Object 28: PGC 43234

Podcast release date: 24 August 2020

Right ascension: 12:48:15.2


Epoch: ICRS

Constellation: Coma Berenices

Corresponding Earth location: About 200 km northeast of Johnston Atoll

PGC 43234 is located at a distance of about 312 million light years (96 Mpc) in the constellation Coma Berenices [1]. The galaxy was discovered around 1975 [2]. At first, it looked like a fairly ordinary and boring galaxy, and people generally ignored the galaxy for a couple of decades.

However, on November 22, 2014, the center of the galaxy suddenly got brighter [3]. This sudden flash of light was first seen by a survey called the All-Sky Automated Survey for Supernovae. The survey has the acronym ASASSN, which is like the word assassin except that it's missing a couple of letters. The survey relies on a network of very small robotically-controlled telescopes distributed around the Earth that try to survey the entire sky every night. Since this event was detected by this survey, it was given the designation ASASSN-14li.

Even though a supernova survey had discovered ASASSN-14li, astronomers somehow did not think that the event was a supernova, although I didn't find anything that explained what people were thinking. Anyhow, they decided to observe the galaxy with a series of X-ray space telescopes, including the Chandra X-ray Observatory, the Swift X-ray Telescope, and XMM-Newton [3,4,5,6]. They found an X-ray source at the center of PGC 43234 that had not been there before, and this demonstrated that something much more exciting than a star exploding had taken place within the galaxy. So, take a couple of seconds just to think about what could be more exciting than an exploding star, and then I'll tell you what happened.

What astronomers think happened is that, at the center of the galaxy, a supermassive black hole with a mass about one million times of the mass of the Sun shredded (or, to use the technical term, tidally disrupted) a Sun-like star that passed too close [3]. The remains of the star did not fall directly into the black hole like you would expect in a science fiction movie like one of the movies that I watched when I was a kid featuring Ernest Borgnine. Instead, the star became a stream of gas that first fell in the direction of the black hole and then went into orbit around it, forming an elliptical disk of gas. In the process of doing this, the gas was compressed by the gravity of the black hole and became very hot, and it produced a huge amount of electromagnetic radiation at all wavelengths.

Astronomers got really excited about all of this not only because this is literally like something out of science fiction but also because nothing like this had ever been seen so close to our galaxy. Hence, this was an excellent opportunity to discover what happens after a black hole tears a star apart.

As I already mentioned, this process caused the center of the galaxy to flare in brightness in all parts of the electromagnetic spectrum, but the center of the galaxy faded over time [7]. However, I found two other results from the subsequent observations particularly interesting. First, observations with radio telescopes found that some of the gas from the newly-formed gas disk may have, through complex physical processes, begun to flow outwards from above and below the poles of the black hole as jets of gas [8]. These types of jets are actually seen originating from above the poles of other supermassive black holes in other galaxies as well, but those are black holes where gas disks formed around the black holes a very long time ago. The black hole in PGC 43234 has a newly-formed gas disk, so it would be really interesting if it formed new jets as soon as it acquired a new gas disk. However, astronomers also question whether these jets just recently formed or if they are left over from a much older time period [9].

The second result which I thought was very interesting was based on observations from the Wide-field Infrared Survey Explorer. That telescope detected a flare in infrared emission that took place about 110 days after the black hole had destroyed the star that got too close to it [11]. Astronomers think that the infrared light came from one or more clouds of interstellar dust located at a distance of 110 light days from the black hole (where a light day is a unit of length equivalent to the distance that light and other forms of electromagnetic radiation can travel in a single day). What astronomers think happened is that, 110 days after the star was torn apart by the supermassive black hole, the flash of electromagnetic radiation from this event reached these dust clouds and then heated up the interstellar dust to a temperature of about 1800 degrees Celsius [11]. This caused the interstellar dust to produce huge amounts of infrared radiation that we on Earth observed 110 days after the main flare. This type of phenomenon is called an echo, and it's very useful for identifying the structure of the interstellar medium around other objects like this supermassive black hole in PGC 43234.


[1] NASA/IPAC Extragalactic Database, 2020

[2] Zwicky, F. et al., Eighth list of compact galaxies., 1975, Astronomical Journal, 80, 545

[3] Miller, Jon M. et al., Flows of X-ray gas reveal the disruption of a star by a massive black hole, 2015, Nature, 526, 542

[4] Maksym, W. P. et al., X-ray Astrometric Confirmation of Association of the Candidate Tidal Disruption Event ASASSN-14li with its Host Nucleus, 2014, The Astronomer's Telegram, 6834, 1

[5] Miller, J. M. et al., X-ray Absorption Lines in the Candidate Tidal Disruption Event ASASSN-14li, 2014, The Astronomer's Telegram, 6825, 1

[6] Holoien, T. W. -S. et al., Swift Observations of the TDE ASASSN-14li Show That It Remains Bright in UV and X-rays, 2015, The Astronomer's Telegram, 8309, 1

[7] Brown, J. S. et al., The Long Term Evolution of ASASSN-14li, 2017, Monthly Notices of the Royal Astronomical Society, 466, 4904

[8] van Velzen, S. et al., A radio jet from the optical and x-ray bright stellar tidal disruption flare ASASSN-14li, 2016, Science, 351, 62

[9] Romero-Cañizales, Cristina et al., The TDE ASASSN-14li and Its Host Resolved at Parsec Scales with the EVN, 2016, Astrophysical Journal Letters, 832, L10

[10] Jiang, Ning et al., The WISE Detection of an Infrared Echo in Tidal Disruption Event ASASSN-14li, 2016, Astrophysical Journal Letters, 828, L14


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: 15HPanska_Ruttner_Jan, afleetingspeck, CalebLopez, dronemachine, ikbenraar, ivolipa, jameswrowles, modularsamples, newagesoup, and Xulie at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)