The Ly$α$ and Continuum Origins Survey. III. Investigating the Link between Galaxy Morphology, Merger Properties and LyC Escape

TL;DR

This study uses the LaCOS dataset to connect galaxy morphology and merger state with LyC escape at $z\sim0.3$, leveraging uniform sub-kpc HST imaging across rest-frame UV and optical bands. It finds robust anti-correlations between $f_{esc}^{LyC}$ and compact, centrally concentrated star-forming regions, as well as UV-clumpiness and asymmetry, indicating LyC leakage is favored by compact UV structures. Visual analyses show a substantial merger fraction among LaCOS galaxies, with robust mergers predominately at advanced interaction stages, yet no statistically robust difference in LyC escape between robust mergers and other systems, suggesting LyC leakage may occur over a narrow merger timescale or via multiple pathways. The results imply mergers can facilitate LyC escape, particularly near coalescence, but larger, multi-wavelength surveys and HI mapping are required to robustly quantify the role of mergers in reionization, especially at high redshift where observational biases dominate.

Abstract

Characterizing the mechanisms and galaxy properties conducive to the emission and escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization, and identify the sources that powered it. The Ly$α$ and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multi-wavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission $f_{esc}^{LyC}$. We find strong anticorrelations between $f_{esc}^{LyC}$ and size ($r_{20}$, $r_{50}$, and $r_{80}$) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, the bluest filter in our dataset, tracing UV continuum and Ly$α$. We find that $\geq48\%$ of LaCOS galaxies, and $\geq41\%$ of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The $f_{esc}^{LyC}$ properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) $f_{esc}^{LyC}$ tends to be larger in galaxies with a small number of compact, centrally-located, UV-emitting star-forming regions, (ii) at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at $z\sim0.3$, $z\sim0$, and (iii) mergers can facilitate the escape of LyC photons from galaxies.

Figures (10)

• Figure 1: Left panel : distribution of visual merger classification votes for LaCOS galaxies. Each galaxy was classified by 24 of the LaCOS collaboration members. There is considerable variation in the votes for any given galaxy, illustrating the importance of a large number of classifiers to obtain a consensus answer. Right panel : Pie chart showing the number of galaxies per category resulting from the classification.
• Figure 2: Heatmap showing the Kendall $\tau$ correlation coefficient and associated p-value between non-parametric morphological estimators and the $f_{esc}^{LyC}$ for LaCOS galaxies in different filters. The Kendall $\tau$ values are shown, with corresponding p-values displayed directly below in italic. Robust correlations ($p<1.35\times10^{-3}$) are indicated in bold black text, tentative correlations ($0.05<p\leq1.35\times10^{-3}$) in bold gray text and non-significant correlations ($p\geq0.05$) in regular gray text.
• Figure 3: Fits between $f_{esc}^{LyC}$ and the morphometrics found to be robustly anti-correlated with $f_{esc}^{LyC}$.
• Figure 4: Comparison of the morphometrics parameter space for LaCOS galaxies as measured in the F850LP filter (black for LyC-emitters and gray for galaxies with upper limits) as compared to COSMOS galaxies with similar redshifts ($z\sim0.3$) and stellar masses ($7.8<\log_{10}\,\rm{M}<11.2$)
• Figure 5: RGB images of LaCOS galaxies, split according to the fraction of merger votes. The top most panels show galaxies with $P_{merg}\geq79\%$, robustly identified as mergers ($\geq3\sigma$), while the galaxies on the bottom panels show galaxies with low fraction of merger votes. In these categories, galaxies are ordered from top to bottom by $f_{esc}^{LyC}$.