Warm Ionized Gas Revealed in the Magellanic Bridge Tidal Remnant: Constraining the Baryon Content and the Escaping Ionizing Photons around Dwarf Galaxies

TL;DR

This study maps and analyzes the warm ionized gas in the Magellanic Bridge, estimating its mass, ionization state, and photon escape fractions, revealing limited ionizing photon escape from dwarf galaxies.

Contribution

First full H-alpha intensity and velocity maps of the Magellanic Bridge, with new estimates of ionized gas mass and photon escape fractions from the Magellanic Clouds.

Findings

Ionized gas mass is between 0.7-1.7 x 10^8 solar masses.

Ionization fraction in the Bridge is 36-52%.

Escape fraction of ionizing photons from LMC and SMC is less than 5%.

Abstract

The Magellanic System includes some of the nearest examples of galaxies disturbed by galaxy interactions. These interactions have redistributed much of their gas into the halos of the Milky Way and the Magellanic Clouds. We present Wisconsin H-alpha Mapper kinematically resolved observations of the warm ionized gas in the Magellanic Bridge over the velocity range of +100 to +300 km/s in the local standard-of-rest reference frame. These observations include the first full H-alpha intensity map and the corresponding intensity-weighted mean velocity map of the Magellanic Bridge across (l, b) = (281.5, -30.0) to (302.5, -46.7). Using the H-alpha emission from the SMC-Tail and the Bridge we estimate that the mass of the ionized material is between (0.7-1.7)x10^8 times the mass of the Sun, compared to 3.3x10^8 times the mass of the Sun for the neutral mass over the same region. The diffuse Bridge is significantly more ionized than the SMC-Tail, with an ionization fraction of 36-52% compared to 5-24% for the Tail. The H-alpha emission has a complex multiple-component structure with a velocity distribution that could trace the sources of ionization or distinct ionized structures. We find that incident radiation from the extragalactic background and the Milky Way alone are insufficient to produced the observed ionization in the Magellanic Bridge and present a model for the escape fraction of the ionizing photons from both the Small and Large Magellanic Clouds. With this model, we place an upper limit of 4.0% for the average escape fraction of ionizing photons from the LMC and an upper limit of 5.5% for the SMC. These results, combined with the findings of a half a dozen results for dwarf galaxies in different environments, provide compelling evidence that only a small percentage of the ionizing photons escape from dwarf galaxies in the present epoch to influence their surroundings.