A new way to measure galactic size
DOI: 10.1063/pt.uxod.ahpe
The compact spiral galaxy IRAS 08339+6517 has active star formation.
Nikole Nielsen/original image from the Hubble Space Telescope, NASA
Where does a galaxy end and intergalactic space begin? The boundary—and thus the size of a galaxy—can be defined in different ways. One common measurement is r90, the radius within which 90% of a galaxy’s total stellar brightness is contained. A determination based on apparent brightness is a problem when factors such as telescope aperture, exposure time, and wavelength can change how much light is detected. Now researchers have used gas to measure an empirical boundary of intergalactic space.
Nikole Nielsen (University of Oklahoma) and her international team targeted IRAS 08339+6517 (IRAS08) to study the starburst galaxy’s distribution of gas. Using the new Keck Cosmic Web Imager on the Keck II telescope in Hawaii, the researchers focused on the singly and doubly ionized oxygen emission lines to trace the cool gas of the galaxy. The instrument allowed Nielsen and colleagues to collect measurements across a wide field of view, which is difficult to do with other methods for studying circumgalactic gas. The researchers were able to measure the ionization of the gas extending to 30 kiloparsecs (kpc) from the galactic center.
When Nielsen and her team measured the surface brightness of IRAS08’s gas at increasing radii, they noticed that the surface brightness profile changed abruptly from an exponential curve to a power law at a radius of 4 kpc. That break radius is consistent with the break in starlight from previous studies, but it is considerably larger than IRAS08’s r90 of 2.4 kpc.
The surface brightness of doubly ionized gas declines according to an exponential function until nearly twice the r90 distance—the most common way to define the edge of a galaxy. Beyond that, the surface brightness falls as a power law. The change in fitting function appears to correspond to whether the gas is part of the galaxy (blue) or part of the circumgalactic medium (purple).
N. M. Nielsen et al., Nat. Astron. (2024), doi:10.1038/s41550-024-02365-x
Nielsen says the team was surprised to see that the line ratios—the relative strengths of emission lines—changed at the break radius as well. The researchers interpreted the changes as having different mechanisms that are responsible for ionization. Gas in the galaxy is likely photoionized by nearby star formation; exterior gas is likely ionized by shocks or the extragalactic UV background.
The transition in the surface brightness profile of the gas is the first robust quantification of the boundary between the interstellar medium inside a galaxy and the circumgalactic medium outside it that doesn’t depend on the light collection ability of an instrument. Nielsen says she hopes to investigate whether the boundary will be at the same relative distance in a less active galaxy that doesn’t have star formation to ionize the surrounding gas. (N. M. Nielsen et al., Nat. Astron., 2024, doi:10.1038/s41550-024-02365-x .)