Object 55: WLM Galaxy

Podcast release date: 06 September 2021

Right ascension: 00:01:57.9


Epoch: ICRS

Constellation: Cetus

Corresponding Earth location: About two-thirds of the distance from Angola to St. Helena in the Atlantic Ocean

The WLM (Wolf-Lundmark-Melotte) Galaxy is a dwarf irregular galaxy in the constellation Cetus. If you've listened to some of my previous episodes, you may have heard me discuss how the ancient Greeks thought that the constellation Cetus looked like a whale or sea monster even thought it doesn't look like much of anything. This demonstrates that the ancient Greeks apparently used lots of mind-altering substances.

Anyway, back to the WLM Galaxy. This galaxy has a rather weird name that belies the weird history of its discovery. The galaxy was originally identified in 1909 by Max Wolf [1], who was a German pioneer in astrophotography, but somehow his discovery was either forgotten or overlooked, maybe because Wolf was busy publishing papers on just about anything that he photographed in the sky and no one noticed that he had published the discovery of an entire new galaxy. In 1926, Philibert Melotte and Knut Lundmark rediscovered the galaxy [2]. Some time after that, people figured out that the thing discovered by Wolf and the thing discovered by Melotte and Lundmark were the same thing, so they started calling it the Wolf-Lundmark-Melotte Galaxy. However, that would get tedious to write on mid-twentieth century typewriters, so they called it the WLM Galaxy for short. Interestingly, this object was discovered before Edwin Hubble and other astronomers had figured out that other galaxies are actually objects located outside our own galaxy, so the WLM Galaxy was originally referred to as a nebula.

Anyway, this is a dwarf irregular galaxy, although it actually does not look that irregular. Most of the stars lie within a slightly distorted oval region that runs north to south as seen on the sky, and a few nebulae are scattered around one end of that oval. This oval region actually appears to be a bar-like structure in three dimensions [3]. Radiowave observations of this galaxy shows that this bar structure sits within a disk of gas that extends about 2.5 times as far from the center of the galaxy as the stars [3].

One of the interesting things about the WLM Galaxy is its location. It lies within the Local Group, which is the gravitationally-bound group of galaxies that includes the Milky Way, the Andromeda Galaxy, the spiral galaxy M33, and many other dwarf galaxies [4]. Most of the Local Group's dwarf galaxies are either in orbit around or gravitationally interacting with either the Milky Way or Andromeda Galaxies, and some of the dwarf galaxies are gravitationally interacting with each other as well. However, at a distance of 3.21 million light years (984 kpc) from Earth [5], the WLM Galaxy is located at the edge of the Local Group where it is relatively undisturbed by everything else that's going on. It's kind of like a quiet kid at the edge of the playground reading a book, while the other dwarf galaxies would be like the kids near the center of the playground shouting about wanting their turn on the swings or pushing other kids off the slide, and I suppose the Milky Way and the Andromeda Galaxy would be the parents trying to manage everything.

Astronomers like to study the WLM Galaxy because it provides insights into what dwarf galaxies are like when they are left undisturbed. For example, gravitational interactions between galaxies can change the orbits of those galaxies' stars and interstellar gas clouds, so the WLM Galaxy shows what the orbits of these stars and gas clouds looks like in an isolated dwarf galaxy [3,6]. In fact, the WLM Galaxy's isolation is probably one of the reasons why it has such a big gas disk. If it was closer to the center of the Local Group, one of the other kids probably would have taken the WLM Galaxy's hat, or actually I mean one of the other dwarf galaxies or even the Milky Way or the Andromeda Galaxy might have gravitationally stripped away the WLM Galaxy's gas disk.

Gravitational interactions between galaxies also tend to cause the interstellar gas clouds within the galaxies to collapse and form new stars, including very bright blue stars that have very short lifespans before they explode as supernovae. I suppose, using the playground analogy, this would be like kids accidentally colliding with each other on the playground and getting bruised, which would be like stars forming maybe, and their kneecaps exploding as supernovae? Anyway, the WLM Galaxy is a really good place for astronomers to look if they want to understand how stars form, evolve, and die in the environment of an undisturbed dwarf galaxy [7,8,9]. This is actually quite important because the elements heavier than hydrogen and helium that we find within interstellar gas clouds within galaxies were initially created by fusion within the interiors of stars and are ejected into space when the stars die. After this, when new stars form out of the gas in the interstellar medium, they will contain some of the heavier elements made by the older stars. For multiple reasons, the stars and interstellar gas in dwarf irregular galaxies contain fewer of these heavy elements than the stars and interstellar gas in larger spiral galaxies. Astronomers are still trying to understand the details of why this happens, so they end up spending a lot of time studying at the WLM Galaxy to observe how stars in undisturbed dwarf galaxies are forming heavy elements [7,10].

Even though the WLM Galaxy is located at the edge of the Local Group, it is still close enough that astronomers using modern telescopes can identify and study individual stars within the galaxy. This allows for all sorts of interesting scientific analyses.

To begin with, astronomers can often measure very precise distances to nearby galaxies if they can identify specific types of stars within those galaxies. The idea is that, if astronomers know exactly what types of stars they are looking at, they know exactly how much total energy those stars radiate, so they can measure how much of that energy reaches the Earth to determine the distances to these stars. Because the WLM Galaxy is so close that astronomers can identify many different types of stars within it, the galaxy has actually become very useful for testing which specific stars can be used for measuring the most accurate distances to other galaxies [4,11,12,13,14,15,16].

Also, as I mentioned before, the WLM Galaxy, like a lot of other dwarf galaxies, contains relatively few elements heavier than hydrogen or helium. Consequently, the WLM Galaxy is not only a good place for studying why dwarf galaxies have fewer heavy elements but also for studying individual stars that contain relatively few heavy elements and learning how they differ from the ones that we find in the Milky Way [12,15,17,18,19,20,21,22,23].


[1] Wolf, M., Über einen größeren Nebelfleck in Cetus, 1909, Astronomische Nachrichten, 183, 187

[2] Melotte, P. J., New nebulæ shown on Franklin-Adams chart plates, 1926, Monthly Notices of the Royal Astronomical Society, 86, 636

[3] Ianjamasimanana, Roger et al., MeerKAT-16 H I observation of the dIrr galaxy WLM, 2020, Monthly Notices of the Royal Astronomical Society, 497, 4795

[4] Karachentsev, I. D., The Local Group and Other Neighboring Galaxy Groups, 2005, Astronomical Journal, 129, 178

[5] Lee, Abigail J. et al., The Astrophysical Distance Scale. III. Distance to the Local Group Galaxy WLM Using Multiwavelength Observations of the Tip of the Red Giant Branch, Cepheids, and JAGB Stars, 2021, Astrophysical Journal, 907, 112

[6] Leaman, Ryan et al., The Resolved Structure and Dynamics of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM, 2012, Astrophysical Journal, 750, 33

[7] Hodge, Paul W. et al., Hubble Space TelescopeStudies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster, 1999, Astrophysical Journal, 521, 577

[8] Dolphin, Andrew E., Hubble Space Telescope Studies of the WLM Galaxy. II. The Star Formation History from Field Stars, 2000, Astrophysical Journal, 531, 804

[9] Albers, Saundra M. et al., Star formation at the edge of the Local Group: a rising star formation history in the isolated galaxy WLM, 2019, Monthly Notices of the Royal Astronomical Society, 490, 5538

[10] Leaman, Ryan et al., The Comparative Chemical Evolution of an Isolated Dwarf Galaxy: A VLT and Keck Spectroscopic Survey of WLM, 2013, Astrophysical Journal, 767, 131

[11] Rejkuba, Marina et al., Deep Hubble Space Telescope STIS Color-Magnitude Diagrams of the Dwarf Irregular Galaxy WLM: Detection of the Horizontal Branch, 2000, Astronomical Journal, 120, 801

[12] Valcheva, A. T. et al., Carbon stars and C/M ratio in the WLM dwarf irregular galaxy, 2007, Astronomy & Astrophysics, 466, 501

[13] Pietrzyński, Grzegorz et al., The Araucaria Project: The Distance to the Local Group Galaxy WLM from Cepheid Variables Discovered in a Wide-Field Imaging Survey, 2007, Astronomical Journal, 134, 594

[14] Gieren, Wolfgang et al., The Araucaria Project: The Distance to the Local Group Galaxy WLM from Near-Infrared Photometry of Cepheid Variables, 2008, Astrophysical Journal, 683, 611

[15] Urbaneja, Miguel A. et al., The Araucaria Project: The Local Group Galaxy WLM-Distance and Metallicity from Quantitative Spectroscopy of Blue Supergiants, 2008, Astrophysical Journal, 684, 118

[16] Tammann, G. A. et al., New period-luminosity and period-color relations of classical Cepheids. IV. The low-metallicity galaxies IC 1613, WLM, Pegasus, Sextans A and B, and Leo A in comparison to SMC, 2011, Astronomy & Astrophysics, 531, A134

[17] Venn, Kim A. et al., The Chemical Composition of Two Supergiants in the Dwarf Irregular Galaxy WLM, 2003, Astronomical Journal, 126, 1326

[18] Lee, Henry et al., Investigating the Possible Anomaly between Nebular and Stellar Oxygen Abundances in the Dwarf Irregular Galaxy WLM, 2005, Astrophysical Journal, 620, 223

[19] Bresolin, Fabio et al., The Araucaria Project: VLT Spectra of Blue Supergiants in WLM- Classification and First Abundances, 2006, Astrophysical Journal, 648, 1007

[20] Battinelli, P. et al., The assessment of the near infrared identification of carbon stars. I. The Local Group galaxies WLM, IC 10 and NGC 6822, 2007, Astronomy & Astrophysics, 474, 35

[21] Jackson, Dale C. et al., A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM, 2007, Astrophysical Journal, 656, 818

[22] Tramper, F. et al., On the Mass-loss Rate of Massive Stars in the Low-metallicity Galaxies IC 1613, WLM, and NGC 3109, 2011, Astrophysical Journal Letters, 741, L8

[23] Tramper, F. et al., The properties of ten O-type stars in the low-metallicity galaxies IC 1613, WLM, and NGC 3109, 2014, Astronomy & Astrophysics, 572, A36


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