Object 57: 3C 147

Podcast release date: 04 October 2021

Right ascension: 05:42:36.1


Epoch: ICRS

Constellation: Auriga

Corresponding Earth location: A mountain within the Altai Mountain range lying within Russia near the border with eastern Kazakhstan

I'll start by saying that this object is a quasar, and it's not interesting just because it is a quasar. It's actually one of the first two objects ever sort-of called a quasar, so it's somewhat important in the history of astronomy. But first, let me describe what quasar are.

Quasars are a subset of galaxies that contain active galactic nuclei (AGN). An active galactic nucleus contains a supermassive black hole that is millions or billions of times more massive than the Sun, a disk of gas that is falling into that black hole, and jets of ionized gas that emerge from above the poles of the black hole. The gas in these jets don't originate from the black hole itself but instead from the gas disk. As the gas approaches the black hole, it gets gravitationally compressed, which causes it to get very hot and very high in pressure, causing it to expand out of the disk. The magnetic fields around the black holes carry the gas outwards towards the poles of the black hole. This gas can potentially fly away from the central black hole at speeds close to the speed of light and can potentially travel outside the galaxy hosting the active galactic nucleus.

Astronomers have many different labels for active galactic nuclei. Things labelled as quasars are generally very bright as seen in many different parts of the electromagnetic spectrum, but they are also very far away. The jets from quasars as seen from Earth appear generally close to along our line of sight. When the jets are pointed perpendicular to the direction in which we see the objects, astronomers call the objects radio galaxies, in large part because these jets of gas produce large amounts of radio waves.

Anyway, let's get back to the history of quasars and the history of the quasar 3C 147. This object was originally discovered in 1953 by R. Hanbury Brown and Cyril Hazard in a survey of radio emission of part of the northern sky [1]. Radio astronomy was still very new in the 1950s, so just detecting radio emission from the sky was really exciting. Interestingly, this survey also detected radio emission from the Andromeda Galaxy, the Whirlpool Galaxy, and the spiral galaxy NGC 891 [1], which, as you would know if you've listened to Episode 42, always causes laughter at professional astronomy conferences. Aside from the fact that the survey that discovered 3C 147 also discovered radio waves from these other nearby galaxies, I find it fascinating that 3C 147 was discovered at Jodrell Bank [1], which I am currently affiliated with (although I don't actually work at the observatory site).

While astronomers in the 1950s could easily associate radiowave emission from some parts of the sky with either nearby galaxies or objects within our own galaxy, they had difficulty figuring out what was happening with radiowave emission from other parts of the sky. This, however, did not stop people from mapping all of the bright radiowave sources in the sky, and starting in 1950, some astronomers at Cambridge University put together a catalog of these sources later called the First Cambridge Catalogue [2]. After this, astronomers at Cambridge continued to periodically publish catalogs of radio sources much in the same way that Universal Studios periodically releases movies in the Fast and Furious franchise, but the Third Reference Catalogue [3] is the most famous of all of these radio source catalogues (which would somehow be like if Tokyo Drift was the best Fast and Furious movie). Objects from the Third Reference Catalogue are often given the label "3C". 3C 147 got it's name because it is the 147th radio source listed in the 3C catalog.

Although astronomers spent a lot of time making lists of radio sources, by the year 1960, they still did not have any good idea as to what some of these objects were. When astronomers looked at photographic plates corresponding to the positions of some of the 3C sources, they found that the radio emission was associated with what looked like blue stars, which was confusing for multiple reasons. First of all, individual stars are generally not very strong sources of radio emission, so it was odd that some blue stars produced lots of radiowaves. Second, it was weird that many of the blue stars associated with radio emission looked slightly fuzzy. Some of the objects even seemed to have small, narrow extension of light coming out of their centers.

To try to make sense of these objects, Maarten Schmidt performed some observations in the early 1960s where he made some spectra of the visible light from these stars. Spectra in astronomy are very useful because individual atoms will absorb or emit light at very specific wavelengths, so astronomers can use spectra to identify which elements and molecules are present either in the atmospheres of stars or in interstellar space. What was very confusing, though, was that the spectra from these radio sources didn't seem to correspond to any elements or molecules found on Earth or anywhere else in space. However, in 1963, Maarten Schmidt had a breakthrough in his observations of the radio source 3C 273, where he figured out that he was looking at hydrogen that was really really redshifted compared to the galaxies that astronomers in the early 1960s were used to looking at [4]. Because of the expansion of the universe, objects that are further from the Milky Way Galaxy appear to be moving faster than objects that are closer, so at the time, 3C 273 was the most distant galaxy in the sky that anyone knew about.

However, he did not stop with just 3C 273. He made a few more observations of other weird radio sources to prove that they too were actually very distant galaxies, and it was in his science paper published in 1964 about the objects 3C 47 and 3C 147 where he first used the term "quasi-stellar radio source", because the objects looked sort of like stars but actually were very distant galaxies that contained very bright radio sources [5]. Later in 1964, this term was shortened to "quasar" by Hong-Yee Chiu in an article in the magazine Physics Today [6], presumably because he did not want to keep on typing "quasi-stellar radio source" on his 1960s era typewriter. The name stuck. So, 3C 147 was sort-of one of the first two objects explicitly called a quasar, which places this quasar in an interesting place in the history of astronomy.

By the way, 3C 147 has recently been measured to be moving away from the Earth at a speed of about 0.41 times the speed of light, which is actually very close to what Maartin Schmidt measured in 1964. This means that all of the wavelengths of light from 3C 147 as observed on Earth appear 55 percent longer than when they were emitted by the quasar [5, 7]. Also, this redshift places the quasar at a distance of roughly 6.7 billion light years (2100 Mpc).

Since Maarten Schmidt's 1964 paper, 3C 147 has been a very popular object to observe with radio telescopes because it is such a bright but small radio source. In fact, it is a popular target to observe with what's called very long baseline interferometry [8, 9, 10, 11, 12, 13, 14, 15, 16]. I'm not going to discuss what this term means, but it basically involves using radio antennae that are spread over really long distances. So, for example, the antennae could be spread across England, or across the United States (including Hawaii and the Virgin Islands), or even across the whole of Europe. Most radio sources in the sky are too extended or too faint for very long baseline interferometry to work, but 3C 147 is so bright and so compact that the radiowaves can be imaged using this technique. When astronomers have observed 3C 147 using very long baseline interferometry, they see a jet of gas that appears to extend about 4800 light years (1.4 kpc) from the center of the galaxy where the supermassive black hole is located [14, 15, 16], although since we are seeing this jet of gas angled towards us, it may actually be longer. Just to remind you, this jet of gas comes from gas that gets too hot as it falls into the central black hole and gets redirected by magnetic fields away from the surface of the black hole. Located at the center of the quasar is something that sort of looks cone-shaped with a bright blob in the center and a fainter blob near the tip of the cone [16]. The fainter blob might actually be from the area around the black hole, although astronomers are still trying to determine if that is actually the case.

The other interesting thing about 3C 147 is that it is used to calibrate radio telescope observations so that astronomers can convert the electronic signal from these telescopes to physical units of measurement that describe how much energy is coming from objects in the sky [17]. This process is called amplitude calibration or flux calibration. 3C 147 is not necessarily astronomers' first choice for radio flux calibration because its brightness is variable. These variations probably come from changes in the rate at which gas in 3C 147 is ejected into the active galactic nucleus's jets. Nonetheless, careful comparisons of 3C 147 to other objects that do not vary in brightness have allowed astronomers to develop mathematical equations for accurately calculating the amount of radio emission from 3C 147, and these days, 3C 147 is very useful for flux calibrating radio astronomy observations.


[1] Hanbury Brown, R. and Hazard, C., A survey of 23 localized radio sources in the northern hemisphere, 1953, Monthly Notices of the Royal Astronomical Society, 113, 123

[2] Ryle, M. et al., A preliminary survey of the radio stars in the Northern Hemisphere, 1950, Monthly Notices of the Royal Astronomical Society, 110, 508

[3] Edge, D. O. et al., A survey of radio sources at a frequency of 159 Mc/s., 1959, Memoirs of the Royal Astronomical Society, 68, 37

[4] Schmidt, M., 3C 273 : A Star-Like Object with Large Red-Shift, 1963, Nature, 197, 1040

[5] Schmidt, Maarten and Matthews, Thomas A., Redshift of the Quasi-Stellar Radio Sources 3c 47 and 3c 147., 1964, Astrophysical Journal, 139, 781

[6] Chiu, Hong-Yee, Gravitational Collapse, 1964, Physics Today, 17, 21

[7] Truebenbach, Alexandra E. and Darling, Jeremy, The VLBA Extragalactic Proper Motion Catalog and a Measurement of the Secular Aberration Drift, 2017, Astrophysical Journal Supplement Series, 233, 3

[8] Phillips, R. B. and Shaffer, D. B., VLBI maps of 3C 147, 3C 286, 3C 380, NRAO 150, CTD 93 and 3C 395 at 2.3 GHz., 1983, Astrophysical Journal, 271, 32

[9] Zhang Fu-Jun et al., VLBI observation at 1.662 GHz of 3C 147., 1989, Acta Astronomica Sinica, 30, 172

[10] Zhang, Fu-jun et al., VLBI observation of 3C 147 at 1.662 GHZ, 1989, Chinese Astronomy and Astrophysics, 13, 386

[11] Akujor, Chidi E. et al., MERLIN images of two compact steep-spectrum sources, 3C 147 and 295., 1990, Monthly Notices of the Royal Astronomical Society, 244, 362

[12] Zhang, Fujun, A MERLIN map of QSO 3C 147 at 1.67 GHz., 1993, Shanghai Observatory Annals, 14, 187

[13] Greve, A. et al., 147 GHz VLBI observations: Detection of 3C 273 and 3C 279 on the 3100 km baseline Metsähovi - Pico Veleta, 2002, Astronomy & Astrophysics, 390, L19

[14] Zhang, H. Y. et al., Parsec-scale rotation-measure distribution in the quasar 3C 147 at 8 GHz, 2004, Astronomy & Astrophysics, 415, 477

[15] Lazio, T. Joseph W. et al., Spatial Variations in Galactic H i Structure on Au-Scales Toward 3C 147 Observed with the Very Long Baseline Array, 2009, Astronomical Journal, 137, 4526

[16] Rossetti, A. et al., VLBA polarimetric observations of the CSS quasar 3C 147, 2009, Astronomy & Astrophysics, 504, 741

[17] Perley, R. A. and Butler, B. J., An Accurate Flux Density Scale from 50 MHz to 50 GHz, 2017, Astrophysical Journal Supplement Series, 230, 7


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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