Dim and isolated, the dwarf irregular galaxy Wolf–Lundmark–Melotte (WLM) could be described as an astronomical “living fossil” as it seems to have developed without ever interacting with any other galaxy since it formed billions of years ago.
First observed in 1909, the WLM galaxy is part of the Local Group, about 10 million light years (3.1 Mpc) across, this is the set of more than 35 galaxies which includes our own Milky Way. About 3 million light years (920 kpc) from the Milky Way, WLM is on the fringe of the Local Group. Its nearest neighbour is the dwarf galaxy IC 1613, more than a million light years away. Compared to our galaxy, WLM is tiny, a mere 8000 light years across and vaguely disc-shaped but otherwise unstructured; there are no majestic spiral arms. I compared this little galaxy to a living fossil because looking at it gives us an insight into a past era in the Universe’s history. By the way, in biology the term living fossil is discredited, but it is still a useful figure of speech so I apologise to all my friends in the life sciences for my loose use of language.
Billions of years ago, all galaxies were small and irregular like WLM. As time passed mutual gravitation brought most of these early galaxies together, merging them into the much larger elliptical and spiral galaxies of today. Essentially galaxies have grown over time by “eating” other galaxies. WLM seems to have escaped this cannibalistic process of amalgamation, being neither absorbed by or consuming other galaxies, instead evolving independently. Today near the centre of WLM we can see the bright blue stars and clouds of pink ionised hydrogen which are the tell-tale signs of active star-forming regions. However research suggests that half of all stars in WLM formed during a starburst phase that began about 13 billion years ago. Less obvious than the bright central stars is WLM’s extended halo of old and dim red stars which is so inconspicuous that it was only discovered in 1996. This halo is probably ancient, unchanged since the little galaxy formed. Most of the stars in this halo, and indeed in this galaxy as a whole, are old Population II stars scarce in heavier elements.
Although it may not be a terribly spectacular sight, WLM’s pristine condition is useful for astronomers investigating galactic evolution to compare to the more typical large galaxies of today.
McConnachie, A. W.; Irwin, M. J.; Ferguson, A. M. N.; Ibata, R. A.; et al. (2005). “Distances and metallicities for 17 Local Group galaxies”
van den Bergh, Sidney (April 2000). “Updated Information on the Local Group”
(Article by Colin Johnston, Science Education Director)