A bottom-up search for Lyman-continuum leakage in the Hubble Ultra Deep Field

Context: Studies of the production and escape of Lyman Continuum from galaxies often rely on array of indirect observational tracers in preselection of candidate leakers. Aims: Here, we investigate how much ionizing radiation might be missed due to these selection criteria by completely removing them and performing a search selected purely from rest-frame LyC emission; and how that affects our estimates of the ionizing background. Methods: We invert the conventional method and perform a bottom-up search for Lyman-continuum leaking galaxies at redshifts $2 < z < 3.5$. Using archival data from HST and VLT/MUSE, we run source finding software on UV-filter HST images from the HUDF, and subject all detected sources to a series of tests to eliminate those that are inconsistent with being ionizing sources. Results: We find 6 new and one previously identified candidate leakers with absolute escape fractions ranging from 36% to 100%. Our filtering criteria eliminate one object previously reported as a candidate ionizing emitter in the literature, while we report non-detection in the rest frame Lyman continuum of two other previously reported sources. We find that our candidates make a contribution to the metagalactic ionizing field of $\log_{10}(\epsilon_{\nu}) = 25.32(+0.25)(-0.21)$ and $25.29(+0.27)(-0.22)$ erg/s/Hz/cMpc$^3$ for the full set of candidates and for the 4 strongest candidates only; both values are higher than but consistent with other recent figures in the literature. Conclusions: Our findings suggest that galaxies that do not meet the usual selection criteria may make a non-negligible contribution to the cosmic ionizing field. We recommend that similar searches be carried out on a larger scale in well-studied fields with both UV and large ancillary data coverage, for example in the full set of CANDELS fields.