We’re Living in an Abnormal Galaxy

Astronomers often use the Milky Way as a standard for studying how galaxies form and evolve. Since we’re inside it, astronomers can study it in detail with advanced telescopes. By examining it in different wavelengths, astronomers and astrophysicists can understand its stellar population, its gas dynamics, and its other characteristics in far more detail than distant galaxies.

However, new research that examines 101 of the Milky Way’s kin shows how it differs from them.

One powerful way to understand things is to compare and contrast them with others in their class, a technique we learn in school. Surveys are an effective tool to compare and contrast things, and astronomical surveys have contributed an enormous amount of foundational data towards the effort. The Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and the ESA’s Gaia mission are all prominent examples.

The Satellites Around Galactic Analogs (SAGA) Survey is another, and its third data release features in three new studies. The studies are all based on 101 galaxies similar in mass to the Milky Way, and each study tackles a different aspect of comparing those galaxies to ours.

• The SAGA Survey. III. A Census of 101 Satellite Systems around Milky Way–mass Galaxies
• The SAGA Survey. IV. The Star Formation Properties of 101 Satellite Systems around Milky Way–mass Galaxies
• The SAGA Survey. V. Modeling Satellite Systems around Milky Way–Mass Galaxies with Updated UniverseMachine

Research shows that galaxies form inside gigantic haloes of dark matter, the elusive substance that doesn’t interact with light. 85% of the Universe’s matter is mysterious dark matter, while only 15% is normal or baryonic matter, the type that makes up planets, stars, and galaxies. Though we can’t see these massive haloes, astronomers can observe their effects. Their gravity draws normal together to create galaxies and stars.

Dark matter haloes are part of the Large-Scale Structure of the Universe, the cosmic web of dark matter and galaxy clusters and superclusters that make up the Universe’s backbone. Simulated Image Credit: Ralf Kaehler/SLAC National Accelerator Laboratory

SAGA is aimed at understanding how dark matter haloes work. It examines low-mass satellite galaxies around galaxies similar in mass to the Milky Way. These satellites can be captured and drawn into the dark matter haloes of larger galaxies. SAGA has found several hundred of these satellite galaxies orbiting 101 Milky Way-mass galaxies.

“The Milky Way has been an incredible physics laboratory, including for the physics of galaxy formation and the physics of dark matter,” said Risa Wechsler, the Humanities and Sciences Professor and professor of physics in the School of Humanities and Sciences. Wechsler is also the co-founder of the SAGA Survey. “But the Milky Way is only one system and may not be typical of how other galaxies formed. That’s why it’s critical to find similar galaxies and compare them.”

The comparison between the Milky Way and the 101 others revealed some significant differences.

“Our results show that we cannot constrain models of galaxy formation just to the Milky Way,” said Wechsler, who is also professor of particle physics and astrophysics at the SLAC National Accelerator Laboratory. “We have to look at that full distribution of similar galaxies across the universe.”

The SAGA Survey’s third data release includes 378 satellites found in 101 MW-mass systems, and the first paper focuses on the satellites. Only a painstaking search was able to uncover them. Four of them belong to the Milky Way, including the well-known Large and Small Magellanic Clouds.

This figure shows how SAGA compares to other efforts to find satellite galaxies. Image Credit: Mao et al. 2024.

“There’s a reason no one ever tried this before,” Wechsler said. “It’s a really ambitious project. We had to use clever techniques to sort those 378 orbiting galaxies from thousands of objects in the background. It’s a real needle-in-the-haystack problem.”

SAGA found that the number of satellites per galaxy ranges from zero to 13. According to the first paper, the mass of the most massive satellite is a strong predictor of the abundance of satellites. “One-third of the SAGA systems contain LMC-mass satellites, and they tend to have more satellites than the MW,” the paper states. The Milky Way is an outlier in this regard, which is one reason it’s atypical.

The second study focuses on star formation in the satellites. The star formation rate (SFR) is an important metric in understanding galaxy evolution. The research shows that star formation is still active in the satellite galaxies, but the closer they are to the host, the slower their SFR. Is it possible that the greater pull of the dark matter halo close to the galaxy is quenching star formation?

“Our results suggest that lower-mass satellites and satellites inside 100 kpc are more efficiently quenched in a Milky Way–like environment, with these processes acting sufficiently slowly to preserve a population of star-forming satellites at all stellar masses and projected radii,” the second paper states.

However, in the Milky Way’s satellites, only the Magellanic Clouds are still forming stars, with radial distance playing a role. “Now we have a puzzle,” Wechsler said. “What in the Milky Way caused these small, lower-mass satellites to have their star formation quenched? Perhaps, unlike a typical host galaxy, the Milky Way has a unique combination of older satellites that have ceased star formation and newer, active ones – the LMC and SMC – that only recently fell into the Milky Way’s dark matter halo.”

This figure from the research shows the SFR (left) and the specific SFR (right) for the satellite galaxies in the study. The specific SFR differs from the SFR in that it’s divided by the total stellar mass of the galaxy. The specific SFR basically tells astronomers how quickly the galaxy is growing relative to its size. It’s used to compare star formation efficiency across different size galaxies. The grey squares the SAGA hosts and the stars are the Large and Small Magellanic Clouds. Image Credit: Geha et al. 2024.

This is another reason that our galaxy is atypical.

What about the smaller dark matter haloes around the satellite galaxies? What role do they play?

“To me, the frontier is figuring out what dark matter is doing on scales smaller than the Milky Way, like with the smaller dark matter halos that surround these little satellites,” Wechsler said.

The third paper compares SAGA’s third data release with computer simulations. The authors developed a new model for quenching in galaxies with less-than-or-equal-to 10⁹ solar masses. Their model is constrained by the SAGA data on the 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 SFRs, and the quenched fractions in the satellites. It also maintained the SFR in more isolated satellite galaxies and observed enhanced quenching in closer satellites.

This figure from the research shows the distribution of stellar mass vs. halo mass, with the grey contours representing 2,500 mock Saga-like hosts. It shows that their model successfully reproduces much of what SAGA found. Image Credit: Wang et al. 2024.

The model needs more testing with observations, and the authors point out that spectroscopic surveys are a logical next step. Those surveys can hopefully answer questions about the role internal feedback plays in the lower-mass satellites, about their mass and gas accretion and the influence dark matter has on them, as well as gas processes specific to the satellites.

“SAGA provides a benchmark to advance our understanding of the universe through the detailed study of satellite galaxies in systems beyond the Milky Way,” Wechsler said. “Although we finished our initial goal of mapping bright satellites in 101 host galaxies, there’s a lot more work to do.”

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