Object 1: Leo IV Dwarf Galaxy

Podcast release date: 12 August 2019

Right ascension: 11:32:57.0


Epoch: ICRS

Constellation: Leo

Corresponding Earth location: Near Kuria and Aranuka Island, part of the Gilbert Islands in the nation of Kiribati in the Pacific Ocean

Leo IV is one of many very ultra faint dwarf galaxies that orbit the Milky Way Galaxy. They are not like, for example, the Large and Small Magellanic Clouds, which can be seen without a telescope. Instead, they are extremely faint and very difficult to see, even though they are very close.

Beth Willman, who is currently as astronomer at the University of Arizona, wrote a great review of the work that has been done just to identify these types of galaxies [1]. Quite a few galaxies were identified by people literally looking at photographic plates in the twentieth century. This is a very inefficient process. With modern digital astronomical images, it has become much easier to identify ultra faint dwarf galaxies. Leo IV is one of many of these types of galaxies; it was identified in data from the Sloan Digital Sky Survey in 2007 [2].

Since its discovery, Leo IV has attracted a lot of attention. Ultra-faint dwarf galaxies like this are thought to have changed very little since they first formed when the universe was very young. Leo IV does not contain very much interstellar gas for forming stars, so what few stars are present are very old. In fact, most of the stars appear to be as old as the oldest stars found in the Milky Way [3, 4]. Multiple generations of stars are needed to produce substantial elements heavier than hydrogen or helium. Because stars have not been forming in Leo IV, the galaxy contains very few heavy elements [3, 5, 6]. Having said that, some evidence was found that some stars (about 2% of the galaxy's mass) formed about 2 billion years ago [3].

These characteristics of Leo IV as well as other similar ultra faint dwarf galaxies have been used to test hypotheses of the formation of the universe. In current models, the universe at first contained nothing but ionized gas, but as it expanded, the gas turned neutral and then formed stars. However, the first stars and active galactic nuclei that formed produced a lot of ultraviolet light that reionized most of the gas in the universe, and this time period is called the epoch of reionization. Leo IV looks like one of the galaxies that was so small when reionization occurred that all of its gas was effectively boiled away after being ionized [6]. Without any gas, it would no longer be able to form new stars, and so it would become the spheroid of old red stars that we hardly see today.

The final thing to say about Leo IV is that, like many other dwarf galaxies, it appears to contain an abnormally large amount of dark matter [8]. Like many other galaxies, the stars in Leo IV are moving too fast compared to the gravity that would be produced by those stars alone, so additional dark matter must be present to stop the galaxy from flying apart. In typical spiral galaxies, the ratio of dark to light matter is around 3 to 5 [9], but in Leo IV, it is around 1000 [6].


[1] Willman, Beth, In Pursuit of the Least Luminous Galaxies, 2010, Advances in Astronomy, 2010, 285454

[2] Belokurov, V. et al., Cats and Dogs, Hair and a Hero: A Quintet of New Milky Way Companions, 2007, Astrophysical Journal, 654, 897

[3] Sand, David J. et al., A Deeper Look at Leo IV: Star Formation History and Extended Structure, 2010, Astrophysical Journal, 718, 530

[4] Brown, Thomas M. et al., The Primeval Populations of the Ultra-faint Dwarf Galaxies, 2012, Astrophysical Journal Letters, 753, L21

[5] Moretti, Maria Ida et al., The Leo IV Dwarf Spheroidal Galaxy: Color-Magnitude Diagram and Pulsating Stars, 2009, Astrophysical Journal Letters, 699, L125

[6] Geha, Marla et al., The Stellar Initial Mass Function of Ultra-faint Dwarf Galaxies: Evidence for IMF Variations with Galactic Environment, 2013, Astrophysical Journal, 771, 29

[7] Brown, Thomas M. et al., The Quenching of the Ultra-faint Dwarf Galaxies in the Reionization Era, 2014, Astrophysical Journal, 796, 91

[8] Simon, Joshua D. and Geha, Marla, The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem, 2007, Astrophysical Journal, 670, 313

[9] van Albada, T. S. and Sancisi, R., Dark Matter in Spiral Galaxies, 1986, Philosophical Transactions of the Royal Society of London Series A, 320, 447


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: craigsmith, Dalibor, ivolipa, jameswrowles, jourblue, shoba, UncleSigmund, and Xulie at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)