Spying on our neighbors: Local galaxies help astronomers understand distant galaxies

To better understand observations of the most distant galaxies, an international team of astronomers has built a sample of local galaxies which can be studied in much higher detail. In a newly published study they show how ...
One of the most informative ways to study in the is through a particular type of ultraviolet called "Lyman alpha." This light is emitted from gas around the hottest stars, and is therefore particularly good for observing galaxies that are highly star-forming.

Unlike other types of light, however, the exact wavelength and direction of traveling depends on many inside and outside the galaxies. Lyman alpha light doesn't just travel directly towards our telescopes, but takes on a complicated path out of the galaxy.
On its way, it travels through regions of different physical conditions which affect not only the path individual light particles take, but also changes their wavelength and even absorb an unspecified fraction of the light.

Some regions are hotter, some are more dusty, some have strong flows of gas clouds, etc. All these makes it notoriously difficult to interpret the Lyman alpha light that we see. On the other hand the reward, if interpreted correctly, is substantial, exactly because we can then learn about the physics of the galaxy.

Two of the 45 galaxies in the sample of local galaxies, observed with the Hubble Space Telescope. The colors indicate infrared, visual, and ultraviolet light from the stars in the galaxies. Credit: Melinder et al. (2023)
Read more

Lyman alpha photons are emitted from gas around stars, but on their way out of the galaxy they hit thousands or even millions of hydrogen atoms. At each interaction, they change direction and wavelength randomly. Credit: Peter Laursen/Cosmic Dawn Center.
Read more

Two galaxies from the LARS sample, here observed through filters that enhance specific physical processes: Green colors show light from the stars. Red shows where the Lyman alpha photons are emitted, and blue shows where they are observed, i.e. where they escape the galaxies (colors "add", so e.g. white means that all three colors are present). It is clear that the Lyman alpha photons do not travel directly toward our telescopes, but are "trapped" inside the galaxies until they are eventually able to escape quite far away, creating a "halo" of Lyman alpha light in the outskirts of the galaxies. Credit: Melinder et al. (2023)
Read more