Written by: Maneth Perera

In 1995, the Hubble Space Telescope took the first Deep Field images. Many astronomers doubted its ability to look for new space objects; to prove them wrong, NASA researchers pointed the telescope at the most barren, darkest part of the night sky. After just 10 days of imaging, over 3,000 galaxies were discovered in a piece of the night sky only as wide as a pin head held at arm’s length. While astronomers have frequently studied the pathways of spiral star-formation and quiescent galaxies, newer classes of galaxies are now being discovered with modern technology. This includes BreakBRDs (Break Bulges in Red Disks), galaxies that create counterintuitive structures with red quenched disks and blue star-forming bulges, offering a contradiction to our proposed theories of galactic evolution.

Figure 1

Different examples of BreakBRD galaxies (not necessarily colored red, but colored to show the intensity of the bulge compared to the intensity of the disk). Quenched disks are red due to the old stars within them while dense bulges are blue due to the high amount of newly formed stars.

Source: Tuttle, S. E., & Tonnesen, S. (2020)

What Sets BreakBRDs Apart

Regular galaxies emerge through hydrogen, helium, and dark matter clumping together due to gravity. The gas swirls into molecular clouds that create star forming regions. Modern galaxies gain their angular momentum from their component gases, but the universe’s first galaxies got their momentum from tidal torques caused by fluctuations in the universe’s density. These galaxies swirl around constantly and form gas-rich arms which allow for high amounts of star formation. Meanwhile, the bulge in the center of the galaxy’s disk is where older stars drift to. This is also generally where active galactic nuclei (objects, usually supermassive blackholes, that create large galactic energy signatures called AGN feedback) form.

The stellar populations of galaxies are usually quantified using their disk g-r and Dn4000 values, where disk g-r is a metric of the green and red color values astronomers see when they image a galaxy while Dn4000, found through spectroscopy imaging, is a metric of how old the stellar population of a galaxy is. Normal spiral galaxies will have low disk g-r values, indicating a blue disk with lots of stellar formation, coupled with low Dn4000 values meaning that stars in the red bulge of the galaxy are relatively young as well. Quiescent, or quenched, galaxies are galaxies that effectively die off due to their star formation completely stopping (which can be due to the galaxy’s gaseous supply being cut off, the environment stripping galactic material, etc.). These galaxies have a high Dn4000 due to their very old stellar populations along with a high g-r since their stellar formation has stopped, leading to a decrease in the number of young stars in the arms of the galaxy and a red bulge / red disk combination.

Figure 2

A graph of the g-r and Dn4000 of normal galaxies (blue), quiescent galaxies (red), and BreakBRDs (green points).

Source: Tuttle, S. E., & Tonnesen, S. (2020)

BreakBRDs are completely different from any other galaxy type we’ve investigated before. They have a high g-r like a quiescent galaxy, meaning that their outer disk is red because of the lack of stellar formation occurring there. Meanwhile, they have a medium to low Dn4000, meaning that their bulge is young and has active stellar formation like in a normal galaxy. This weird combination of a quenched disk and a normal bulge means that the distribution of star formation is completely different in a BreakBRD, leading to all sorts of interesting galactic properties that scientists are still trying to investigate.

Origins and Properties of BreakBRDs

Pinpointing the formation and evolution of a BreakBRD galaxy is even harder than figuring out the life cycle of a normal galaxy. Researchers are already unsure of the exact origins of galaxies due to the extra gravity needed to form them that aren’t accounted for by our calculations of galactic mass. This problem is exacerbated for BreakBRD formation due to their abnormal mass distributions. As a result, some astronomers have suggested that BreakBRDs form from quiescent galaxies having their dead bulges rejuvenated through some sort of galactic process. Proposed theories would have to include mechanisms that remove gas from the disk and insert cold gas into the bulge to accurately explain the structure of a BreakBRD. Additionally, AGN feedback should theoretically occur due to the concentration of gas within the bulge but every BreakBRD so far discovered has extremely weak AGN feedback. Some of these theories include galactic structures such as bars, or elliptical rings of stars inside galaxies, that push gas towards the center or include the merging of galaxies that can lead to tidal interactions and gas inflows.

Figure 3

A graph of the stellar growth rate of normal galaxies (blue), quiescent galaxies (red), and BreakBRDs (green points).

Source: Tuttle, S. E., & Tonnesen, S. (2020)

Further complicating this area of research are the strange properties that BreakBRDs have. Their origin can’t be tied to galactic mass or lifetime, as BreakBRDs occur in almost every class of galaxy. They pop up anywhere and everywhere in galactic populations, indicating that they have no environmental dependence either. However, their stars still form similarly to the stars within other galaxies, just that the location of their formations is now in the bulge. This calls into question whether BreakBRDs are a transitional stage that galaxies can enter or leave due to a combination of other factors. Previous studies have shown interesting gas kinematics within BreakBRDs that create counter-rotating or highly distorted gas velocity fields. Additionally, many BreakBRDs are morphologically asymmetrical and have HI asymmetry (abnormal distributions of partially ionized hydrogen gas).

Figure 4

A graph of the environmental factors of normal galaxies (blue), quiescent galaxies (red), and BreakBRDs (green points).

Source: Tuttle, S. E., & Tonnesen, S. (2020)

Figure 5

A graph of the gas and star velocity profiles within different sizes of BreakBRDs.

Source: Stark, D. V., Tuttle, S., Tonnesen, S., Tu, Z., & Fillingham, S. P. (2024)

Conclusion

BreakBRDs are a relatively rare type of galaxy, but by investigating their mysteries, we stand to learn much more about the types of galaxies that populate our universe. Through a mix of large scale observational surveys, molecular spectroscopy, and computer modeling, researchers continue to pursue the answers to the questions raised about these galaxies. Only through a willingness to look into the abnormal and strange can we unlock the secrets of the universe.

References

National Aeronautics and Space Administration. (2025, April 14). Hubble’s Deep Fields. NASA. https://science.nasa.gov/mission/hubble/science/universe-uncovered/hubble-deep-fields/

Perimeter Institute for Theoretical Physics. (2024, July 26). Galaxy Formation Explained. YouTube. https://www.youtube.com/watch?v=O9UdJ-hciZ0

Stark, D. V., Tuttle, S., Tonnesen, S., Tu, Z., & Fillingham, S. P. (2024). BreakBRD Galaxies: Evolutionary Clues through an Analysis of Gas Content. The Astrophysical Journal, 971(1), 116–116. https://doi.org/10.3847/1538-4357/ad54af

Tonnesen, S., Stark, D., & Tuttle, S. (2024). The cold gas properties of BreakBRD galaxies from new deep GBT 21cm observations. American Astronomical Society Meeting Abstracts #243, 56(2), 262.10. https://ui.adsabs.harvard.edu/abs/2024AAS…24326210T/abstract

Tuttle, S. E., & Tonnesen, S. (2020). BreakBRD Galaxies. I. Global Properties of Spiral Galaxies with Central Star Formation in Red Disks. The Astrophysical Journal, 889(2), 188. https://doi.org/10.3847/1538-4357/ab5dbb