New model reveals what the color of a galaxy tells about its distance

A team from Ludwig Maximilian University (LMU) has presented a model that shows that the colour of galaxies can tell us their distance, which can then be used to measure the structure of the universe.

our universe The Earth is about 13.8 billion years old. During this long period of time, the initial slight asymmetry Large-scale structures Through telescopes we can see into the night sky: galaxies like our own Milky Way, clusters of galaxies, and even larger collections of matter and strings of gas and dust.

How fast this growth happens, at least in today’s universe, depends on a kind of wrestling contest between the forces of nature: Can dark matter, which holds everything together with gravity and pulls matter even closer, compete with dark energy, which pulls the universe even further apart?

“If we could precisely measure the structure of the sky, we could observe this struggle,” says LMU astrophysicist Daniel Grün.

That’s where telescope-based observational projects come in, capturing incredibly accurate images of large swaths of the sky, such as the Dark Energy Survey at the Blanco Telescope in Chile and the recently operational Euclid satellite. LMU scientists have been involved in both projects, including in leadership roles, for many years.

The largest dataset evaluated to date

It’s not always easy to determine exactly how far away individual structures and galaxies are from us, but it’s crucially important. After all, the farther away a galaxy is, the longer it has taken for its light to reach us, and the older the snapshot of the Universe that an observation reveals. A key source of information is the observed color The radius of the galaxy is measured by ground-based telescopes like Blanco and satellites like Euclid.

The new study, by a team led by Jamie McCullough and Daniel Gruen, Published In the journal Monthly Bulletin of the Royal Astronomical Societyhas analysed the largest data set to date to reveal what the colours of different galaxies tell us about their actual distances.

In principle, the distance to a galaxy could be measured precisely by spectroscopy, which involves measuring the spectral lines of distant galaxies. Because the whole Universe is expanding, the farther a galaxy is away from us, the longer the wavelength of its spectral lines. This is because the light waves of distant galaxies are stretched on the long journey to reach us.

This effect, called redshift, also changes the apparent color that instruments measure in images of the galaxy, making it appear redder than it actually is – similar to the Doppler shift in the pitch of an ambulance siren we hear as it drives past us.

No two galaxies are the same

Jamie McCullough is a postdoctoral researcher at LMU and Stanford University. For her analysis, she combined spectroscopic measurements from the Dark Energy Spectroscopic Instrument (DESI) with the largest dataset to date to precisely measure the colors of galaxies (KiDS-VIKING).

Specifically, the authors combined DESI’s spectroscopic data for 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 the distance to a galaxy and its observed color and brightness. Although no two galaxies in the universe are alike, each class of similar galaxies has a special relationship between observed color and redshift.

“If we can combine distance information with measurements of the galaxy’s shape, we can infer large-scale structure from the distortion of light,” says Jamie McCullough.

The results of this study make it possible to statistically determine the actual distance of each galaxy observed in images taken by Euclid and the Dark Energy Survey.

By analysing the distortions in observed images of galaxies, scientists could learn something about the behaviour of galaxies. Space Structure It delves into and deepens our understanding of the Universe as it is today and as it was billions of years ago, which gives us insight into the evolutionary history of the Universe.

To observe how structures form over time, we don’t need to wait billions of years – we just need to measure structures at different distances from Earth. With images alone, this is nearly impossible, because we can’t determine how far a galaxy is from our own galaxy based on what it looks like in the image.

Jamie McCullough’s work holds the key to this question by providing a model of what a galaxy’s apparent ‘colour’ tells us about that galaxy. distance From us.

Observing the battle between dark matter and dark energy

The main purpose of this precise observation and distribution is to Galaxy The aim is to gain insight into the great melee between the forces of nature – dark matter and dark energy – at various distances.

“To really know what’s going on, we need to be able to observe each individual round of this match,” Grün says, because that could mean dark energy could catch up and completely block the formation of larger accumulations of mass in the universe.

“Only then can we Dark matter and Dark Energy What is it really and who will ultimately win?”