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UMass student Merdith Stone may have discovered how galaxies evolve

An undergraduate student at the University of Massachusetts (UMass), Amherst made significant contributions to our understanding of the relationship between star formation and black holes. Hopefully, this breakthrough will help the James Webb Space Telescope (JWST) to more effectively clarify exactly how galaxies work, thanks to this new information.

At present, astronomers are aware that two processes, the expansion of supermassive black holes at the center of each galaxy 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 clarify this.

The student in question, Meredith Stone, who graduated from the UMass Amherst School of Astronomy in May 2022, will now help researchers understand their connections.

“We know that galaxies grow, collide, and change throughout their lifetimes,” says Stone, who completed the research under the direction of Alexandra Pope, professor of astronomy at the University of Massachusetts at Amherst and lead author of a new article, recently published in The Astrophysical Journal.

“And we know that the growth of black holes and star formation play crucial roles. We think the two are linked and that they regulate each other, but until now it’s been very difficult to see exactly how,” she added.

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

It’s been difficult to understand how black holes and stars interact because we can’t really observe these interactions because they happen behind huge clouds of galactic dust. According to Pope, more than 90% of the visible light from galaxies actively generating stars can be absorbed by dust. This dust also absorbs visible light.

There is a workaround, however: dust heats up when it absorbs visible light, and while the naked eye can’t detect heat, infrared telescopes can.

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

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

The problem is that these fingerprints are so incredibly faint that they are virtually indistinguishable from background infrared noise. “What Meredith did,” says Pope, “is calibrate the measurements of these tracers to be more distinct”

Once scientists had these clearer views, they were able to observe that star formation and black hole expansion do indeed occur simultaneously in the same galaxies and appear to influence each other. Stone was able to determine the ratio that illustrates the link between the two phenomena.

James Webb
Artist’s impression of the James Webb Space Telescope. Source: NASA/Adriana Manrique Gutierrez

It’s not just a fascinating scientific breakthrough in itself. Yet the JWST can also leverage Stone’s study to focus much more intensely on 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 consult the study which was recently published in The Astrophysical Journal.

Summary of the study:

We present the results of a stacking analysis performed on high-resolution mid-infrared (mid-IR) spectra from the Spitzer/Luminous Infrared Galaxy Infrared Spectrograph (LIRG) as part of the LIRG All-Sky Survey of the Great Observatories. By binning against the active galactic nucleus (AGN) fraction in the mid-IR 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 plot AGN activity, throughout the sample. We find that the [Ne ii] the brightness is fairly constant across all AGN fraction compartments, while the [O iv] and [Ne v] the luminosities increase by more than an order of magnitude. Our measured average line ratios, [Ne v]/[Ne ii] and [O iv]/[Ne ii], low AGN fraction are similar to H II galaxies, while high AGN fraction line ratios are similar to LINER and Seyferts. We decompose the [O iv] brightness in star formation and AGN components by adjusting the [O iv] brightness depending on the [Ne ii] luminosity and AGN mid-IR 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 year−1 at low AGN fractions down to 0.2 M year−1 at the highest AGN fractions. We find that using the [O iv] brightness, without star formation correction, can lead to an overestimation of BHAR by up to a factor of 30 in star-dominated LIRGs. Finally, we show that the BHAR/star formation rate ratio increases by more than three orders of magnitude as a function of the mid-IR AGN fraction in LIRGs.”