For us, galaxies are where most of the interesting stuff happens in the cosmos. Stars, planets, and black holes are all usually found inside these bustling cosmic neighborhoods. But space isn't just galaxies. In between galaxies are vast expanses of interstellar space — galactic deserts where you might be lucky to stumble across a few hydrogen atoms or, at best, a wandering rogue planet ejected from its home galaxy.
So, where does a galaxy end and interstellar space begin? There seem to be a number of plausible ways astronomers might define the 'edge' of a galaxy — perhaps where concentrations of gas and dust drop below a certain threshold or even via a galaxy's gravitational influence.
Defining the Edge of a Galaxy
Nushkia Chamba is a postdoctoral fellow at NASA's Ames Research Center who specializes in studying the outskirts of galaxies, as well as galaxy formation and evolution in general. Here, she helps explain how astronomers have defined the edge of a galaxy and what factors contribute to where a given galaxy's edge is likely to be situated.
"The short answer is that the ‘boundary’ of a galaxy has been traditionally defined using fixed levels of brightness (or technically called surface brightness isophotes)," says Chamba. However, Chamba has spearheaded recent efforts to more robustly define the edge of a galaxy. "Our work in 2020 and 2022 was the first to discuss an alternative idea of “edge of a galaxy” as a definition of size."
