Stunning Mosaic Unveiled: Hubble’s Captivating Views of the Andromeda Galaxy!

Astronomers are rejoicing over the completion of a 2.5-billion-pixel panoramic image depicting the entirety of the Andromeda Galaxy. The group consists of various researchers from UC Santa Cruz who made substantial contributions to the extensive photomosaic that integrates approximately 600 images captured by the Hubble Space Telescope throughout more than a decade and 1,000 orbits.

This astronomical panorama represents a significant achievement, as Andromeda is the “nearest” major galaxy to our own, located at a distance of 2.5 million light years from Earth—equivalent to the diameter of the Milky Way disk multiplied by 25. Besides narrating the tumultuous history of the Andromeda Galaxy, the mosaic will now enable astronomers to gain deeper insights into our similarly structured Milky Way.

Without Andromeda serving as a proxy for spiral galaxies across the cosmos, astronomers would possess substantially less understanding of the structure and development of the Milky Way, according to the Space Telescope Science Institute (STScI). “That’s because we are situated within the Milky Way,” STScI remarked in its January 16 announcement. “This is akin to attempting to comprehend the layout of New York City by being positioned in the center of Central Park.”

Professor Puragra “Raja” GuhaThakurta and Ph.D. candidate Douglas Grion Filho were amongst the astronomers from UC Santa Cruz who participated in this endeavor, culminating in the largest mosaic of Hubble Space Telescope imagery ever assembled—identifying over 200 million stars in the Andromeda galaxy.

The UC Santa Cruz team also investigated the collection of stars located within the disk-shaped segment of Andromeda using the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) at the W.M. Keck Observatory. According to GuhaThakurta, Hubble’s images captured the count and diversity of stars constituting Andromeda’s disk, whereas the Keck DEIMOS spectra permitted his team to assess the velocity of each star, in addition to detailing stellar attributes such as surface temperature and gravity, chemical makeup, age, or evolutionary status.

“It’s similar to observing an aerial photograph of a beach. However, this remarkable data set enables us to zoom in and observe and measure the characteristics of individual stars, which is somewhat analogous to counting and measuring the properties of individual grains of sand on a beach,” he stated. “This data set is transforming our comprehension of how Milky Way-like galaxies form and develop.”

Divide and Capture

This panorama originated with the Panchromatic Hubble Andromeda Treasury (PHAT) initiative nearly a decade ago. Images were captured in near-ultraviolet, visible, and near-infrared wavelengths utilizing the Advanced Camera for Surveys and the Wide Field Camera 3 aboard Hubble to photograph the northern hemisphere of Andromeda.

This initiative was succeeded by the Panchromatic Hubble Andromeda Southern Treasury (PHAST), recently published in the Astrophysical Journal and led by Zhuo Chen at the University of Washington, which incorporated images of approximately 100 million stars in the southern hemisphere of Andromeda.

The collective efforts of these programs comprehensively cover the entirety of Andromeda’s disk, which is observed almost edge-on—tilted at 77 degrees relative to Earth’s perspective. Hubble can solely detect stars brighter than our Sun, capturing just a small fraction of Andromeda’s estimated total population of 1 trillion stars. This galaxy can be observed with the naked eye on a very clear autumn evening as a faint cigar-shaped object roughly the same angular diameter as our Moon.

When Grion Filho commenced his studies at UC Santa Cruz as a graduate student, GuhaThakurta’s group was engaged in measuring velocities for a subsample of the stars found in PHAST. Over the recent years, Filho assisted in selecting targets from that star cohort, and also conducted some observations at Keck.

“Essentially, we gauge the line-of-sight velocities of the stars through their spectra and—by accumulating numerous measurements—we can obtain a reliable understanding of how Andromeda as a whole is rotating and moving,” Grion Filho mentioned.

Additionally, this representative map of the line-of-sight velocities enabled the team to “reverse the clock,” Filho elucidated. “It aids in understanding Andromeda’s historical background—whether it experienced prior mergers, when those mergers took place, and what effects they imposed on the galaxy. Andromeda evidently has a considerably more dynamic merger history compared to the Milky Way, which is clearly evident in the velocity plane.”

A Galactic ‘Train Wreck’

Although the Milky Way and Andromeda presumably formed around the same period several billion years ago, observational data indicates that they possess significantly different evolutionary trajectories, even though they originated within the same cosmological vicinity. Researchers suggest that Andromeda appears to be more densely populated with younger stars and exhibits extraordinary features like coherent streams of stars. This suggests a more active recent history of star formation and interactions compared to the Milky Way.

“Andromeda’s a train wreck. It seems to have gone through some kind of event that triggered it to generate a considerable number of stars and then subsequently ceased,” stated Daniel Weisz, an associate professor of astronomy at UC Berkeley. “This was likely caused by a collision with another galaxy in the vicinity.”

A potential culprit is the compact satellite galaxy Messier 32, resembling the stripped core of a once-spiral galaxy that may have interacted with Andromeda previously. Computer simulations indicate that when a close encounter with another galaxy depletes all accessible interstellar gas, star formation declines.

“Andromeda appears to be a transitional category of galaxy that lies between a star-forming spiral and a kind of elliptical galaxy dominated by aging red stars,” Weisz noted. “We can observe that it has this significant central bulge of older stars alongside a star-forming disk that is less active than one might expect given the galaxy’s mass.”

Astronomers designate the Andromeda Galaxy as M31 due to its classification as the 31st object listed in the Messier Catalog, a compilation of astronomical objects created by French astronomer Charles Messier.