Lyman Continuum escaping from in-situ formed stars in a tidal bridge at z = 3

TL;DR

This study investigates the origin of Lyman Continuum escape in the high-redshift universe by combining JWST/NIRSpec IFU spectroscopy with HST imaging of the z = 3 Lyman-Continuum emitter LACES104037. The analysis demonstrates that LyC photons escape from an in-situ formed star cluster embedded in a tidal bridge toward an interacting companion, rather than from the galaxy core, and estimates a global LyC escape fraction of $f_{\text{esc}}^{\text{LyC}} = 0.57 \pm 0.05$ with an estimated stellar age of $\sim 6$–$7$ Myr. The interacting partner lies at a projected distance of $\sim 12.5$ kpc and is blueshifted by $\sim 450$ km s$^{-1}$, yielding interaction timescales of order $15$–$45$ Myr, consistent with the tidal-bridge scenario. This result suggests that environmental processes, such as tidal stripping and in-situ star formation in tidal features, can play a major role in LyC leakage at $z \gtrsim 2$, potentially revealing a substantial, previously underrepresented fraction of LCEs and a higher cosmic LyC budget than inferred from low-redshift calibrations.

Abstract

In order to account for reionization of the early Universe, galaxies at that time must have had significantly higher escape fractions of Lyman Continuum (LyC) than observed in the present Universe. Any explanation invoked to explain LyC escape must agree with this dramatic cosmic evolution. Galaxy mergers are often suggested as such a regulating mechanism. They occur an order or magnitude more frequently at $z \ge 3$ than in the local Universe, and they can trigger LyC escape either by inducing strong nuclear starbursts, or by tidally displacing the neutral ISM from the bulk of the stars. In the local Universe, LyC escape has been found to correlate with a range of physical and observable properties closely associated with strong star formation. For this reason, interest in mergers as drivers of LyC escape have been mainly focused on their capacity to induce strong star formation. However, at $z \ge 2$, these correlations are weaker, and we observe a much more diverse Lyman Continuum Emitter (LCE) population. This suggests that processes external to the LCE galaxies are more important for facilitating the escape at higher redshifts, which makes tidal displacement an interesting explanatory model; however, this has only been conclusively observed once before. In this letter, we present archival JWST/NIRSpec IFU and HST UVIS and IR imaging observations of the z = 3 Lyman-Continuum emitter LACES104037. We find that its Lyman-Continuum escape originates in a tidal bridge in the direction towards an interacting companion galaxy first identified in this work. LyC escape from tidal stripping or in-situ formed stars in tidal features could help explain both the higher cosmic LyC escape fraction and the greater diversity of LCE galaxy properties at higher redshifts.

Figures (2)

• Figure 1: Archival Hubble observations in F160W probing the rest-frame B band (left) and F336W showing rest-frame LyC (center), along with a wave-axis median image of the JWST/NIRSpec IFU cube showing the rest-frame Optical (right), showing LACES104037 and its immediate surroundings. Green dashed squares in the HST images show the approximate footprint of the JWST/NIRSpec observations. Insets show a zoom in on the LACES104037 itself, centered on the LCE.
• Figure 2: Kinematics and [Oiii] line emission in LACES104037 and the companion LACES104037s. The Redshift is masked to only include spaxels where S/N([Oiii]) > 3 and the line fitting succesfully converged on a solution. This map shows that LACES104037s is close to LACES104037 in velocity space and most likely an interacting companion. The green circle shows the approximate location of the escaping LyC, and the white contours in the redshift map show stellar continuum levels from the left panel.