The Ghost of Mirach

Contributed by
Dec 30, 2008

Most objects in the sky are fairly well scattered, separated from each other. Through a telescope, which magnifies images, this becomes even more apparent. Bright, big, nearby galaxies usually float alone in the image, unless accompanied by satellite galaxies (or they live in a cluster).

But there are exceptions. Here's a good one:

On the left is a visible light image of Mirach, a medium-bright naked-eye star 200 light years away in the constellation of Andromeda. It's a red giant, a star more massive than the Sun that is nearing the end of its life. It's hugely over-exposed in the image. Look just above it and to the right: see that smudge? It's The Ghost of Mirach (technically NGC 404*), a galaxy 11 million light years away overwhelmed by the light of the much closer star. If that galaxy were by itself someplace, it would be a real showpiece, easily seen in small telescopes. As it is, it's like a firefly sitting on the edge of a spotlight.

But wait! That's visible light, the kind our eye is sensitive to. What if we look at other wavelengths?

In infrared things would be worse; Mirach pours out long wavelength light, being a red giant at all. But what about bluer light? Like ultraviolet, say? Well, that's the image on the right. Taken by the Galaxy Evolution Explorer, or Galex, it's a UV image of the same region of sky. Mirach is gone! For all its sound and fury in the red, a star like Mirach doesn't put out much UV, so it essentially vanishes, and we can see past it to the Ghost beyond.

What is seen is a bit surprising. The Ghost is a lenticular, a lens-shaped disk galaxy. These are weird galaxies; they are flattened disks like our Milky Way, but generally are devoid of gas, so they don't have any ongoing star formation in them (like elliptical galaxies). Usually, UV light from a galaxy is a product of gas actively forming stars; massive stars get formed which blast out UV radiation that in turn makes the gas clouds glow. Lenticulars, being gas-poor, shouldn't be bright in the UV.

But look at the Ghost of Mirach! It's clearly seen putting out UV light, and in fact you can trace a ring pattern too. Rings usually indicate a galaxy got disturbed somehow, usually by the gravity of a nearby galaxy passing it or colliding with it. And, as it happens, earlier observations of The Ghost using radio telescopes indicated a ring of hydrogen just where the UV ring is, and the authors of that study think that this galaxy did suffer a collision, nearly a billion years ago. Perhaps back then there was more gas in the galaxy, and the collision triggered a massive wave of star formation that used up almost -- but not quite all -- of the gas. All that remains now is this semi-stable ring, still making stars, but at much lower rates.

However it happened, NGC 404 is an object worthy of study, even if sitting in a natural source of light pollution. Astronomers prefer the dark, but sometimes, like anyone else searching for something, we're forced to look where the light is bright.

* You gotta love the punning and appropriate catalog number for this galaxy. NGC 404, for a galaxy that is hard to see and therefore usually not found?

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