Astronomers often use the Milky Way as a standard for studying how galaxies form and evolve. Because we are inside it, astronomers can study it in detail with modern telescopes. By studying it at different wavelengths, astronomers and astrophysicists can understand its stellar population, gas dynamics, and other properties in much more detail than in distant galaxies.

But a new study examining 101 relatives of the Milky Way shows how different the Milky Way is from them. A powerful way to understand things is to compare them to other techniques we learn in class; This is a technique we learned in school. Surveys are an effective tool for comparing and contrasting things, and astronomical surveys have contributed an enormous amount of background data to this effort.

Prominent examples include the Sloan Digital Sky Survey (SDSS), the Two-Micron All-Sky Survey (2MASS) and ESA’s Gaia mission. The Satellites Around Galactic Analogues (SAGA) survey is another and third release of data contained in three new surveys. All studies are based on 101 galaxies with masses similar to the Milky Way, and each study examines a different aspect of how these galaxies compare to ours.

Research shows that galaxies form within giant halos of dark matter, an elusive substance that does not interact with light. 85% of the matter in the universe is the mysterious dark matter, with only 15% being the normal or baryonic matter that forms planets, stars and galaxies. Although we cannot see these giant halos, astronomers can observe their effects. Their gravitation pulls the normals together, creating galaxies and stars.

SAGA aims to understand how dark matter halos work. It investigates low-mass satellite galaxies around galaxies similar in mass to the Milky Way.

These moons can be captured and pulled into the dark matter halo of large galaxies. SAGA found hundreds of such satellite galaxies orbiting 101 Milky Way-mass galaxies.

“The Milky Way has been an incredible physics laboratory, especially in terms of the physics of galaxy formation and the physics of dark matter,” said Risa Wexler, professor of physics and professor of humanities in the College of Humanities and Sciences. Wechsler is also the co-founder of SAGA Survey.

“But the Milky Way is just one system and may not be unique to the formation of other galaxies. That’s why it’s so important to find and compare similar galaxies.”

Comparing the Milky Way to 101 others revealed some important differences.

“Our results show that we cannot limit galaxy formation models to the Milky Way,” said Wechsler, a professor of particle physics and astrophysics at SLAC National Accelerator Laboratory.

“We need to look at the exact distribution of similar galaxies in the universe.”

The third edition of SAGA Survey data includes 378 satellites in 101 MW systems, and the first article is devoted to satellites. Only a diligent search could find them. Four of them belong to the Milky Way, including the famous Large and Small Magellanic Clouds.

“There’s a reason no one has tried this before,” Wexler said. “This is a truly ambitious project. We had to use clever techniques to separate these 378 orbital galaxies from thousands of objects in the background. This is a real needle in a haystack problem.”

SAGA found that the number of moons per galaxy ranges from zero to 13. According to the first paper, the mass of the largest moon is a strong predictor of lunar abundance.

“A third of SAGA systems contain LMC mass satellites, and they tend to have more satellites than MWs,” the paper says. The Milky Way is an exception in this regard, and this is one of the reasons why it is atypical.

The second work is devoted to the formation of stars in satellites. Star formation rate (SFR) is an important indicator for understanding the evolution of galaxies. The study shows that star formation is still active in companion galaxies, but the closer stars are to their hosts, the slower their SFRs. Is it possible that the stronger gravity of the dark matter halo near the galaxy is preventing star formation?

“Our results show that low-mass satellites and satellites within 100 kpc are quenched more efficiently in a Milky Way-like environment, with these processes acting slowly enough to maintain a population of star-forming satellites at all stellar masses and projected radii,” the second state paper states.

But in the satellites of the Milky Way, only the Magellanic Clouds form stars, and radial distance also plays a role.

“Now we have a conundrum,” Wexler said.

“What caused these low-mass small moons in the Milky Way to stop star formation? Perhaps unlike a typical host galaxy, the Milky Way contains older moons that have stopped star formation, as well as newer, active LMCs that have recently entered the Milky Way’s dark matter halo and It has a unique combination of SMCs.”

This is another reason for the unusualness of our galaxy.

What about the smaller dark matter halos around companion galaxies? What role do they play?

“The frontier for me is finding out what dark matter does on scales smaller than the Milky Way, for example on the smaller dark matter halos surrounding these small moons,” Wechsler said.

A third paper compares the third version of the SAGA data with computer simulations. The authors developed a new extinction model in galaxies with solar masses less than or equal to 10.⁹.

Their model was constrained to SAGA data for 101 galaxies, and the researchers then compared it to isolated field galaxies from the Sloan Digital Sky Survey. The model successfully reproduced the stellar mass function of the satellites, their average SFR, and the extinction fraction in the satellites. He also supported SFRs in more isolated satellite galaxies and observed increased extinction in closer satellites.

The model needs further observational testing, and the authors note that spectroscopic studies are the logical next step. We hope that these studies can answer questions about the role of internal feedback in low-mass satellites, their mass and gas accumulation, the influence of dark matter on them, and gas processes specific to satellites.

“SAGA provides a reference point for advancing our understanding of the universe by studying satellite galaxies in systems beyond the Milky Way in detail,” Wechsler said. he said. “Although we have completed our initial goal of mapping bright moons in 101 large galaxies, there is still much work to be done.”