Studying local galaxies to understand distant galaxies’ light emissions

An international team of astronomers has created a sample of local galaxies to gain deeper insights into observations of the most distant galaxies. By studying the amount of light that escapes from a galaxy and its physical properties, they have revealed how this is connected. This discovery has significant implications for our understanding of galaxies in the early universe.

To examine galaxies in the early universe, ultraviolet light known as “Lyman alpha” is an informative tool. This light is emitted from gas surrounding the hottest stars, which makes it ideal for observing highly star-forming galaxies.

However, unlike other forms of light, Lyman alpha light’s wavelength and direction of travel are determined by various physical processes inside and outside galaxies. As it exits the galaxy, it takes a complex path, passing through regions with different physical conditions that alter its wavelength, absorb some of its light, and even change its direction.

These regions can be hot, dusty, or contain gas cloud flows, among other factors. Such conditions make it challenging to interpret the Lyman alpha light we observe, but the potential benefits of correctly interpreting it are substantial, particularly in gaining insights into the galaxy’s physics.

Spying on our neighbors

Observing distant galaxies can be challenging due to their small size and faintness. To overcome this obstacle, an international team of astronomers built a “reference” sample of galaxies in our local neighborhood, known as the Lyman Alpha Reference Sample (LARS). Despite being hundreds of millions of lightyears away, these galaxies are near enough to be studied in great detail using various telescopes worldwide and in space.

In a recent study published in The Astrophysical Journal Supplement Series and led by Jens Melinder, Senior Researcher at the University of Stockholm, the LARS has revealed many interesting properties of galaxies that are useful for observing distant ones. The team deduced the amount of Lyman alpha light that escapes the galaxy and how this fraction correlates with the galaxy’s physical properties.

Melinder explains, “We have established a connection between the amount of Lyman alpha that escapes galaxies and several of their physical properties. For example, there is a clear correlation between the amount of cosmic dust a galaxy contains and how much Lyman alpha light escapes it. This correlation was expected since dust absorbs light, but we have now quantified its effect.”

The team also found a connection between the escaping light and the total mass of stars in the galaxy, although this correlation was less clear. However, other properties like the amount of new stars formed by the galaxy did not appear to correlate with the amount of Lyman alpha light that escapes it.

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)

Observing galaxies in Lyman alpha has revealed another interesting finding: these galaxies appear larger than when observed in other wavelengths. This phenomenon is not new and aligns with theoretical expectations. The same effect has been observed in computer simulations of galaxies, confirming that the physics involved is well understood.

This result is essential when observing distant galaxies where the light from the edges of the galaxy is too faint to detect or lies outside the detectors’ range. Quantifying this effect will be useful in future observations of the earliest galaxies using the Hubble and James Webb Space Telescopes.

Melinder states that these results will aid in understanding the astrophysics of these galaxies and developing theories on how the first galaxies evolved and formed. Knowing the precise astrophysics of such galaxies is critical in comprehending observations of similar, distant galaxies.

Source: Niels Bohr Institute

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