Object 22: NGC 55

Podcast release date: 01 June 2020

Right ascension: 00:14:53.6


Epoch: ICRS

Constellation: Sculptor

Corresponding Earth location: About halfway between Cape Town, South Africa, and Gough Island in the Atlantic Ocean

NGC 55 is called a Magellanic-type irregular galaxy [1]. This class of galaxies is named after the Magellanic clouds, a pair of irregular galaxies that are orbiting the Milky Way so close that they are visible without a telescope, and if they weren't visible only from the Southern Hemisphere, everybody would already know exactly what I am talking about. NGC 55 is like the Magellanic Clouds located further away at a distance of 6.5 million light years (2.0 Mpc) [2,3]. The galaxy lies within a group of galaxies called the Sculptor Group, a nearly gravitationally bound system that also contains the galaxies NGC 247, NGC 253, NGC 300, and NGC 7793 as well as lots of other dwarf galaxies [4]. The Scuptor Group is a close neighbor of the Local Group, which is the gravitationally-bound system that contains our galaxy, the Andromeda Galaxy, M33, and lots of extra dwarf galaxies.

NGC 55 has the interesting distinction of being the closest galaxy that we see edge-on from Earth. Even though the galaxy is irregularly-shaped and looks asymmetric, with a bright blob of stars on one side on the galaxy, it still appears flat as seen from the side. As a consequence of this, people like to go observe NGC 55 to understand how various things are distributed vertically in galaxies. Galaxies seen edge-on like this are very important for this type of study because we can't do this very easily with other galaxies. When looking at our own galaxy, we have difficulty figuring out what's inside the disk and whats above or below the disk because we are sitting somewhere in the middle of the disk looking through it. We also can't do this with many nearby galaxies where we are looking down onto the disk, like the Magellanic Clouds or M33, or galaxies that are just slightly inclined as seen from Earth, like the Andromeda Galaxy, because when we look at these galaxies, it's tough to figure out whether we are looking at something inside the disk, in front of the disk, or behind the disk. Since we see NGC 55 edge-on, it's easy to tell what stuff is inside the disk or above the disk or below the disk. (The difference between what counts as "above" and what counts as "below" is kind of arbitrary here, but it sounds better than just saying "outside" the disk. I could say "outside the plane of the galaxy", but that somehow that doesn't convey the same mental image as "above and below the plane of the galaxy".) Anyhow, since NGC 55 is seen edge-on, we can see things such as how gas and stars are distributed vertically or how stars slowly end up moving out of the plane of the galaxy over time [2,5,6].

NGC 55 also attracts a lot of attention because it is a nearby dwarf galaxy and because, as compared to larger spiral galaxies like the Milky Way, dwarf galaxies generally contain few heavy elements relative to hydrogen and helium [7,8,9]. These heavier elements are created by a combination of fusion processes in the centers of stars and the outer atmospheres of dying stars, so we know that the reason why dwarf galaxies contain fewer heavy elements is somehow linked to either what the timelines are for the creation of these heavy elements and the stars that produce them or what happens to the gas from the stars when they die. NGC 55 is like other dwarf galaxies in that it contains relatively few heavy elements, but what's a little intriguing is that people are debating whether more heavy elements are found in the center of NGC 55 as compared to the outside or if the heavy elements are evenly distributed throughout the galaxy. If they are evenly distributed, it suggests that the heavy elements that are blown out of stars when they die are distributed over a broader area than would be the case for a spiral galaxy [8]. This could happen because NGC 55 is less massive, so the galaxy exerts less gravitation force on the gas expelled by dying stars, and the gas ends up all over the place like a bag of exploding microwave popcorn.

Also, very interestingly, a couple of regions located above or below the disk (I'm not sure which to call it) have recently formed stars, and these regions have very very few heavy elements compared to the disk, which was already low in heavy elements to begin with [7]. This implies that the stars formed out of the gas located above or below the plane of the galaxy rather than forming inside the disk of the galaxy and then getting ejected from the disk. I study star formation in galaxies, and I think it's just weird to find star formation taking place in these locations.

NGC 55 also contains a notable ultraluminous X-ray source (ULX). This object is unimaginatively named NGC 55 ULX. If you think that the part of the sky that is above the North Pole as seen from Earth is the direction up, then NGC 55 ULX is located slightly above the plane of NGC 55 and slightly off to the left from the center of the galaxy.

Ultraluminous X-ray sources are defined as objects that produce more X-rays than 1039 erg/s, and they also need to be located outside the center of the galaxy [10]. An erg per second is a unit of power equivalent to 1 ten millionth of a Watt, which makes it a stupidly small unit of measurement to use in astronomy or even to use when discussing light bulbs. A clearer definition is that ultraluminous X-ray sources produce an amount of X-ray emission that is about 324000 times the total amount of energy emitted by the Sun.

While this definition makes it easy to label these types of objects, astronomers are still trying to figure out what these objects actually are. If you go back to episode 17, I described an X-ray binary star system in our own galaxy that consists of a rather ordinary star and a relatively small black hole about 3 times the mass of the Sun. The black hole is stripping gas from the other star. The gas forms a disk as it slowly falls into the black hole, and it gets extremely hot near the edge of the black hole, which is how it produces X-ray emission. ULXs are thought to be similar except that they are too bright for gas to be falling normally onto a black hole (as normal as that may seem). Instead, ULXs could be binary star systems where gas is being stripped from ordinary stars into neutron stars with extremely strong magnetric fields, or they could be just like normal X-ray binary star systems with small black holes except that huge amounts of gas are falling all at once into the black holes, or they could be much larger black holes (that no longer need to be in binary star systems) that are somewhere between 1000 and 100000 times the mass of the Sun [10].

NGC 55 ULX happens to be a relatively nearby ULX, so people who want to learn more about these types of objects are going to spend a lot of time looking at it. You would think that astronomers would use X-ray observatories to probe whether NGC 55 ULX contains a neutron star or a black hole, but instead, they used X-ray observatories to answer a completely different science question.

Astronomers noticed that the X-ray emission from NGC 55 ULX varies on timescales of minutes and that the higher-frequency X-ray emission seems to be more strongly affected [11]. The gas is still expected to be falling into either a black hole or neutron star in a disk in this ULX. What astronomers suspect is that we are actually viewing the edge of the disk of the infalling gas just like we are viewing the edge of the disk for NGC 55 itself and that clouds in the outer disk in NGC 55 ULX occasionally block the X-ray emission from the center just like clouds of interstellar dust in the host galaxy block starlight from its center [11].

This would link NGC 55 ULX to another class of X-ray sources called ultraluminous supersoft sources (or ULSs) [11]. These sound like they should be the cuddly, plush version of ULXs that you can buy in toy stores, but ULSs are actually defined as objects that also produce more than 1039 erg/s of X-ray radiation, but the radiation consists mainly of very low-frequency X-rays [10]. Astronomers have proposed that ULSs and normal ULXs are the same types of objects but that normal ULXs are objects where we are above the disks and are able to see the X-rays coming from where gas is falling into the central neutron star or black hole while ULSs are objects where we are looking at the edges of the disks and where clouds in the disks block the strongest X-ray emission from the central regions. NGC 55 ULX just happens to be oriented in such a way that we sometimes see the central region, in which case it looks like a normal ULX, and sometimes a random cloud passed in front and we don't see the center, in which case it looks more like a ULS [11].

As a final note, NGC 55 is sufficiently close to Earth that it can be seen rather easily with amateur telescopes, although you have to go to the Southern Hemisphere or at least the tropics to be able to see it. It isn't really located near any distinct stars. If you can find the constellation Phoenix, imagine a line drawn running southeast to northwest through the two brightest stars in that constellation. NGC 55 is located close to the same line at a distance from the northwest star equivalent to about half the distance between the two stars themselves. With a telescope with a diameter of about 15 cm (6 inches), the center of the galaxy itself will be visible, and it will even be possible to see the dust lanes within the plane of the galaxy [12]. With a larger telescope, it's possible to see the full extent of the galaxy, which is about the same width as the Moon [12].


[1] de Vaucouleurs, Gerard, Southern Galaxies. I. Luminosity, Rotation, and Mass of the Magellanic System NGC 55., 1961, Astrophysical Journal, 133, 405

[2] Tanaka, Mikito et al., Structure and Population of the NGC 55 Stellar Halo from A Subaru/Suprime-Cam Survey, 2011, Astrophysical Journal, 738, 150

[3] 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

[4] Karachentsev, I. D. et al., Distances to nearby galaxies in Sculptor, 2003, Astronomy & Astrophysics, 404, 93

[5] Davidge, T. J., Seeing Red in NGC 1978, NGC 55, and NGC 3109, 2018, Astrophysical Journal, 856, 129

[6] Politakis, Babis et al., Vertical distribution of HMXBs in NGC 55: constraining their centre-of-mass velocity, 2020, Monthly Notices of the Royal Astronomical Society, 493, 5369

[7] Kudritzki, R. P. et al., A Spectroscopic Study of Blue Supergiant Stars in the Sculptor Galaxy NGC 55: Chemical Evolution and Distance, 2016, Astrophysical Journal, 829, 70

[8] Magrini, Laura et al., NGC 55: a disc galaxy with flat abundance gradients, 2017, Monthly Notices of the Royal Astronomical Society, 464, 739

[9] Patrick, L. R. et al., Physical properties of the first spectroscopically confirmed red supergiant stars in the Sculptor Group galaxy NGC 55, 2017, Monthly Notices of the Royal Astronomical Society, 468, 492

[10] Kaaret, Philip et al., Ultraluminous X-Ray Sources, 2017, Annual Reviews of Astronomy and Astrophysics, 55, 303

[11] Pinto, C. et al., From ultraluminous X-ray sources to ultraluminous supersoft sources: NGC 55 ULX, the missing link, 2017, Monthly Notices of the Royal Astronomical Society, 468, 2865

[12] Eicher, David J., The Universe from Your Backyard, 1988


Podcast and Website: George J. Bendo

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