Confirmation and Refutation of Lyman Continuum Leakers at $z\sim3$ with JWST NIRSpec/IFU

Abstract

Our understanding of the physical mechanisms and environments conducive to the escape of Lyman-continuum (LyC) radiation within the first 2 Gyr of cosmic history remains limited. Here we present a detailed analysis of JWST/NIRSpec medium-resolution IFU observations of two LyC-leaker candidates, LACES-94460 and LACES-104037 at z = 3.1, selected from deep HST/WFC3 F336W imaging and supported by ground-based spectroscopy. We first rule out LACES-94460 as a genuine LyC leaker, demonstrating that its apparent F336W signal originates from a nearby low-redshift interloper at z = 1.6, unambiguously identified through IFU spectroscopy. In contrast, for LACES-104037 we spectroscopically confirm bona fide LyC emission arising from a tidal-tail structure during the early stage of a galaxy merger, dubbed LACES104037-LyC. LACES104037-LyC exhibits extremely low rest-frame optical emission-line equivalent widths together with an exceptionally strong LyC flux. Within a picket-fence model framework, we reproduce its observed spectral and photometric properties with a young stellar population of age $\sim5$ Myr and a LyC escape fraction of $f_{\mathrm{esc}} \sim 99\%$. Our identification and detailed modeling of LACES104037-LyC provide one of the first compelling observational demonstrations for merger-driven LyC escape, indicating that galaxy mergers may represent an important and previously underappreciated contributor to the ionizing photon budget relevant for cosmic reionization. Furthermore, our analysis highlights the critical role of sub-kiloparsec resolution spectroscopy in securely identifying LyC leakers, removing contamination from closely projected low-redshift interlopers, and pinpointing the physical regions responsible for LyC leakage.

Figures (8)

• Figure 1: HST and JWST observations of LACES104037. Top panels: HST/WFC3 imaging obtained by the LACES program Fletcher_2019 and NIRSpec/IFU maps from the G235M/F170LP observations acquired by JWST-GO-1827 (PI: Kakiichi). From left to right, we show the LyC-band (F336W), the NUV continuum (F160W), the optical continuum reconstructed from the IFU observations (with emission features masked), and the flux maps of [Oiii] and H$\alpha$. The black box marks the IFU FoV, while the black circle with a radius of 0.2 indicates the region of significant LyC leakage. The IFU maps are trimmed at the edges to exclude regions strongly affected by instrumental artifacts. Bottom panels: three-color composite image and optimally extracted 1D spectra for the bulk of the system (LACES104037-bulk), the LyC-leaking region (LACES104037-LyC), and two mergers (LACES104037-s and LACES104037-s2), all spectroscopically confirmed at $z=3.06$. The green and red shaded regions indicate the wavelength ranges corresponding to two of the color channels in the composite image, while the blue shaded region marks the wavelength coverage of the [Oiii] and H$\alpha$ emission-line maps.
• Figure 2: HST and JWST observations of LACES94460. Left: HST WFC3/UVIS F336W image obtained by the LACES program Fletcher_2019. The black box marks the FoV of the JWST NIRSpec/IFU G235M/F170LP observations acquired by JWST-GO-1827 (PI: Kakiichi). Middle: three-color composite image with the HST F160W imaging as the blue channel and the JWST IFU data as the green and red channels. Right: optimally extracted 1D spectra at $z=3.072$ (LACES94460-a), $z=1.597$ (LACES94460-b), and $z=2.629$ (LACES94460-c), with the corresponding emission lines highlighted. LACES94460 was previously identified as a silver LyC-leaking candidate at $z\sim3.1$ by Fletcher_2019, based on its highly significant F336W detection spatially offset from UV continuum and its strong rest-optical oxygen lines confirmed by ground-based spectroscopy Nakajima_2020. However, our analysis demonstrates that the apparent F336W signal instead originates from a nearby low-redshift interloper, LACES94460-b at $z=1.6$, located within 0.5". This result highlights the critical importance of sub-kiloparsec resolution spectroscopy for reliably identifying LyC leakers at high redshifts and for removing contamination from closely projected low-redshift sources.
• Figure 3: H$\alpha$ velocity, EW, and Balmer decrement maps of LACES104037. The left panel shows the velocity distribution of the LACES104037 merger system, primarily consisting of three distinct components. We adopt the redshift of LACES104037-bulk as the zero point and compute the velocity field of the entire system, which clearly reveals the relative velocity offsets among the three components. The black box indicates the JWST/NIRSpec IFU FoV, while the green box marks the spatial coverage of the panels shown on the right. The top right panel presents the spatial distribution of H$\alpha$ EW in LACES104037. The H$\alpha$ EW distribution within LACES104037-bulk is highly inhomogeneous, suggesting substantial differences in stellar population properties among individual star-forming clumps. The LACES104037–LyC region (marked by the red circle) exhibits an apparently much lower H$\alpha$ EW than the bulk component, which is caused by the significant escape of ionizing photons, breaking Case B recombination conditions. The bottom right panel shows the Balmer decrement map. Only pixels with H$\beta$ S/N $>$ 3 are included in the calculation. The H$\alpha$ and H$\beta$ maps are smoothed with a $2 \times 2$ box filter prior to computing the Balmer decrement. A pronounced spatial inhomogeneity is evident across the system. We note that even after stacking all pixels within the LACES104037-LyC (marked by the red circle), the H$\beta$ S/N remains below the $3\sigma$ level.
• Figure 4: Best-fit SED model of LACES104037 derived using PROSPECTOR, fitted to the existing broad-band photometry and IFU spectroscopy. Our spectral fitting results are shown in Table \ref{['tab:laces104037_phys_all']}. The best-fit SED model yields a reduced chi-square of $\chi^2 = 2.93$. LACES104037 is characterized by a very young stellar population, with its SED dominated by a moderately evolved component of age $\sim16$ Myr, while an additional recent starburst within the past $\sim1$ Myr is required to reproduce the observed spectroscopic and photometric properties.
• Figure 5: Relation between $f_{\mathrm{esc}}$ and observable quantities (emission-line EW and UV slope) in the picket-fence model. Top panel shows the variation of $f_{\mathrm{esc}}$ in the UV slope--EW(H$\alpha$) parameter space for different stellar ages, with the observational result of LACES104037-bulk overplotted. Bottom panel illustrates that $f_{\mathrm{esc}}$ and stellar age are fully degenerate when only emission-line EWs are considered. H$\alpha$ and [Oiii] are the two emission lines observable for LACES104037-LyC, and in the EW-based parameter space, variations in age and $f_{\mathrm{esc}}$ trace the same evolutionary locus. The pixel-by-pixel measurements of LACES104037-bulk (with emission-line S/N $>10$ per pixel) and the observational result of LACES104037-LyC are both overplotted, revealing a pronounced gap between the two. We note that our models include only a young stellar population, while the observed optical continuum may contain a contribution from an older stellar population (see the stellar population parameters derived from SED fitting in Table \ref{['tab:laces104037_phys_all']}). As a result, the observed EWs should be regarded as lower limits relative to the intrinsic EWs of the young stellar component.