Galaxies, which are akin to cities hosting billions of stars like our Sun, come in a variety of flavors, including different sizes, shapes, and formation histories. Less massive galaxies typically contain millions or billions of stars while the most massive galaxies can house upwards of trillions of stars. Galaxies also come in a range of shapes, from being smooth and elliptical to being similar in shape to a flat pancake with clumpy spiral arms. Galaxies can also be categorized based on how much star formation is occurring: astronomers refer to galaxies that are no longer actively forming new stars as quenched or dead (Figure 1). Whereas star forming galaxies, like our own Milky Way, form about one new star per year, a dead galaxy with a similar mass would form one star every ten years.


The Hubble Tuning Fork, the classification scheme created by Edwin Hubble in 1926 to categorize galaxies based on their morphology.

The Hubble Tuning Fork, the classification scheme created by Edwin Hubble in 1926 to categorize galaxies based on their morphology (i.e., their shape). The elliptical galaxies on the left range from circular (E0) to oval-like (E5), while spiral galaxies on the right are split into two categories depending on if they have a bar (bottom) or not (top). Spiral galaxies are actively forming stars while elliptical galaxies tend to be more dormant.


In the distant universe, massive galaxies have been better studied than their lower mass counterparts, as they are brighter and more easily detected by instruments like the Hubble Space Telescope. Astronomers have known for years that the size and mass of these galaxies is directly correlated. This means that physically-larger galaxies are more massive, a relation which has been confirmed by countless observations of massive galaxies. Thus one would naturally expect that the same size-mass relationship holds for distant low mass galaxies, however this is not the case. Astronomers are puzzled by this notion and wonder if this is an innate property of the universe or if it is an observed bias that occurs because our instruments are more prone to detecting the biggest, brightest galaxies. Instrumental limitations on sensitivity have essentially skewed our understanding of galaxies to be “biased bright”.

In his recent paper, Sam Cutler, a fifth year PhD candidate at UMass, investigates the size-mass relation of low mass dead galaxies with the use of the much more powerful James Webb Space Telescope, under the watchful eye of Prof. Kate Whitaker (UMass). Sam and his international team of collaborators select a sample of about 300 galaxies that come from two extragalactic sky surveys that have been observed by the Webb Telescope: UNCOVER which targets a cluster of galaxies known as Pandora’s Cluster and PRIMER which aims at two separate deep fields of the sky. Deep fields target what appear to be blank patches of sky but upon deeper inspection reveal thousands of galaxies. Using the near-infrared observations taken by the Webb Telescope, the team estimates the size and mass of each galaxy in their sample and confirm that for these low mass galaxies, the size and mass are independent of each other.

Sam Cutler

Sam Cutler presenting his work on galaxy evolution and structure at the 243rd meeting of the American Astronomical Society in January 2024, which was held in New Orleans.

Digging a bit deeper, Sam and collaborators wanted to see if the morphology or shape of a galaxy had any connection to the size-mass relationship. Using the axis ratio, which describes how flat a galaxy is, they find that there are two distinct populations of galaxies in their sample: low mass disk-like galaxies and high mass elliptical galaxies. Sam was so surprised by this result that he shifted the focus of the paper, that has been submitted to the Astrophysical Journal Letters, on it and said, “This result of these two distinct populations was so interesting that we made that front and center in the title of the paper”. Surprisingly, the low mass population was also younger in age on average than the higher mass population. These two separate populations imply that they have different evolutionary paths suggesting that the histories of galaxies can dramatically differ from one another. They believe the high mass galaxy population stops forming stars because of compaction from gas inflows or shockwaves that are sent out from stars that explode. On the other hand, they think the low mass galaxies die out because the surrounding environment strips gasses, which are the building blocks of stars, from the galaxy while leaving its disk structure intact. This leaves the low mass disk galaxies frozen in their youthful appearance for a prolonged period.

Open access to the full text of the original paper is available.

About the Author:

Moiz Khalil is a senior undergraduate studying astronomy at UMass Amherst. He currently studies the atmospheres and colors of brown dwarfs and wants to pursue a career in science communication. Outside of science, he enjoys listening to music and reading novels.