Podcaster: George Bendo
Title: George’s Random Astronomical Object (Object 135: Feedback Time)
Organization: Jodrell Bank Centre for Astrophysics, The University of Manchester
Link: https://www.randomastronomicalobject.com
Description: George’s Random Astronomical Object presents the galaxy NGC 3801, which is one of the very few nearby galaxies where astronomers can see jets from an active galactic nucleus disrupting star formation in the galaxy in a process known as feedback.
Bio: George Bendo is an astronomer who specializes in studying interstellar dust and star formation in nearby galaxies. He currently works at the Jodrell Bank Centre for Astrophysics at the University of Manchester, and his primary role is to support other astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA). He has been creating biweekly episodes of George’s Random Astronomical Object since 2019.
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Transcript:
Welcome to episode 135 of George’s Random Astronomical Object, or, if you prefer Roman numerals, episode CXXXV. Every episode, I run a random number generator to select random astronomical coordinates in the sky, and I then search for an astronomical object near those coordinates and talk about what makes that object so interesting to astronomers. So let’s now turn on the random number generator.
The coordinates for this episode are 11:40:16.9 right ascension and +17:43:41 declination. This points to the galaxy NGC 3801, which is located at a distance of roughly 176 million light years (54 Mpc)from Earth in the constellation Leo. It’s a little hard to describe the appearance of this galaxy because it looks a bit unusual. It’s officially classified as a peculiar lenticular galaxy. A typical lenticular galaxy looks like something halfway between a spiral galaxy and an elliptical galaxy, with a very large bulge of stars in the center and a smooth, featureless disk of stars bisecting that bugle. Lenticular galaxies normally contain very little interstellar gas or dust, and what is present would be located within the inner parts of the disk in the center of the galaxy. NGC 3801, however, is a bit different. In the visible part of the electromagnetic spectrum, it’s possible to see one filament of interstellar dust extending from the center of the galaxy to one end of the disk and another shorter dust lane extending through the bulge of the galaxy perpendicular to the disk. At radio and millimeter wavelengths, it’s possible to see that the galaxy has an outer gas disk aligned with the stellar disk and a smaller inner gas disk that is oriented perpendicular to the stellar disk but parallel to the shorter dust lane.
So, NGC 3801 is officially weird. People have determined that it formed as the result of two disk-shaped galaxies merging together. These galaxies were probably different sizes, with the smaller one colliding in a perpendicular direction with the bigger one. These galaxies also probably contained a lot of gas, and during the merger process, the orbits of the gas clouds would have become very messy, and a lot of these gas clouds could have collided with each other. While some of these gas clouds ended up in either the extended outer gas disk or the really weird perpendicular inner gas disk within NGC 3801, some of the other gas clouds may have fallen into the very center of the galaxy. The various collisions would have caused the gas clouds to collapse, and when interstellar gas clouds collapse, they form new stars. In fact, it looked like NGC 3801 underwent an intense period of forming new stars between 100 and 500 million years ago, but it also looks like the rate at which stars are forming has been steadily decreasing since.
However, this is not the only thing that happened when these gas clouds collided. It looks like at least one of the two galaxies involved in the collision contained a supermassive black hole that I don’t know the mass of but that is going to be millions or billions of times the mass of the Sun. (If both colliding galaxies had black holes, that would have been quite interesting when the black holes merged together, but I’m just going to pretend that only one supermassive black hole was present.) Anyway, that black hole is now at the center of NGC 3801, and about 2.4 million years ago, some gas must have fallen onto the black hole, because it has become what astronomers would call a radio galaxy, which is a type of active galactic nucleus (AGN). The infalling gas would have formed a disk around the supermassive black hole, but a lot of that gas would have become very hot and ionized and then would have been deflected from the event horizon of the black hole by the magnetic fields around the system. That deflected gas would fly away from the poles of the system at speeds close to the speed of light to form jets moving perpendicular to our line of sight, and those jets of gas produce radio emission, which is why this is called a radio galaxy.
While the jets have not yet travelled very far from the center of the AGN, they are currently plowing through the surrounding gas within the galaxy, maybe sort of like a hot knife through butter. (I should try that with a hot knife and a stick of butter in my kitchen sometime.) Anyway, this gas was previously very cold and could have potentially collapsed to form new stars, but the collision with the jets from the AGN are heating everything up, and when interstellar gas gets too hot, it will cease to collapse to form new stars.
This process is called feedback. Some people, when they first hear the word feedback in an audio podcast, might think of a microphone near a speaker. However, that is positive feedback. What’s happening in NGC 3801 is negative feedback, which would be like going to the contact page on my website at www.randomastronomicalobject.com and leaving negative reviews about how my haircut looked while I recorded this podcast. That would make me less likely to cut my hair this way. With NGC 3801, the feedback from the AGN is causing the formation of stars, which had already slowed down from 100 million years ago, to stop completely.
This phenomenon is something that astronomers have seen very frequently in more distant galaxies, which are effectively galaxies that we are seeing when the universe was billions of years younger. It’s known to be a major stage in the evolution of galaxies in the universe, but it has been very rare to actually see it happening in nearby galaxies. NGC 3801 is apparently among the very few nearby galaxies where this feedback process has been positively identified. Understanding exactly how the feedback process works is going to be critical to being able to model the evolution of other galaxies in the universe, and NGC 3801 provides a unique opportunity for astronomers to study the feedback process up close. This is why NGC 3801 is so important in extragalactic research.
That finishes up my summary of NGC 3801, and the location on the Earth’s surface corresponding to the position of NGC 3801 in the sky is about 900 km east southeast of Wake Island in the Pacific Ocean. This could best be described as a place where you would find random fishing boats, but that description could be applied to many other locations in the Pacific. It’s at least slightly more interesting than those places in the ocean without fishing ships.
What’s also more interesting than a random patch of the Pacific Ocean is the website for my podcast at www.randomastronomicalobject.com. You can visit the website to read information about the astronomical objects, view images of those astronomical objects, look up additional reference information, and send me positive and negative random feedback, and just to remind people, this podcast also has social media pages of Facebook, Instagram, and Threads.
The audio was recorded and edited by George Bendo. The sound effects are from The Freesound Project at www.freesound.org. Thanks for listening.
End of podcast:
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