Influence of mergers on LyC escape of high redshift galaxies

Abstract

Aims: We investigate the impact of galaxy mergers on the Lyman Continuum (LyC) radiation escape, fesc, from high-redshift galaxies. Methods: We post-process ~ 6e5 galaxies (redshift 5.2 < z < 10) extracted from the TNG50 cosmological simulation using a physically motivated analytic model for LyC escape. Results: Galaxies that have not experienced a merger for the last ~ 700 Myr have an average fesc ~ 3%, which increases to up to 14% immediately following a merger. The strongest effect can be observed in galaxies with stellar masses of ~ 1e7 Msun. We attribute the increase in the escape fraction to two main factors: (i) accretion of metal-poor gas onto the central region of a galaxy, which feeds star formation and LyC emission; and (ii) displacement of neutral gas relative to star-forming regions, which reduces the optical depth to LyC photons. We additionally examine how proximity to other galaxies influences LyC escape, finding that galaxies with more neighbors tend to have more frequent mergers, and thus a higher LyC leakage. However, galaxies in overdense regions tend to have a larger LyC escape fraction independently from mergers, because of their higher gas inflow, and consequent increase in the star formation rate. The increase in both mergers and gas inflow could contribute to low-mass galaxies ionizing proximity zones of high-z Ly-alpha leakers recently observed with JWST.

Figures (6)

• Figure 1: Average LyC escape fraction as a function of time elapsed since the last major merger for galaxies at various redshifts, as indicated by the colors. The black line refers to all redshift combined.
• Figure 2: Average LyC escape fraction as a function of time elapsed since the last major merger for galaxies at all examined redshifts with different stellar masses, as indicated by the colors. The black line refers to all masses combined and is the same as in fig. \ref{['fig:redshift']}.
• Figure 3: Average LyC escape fraction as a function of time elapsed since the last major merger for galaxies with gas scale height $H<10^{21}\mathrm{cm}$ ( ext-mode of escape) and $H>10^{21}\mathrm{cm}$ ( loc-mode), as well as the full sample. The lines refer to galaxies at all redshift and stellar masses.
• Figure 4: Histogram of the average escape fraction as a function of the time since the last major merger and the gas scale height for galaxies with $H> 10^{21}$cm ( loc-mode; upper panel) and $H < 10^{21}$cm ( ext-mode; lower panel). The dashed and dotted lines refer respectively to the one and two sigma distribution of galaxy counts of the respective samples, while the green line corresponds to the average gas scale height of all galaxies in a given $t_\mathrm{merger}$ bin.
• Figure 5: Histograms showing the average escape fraction (color bar) as a function of the time since the last major merger for galaxies in the loc-mode with stellar masses $M_\star = (10^{6.8}-10^{7.2})\mathrm{M}_\odot$. The panels refer to dependance on the gas flow $\mathcal{F}_\mathrm{gas}$ (top-left), the specific SFR (top-right), the ratio of galactic gas and stellar mass (center-left), the ratio of the peak rotational velocity and the one dimensional velocity distribution (center-right), the metallicity of the galaxy normalized by the stellar mass (bottom-left) and the distance between the center of mass of the galactic gas and the center of mass of the star forming gas (bottom-right). The dashed and dotted lines refer respectively to the one and two sigma distribution of galaxy counts, while the green line represents the average value of the given quantity of all galaxies in a given $t_\mathrm{merger}$ bin.