Re: Dark galaxies, how big and where they could be?

Hi, Yevgeniy,
You can read at space.com about how the "dark galaxy" VIRGOHI21 was discovered, even though it is "dark": it is emitting radio waves with a wavelength of 21 cm. This particular radio wavelength comes from free, neutral hydrogen atoms (not bound in H₂ molecules, like hydrogen you'd make in a chemistry lab on earth, nor ionized like the hydrogen in the sun.) The strange thing about this galaxy is that it doesn't emit visible light; this means it has very few stars. We have long expected that small "dark galaxies" should exist, especially in little clumps orbiting ordinary galaxies. (These are sometimes called "subhalos.") This one is bigger than the subhalos I usually think about, so it seems to be an entirely new phenomenon.

Since the galaxy is visible to radio telescopes, you might think that we would have seen one if it were nearby; after all, radio telescopes have surveyed the whole sky many times over. Sometimes, though, a nearby object is harder to see than a faraway object---the Gum Nebula is a good example in visible light. The strength of the signal in a radio telescope usually depends on the "surface brightness," the amount of light per square arcsecond in your field of view. Interestingly, the surface brightness of an object stays the same no matter how far away you are! If VIRGOHI21 were only 1/3rd as far away as it is, it would be nine times brighter, but its angular extent would be nine times greater, so the surface brightness would be the same! So, we could imagine that if a very low surface brightness dark galaxy is present nearby, and takes up a large area of the sky (like the Andromeda galaxy, which is more than a degree across, or the even closer Magellanic Clouds), it might be quite difficult to observe! You would need to use a wide-field telescope to look at a large patch of sky, and it must be sensitive enough to see very dim objects. When you look at an image from this telescope, you'll see thousands or millions of other sources: stars and nebulae in the foreground, galaxies in the background, dwarf galaxies and whatnot in between. You might not see the very dim, foggy object spanning half of your field of view. So, it is perhaps possible that we have "missed" something very dim and very close by.

Could such an object disrupt the Milky Way? Well, that will happen anyway! The Milky Way has already absorbed several small galaxies; the Sagittarius dwarf galaxy is colliding with us right now, and we have stripped a "tidal tail" from it. The Milky Way will eventually collide with Andromeda. Both of us are falling into the Virgo Cluster, and will probably merge with some of the many galaxies there---maybe even VIRGOHI21 itself. And all of these are slowly falling in to a mysterious region called the "Great Attractor".

There is one very interesting thing about galaxy collisions: they very often initiate star formation. A featureless, smooth cloud of gas might persist for billions of years if nothing disturbs it; however, tidal forces and shock waves from a galactic collision (or even a near miss) often seem to "seed" new stars. This may just be a matter of making the gas density higher and more irregular, so that the smooth gas suddenly has somewhere to fall together. We don't exactly understand the mechanism, but we often see bursts of UV light, Type-II supernovae, and T-tauri stars in merging galaxies; these are all indications that new stars are being born. This new dark galaxy is in the dense Virgo cluster, and presumably has survived some sort of galaxy-galaxy interactions without performing a lot of star formation. However, I suspect that a "dark galaxy" crashing into the Milky Way (where it would be close enough for individual stars to be visible) would stick out like a sore thumb, because young stars would form within it. (On the other hand, this could all happen on the "far side" of the galaxy, which we cannot see because of all the dust in the way ....)

If you want to learn more, you may enjoy the actual research article by Robert Minchin et. al. If you're already reading space.com, I suspect you also know about the Astronomy Picture of the Day.

Cheers,

-Ben Monreal