A 'saga' about 101 galaxies like the Milky Way and their companions

Is our home galaxy, the Milky Way Galaxy, a special place? A team of scientists started a journey to answer this question more than a decade ago. Commenced in 2013, the Satellites Around Galactic Analogs (SAGA) Survey studies galaxy systems like the Milky Way. Now, the SAGA Survey just published three new research articles that provide us with new insights into the uniqueness of our own Milky Way Galaxy after completing the census of 101 satellite systems similar to the Milky Way’s.   

These “satellites” are smaller galaxies in both mass and size which orbit a larger galaxy, usually called the host galaxy. Just as with smaller satellites that orbit the Earth, these satellite galaxies are captured by the gravitational pull of the massive host galaxy and its surrounding dark matter. The Milky Way Galaxy is the host galaxy of several satellite galaxies, of which the two largest are the Large and Small Magellanic Clouds (LMC and SMC). While LMC and SMC are visible to the naked eye from the Southern Hemisphere, there are many other fainter satellite galaxies orbiting around the Milky Way Galaxy that can only be observed with a large telescope.  

The goal of the SAGA Survey is to characterize satellite systems around other host galaxies that have similar stellar masses as the Milky Way Galaxy. Yao-Yuan Mao, a University of Utah faculty member in the Department of Physics and Astronomy, is co-leading the SAGA Survey with Marla Geha at Yale University and Risa Wechsler at Stanford University. Mao is the lead author of the first article in the series of three that have all been accepted by the Astrophysical Journal. This series of articles reports on the SAGA Survey’s latest findings and makes the survey data available to other researchers worldwide.  

 An outlier galaxy? 

An image of a Milky Way-like galaxy and its system of satellite galaxies. The SAGA survey identified six small satellite galaxies in orbit around this Milky Way analog. Credit: Yasmeen Asali (Yale), with images from the DESI Legacy Surveys Sky Viewer https://www.legacysurvey.org/acknowledgment/

 In the first study led by Mao, the researchers highlighted 378 satellite galaxies identified across 101 Milky Way-mass systems. The number of confirmed satellites per system ranged from zero to 13 — compared to four satellites for the Milky Way. While the number of satellite galaxies in the Milky Way system is on par with the other Milky Way-mass systems, “the Milky Way appears to host fewer satellites if you consider the existence of the LMC,” Mao said. The SAGA Survey has found that systems with a massive satellite like the LMC tend to have a higher total number of satellites, and our Milky Way seems to be an outlier in this regard. 

An explanation for this apparent difference between the Milky Way and the SAGA systems is the fact that the Milky Way has only acquired the LMC and SMC quite recently, compared with the age of the universe). The SAGA article explains that if the Milky Way Galaxy is an older, slightly less massive host with the recently added LMC and SMC, one would then expect a lower number of satellites in the Milky Way system not counting other smaller satellites that LMC/SMC might have brought in.  

This result demonstrates the importance of understanding the interaction between the host galaxy and the satellite galaxies, especially when interpreting what we learn from observing the Milky Way. Ekta Patel, a NASA Hubble Postdoctoral Fellow at the U but not part of the SAGA team, studies the orbital histories of Milky Way satellites. After learning about the SAGA results, Patel said, “Though we cannot yet study the orbital histories of satellites around SAGA hosts, the latest SAGA data release includes a factor of ten more Milky Way-like systems that host an LMC-like companion than previously known. This huge advancement provides more than 30 galaxy ecosystems to compare with our own, and will be especially useful in understanding the impact of a massive satellite analogous to the LMC on the systems they reside in.”  

Why do galaxies stop forming stars? 

The second SAGA study of the series is led by Geha, and it explores whether these satellite galaxies are still forming stars. Understanding the mechanisms that would stop the star formation in these small galaxies is an important question in the field

Yao-Yuan Mao

of galaxy evolution. The researchers found, for example, that satellite galaxies located closer to their host galaxy were more likely to have their star formation “quenched,” or suppressed. This suggests that environmental factors help shape the life cycle of small satellite galaxies.  

The third new study is led by Yunchong (Richie) Wang, who obtained his PhD with Wechsler. This study uses the SAGA Survey results to improve existing theoretical models of galaxy formation. Based on the number of quenched satellites in these Milky Way-mass systems, this model predicts quenched galaxies should also exist in more isolated environments — a prediction that should be possible to test in the coming years with other astronomical surveys such as the Dark Energy Spectroscopic Instrument Survey.  

Gift to the astronomy community 

In addition to these exciting results that will enhance our understanding of galaxy evolution, the SAGA Survey team also brings a gift to the astronomy community. As part of this series of studies, the SAGA Survey team published new distance measurements, or redshifts, for about 46,000 galaxies. “Finding these satellite galaxies is like finding needles in a haystack. We had to measure the redshifts for hundreds of galaxies to just identify one satellite galaxy,” Mao said. “These new galaxy redshifts will enable the astronomy community to study a wide range of topics beyond the satellite galaxies.”  

The SAGA Survey was supported in part by the National Science Foundation and the Heising-Simons Foundation. Other authors of these three SAGA studies include Yasmeen Asali, Erin Kado-Fong, Nitya Kallivayalil, Ethan Nadler, Erik Tollerud, Benjamin Weiner, Mia de los Reyes, John F. Wu, Tom Abel, and Peter Behroozi. 

Through a small telescope, it looks no different from other so-called globular clusters; a spherical stellar collection so dense towards the center that it becomes impossible to distinguish individual stars. But a new study, led by researchers from the University of Utah and the Max Planck Institute for Astronomy, confirms what astronomers had argued about for over a decade: Omega Centauri contains a central black hole.The black hole appears to be the missing link between its stellar and supermassive kin—stuck in an intermediate stage of evolution, it is considerably less massive than typical black holes in the centers of galaxies. Omega Centauri seems to be the core of a small, separate galaxy whose evolution was cut short when it was swallowed by the Milky Way.

“This is a once-in-a-career kind of finding. I’ve been excited about it for nine straight months. Every time I think about it, I have a hard time sleeping,” said Anil Seth, associate professor of astronomy at the U and co-principal investigator (PI) of the study. “I think that extraordinary claims require extraordinary evidence. This is really, truly extraordinary evidence.” A clear detection of this black hole had eluded astronomers until now. The overall motions of the stars in the cluster showed that there was likely some unseen mass near its center, but it was unclear if this was an intermediate-mass black hole or just a collection of the stellar black holes. Maybe there was no central black hole at all.

A medium Level panel zoom of the Omega Centauri star cluster’s intermediate black hole likely position. PHOTO CREDIT: ESA/HUBBLE & NASA, M. HÄBERLE (MPIA)

“Previous studies had prompted critical questions of ‘So where are the high-speed stars?’ We now have an answer to that, and the confirmation that Omega Centauri contains an intermediate-mass black hole. At about 18,000 light-years, this is the closest known example for a massive black hole,” said Nadine Neumayer, a group leader at the Max Planck Institute and PI of the study. For comparison, the supermassive black hole in the center of the Milky Way is about 27,000 light-years away.

A range of black hole masses

In astronomy, black holes come in different mass ranges. Stellar black holes, between one and a few dozen solar masses, are well known, as are the supermassive black holes with masses of millions or even billions of suns. Our current picture of galaxy evolution suggests that the earliest galaxies should have had intermediate-sized central black holes that would have grown over time, gobbling up smaller galaxies done or merging with larger galaxies.

Such medium-sized black holes are notoriously hard to find. Although there are promising candidates, there has been no definite detection of such an intermediate-mass black hole—until now.

“There are black holes a little heavier than our sun that are like ants or spiders—they’re hard to spot, but kind of everywhere throughout the universe. Then you’ve got supermassive black holes that are like Godzilla in the centers of galaxies tearing things up, and we can see them easily,” said Matthew Whittaker, an undergraduate student at the U and co-author of the study. “Then these intermediate-mass black holes are kind of on the level of Bigfoot. Spotting them is like finding the first evidence for Bigfoot—people are going to freak out.”

Read more about the Discovery @TheU.

Neutrinos are also incredibly small and light. They have some mass, but not much and they rarely interact with other matter. They come in three types or "flavors": electron, muon, and tau.

Cosmic rays travel through space then crash into the earth's atmosphere and produce  air showers that Include neutrinos and many other types of particles. When neutrinos are produced and start traveling, they can change from one flavor to another. The atmospheric neutrinos are then detected by DeepCore, a denser array of sensors in the center of the IceCube detector at the South Pole.This process is called neutrino oscillation and the IceCube Detector, a massive neutrino detector buried deep in the ice at the South Pole, has a special area called DeepCore that can detect lower-energy neutrinos.

Scientists at the IceCube Neutrino Observatory in Antarctica have made a breakthrough in measuring neutrinos. Using advanced computer techniques, they've achieved the most precise measurements to date of how these particles change as they travel through space, helping us understand fundamental properties of the universe that could lead to new discoveries in physics.

Shiqi Yu

Shiqi Yu, a research assistant professor in the Department of Physics & Astronomy at the University of Utah and others who published their findings recently in Physical Review Letters analyzed data from over 150,000 neutrino events collected over nine years (2012-2021). They used advanced computer programs called convolutional neural networks (CNNs) to process this data. The team made the most precise measurements ever of two important properties related to neutrino oscillation: Delta m²₃₂ and sin²(θ₂₃). These numbers help describe how neutrinos change as they travel.

“We also carefully studied the systematic uncertainties that arise from our imperfect knowledge of our models and chose some to use as free nuisance parameters that fit together with the physics parameters for our data,” says Yu.

Using CNNs, which use three-dimensional data for image classification, Yu and co-lead of the study Jessie Micallef first developed use cases for the CNNs to focus on the DeepCore region and trained them to reconstruct different properties of particle interactions in the detector. They then used the CNN reconstructions to select qualified neutrino interactions that happened in or near the DeepCore region to produce a neutrino-dominated dataset with well-reconstructed energies and zenith angles.

Jessie Micallef

Yu notes that the CNN-reconstructed analysis-level dataset is already being used for other neutrino oscillation analyses, such as determining the neutrino mass ordering and non-standard neutrino interactions and for atmospheric tau neutrino appearance analyses.

“The atmospheric neutrino dataset from DeepCore exhibits relatively high energies in the oscillation analyses, which is unique compared to existing accelerator-based experiments,” says Yu. “Given our dataset and independent analysis, it is interesting to see agreement and consistency in physics parameter measurements.”

This research helps confirm and refine our understanding of how neutrinos — fundamental particles that can tell us a lot about the universe — behave. The techniques developed here, animated by machine learning, can be used in future studies to learn even more about neutrinos and the universe. Those future studies will be informed by IceCube which is planning an upgrade in 2025-2026 that will allow for even more detailed measurements of neutrinos.

By studying neutrino detection and the phenomenon of neutrino oscillation, scientists like Shiqi Yu hope to answer big questions about the nature of matter, energy and the cosmos.