Object 32: NGC 6251

Podcast release date: 19 October 2020

Right ascension: 16:32:32.0


Epoch: ICRS

Constellation: Ursa Minor

Corresponding Earth location: Slightly over 300 km northwest from Ellesmere and Axel Heiberg Islands in the Arctic Ocean

NGC 6251 is a large elliptical galaxy found in the constellation of Ursa Minor. Everything about this galaxy is large except the constellation that it's located in.

The most interesting thing about NGC 6251 is its active galactic nucleus (AGN). An AGN consists of a supermassive black hole, a disk of gas falling into that black hole, and jets of gas that emerges from above and below the black hole. The jets do not come out of the black hole itself but instead come from gas that was originally falling inwards towards the black hole but did not actually enter the black hole itself. The reason why the gas does this is that it gets compressed as it falls towards the black hole, which makes it hot and causes the gas to expand out of the plane of the disk, thus producing something like wind in space. Magnetic fields in the environment around a supermassive black hole will deflect these winds so that they flow outwards above the poles of the black hole, thus producing the jets [1].

So, NGC 6251, like a lot of AGN, has two jets of gas emerging from opposite sides of its nucleus. When we look at this galaxy in the visible part of the electromagnetic spectrum, we mostly just see a giant sphere of stars; the jets are virtually invisible. However, the jets produce very strong radiowave emission, which has led astronomers to refer to NGC 6251 and galaxies with similar jets as radio galaxies. The two jets extend beyond the sphere of stars to both the northwest and southeast of the galaxy (as viewed in the Earth's night sky) [2,3,4,5,6]. The jet to the northwest looks like a narrow stream of gas sort of like the stream of water from a faucet or a hose, but the jet then terminates in a giant cloud of gas called a lobe. This is where the gas from the jet slams into the very very thin gas between galaxies called the intergalactic medium. The jet to the southeast looks similar to the one to the northwest except that it doesn't have the narrow stream of gas from the center of the galaxy. Also, the lobe to the southeast is twice as far from the center of the galaxy as the lobe to the northwest.

The jets are surprisingly huge. NGC 6251 actually gets placed in a subcategory of radio galaxies called giant radio galaxies, although I think supersized or titanic or ginourmous or another synonym with more syllables than giant would be better. As seen from Earth, the jets extend about one degree in the sky [2,4,5]. For comparison, the Moon covers only half a degree on the sky, so the jets appear twice as wide as the Moon. However, the Moon is very close to the Earth, whereas NGC 6251 is located at a distance of 318 million light years (97.6 Mpc) [7]. Applying simple geometry, this means that the jets are more than 5.5 million light years (1.7 Mpc) in size. To put that number into context, the distance from the Earth to the Andromeda galaxy, which is the closest spiral galaxy to our own, is less than half the size of these jets [8,9]. It's also worth pointing out that, if we look at the stars in NGC 6251 in the visible part of the electromagnetic spectrum, they only extend over a diameter that is less than one-thirtieth the size of the jets [10]. Because the jets are so immense and because nothing travels at the speed of light (although most things don't get even that close), it took a while for these jets to form. A recent analysis by a group of astronomers including people that I work with at the University of Manchester indicate that the jets took at least somewhere between 200 and 250 million years to form [11].

However, it's not just the jets in this galaxy that are enormous. The supermassive black hole in the AGN is also, as implied by the name, very massive. A pair of astronomers using the Hubble Space Telescope were able to detect ionized gas in orbit around the very center of the galaxy, and they inferred that this gas was orbiting the central black hole. By applying some really basic physics, they determined that the central black hole was somewhere between 400 and 800 million times the mass of the Sun [12]. Now, I really wanted to say that this was a jumbo-sized supermassive black hole, and while it is gigantic compared to the supermassive black hole at the center of our galaxy, its not actually that colossal compared to many of the others that have been found at the centers of other nearby galaxies [13].

So even though this isn't the bulkiest supermassive black hole, it's still a very useful measurement in the overall discussion of supermassive black holes in the universe. As I have explained in a couple of episodes, including most recently in episode 30, the mass of the supermassive black hole at the center of a galaxy is directly proportional to either, for spiral galaxies, the size of the bulge of stars at the center of the galaxy or, for elliptical galaxies, the size of the galaxy overall (because elliptical galaxies are effectively all bulge and no disk) [13]. Every mass measurement of a supermassive black hole like the one in NGC 6251 is another data point that illustrates the existence of this relation.


[1] Blandford, Roger et al., Relativistic Jets from Active Galactic Nuclei, 2019, Annual Reviews of Astronomy and Astrophysics, 57, 467

[2] Waggett, P. C. et al., NGC 6251, a very large radio galaxy with an exceptional jet., 1977, Monthly Notices of the Royal Astronomical Society, 181, 465

[3] Saunders, R. et al., The radio jet in NGC 6251, 1981, Monthly Notices of the Royal Astronomical Society, 197, 287

[4] Willis, A. G. et al., Recent WSRT and VLA observations of the jet radio galaxy NGC 6251., 1982, in Extragalactic Radio Sources, 97, 141

[5] Perley, R. A. et al., High-resolution VLA observations of the radio jet in NGC 6251., 1984, Astrophysical Journal Supplement Series, 54, 291

[6] Jones, Dayton L. et al., High Dynamic Range VLBI Observations of NGC 6251, 1986, Astrophysical Journal, 305, 684

[7] Theureau, G. et al., Kinematics of the Local Universe. XIII. 21-cm line measurements of 452 galaxies with the Nançay radiotelescope, JHK Tully-Fisher relation, and preliminary maps of the peculiar velocity field, 2007, Astronomy & Astrophysics, 465, 71

[8] de Grijs, Richard and Bono, Giuseppe, Clustering of Local Group Distances: Publication Bias or Correlated Measurements? II. M31 and Beyond, 2014, Astronomical Journal, 148, 17

[9] Bhardwaj, Anupam et al., Large Magellanic Cloud Near-infrared Synoptic Survey. II. The Wesenheit Relations and Their Application to the Distance Scale, 2016, Astronomical Journal, 151, 88

[10] de Vaucouleurs, Gerard et al., Third Reference Catalogue of Bright Galaxies, 1991

[11] Cantwell, T. M. et al., Low-frequency observations of the giant radio galaxy NGC 6251, 2020, Monthly Notices of the Royal Astronomical Society, 495, 143

[12] Ferrarese, Laura et al., Nuclear Disks of Gas and Dust in Early-Type Galaxies and the Hunt for Massive Black Holes: Hubble Space Telescope Observations of NGC 6251, 1999, Astrophysical Journal, 515, 583

[13] Kormendy, John and Ho, Luis C., Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies, 2013, Annual Reviews of Astronomy and Astrophysics, 51, 511


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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