Spitzer gives scientists insights into black hole eating habits

by Haygen Warren

Using data from NASA’s retired Spitzer Space Telescope, scientists have gained new insights into the eating habits of supermassive black holes at the centers of galaxies throughout the universe.  These black holes often fluctuate in brightness from the massive clumps of cosmic material falling into them.

However, this is not true for the black holes located at the centers of the Milky Way and the Andromeda galaxies. Instead, they remain fairly quiet and rarely ever vary in brightness. To find the cause of the decreased activity around the black holes, a new study used observations from Spitzer and Hubble to model the black hole and the material surrounding it at the center of the Andromeda galaxy.

In images from Spitzer, long streams of dust that span thousands of light-years in length can be seen flowing into the supermassive black hole at the center of the Andromeda galaxy — the closest major galaxy to Earth, located at about 2.5 million light-years away.

As cosmic gas and dust fall into supermassive black holes, such as the one located in Andromeda, the material heats up and begins to glow, creating light shows around the black hole that can glow brighter than entire galaxies. However, this material isn’t absorbed all at once. Instead, the material is consumed in clumps that vary in size, causing the brightness of the black hole to fluctuate.

Close-up view of the center of the Andromeda galaxy, taken by the Spitzer Space Telescope. The blue dotted lines highlight the path of two streams that flow into the black hole at the center of the galaxy. (Credit: NASA/JPL-Caltech)

Interestingly, the supermassive black holes located at the center of the Milky Way and Andromeda are among the quietest known black holes in the universe when it comes to consuming cosmic material (or “eating”). When light is emitted from the black holes, the light doesn’t significantly vary in brightness, which could mean the black holes are feeding off of a small and steady stream of cosmic material rather than different-sized clumps of material.

Earlier this year, a team of scientists applied the hypothesis of a black hole feeding on a small steady stream of cosmic material to the Andromeda galaxy and simulated how gas and dust around Andromeda’s black hole would behave over time. The simulation revealed that a small disk of hot gas could form near the black hole and continuously provide the black hole with a flow of cosmic material. The disk can constantly provide the material due to the disk being replenished by numerous streams of gas and dust.

However, the team also found that the streams replenishing the disk must remain within a particular size and flow rate. If they become too big or too small, the material would fall into the black hole in clumps of various sizes, leading to the black hole fluctuating in brightness — which the Milky Way and Andromeda black holes do not do.

When looking back at previous observations of Andromeda from NASA’s Hubble Space Telescope and Spitzer, the scientists found spirals of dust that fit the constraints highlighted by the simulation. Using these images, the team concluded that the spirals were indeed feeding the supermassive black hole at the center of Andromeda. This result also means that a similar process is likely taking place at the center of the Milky Way, given that the two black holes exhibit similar behaviors and characteristics.

“This is a great example of scientists reexamining archival data to reveal more about galaxy dynamics by comparing it to the latest computer simulations. We have 20-year-old data telling us things we didn’t recognize in it when we first collected it,” said co-author Almudena Prieta of the Institute of Astrophysics of the Canary Islands and the University Observatory Munich.

Image of the center and outer regions of the Andromeda galaxy, taken by Hubble in visible light. (Credit: NASA/ESA/J. Dalcanton (University of Washington, USA)/B. F. Williams (University of Washington, USA)/L. C. Johnson (University of Washington, USA)/the PHAT team/R. Gendler)

As mentioned, images from Spitzer were used to confirm the scientists’ hypothesis. Spitzer was launched in August 2003 atop a Delta II from Cape Canaveral Air Force Station and was the third telescope dedicated exclusively to observing the universe in infrared. The joint NASA, European Space Agency, and Canadian Space Agency James Webb Space Telescope is another telescope that exclusively observes in infrared. Observing in infrared has many advantages, most notably that it gives scientists the capability to see through thick layers of dust that are present in galaxies and other cosmic objects, as well as the capability to see the very early universe.

Spitzer’s observations of Andromeda were performed using different wavelengths, each revealing different features of the galaxy like stars and dust structures. By separating the wavelengths and solely looking at the dust, the scientists were able to view the “skeleton” of the galaxy or regions where gas has coalesced and cooled, creating stellar nurseries where young stars can form.

Viewing the galaxy in this way surprised the scientists. One surprise was that Andromeda is dominated by a large dust ring rather than conventional distinct arms that circle the center of the galaxy. Additionally, a large hole was found within the ring where a dwarf galaxy passed through.

Infrared images from Spitzer showing the dust within Andromeda. (Credit: NASA/JPL-Caltech/University of Arizona)

Even though Spitzer has a field of view that is wider than Hubble’s, the telescope still had to take around 11,000 images of the galaxy to create the image used in the study.

The team’s results were published in The Astrophysical Journal.

(Lead image: The Andromeda galaxy seen in infrared by the Spitzer Space Telescope. Credit: NASA/JPL-Caltech/University of Arizona)

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