A finding made by a cohort spearheaded by scholars from the University of Michigan challenges some fundamental cosmic concepts we believed we understood from our own galaxy.

This newfound insight emerges from the peripheries of Andromeda, the Milky Way’s closest significant galactic neighbor, where astronomers have identified the smallest and faintest satellite galaxy within the system so far.

This dwarf galaxy, designated Andromeda XXXV and situated around 3 million light-years distant, is compelling astronomers to reconsider the processes governing galaxy evolution across different cosmic settings and the survival of galaxies through various epochs of the universe.

While the discovery raises numerous inquiries rather than providing answers, this is typical in the pursuit of understanding the universe, remarked Marcos Arias, the lead author of the study published in Astrophysical Journal Letters that elucidates the finding.

The universe continues to conceal many enigmas, he stated, yet this revelation aids in reassessing what we comprehend and unveils further about what remains unknown.

“We still have a plethora to explore,” Arias expressed, who conducted this research as an undergraduate in the Astronomy Department and is presently a post-baccalaureate research fellow in astronomy and physics.

“Many aspects about galaxy formation, evolution, and structure must still be learned—even concerning what is nearby—before we can effectively reverse-engineer the universe’s history and grasp how we arrived at our current state.”

Intergalactic and quite diminutive

Our Milky Way system also accommodates numerous satellite or companion galaxies, which helps explain why their narrative is still under construction. These companions are separate from their massive, central host galaxy yet are sufficiently close to be captivated by its gravitational influence. The satellites are also significantly smaller.

“These are fully operational galaxies, but they measure about one-millionth the size of the Milky Way,” commented the study’s senior author, Eric Bell, U-M professor and associate chair of astronomy. “It’s akin to possessing a fully functional human that is the dimension of a grain of rice.”

Due to their significantly smaller size, these satellite galaxies are also considerably dimmer and more challenging to detect. It’s only in the last few decades that astronomers have developed technology sensitive enough to uncover most of the Milky Way’s known satellites. Presently, it is unfeasible for observers to identify extremely faint satellites orbiting hosts situated farther away than Andromeda, Bell noted.

Because they are in proximity to us, the satellites of the Milky Way have been our sole source of knowledge regarding these minuscule galaxies. Although previously discovered satellite galaxies in Andromeda exist—that’s the reason for its designation as Andromeda XXXV instead of Andromeda I—they have been excessively large and bright to effectively challenge the insights we’ve gained from the Milky Way.

“This emphasizes why Marcos’ discovery is so significant. Such a type of galaxy was only identifiable around one system, the Milky Way, in the past,” Bell remarked. “Now we have the capability to examine one around Andromeda, marking the first instance we’ve accomplished this outside our system.”

To uncover Andromeda XXXV, Arias initially sifted through extensive observational datasets to seek indicators of potential companions. After he compiled a list of promising options, he and Bell obtained time on the Hubble Space Telescope to conduct a closer examination.

“The chances for impactful research as an undergraduate in the astronomy department at U-M are abundant,” Arias stated. The endeavor was backed by NASA and the National Science Foundation.

Utilizing Hubble, the researchers revealed that not only was Andromeda XXXV a satellite galaxy, but its size was small enough to reshape some of our understanding regarding galaxy evolution, such as their lifespan for star formation.

“It was incredibly surprising,” Bell commented. “It’s the faintest entity you can find around, so it’s quite an intriguing system. Yet, it’s also unexpected in many respects.”

The group also comprised scholars from the University of Chicago, Utah Valley University, the Vatican Observatory, the University of La Serena in Chile, the University of Alabama, Montana State University, and the Leibniz Institute for Astrophysics Potsdam in Germany.

A cosmic mystery

Despite the unexpected nature of the finding, Bell emphasized that it’s not uncommon for concepts in astronomy to become more intricate as we venture beyond our immediate environment. When you are limited to analyzing a single system, you can’t ascertain which features are universally applicable and which are peculiarities, he remarked.

Now, Andromeda XXXV has presented sufficient robust evidence to begin reliably identifying those characteristics. The most noticeable distinction between satellites in the Milky Way and those in Andromeda was the timing of when they ceased star formation.

“The majority of the Milky Way satellites possess extremely ancient star populations. They ceased star formation approximately 10 billion years ago,” Arias stated. “What we observe is that comparable satellites in Andromeda can continue to form stars until about a few billion years ago—approximately 6 billion years.”

This insight also aided the researchers in unraveling what Bell described as a murder mystery in Andromeda.

Regardless of a galaxy’s magnitude, it must possess a reservoir of gas that can condense into stars. Once that gas is depleted, star formation halts. A natural inquiry to make is whether this cessation occurs because a galaxy’s gas supply exhausts itself, or if it is drained away by the larger host.

For the satellites of the Milky Way, their earlier shutdown corresponds with the self-sufficient option. Given that star formation persists longer in Andromeda’s smaller companions, it appears to be the latter scenario.

“It’s a bit murky, but the question remains did they fall or were they pushed? These galaxies seem to have been nudged,” Bell remarked. “Thus, we’ve gained qualitatively new insights into galaxy formation from them.”

This prolonged star formation is even more fascinating when contextualized within the scale of Andromeda XXXV and the history of the universe.

Heated cosmology

The universe initially existed in a state of incredible heat and density, but by the time it celebrated its first billionth anniversary, it had expanded and cooled to a temperature reminiscent of a pleasant spring day. This ideal temperature was also favorable for the condensation of gas into stars, which then grouped into galaxies.

However, as these stars emitted energy and black holes emerged to swallow matter, the universe experienced another surge in heat. For diminutive galaxies—those with a mass less than about 100,000 suns—this was believed to represent a fatal blow to star formation. The heat would effectively vaporize the gas necessary for star seeding.

“We believed that they would essentially be roasted since the entire universe transformed into a pot of boiling oil,” Bell explained. Yet, Andromeda XXXV was not subjected to such a fate.

“We presumed it would completely deplete its gas, but evidently that’s not the case, because this entity is around 20,000 solar masses and still managed to form stars effectively for a few additional billion years.”

How it managed to survive poses an intriguing question.

“I don’t possess an answer,” Bell acknowledged. “It remains true that the universe did heat up; we’re simply discovering that the repercussions are more intricate than we initially believed.”

Organizations such as NASA are gearing up to launch missions aimed at identifying more satellite galaxies in the coming years, which will assist in piecing together new answers.

While Arias and Bell are undoubtedly eager for these developments and to extract as much knowledge as they can from the resources currently available to them, they are also content with the uncertainties. For Arias, it’s the aspect that attracted him to the field.

“It’s the universe,” he declared. “There will eternally be something novel to uncover.”