To better understand observations of distant galaxies, an international team of astronomers has built a sample of local galaxies that can be studied in greater detail. In a newly published study, they show how the amount of light escaping from a galaxy is related to its physical properties. The result has an implication for how we interpret observations of galaxies in the early universe.
One of the most useful ways to study galaxies In the early universe by a certain type of ultraviolet radiation a light It’s called Lyman Alpha. This light is emitted by the gas around the hottest stars, and is therefore particularly good for observing galaxies of high star formation.
Unlike other types of light, the exact wavelength and direction of travel depends on many physical processes inside and outside galaxies. Alpha Lyman light does not travel directly toward our telescopes, but takes a convoluted path out of the galaxy.
On its way, it travels through regions with different physical conditions that not only affect the path taken by individual particles of light, but also change the wavelength and even absorb an indefinite part of the light.
Some regions are hotter, some are more dusty, some have strong outflows of gas clouds, etc. All of these things physical condition It makes it difficult to interpret the Lyman alpha light we see. On the other hand, the reward, if interpreted correctly, is great, precisely because we can then learn about the physics of the galaxy.
Spying on our neighbors
Galaxies in the distant universe are faint and small, and thus are particularly difficult to observe. So an international team of astronomers set out to build a “reference” sample of galaxies in our local neighborhood. Although it is still hundreds of millions of light-years away, it is close enough that it can be studied in great detail, using many different telescopes around the world and in space.
This Lyman Alpha Reference Sample, or LARS, revealed many interesting properties of galaxies that are most useful when observing more distant galaxies. In the latest study, led by Jens Millinder, a senior researcher at Stockholm University, and published in the journal The Astrophysical Journal Supplement Seriesastronomers deduced how much Alpha Lyman light escapes from the galaxy, and whether or not this part is related to the different physical properties of the galaxy.
“Through the new observations, we have established a link between the extent of Alpha Lyman’s escape galaxies, and many more physical properties “For example, there is a clear relationship between the amount of cosmic dust a galaxy has and how much Lyman gets out of these galaxies. This was expected, because dust absorbs light, but now we have quantified the effect,” Melinder explains.
Astronomers have also found a relationship between the escaping light and the total mass of all the stars in the galaxy, although it is less clear. On the other hand, other properties, such as the amount of new stars forming in the galaxy, do not appear to be related to the amount of Lyman-alpha escaping from the galaxy.
Another interesting result is that the galaxies observed in Lyman Alpha appear much larger than they do at other wavelengths. This effect has been observed before, and is in line with theoretical expectations.
“We see the same effect in computer simulations of galaxies with calculations of how Lyman’s alpha travels through gas clouds in interstellar spaceexplains Peter Laursen of the Cosmic Dawn Center who was also involved in the study. This confirms that we have a fairly good theoretical understanding of the physics at play. ”
It is important to take this effect into account when observing distant galaxies where the light from the edges of galaxies is either too faint to detect or falls outside the detectors.
A quantification of the effect will be useful in future observations of more distant and therefore older galaxies:
“These findings will help explain observations of very distant, yet similar, galaxies made by the Hubble and James Webb Space Telescopes,” says Millinder. “Understanding the detailed astrophysics of this type of galaxy is critical to developing theories about how the first galaxies formed and evolved.”
Jens Melinder et al, the reference sample for Lyα. fourteenth. Lyα imaging of 45 low redshift star-forming galaxies and conclusions about global emissions, The Astrophysical Journal Supplement Series (2023). DOI: 10.3847/1538-4365/acc2b8
Niels Bohr Institute
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