Radio Galaxy Zoo

by Zachary Lee and Darshan Chakrabarti

Adler Zoo Nyverse: Greetings! Welcome to the first post of a 3 week mini-series dedicated to exploring Radio Galaxy Zoo, a new citizen science project. My name is Adler Zoo Nyverse, and I will be your host for these next couple of weeks. I would like to welcome Professor Hubble and Professor Kepler. Gentlemen, welcome, and thank you for being with us today.


P. Kepler: The pleasure is all mine.


P. Hubble: Likewise.


Adler Zoo Nyverse: So let’s start off with this concept of gravity, one that is confusing for many people. What IS GRAVITY?


P. Kepler: Commonly, gravity is considered to be an interaction which occurs between any two things that are made out of matter. Gravity is an attractive force which means that it pulls things together. When objects are closer, the force between them is stronger.  Sometimes we refer to the gravitational fields of an object. The gravitational field of an object increases with its mass and decreases with its size.


P. Hubble: For example, the Earth’s mass and size creates a gravity that pulls things to the ground. Since the Earth’s mass is relatively big (a modest 5.97219 x 10^24 kilograms) it takes powerful engines to break free from the grip of the Earth’s gravity and make it to space. The speed need to break free from Earth’s gravitational field is known as escape velocity. You can almost think of the situation as pedaling up a steep hill: if you don’t start with a high enough speed, you won’t be able to get past the hill and will fall back down again.



A black hole’s escape velocity is greater than the speed of light. This means that even light cannot escape a black hole; since the speed of light is the universal speed limit, nothing is fast enough to escape the field of a black hole. Note that if the Earth was crushed to a very small size but kept the same mass, that the escape velocity becomes higher than the speed of light, making Earth a black hole. Not to worry though, Earth is very stable and won’t collapse into a black hole.


Adler Zoo Nyverse: So we all know about black holes and how they are really powerful. Can you explain what role gravity plays in the properties of a black hole?


P. Kepler: Black holes’ have very strong gravitational fields because they are so dense meaning that they are very massive but take up very little space. When a black hole has a companion star nearby it, matter from the companion star often gets sucked in by the black hole and forms a disk around it. This disk tends to emit radiation due to immense amounts of friction within the disk. If parts of the star fall on the disk, large jets of electrically charged particles can be produced.


 Here is an artist’s depiction of the jets coming from a black hole in the center of a galaxy .


Adler Zoo Nyverse: Fascinating! But how can we see these wonderful acts of nature firsthand? Is it possible for powerful telescopes to detect them?


P. Hubble: Well unfortunately, it is impossible to actually SEE a black hole. However, if one were to find themselves in the possession of a KG Jansky Very Large Array (NRAO) or an Australia Telescope Compact Array (CSIRO) then one could observe the jets that a black hole emits.


Adler Zoo Nyverse: So all this is great, but I don’t think most of us have such fancy telescopes lying around in our living rooms. So how can people like us pitch in and help? You know…the average science-enthusiast?


P. Kepler: Zooniverse, a citizen science web portal designed to allow citizens to help scientific research, has a project known as Radio Galaxy Zoo. This project is interested in learning more about supermassive black holes, which are possibly formed when different galaxies collide.  These black holes tend to exist in the center of galaxies and are billions of times more massive than our own sun. Radio Galaxy Zoo needs the help of citizen scientists to identify jets that massive black holes tend to form, and associate them with the host galaxies of the black holes. Humans are required for this task because jets and galaxies can sometimes overlap and form complex patterns, and humans are much better at being able to interpret patterns than computers.


P. Hubble: Here are some screenshots from the project:

r1 © Radio Galaxy Zoo 2014


Radio images (like that above) are used to identify the jets since the jets produce radio waves and are thus detectable by radio telescopes. The contours (in white) help emphasize the jets of the black holes and between the two jets, one should be able to trace the origin of the black hole.


© Radio Galaxy Zoo 2014

In order to help identify the associated galaxies that the black hole sits in we look at infrared images (like that above), since the dust in galaxies emanate infrared radiation. Infrared images show us signs of the galaxy. An infrared source between the contours is clearly seen here, whereas one could not see it in the radio images, showing the need for both kinds of telescopes.


© Radio Galaxy Zoo 2014

Notice that the look of black holes is all a matter of perspective. The  image above is combined infrared and radio light and the contours tracing the jets of a black hole are highlighted when being oriented by like the artist’s interpretation (see above). Compare this with the image below.


 © Radio Galaxy Zoo 2014

While black holes usually have a pair of jets, sometimes depending on what point of view the image was taken from, there appears to be only one jet, as is the case in this image. In reality, there are always two. Here, the image shows only one jet, but that is because one is looking at the jet head on, rather than from a different angle.


Adler Zoo Nyverse: Fascinating! Well it has been a pleasure to hear from bothof you. Thank you to everyone who has been reading! Check back next week for the next installment!


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