George's Random Astronomical Object

Object 43: NGC 1052-DF2

Podcast release date: 22 March 2021

Right ascension: 02:41:46.5

Declination: -08:24:08

Epoch: J2000

Constellation: Cetus

Corresponding Earth location: Slightly less than 90 km southeast of Mafia Island off the coast of Tanzania

NGC 1052-DF2 is located at a distance of about 61 million light years (18.65 Mpc) in the constellation Cetus [1,2]. The galaxy is located within the NGC 1052 Group, which is where the "NGC 1052" part of the name comes from, while I think the "DF2" part of the name is supposed to indicate that this was the second of a series of faint galaxies located within that group that was imaged in a survey using the Dragonfly Telephoto Array [3]. The NGC 1052 Group itself is named after its largest and brightest galaxy, the elliptical galaxy NGC 1052. Unfortunately, having one galaxy named NGC 1052-DF2, another galaxy named just NGC 1052, and a group called the NGC 1052 Group could be a little confusing, but fortunately, I don't need to mention the other two objects very often. I can just focus on NGC 1052-DF2.

So, back to that galaxy. Although NGC 1052-DF2 is not that far from Earth in relative terms, it was only discovered around the year 2000 [4], and this is because it is one of a relatively newly-defined class of faint galaxies called ultra-diffuse galaxies [1,5]. These are ellipsoidal galaxies that are similar in size to the Milky Way or other spiral galaxies in terms of their diameters but that contain about as many stars as are found in dwarf galaxies. In the case of NGC 1052-DF2 specifically, it contains within a diameter of about 50000 light years (15.2 kpc) about 200 million stars (or, more accurately, a mass of stars equivalent to 200 million times the mass of the Sun) [6]. For comparison, the Milky Way contains about 100 billion stars within a diameter of about 100000 light years, or, in other words, NGC 1052-DF2 contains 500 times fewer stars than the Milky Way even though it is only half as wide. Because it contains so few stars spread over such a broad area, NGC 1052-DF2 looks like a very faint, hazy oval patch in the sky, even when imaged by professional telescopes. What's really freaky is that, if you look at the Hubble Space Telescope image of this galaxy, you can actually see other more distant galaxies on the other side. This just seems really weird to me. Normal elliptical galaxies that have the same diameter as NGC 1052-DF2 have such a large number of stars packed so close together that they outshine the other galaxies behind them, so NGC 1052-DF2 looks strange in comparison.

However, this is not what astronomers find to be the most interesting thing about NGC 1052-DF2. No, what's astronomers find really interesting about this galaxy is that it may contain virtually no dark matter whatsoever. I'm sure a lot of people who follow astronomy or science fiction have probably heard of dark matter at some point. This is basically matter that neither absorbs nor emits any form of electromagnetic radiation. Just to be clear, dark matter is not just material in space that we can't see in the visible part of the electomagnetic spectrum. We also can't see it in radio waves, microwaves, infrared light, ultraviolet light, X-rays, or gamma rays. We only know that galaxies contain dark matter by watching how stars and interstellar gas orbit the galaxies. By looking at the redshifting and blueshifting of light from the stars and gas clouds as they move towards or away from us, we can measure the velocities of those things, and we generally find that everything is moving so fast that the galaxies would basically fly apart if they did not contain extra dark matter that produces extra gravitational forces to keep everything together.

For the past few decades, virtually everyone in professional astronomy has accepted that all galaxies contain this type of dark matter that we otherwise can't see, or I should say almost all galaxies contain dark matter, because NGC 1052-DF2 is an exception. A couple of years ago, a group of astronomers led by Pieter van Dokkum decided to look at how star clusters orbit this galaxy, and they published a paper in 2018 where they stated that the clusters orbit the galaxy sufficiently slowly that NGC 1052-DF2 might contain no dark matter whatsoever [6]. The stars that are visible in the astronomical images produce enough gravitational force to hold the galaxy together.

This claim was very controversial. It basically led to an inane argument about whether something that we cannot see actually is not there in the first place. If you go look at the science publications about this galaxy, you will see a bunch of paper claiming that, if you analyze the data correctly or look at the galaxy in such-and-such a way, dark matter is actually present [7,8,9,10,11,12,13], and then you'll also see a bunch of other papers with rebuttals claiming that those other people's analyses have mistakes in them or papers stating that, if you look at some new data or analyze the data in a new way, the galaxy actually still contains no dark matter [1,2,14,15,16,17]. One person very confusingly first published a paper casting doubt on the claims that NGC 1052-DF2 contained no dark matter and then published another paper explaining how it was possible to actually remove the dark matter from this galaxy [11,18].

Anyhow, if NGC 1052-DF2 actually contains no dark matter, then the interesting question is how did it get this way. We still find that most other galaxies need to contain dark matter or else they would fly apart; that hasn't changed. Something special had to happen to NGC 1052-DF2 to make it different. One proposal is that NGC 1052-DF2 gravitationally interacted with other galaxies within its group, including the elliptical galaxy NGC 1052, and that these gravitational interactions stripped the dark matter away from NGC 1052-DF2, leaving just the stars [18,19]. Another explanation is that NGC 1052-DF2 formed when two dwarf galaxies collided at high speeds, separating the dark matter from the stars and scattering the stars over a really broad spherical area. It's still not entirely clear what happened to NGC 1052-DF2, and astronomers are going to be spending much more time trying to understand exactly what happened to the galaxy. You could say that NGC 1052-DF2 really matters to a lot of astronomers.


[1] van Dokkum, Pieter et al., The Distance of the Dark Matter Deficient Galaxy NGC 1052-DF2, 2018, Astrophysical Journal Letters, 864, L18

[2] Blakeslee, John P. and Cantiello, Michele, Independent Analysis of the Distance to NGC\,1052-DF2, 2018, Research Notes of the American Astronomical Society, 2, 146

[3] Cohen, Yotam et al., The Dragonfly Nearby Galaxies Survey. V. HST/ACS Observations of 23 Low Surface Brightness Objects in the Fields of NGC 1052, NGC 1084, M96, and NGC 4258, 2018, Astrophysical Journal, 868, 96

[4] Karachentsev, I. D. et al., Dwarf galaxy candidates found on the SERC EJ sky survey, 2000, Astronomy & Astrophysics Supplements, 145, 415

[5] van Dokkum, Pieter G. et al., Forty-seven Milky Way-sized, Extremely Diffuse Galaxies in the Coma Cluster, 2015, Astrophysical Journal Letters, 798, L45

[6] van Dokkum, Pieter et al., A galaxy lacking dark matter, 2018, Nature, 555, 629

[7] Hayashi, Kohei and Inoue, Shigeki, Effects of mass models on dynamical mass estimate: the case of ultradiffuse galaxy NGC 1052-DF2, 2018, Monthly Notices of the Royal Astronomical Society, 481, L59

[8] Martin, Nicolas F. et al., Current Velocity Data on Dwarf Galaxy NGC 1052-DF2 do not Constrain it to Lack Dark Matter, 2018, Astrophysical Journal Letters, 859, L5

[9] Wasserman, Asher et al., A Deficit of Dark Matter from Jeans Modeling of the Ultra-diffuse Galaxy NGC 1052-DF2, 2018, Astrophysical Journal Letters, 863, L15

[10] Laporte, Chervin F. P. et al., Reconciling mass estimates of ultradiffuse galaxies, 2019, Monthly Notices of the Royal Astronomical Society, 484, 245

[11] Nusser, Adi, Towards a higher mass for NGC 1052-DF2: an analysis based on full distribution functions, 2019, Monthly Notices of the Royal Astronomical Society, 484, 510

[12] Trujillo, Ignacio et al., A distance of 13 Mpc resolves the claimed anomalies of the galaxy lacking dark matter, 2019, Monthly Notices of the Royal Astronomical Society, 486, 1192

[13] Lewis, Geraint F. et al., The globular cluster population of NGC 1052-DF2: evidence for rotation, 2020, Monthly Notices of the Royal Astronomical Society, 491, L1

[14] Wasserman, Asher et al., A Deficit of Dark Matter from Jeans Modeling of the Ultra-diffuse Galaxy NGC 1052-DF2, 2018, Astrophysical Journal Letters, 863, L15

[15] Danieli, Shany et al., Still Missing Dark Matter: KCWI High-resolution Stellar Kinematics of NGC1052-DF2, 2019, Astrophysical Journal Letters, 874, L12

[16] Emsellem, Eric et al., The ultra-diffuse galaxy NGC 1052-DF2 with MUSE. I. Kinematics of the stellar body, 2019, Astronomy & Astrophysics, 625, A76

[17] Dutta Chowdhury, Dhruba et al., On the Evolution of the Globular Cluster System in NGC 1052-DF2: Dynamical Friction, Globular-Globular Interactions, and Galactic Tides, 2020, Astrophysical Journal, 903, 149

[18] Nusser, Adi, A Scenario for Ultradiffuse Satellite Galaxies with Low Velocity Dispersions: The Case of [KKS 2000]04, 2020, Astrophysical Journal, 893, 66

[19] Ogiya, Go, Tidal stripping as a possible origin of the ultra diffuse galaxy lacking dark matter, 2018, Monthly Notices of the Royal Astronomical Society, 480, L106

[20] Silk, Joseph, Ultra-diffuse galaxies without dark matter, 2019, Monthly Notices of the Royal Astronomical Society, 488, L24

[21] Shin, Eun-jin et al., Dark Matter Deficient Galaxies Produced via High-velocity Galaxy Collisions in High-resolution Numerical Simulations, 2020, Astrophysical Journal, 899, 25

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Last update: 21 March 2021