Undergraduate student discovers missing link in galaxy evolution

The findings of UMass student could be used to help the James Webb Space Telescope too.
Christopher McFadden

An undergraduate student from the University of Massachusetts (UMass), Amherst has substantially contributed to our understanding of the relationship between the formation of stars and black holes. Hopefully, this breakthrough will help the James Webb Space Telescope (JWST) more effectively clarify how exactly galaxies function, thanks to this new information.

At present, astronomers are aware that two processes, the expansion of supermassive black holes at each galaxy's center and the creation of new stars, are responsible for the evolution of galaxies. How these processes are connected has remained a mystery, and the recently launched James Webb Space Telescope (JWST) is currently being used to help clear this up.

The student in question, Meredith Stone, who graduated from the astronomy school at UMass Amherst in May 2022, will now aid researchers in understanding their connections.

"We know that galaxies grow, collide and change throughout their lives," says Stone, who completed this research under the direction of Alexandra Pope, professor of astronomy at the University of Massachusetts Amherst and senior author of a new paper, recently published in The Astrophysical Journal.

"And we know that black hole growth and star formation play crucial roles. We think that the two are linked and that they regulate each other, but until now, it's been very hard to see exactly how," she added.

how galaxies form
The new research will help understand how galaxies evolve over time. Source: ESA/Hubble & NASA

It has been challenging to understand how black holes and stars interact since we can't truly observe these interactions because they occur behind massive clouds of galactic dust. According to Pope, more than 90 percent of the visible light from galaxies actively generating stars can be absorbed by dust. This dust also absorbs visible light.

There is a workaround, though: The dust heats up when it absorbs visible light, and while the naked eye cannot sense heat, infrared telescopes can.

"We used the Spitzer Space Telescope," says Stone, who will begin her graduate studies in astronomy at the University of Arizona this fall, "collected during the Great Observatories All-sky LIRG Survey (GOALS) campaign, to look at the mid-infrared wavelength range of some of the brightest galaxies that are relatively close to Earth." 

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Stone and her co-authors searched for distinctive tell-tale tracers, which are the fingerprints of black holes and stars still forming.

The problem is that these fingerprints are so incredibly weak that it is practically impossible to tell them apart from the background infrared noise. "What Meredith did," says Pope, "is to calibrate the measurements of these tracers so that they are more distinct"

Once the scientists had these clearer views, they could observe that star formation and black hole expansion are indeed occurring simultaneously in the same galaxies and that they appear to be impacting one another. Stone was able to determine the ratio that illustrates the connection between the two phenomena.

james webb
An artist's impression of the James Webb Space Telescope. Source: NASA/Adriana Manrique Gutierrez

This is not just a fascinating scientific breakthrough on its own. Still, the JWST can also leverage Stone's study to focus much more intently on the unanswered problems thanks to its unique access to light in the mid-infrared spectrum. Although Jed McKinney, a Ph.D. student in astronomy at UMass Amherst, and Stone calculated the relationship between black holes and stars in the same galaxy, the reason for this relationship is still unknown.

You can view the study that was recently published in The Astrophysical Journal.

Study abstract:

"We present the results of a stacking analysis performed on Spitzer/Infrared Spectrograph high-resolution mid-infrared (mid-IR) spectra of luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey. By binning in relation to mid-IR active galactic nucleus (AGN) fraction and stacking spectra, we detect bright emission lines [Ne ii] and [Ne iii], which trace star formation, and fainter emission lines [Ne v] and [O iv], which trace AGN activity, throughout the sample. We find that the [Ne ii] luminosity is fairly constant across all AGN fraction bins, while the [O iv] and [Ne v] luminosities increase by over an order of magnitude. Our measured average line ratios, [Ne v]/[Ne ii] and [O iv]/[Ne ii], at low AGN fraction are similar to H II galaxies, while the line ratios at high AGN fraction are similar to LINERs and Seyferts. We decompose the [O iv] luminosity into star formation and AGN components by fitting the [O iv] luminosity as a function of the [Ne ii] luminosity and the mid-IR AGN fraction. The [O iv] luminosity in LIRGs is dominated by star formation for mid-IR AGN fractions ≲0.3. With the corrected [O iv] luminosity, we calculate black hole accretion rates (BHARs) ranging from 10−5 M yr−1 at low AGN fractions to 0.2 M yr−1 at the highest AGN fractions. We find that using the [O iv] luminosity, without correcting for star formation, can lead to overestimation of the BHAR by up to a factor of 30 in starburst-dominated LIRGs. Finally, we show that the BHAR/star formation rate ratio increases by more than three orders of magnitude as a function of mid-IR AGN fraction in LIRGs."

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