Lyman-continuum leakage as dominant source of diffuse ionized gas in the Antennae galaxy

Abstract

The "Antennae Galaxy" (NGC 4038/39) is the closest major interacting galaxy system and therefore often taken as merger prototype. We present the first comprehensive integral field spectroscopic dataset of this system, observed with the MUSE instrument at the ESO VLT. We cover the two regions in this system which exhibit recent star-formation: the central galaxy interaction and a region near the tip of the southern tidal tail. In these fields, we detect H ii regions and diffuse ionized gas to unprecedented depth. About 15% of the ionized gas was undetected by previous observing campaigns. This newly detected faint ionized gas is visible everywhere around the central merger, and shows filamentary structure. We estimate diffuse gas fractions of about 60% in the central field and 10% in the southern region. We are able to show that the southern region contains a significantly different population of H ii regions, showing fainter luminosities. By comparing H ii region luminosities with the HST catalog of young star clusters in the central field, we estimate that there is enough Lyman-continuum leakage in the merger to explain the amount of diffuse ionized gas that we detect. We compare the Lyman-continuum escape fraction of each H ii region against ionization-parameter sensitive emission line ratios. While we find no systematic trend between these properties, the most extreme line ratios seem to be strong indicators of density bounded ionization. Extrapolating the Lyman-continuum escape fractions to the southern region, we conclude that just from the comparison of the young stellar populations to the ionized gas there is no need to invoke other ionization mechanisms than Lyman-continuum leaking H ii regions for the diffuse ionized gas in the Antennae.

The MUSE Atlas of Disks (MAD): resolving star formation rates and gas metallicities on <100 pc scales†

“…In other words, we find a "retired" RSFMS for the DIG component. As our study focuses on the regions ionized by hot/young stars, this result helps to understand the origin of the DIG at sub-kpc scales and agrees with Weilbacher et al (2018), where they found that the leaking UV photons from Hii regions can explain the ionization of the DIG. Our study cannot rule out the possibility that the hot old stars can ionize part of the DIG because we do not study the LINER-like ionized regions.…”

“…This component can be pushed above the galactic disk by superbubbles created by supernova events (e.g., Wood et al 2010). Weilbacher et al (2018) recently found that the UV photons leaked by the Hii regions in the Antennae can explain the ionization of the DIG there.…”

The Gas–Star Formation Cycle in Nearby Star-forming Galaxies. I. Assessment of Multi-scale Variations

“…Unfortunately, DIG can contribute 50% or more to the observed Hα flux of a galaxy (e.g., review by Haffner et al 2009). The origin of the ionizing photons producing the DIG is still debated, with some or indeed all of these photons originating from massive young stars (e.g., due to Lyman-continuum leakage from H II regions; Weilbacher et al 2018). For our purposes, the key issue is that the DIG emission is not necessarily co-spatial with the young stellar population powering the Hii regions (see, e.g., Kreckel et al 2016).…”

KUG 0200-096: Dwarf Antennae Hosting a Tidal Dwarf Galaxy

“…NGC 4038/39 system is more massive and the TDG has a 15 kpc diameter whereas KUG 0200-096 TDG diame-ter is 2.5 kpc. Weilbacher et al (2018) identify multiple sub-clumps in the NGC 4038/39 TDG and detected multiple HII regions. In that sense, KUG 0200-096 TDG is morphologically more similar to BCDs, typically xBCDs (Drinkwater & Hardy 1991) and in contrast morphological properties of NGC 4038/39 TDG is comparable to a typical dwarf irregular galaxy (dIr).…”