The new model reveals what a galaxy’s color tells us about its distance

A team from the Ludwig Maximilian University (LMU) has provided a model to discover what the color of a galaxy tells about its distance, to be used to measure cosmic structures.

Our universe is about 13.8 billion years old. Over the infinity of this time, the smallest initial asymmetries have grown into the large-scale structures we can see through our telescopes in the night sky: galaxies like our Milky Way, galaxy clusters, and even larger clumps of matter or filaments. . of gas and dust.

How quickly this growth happens depends, at least in today’s universe, on a kind of struggle between natural forces: Can dark matter, which holds everything together through its gravity and attracts additional matter, hold up against the energy of dark, which pushes the universe ever further?

“If we can precisely measure the structures in the sky, then we can observe this fight,” says LMU astrophysicist Daniel Grün.

This is where telescopic observation projects come in, capturing large swathes of the sky with great precision in images. For example, there is the Dark Energy Survey with the Blanco telescope in Chile and the recently commissioned Euclid satellite. LMU scientists have been involved in both projects, including in leadership roles, for years.

Largest dataset evaluated to date

Although accurately determining the distances of individual galaxies and structures from us is not always easy, it is extremely important. After all, the farther away a galaxy is, the longer its light has traveled toward us, so the picture of the universe revealed by observing it is older. An important source of information is the observed color of a galaxy, which is measured by ground-based telescopes such as Blanco or satellites such as Euclid.

A new study by a team led by Jamie McCullough and Daniel Grün, which is published in the journal Monthly Notices of the Royal Astronomical Societyanalyzed the largest data set to date and shed light on what the color of different galaxies actually says about their true distance.

In principle, the distance of a galaxy can be precisely determined by spectroscopy. This involves measuring the spectral lines of distant galaxies. As the universe as a whole is expanding, these appear to have a longer wavelength the farther away a galaxy is from us. This is because the light waves of distant galaxies are stretched on the long journey towards us.

This effect, known as redshift, also changes the apparent colors that the instruments measure in the image of the galaxy. They look redder than they actually are. This is similar to the Doppler effect we hear in the apparent sound of an ambulance siren as it passes us and leaves.

No two galaxies are alike

Jamie McCullough is a doctoral researcher at LMU and Stanford University. For her analysis, she used spectroscopic measurements from the Dark Energy Spectroscopic Instrument (DESI) in conjunction with the largest data set to date for precise galaxy color measurement (KiDS-VIKING).

Specifically, the authors combined spectroscopic data from DESI of a total of 230,000 galaxies with the colors of these galaxies in the KiDS-VIKING survey and used this information to determine the relationship between a galaxy’s distance from us and its color and luminosity. observed. No two galaxies in the universe are alike, but for each class of similar galaxies, there is a particular relationship between observed color and redshift.

“If we can combine distance information with measurements of the shape of galaxies, we can infer large-scale structures from light distortions,” says Jamie McCullough.

The results of the study make it possible to statistically determine the true distance of each galaxy observed in the images obtained by Euclid or the Dark Energy Survey.

By analyzing the observed distortions of galaxy images, scientists will be able to learn something about the behavior of cosmic structures today and billions of years ago and understand them better. This will provide insight into the evolutionary history of the universe.

To be able to observe the course of structure formation over time, you do not need to wait billions of years; it is enough to measure the structure at different distances from the Earth. With images alone, this is almost impossible, as you cannot tell the distance of a galaxy from our own by its appearance in an image.

Jamie McCullough’s study holds the key to this problem by providing a model for what a galaxy’s apparent “color” tells us about its distance from us.

Observing how dark matter and dark energy fight

The main purpose of this observation and precise distribution of galaxies at different distances is to derive insights into the great wrestling match between the natural forces of dark matter and dark energy.

“To really see what’s going on, you have to be able to watch the individual rounds of this match,” Grün says. This is because dark energy is poised to reach and potentially stop the formation of the largest accumulations of mass in the universe.

“Only then will we understand what dark matter and dark energy really are and which one will ultimately prevail.”