncertainty was corrected for the correlated noise introduced in the drizzling process (Casertano et al. 2000). Finally, we corrected our photometry for Galactic extinction (0.041 magnitude; Schlafly & Finkbeiner 2011). The HST imaging clearly solved the problem of the close neighbour and the LyC emission has been unambiguously confirmed at S/N = 10 with magnitude m(F336W) = 27.57 ± 0.11 (see Fig. 1). It also provides the spatial mapping of ionizing radiation from a galaxy for the first time. From deep multi-wavelength observations, including Chandra 6Ms image, HST optical (GOODS, Giavalisco et al. 2004) and nearinfrared (CANDELS, Grogin et al. 2011), Spitzer (3.6, 4.5, 5.8, 8.0, 24µm) and wide spectroscopic coverage from U to the K band (VLT and Keck), we found Ion2 to be a low-mass (≤ 10 9 M ), low-metallicity (∼ 1/6Z ) star-forming galaxy (de Barros et al. 2016).

LyC morphology
The LyC leakage is co-spatial with the component A and nothing is detected from component B, where A and B refer to the brighter and the fainter blobs, respectively (see Fig. 1, following the Vanzella et al. (2015) nomenclature). Ion2 is a compact but well resolved galaxy in the F435W, F606W, F775W and F850LP ACS images, corresponding to the restframe far-UV from 1030 to 2020 Å. The galaxy, however, does not appear to be resolved in the WFC3 F336W image. We have a well-exposed nearby bright star and inserted in a blank area of the image at random position to simulate a number (N = 50) of realizations of the PSF, after rescaling it to the same flux as Ion2. We have then registered the star at the same position of the light centroid of Ion2 and subtracted it, in each case obtaining residuals consistent with the sky background (an example is shown in Fig. 3). We have also compared the morphology of Ion2 in the F336W and F435W bands to test the possibility that the lower SNR in the former band is the reason of the apparent unresolved morphology, adopting the following procedure. Basic PSF-corrected morphological information from both components were extracted by Vanzella et al. (2015) using GALFIT (Peng et al. 2010). Given their regular (symmetric) morphology, the GALFIT modeling with a Gaussian light profile well reproduces both regions in all the ACS images and shows they are spatially resolved with effective radii of a few hundred parsecs. In particular, we show in Fig. 4 the result obtained at 1000Å rest-frame (F435W band) adopting a Gaussian shape. The best solution produces an effective radius of R e = 340 ± 25pc, suggesting the stellar radiation emerges from a compact and resolved region; the error has been calculated by running GALFIT on simulated images obtained by inserting the B-band model in random and free regions of the F435W-band image. GALFIT fitting on the LyC image does not produce resolved solutions. In order to characterize the LyC morphology we perform a simple test by subtracting the best-fit B435-band model from the F336W image. After normalizing the two images at the same flux peak and checking the residuals also over a grid of dimming factors, it turns out that the LyC emission arises from a region smaller than what has been inferred from the F435W band (see Fig. 3), in particular the spatially unresolved detection in F336W corresponds to an effective radius < 200pc. The negative residual flux seen in Fig. 3 extends at least 5 pixels radially from the center, and can't be explained by the small variations in the PSFs at the two wavelengths. The observed compactness of the LyC emission is probably not surprising if we compare our HST resolution to the size of the superstar clusters observed in local starbursts, in which the O-type stars are spatially segregated toward the center within tens of parsecs (e.g., Annibali et al. 2015;James et al. 2016).
Another possible interpretation is that the LyC emission of Ion2 could originate from an AGN. This is unlikely and is discussed in Sect. 5.1. This comparison suggests the LyC emitting region is more compact than the resolved emission measured in the F435W band. In the right panel the residuals after subtracting a star rescaled to the source flux is shown. The size of the thumbnails is 1.2 × 1.0 .

DISCUSSION
Independent from the nature of ionizing photons, the LyC detection implies a low column density of neutral gas along the line of sight, lower than 10 17.2 cm −2 (corresponding to τ (LyC) < 1). de Barros et al. (2016) has shown for the first time empirical evidences linking high f esc (LyC) with compactness of the star-forming region, the large [OIII]/[OII] line ratio, the weak ultraviolet interstellar absorption lines, and the emerging Lyα emission close to the systemic redshift, as predicted by photoionization and radiative transfer models when the medium is considered optically thin (Verhamme In the following, we summarize a few key elements (de Barros et al. 2016) and also add new empirical and theoretical evidence that favors the stellar origin of the ultraviolet light.

No high-ionization emission lines like NVλ1240,
CIVλ1550, HeIIλ1640 have been detected at more than 3σ level from the VLT/VIMOS MR spectrum (Fig. 5), while the CIII]λ1909 line is clearly detected. From CIVλ1550/CIII]λ1909 < 0.15 and CIII]λ1909/HeIIλ1640 > 4.0 line ratios and following Feltre et al. (2016), this source is classified as a starforming galaxy, lying in the same region occupied by low-metallicity galaxies of Stark et al. (2014).
2. Ion2 is spatially resolved in all of the ACS bands (340 ± 25pc) implying the stellar emission is detected and dominating the observed range 1000Å-2000Å restframe.
3. The narrow width of the [OIII]λ5007 emission line (σ = 65km s −1 ) is compatible with lower redshift starforming galaxies (Maseda et al. 2014), while the AGN population typically show higher velocity dispersions up to several hundreds or thousand km s −1 .
4. Ion2 is not detected in the X-ray in the 4 Ms CDFS (Cappelluti et al. (2015)) and no evidence of emission is inferred from the recent Chandra pointings publicly available on the Chandra website 13 , which increased the exposure time to ∼ 6Ms (see Fig. 5). This places an X-ray luminosity limit of L X 3 × 10 42 ergs −1 at 1σ limit. If Ion2 were a type I AGN, and assuming that the L X − L [OIII] relation observed locally is valid at z = 3, the measured [OIII]λ5007 luminosity (de Barros et al. 2016) would imply a detection with S/N > 100 at the Ion2 position in the 6Ms X-ray image, corresponding to L X 10 45 ergs −1 (Panessa et al. 2006;Ueda et al. 2015). The measured [OIII]λ5007 line luminosity (2 × 10 43 ergs −1 ) and the non detection in the 6Ms X-ray would imply a high obscuration (if it were an AGN), formally with an equivalent column density N H > 10 25 cm −2 (assuming solar metallicity). The estimated stellar mass from the LyC source is ∼ 10 8 M (see Section 5.2) and, following Maseda et al. (2014), the dynamical mass from the [OIII]λ5007 line width (σ = 65 kms −1 ) is ∼ 10 9 M . A lower limit on the BH mass of 10 8 M can be estimated by converting the [OIII]λ5007 line luminosity into an AGN bolometric luminosity (Panessa et al. 2006;Lusso et al. 2012) and then assuming that the AGN is radiating at the Eddington limit, that imply a very unusual and probably unrealistic ratio of M BH /Total mass ≥ 0.1 for this object.
5. Adopting the expected L X (from [OIII]λ5007 reported above), we expect a clear detection at 6µm rest-frame (i.e., at 24µm observed with Spitzer/MIPS) by assuming valid the relation between L X and L(6µm) (Stern 2015). In particular even adopting a L X > 10 44 erg s −1 we expect L(6µm)> 10 44 erg s −1 that corresponds to mag ( It is worth noting a photometric excess (S/N > 5) in the second IRAC channel at 4.5µm (Fig. 5), that we ascribed to possible HeIλ10830+Paγ lines with rest-frame equivalent width of ∼ 1100Å. While these lines can only be observed with JWST, such a strong HeIλ10830 emission up to 1000Å equivalent width has been observed in local compact HII regions by Izotov et al. (2014) and in a local LyC emitter candidate (Verhamme et al. 2015). Therefore we conclude that the signal observed in the HST/F336W band is very likely due to stellar emission; the only (unlikely) alternative would be the presence of a heavily obscured AGN, located at the center of the star-forming region and hidden by gas and dust at all wavelengths but visible in the ionizing continuum, a possibility we deem very contrived. An intrinsically faint AGN could co-exist with a dominating star formation activity that controls the transparency of the medium.

Constraining the burst of star formation
The measured ionizing photon production rate and the comparison with Starburst99 models (Leitherer et al. 1999) adopting instantaneous burst, Z = 0.004 and Salpeter IMF, provide a stellar mass involved in the starburst event in the range  Barros et al. 2016). In panel E a schematic view of the spectral SED fitting indicating the available observations (VLT, Keck, HST, Spitzer) is shown. JWST can cover the part at 2.4-5µm, including lines as Hα and HeIλ10830, not accessible with current instruments (panel D). Panel F shows the 6Ms X-ray 0.5-7 keV Chandra cutout (10 ×10 ) around Ion2 obtained from the public Chandra observation (the circle of diameter 3 marks the position of Ion2). The source is off-axis thus the PSF of the source would cover the entire circle symbol. 5 × 10 6 − 5 × 10 7 M (depending on the IGM transmission), with the young stellar component (< 10Myr) dominating the ionizing radiation, in which the number of O-type stars ranges between 2 × 10 4 − 2 × 10 5 . The ionizing photon production rate is similar to the one derived at lower redshift for a recently discovered LyC emitter (Izotov et al. 2016).
Such a young starburst event can generate substantial ionizing radiation and produce ionized cavities, and recent supernovae and stellar winds may have carved holes in the ISM that favour LyC photon escape into the intergalactic medium.  (James et al. 2016) in which the younger stellar component containing O-type stars is identified in the core of the starburst (similarly to what is inferred here) and can photoionize regions out to hundreds of parsecs, as observed also in a local starburst by Annibali et al. (2015). It is worth noting that the differential depression of nebular emission among Balmer and metal lines when a substantial leakage of ionizing radiation is present could strongly affect the usual diagnostic diagrams that separate star-forming vs. AGN emission (e.g., BPT, Baldwin et al. 1981); in partic-ular, a LyC emitter would move toward the AGN cloud if the Balmer lines are attenuated first.

CONCLUSIONS
While the LyC emitter reported here is rare among the sources with similar luminosity (Vanzella et al. 2010b;Grazian et al. 2016), the non-ionizing multi-frequency properties observed in our galaxy and the confirmed LyC emission provide valuable prospects for the characterization of similar or fainter sources in higher redshift domains. In particular, the confirmed z > 7.5 galaxies (Finkelstein et al. 2013;Oesch et al. 2015;Zitrin et al. 2015) show particularly strong Oxygen and Balmer structure ([O III]λλ4959, 5007 + Hβ), at the same level reported here (equivalent widths larger than 800-1000Å rest-frame). It is premature to address if those sources have effectively an f esc (LyC) > 0, as optical line ratios are needed to perform a direct comparison and this is postponed until JWST launch. Our result is currently a unique highredshift reference, both in terms of large Oxygen line ratio and large line equivalent width, and needs to be extended to statistically significant samples, especially investigating the faint luminosity domain. It also outlines the feasibility of the identification of ionizing sources during the reionization epoch.
We thank F. Annibali and D. Schaerer for useful discussions. Part of this work has been funded through the INAF grants (PRIN INAF 2012). MB acknowledges support from the FP7 Grant "eEASy" (CIG 321913).