Object 82: NGC 3593

Podcast release date: 19 September 2022

Right ascension: 11:14:37.0


Epoch: ICRS

Constellation: Leo

Corresponding Earth location: An area of the Pacific Oean at the north end of the Marshall Islands

NGC 3593 is a galaxy located at a distance of 29.2 million light years (8.95 Mpc) [1] in the constellation Leo that is officially listed as "peculiar", as in it does not fit into standard galaxy classifications [2, 3]. Nominally, it is supposed to be very similar to a lenticular galaxy, which is a type of galaxy halfway between an elliptical galaxy and a spiral galaxy. Like other lenticular galaxies, NGC 3593 has both a really large bulge of stars and a smooth disk of stars that bisects the bulge. However, while lenticular galaxies usually look like they are relatively devoid of interstellar material, the disk of NGC 3593 contains an unusual amount of gas and dust near its center that, because we are looking at the disk nearly edge-on, looks like a dark cloud crossing in front of the stars in the galaxy [2].

However, while this leads to the shape (or, to use the astronomy term, morphology) of NGC 3593 being officially designated as "peculiar", it is only a small part of what makes this galaxy "peculiar" as in the everyday synonym for weird, or strange, or bizarre. The most peculiar or weird or strange or bizarre thing about NGC 3593 is that some of the stars in the disk of the galaxy are travelling in the opposite direction from most of the other stars in the galaxy [4, 5].

In most galaxies with disks, including the Milky Way, virtually all of the stars and interstellar gas clouds in the galaxies' disks orbit the centers of their galaxies in the same direction. This is because when galaxies with disks initially formed out of intergalactic gas, the gas in these disks would have been left with a little bit of residual rotation (or, to use the physics term, angular momentum) left over from the motions of the gas clouds that they formed out of. With nothing to stop the gas and the stars rotating in the same direction, they will all continue to rotate in the same direction forever.

However, in NGC 3593, roughly one-fifth of the stars are travelling in the opposite direction from the other stars [5]. The term used to describe this is counterrotation. The counterrotating stars are found mostly within the inner 1600 light years of the galaxy [5]. Additionally, all of that interstellar gas near the galaxy's center that I mentioned before is also travelling in the same direction as the counterrotating stars [5, 6].

What seems to have happened in NGC 3593 is that something containing a lot of gas, possibly a dwarf galaxy or some sort of primordial intergalactic gas cloud, fell into the center of the galaxy about 2 billion years ago [5]. The object fell into the center of NGC 3593 in such a way that it ended up orbiting in the wrong direction from everything else, and the gas would have later formed stars that would have continued to travel in the wrong direction from the older stars in NGC 3593.

Counterrotating stars are frequently seen in many lenticular and elliptical galaxies, and, in fact, I discussed conterrotation in the lenticular galaxy NGC 5102 in Episode 23. However, NGC 3593 is one of the most prominent examples of this phenomenon, and the detailed scientific studies that have been performed on NGC 3593 have provided detailed information on both the origins of counterrotating stars and gas and how it affects galaxies' appearances.

And now on to the science bonus round. At the moment, astronomers have the expectation that the average large spiral, lenticular, or elliptical galaxy contains a supermassive black hole that has a mass millions or billions of times larger than the mass of the Sun. In fact, astronomers have found a relation where the size of a galaxy's central bulge of stars is directly related to the size of the galaxy's central supermassive black hole, which implies that the processes that make the bulge larger also make the black hole larger [7].

In the past decade, many astronomers have been using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to look at how quickly gas rotates around galaxies' nuclei to try to determine whether the locations contain supermassive black holes and, if they do, to try to measure the masses of the black holes. I mentioned ALMA specifically because I work as a Contact Scientist for ALMA, and while I have not been part of any collaboration using ALMA to measure black hole masses, I have been responsible for providing behind-the-scenes support for multiple UK-based observing projects using ALMA to make these types of measurements, so I am quite familiar with the science. Having said that, I had no involvement in the ALMA observations of NGC 3593 simply because the project was led by Dieu Nguyen, who is from outside the UK.

Dieu Nguyen's group did find a supermassive black hole at the center of NGC 3593, and the mass of the black hole was estimated to be roughly 2.4 million times the mass of the Sun [8]. This might sound big, but I actually know of galaxies with supermassive black holes at least a couple of hundred times larger than this [9, 10]. Still, it's a really impressive measurement, and it's going to be very important for refining the relation between supermassive black hole mass and galaxy bulge size.


[1] Tully, R. Brent et al., Cosmicflows-2: The Data, 2013, Astronomical Journal, 146, 86

[2] Buta, Ronald J. et al., The de Vaucouleurs Atlas of Galaxies, 2007

[3] Buta, Ronald J., The systematics of galaxy morphology in the comprehensive de Vaucouleurs revised Hubble-Sandage classification system: application to the EFIGI sample, 2019, Monthly Notices of the Royal Astronomical Society, 488, 590

[4] Bertola, Francesco et al., Counterrotating Stellar Disks in Early-Type Spirals: NGC 3593, 1996, Astrophysical Journal Letters, 458, L67

[5] Coccato, L. et al., Spectroscopic evidence of distinct stellar populations in the counter-rotating stellar disks of NGC 3593 and NGC 4550, 2013, Astronomy & Astrophysics, 549, A3

[6] García-Burillo, S. et al., Anatomy of the counterrotating molecular disk in the spiral NGC 3593. 12CO(1-0) interferometer observations and numerical simulations, 2000, Astronomy & Astrophysics, 363, 869

[7] 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

[8] Nguyen, Dieu D. et al., The MBHBM* Project - II. Molecular gas kinematics in the lenticular galaxy NGC 3593 reveal a supermassive black hole, 2022, Monthly Notices of the Royal Astronomical Society, 509, 2920

[9] Poci, Adriano et al., Combining stellar populations with orbit-superposition dynamical modelling: the formation history of the lenticular galaxy NGC 3115, 2019, Monthly Notices of the Royal Astronomical Society, 487, 3776

[10] Jardel, John R. et al., Orbit-based Dynamical Models of the Sombrero Galaxy (NGC 4594), 2011, Astrophysical Journal, 739, 21


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