Object 104: J16021+3326

Podcast release date: 07 August 2023

Right ascension: 16:02:07.3


Epoch: ICRS

Constellation: Corona Borealis

Corresponding Earth location: Just offshore from Los Angeles near San Nicolas Island

This is another one of those objects where the name is based on the coordinates of the object. The J at the beginning of the name stands for Julian and indicates that this is based on the coordinate system for what is called the Julian epoch, which corresponds to the year 2000. This object is a very distant galaxy that contains an active galactic nucleus (AGN). AGN seem to have been coming up a lot in my podcast recently. An AGN contains a supermassive black hole millions or billions of times the mass of the sun, a disk of gas and dust falling into the black hole, and one or two jets of gas that appear above the poles of the black hole that originate from hot infalling gas that was deflected away from the black hole by the strong magnetic fields in the system. These jets, which can extend for thousands of light years from the centers of the AGN where they originate, produce a huge amount of electromagnetic radiation, but they are particularly bright in terms of radio emission.

AGN that are very bright but very distant generally fall into one of a couple of categories based on their appearance. If the jets are pointed sideways as seen from Earth, it's possible to see the full extent of the jets, and because the jets produce strong radio emission, the object would be referred to as a radio galaxy. If one of the jets is tilted towards the Earth but not necessarily directly at the Earth, the object looks relatively compact, but it's still possible to see the jet extending outwards (although it looks kind of short), and the object is typically called a quasar.

J16021+3326, however, looks very point-like. This is often a situation where one of the jets is aimed directly at the Earth, which would make the AGN look abnormally bright. These types of AGN are called blazars. However, it was possible that J16021+3326 was a different type of AGN called a compact symmetric object where the jets from the AGN are plowing into really thick interstellar gas within the galaxy rather than flowing outwards, and since these galaxies are very far away, the radio emission looks very compact [1].

One way to determine whether an AGN is a blazar or a compact symmetric object is to make an image of the radio emission. A blazar will typically have one very bright jet and will therefore look asymmetric, whereas a compact symmetric object will have two jets and look symmetric (as is also indicated by the name). However, the radio emission comes from a very small region on the sky that is difficult to image using most conventional telescopes.

So, to properly identify what type of AGN that J16021+3326 was, it was necessary to use a radio telescope called the Very Long Baseline Array, which consists of 10 radio antennas spread across the United States. One of the antennas is in New Hampshire, one is in Hawaii, and one is in the Virgin Islands. It's literally a telescope as large as the United States. For various reasons related to its size, it has problems seeing extended things, like most galaxies and nebulae, and it lacks the sensitivity to see faint radio sources, but it is excellent for imaging very small but very bright radio sources like blazars and compact symmetric objects. (As an aside, I heard a joke as a PhD student that the Very Long Baseline Array can only observe 100 astronomical objects. That might be an accurate assessment.)

Anyway, a group led by S. E. Tremblay (where I think the "S" stands for Steven but I'm not absolutely certain) published a paper in 2010 describing the results from observing J16021+3326 with the Very Long Baseline Array, and they found that the object is indeed asymmetric [1]. These results along with some additional data from other telescopes clearly indicated that J16021+3326 is a blazar. While it may not be the best known example of a blazar, blazars are still relatively rare. The object's identification as a blazar is still useful for astronomers who want to perform surveys of these objects; those astronomers now know that J16021+3326 is indeed like other blazars. Additionally, those astronomers know that it is not an example of a compact symmetric object, so they shouldn't group it together with real compact symmetric objects if they want to understand that class of AGN better.


[1] Tremblay, S. E. et al., J16021+3326: New Multi-Frequency Observations Of a Complex Source, 2010, Astrophysical Journal, 712, 159


Podcast and Website: George J. Bendo

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