Andromeda Unleashed: A Cosmic Showcase of Galactic Grandeur

It may be described as a “train wreck,” according to astronomer Dan Weisz, but it’s a stunning train wreck.

Weisz, serving as an associate professor of astronomy at the University of California, Berkeley, is alluding to the Andromeda galaxy, the closest substantial galaxy to our Milky Way and the nearest one astronomers can investigate for insights into our galaxy’s development.

A mosaic depiction of the complete Andromeda galaxy (Messier 31, or M31), located 2.5 million light years away yet six times larger than the moon in the night sky, was unveiled today (Jan. 16) by the Space Telescope Science Institute based in Maryland. The galaxy’s striking yellow core is enveloped by an ethereal blue glow, with stars resembling grains of sand on a shore.

The latest image comprises 2.5 billion pixels and was created from approximately 600 images taken by NASA’s Hubble Space Telescope throughout the past decade. When combined into a single image, the mosaic would encompass around 75 8K ultra-high-definition video displays.

According to Weisz, what it unveils is that the galaxy has experienced collisions with another galaxy within the last 5 billion years, leaving behind trails of star formation that astronomers can trace back through these past encounters. Weisz was a co-principal investigator in the initial segment of the Andromeda survey, known as the Panchromatic Hubble Andromeda Treasury (PHAT) program, which generated a high-resolution image of the galaxy’s northern section in 2015. He also serves as a co-investigator on the subsequent and concluding phase, the Panchromatic Hubble Andromeda Southern Treasury (PHAST). The mosaic of the combined images was revealed at the annual convention of the American Astronomical Society in National Harbor, Maryland.

“This is the most intricate view of a galaxy beyond the Milky Way,” Weisz asserted. “In fact, one could contend it’s even more detailed than the Milky Way because we cannot visibly perceive most of the Milky Way — it’s hidden from our view on Earth. We’re resolving 250 million stars in M31 — it’s astonishing. No other galaxy allows us to achieve such resolution.”

“With Hubble, we can acquire immense detail concerning the events occurring on a comprehensive scale throughout the galaxy’s entire disk,” remarked Ben Williams from the University of Washington. Williams is the principal researcher for the PHAST initiative.

The individual images were captured at near-ultraviolet, visible, and near-infrared wavelengths utilizing Hubble’s Advanced Camera for Surveys and the Wide Field Camera. Only stars somewhat brighter than our sun were resolvable by the Hubble cameras — about 0.02% of all stars in Andromeda.

Of star clusters and companion galaxies

The previous mosaic of the northern section of Andromeda illuminated much about star formation within the galaxy and how it compares to that occurring within the Milky Way. Concentrating on Andromeda’s star clusters — groups of stars that formed collectively from the same cloud of neutral hydrogen gas, with a mix of dust and heavier elements — Weisz spearheaded a significant team that discovered in 2015 that the arrangement of stars within these clusters was astonishingly uniform. The ratio of stars of various sizes, ranging from the largest blue supergiants to the tiniest red dwarf stars, was surprisingly consistent throughout the galaxy and similar to the so-called initial mass function (IMF) found in star clusters within the Milky Way.

“It’s difficult to fathom that the IMF is so uniform throughout our neighboring galaxy, considering the intricate physics of star formation,” Weisz stated during that period.

This is significant because it is believed that the majority of stars, including our sun, initially form within star clusters before dispersing across the galaxy.

“We theorize that most stars form in clusters and subsequently somewhat evaporate from the cluster to populate the field, which means most of the stars you observe,” he noted. “By examining star clusters, you effectively track the process through which star clusters populate the whole galaxy. Our sun likely originated in a cluster closer to the galaxy’s center and has since been displaced to greater distances.”

The newly disclosed image and stellar data concerning roughly 100 million new stars nearly doubles

The quantity of star clusters for Weisz and colleagues to examine, which will assist in discerning how star clusters vary across different regions of the galaxy.

Weisz is additionally curious about the small satellite galaxies that revolve around Andromeda, and how these contrast with the satellites of the Milky Way. Most of them are dwarf galaxies hosting fewer than a billion stars.

“Considering that we recognize Andromeda is somewhat chaotic, and it’s distinct from the Milky Way, we aim to comprehend what influence this had on the satellite galaxies, as they serve as highly sensitive indicators of galaxy evolution,” he stated. “Minor disturbances in larger galaxies create significant effects for smaller ones.”

Andromeda’s Past and Future

Even though Andromeda and the Milky Way likely originated around the same period, the considerably larger Andromeda — possessing approximately a trillion stars compared to the Milky Way’s 300 billion — has developed differently. It features younger stars and peculiar characteristics, like organized streams of stars, which suggest that it has undergone a more vigorous recent star formation and interaction history than the Milky Way.

“It appears to have undergone an event that prompted it to produce a slew of stars and then abruptly cease,” Weisz remarked. “This was likely the result of a collision with another galaxy nearby. We observe bursts of star formation between 2 and 4 billion years ago, and the star counts from that era imply that this period coincides with Andromeda’s major collision.”

A potential offender is the compact satellite galaxy M32, which resembles the stripped core of a former spiral galaxy that may have interacted with Andromeda in its past. Simulations on computers indicate that a close encounter with another galaxy can deplete all available interstellar gas, leading to a decline in star formation.

“Andromeda appears to be a transitional variety of galaxy, positioned between a star-forming spiral and a sort of elliptical galaxy dominated by aging red stars,” noted Weisz. “We can observe that it contains this substantial central bulge of older stars and a star-forming disk that isn’t as active as one might anticipate, given the galaxy’s mass.”

Conversely, the Milky Way has not experienced a collision with another galaxy for around 8 to 10 billion years, he stated.

Nevertheless, the two galaxies share a connected destiny.

“In 5 billion years, Andromeda will merge with the Milky Way, resulting in the formation of a single large elliptical galaxy remaining in the local group,” he concluded.

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