For instance, the redshift of the other galaxies is around 4000-4500 km/s, while that of the background spiral is around 20,000 km/s.
Definition of redshift: It's an electromagnetic Doppler effect. If a light-emitting object (star, galaxy, nebula) is moving either toward or away from us, its spectrum (think prism here) is shifted either toward the blue end or red end of the spectrum. The amount of redshift (blueshift) determines how fast the object is receding (approaching). The only cosmic objects with blueshift are galaxies in our Local Group, and some stars within the Milky Way.
The theory that the universe is expanding seems to explain a lot, but there's a problem with it concerning Seyfert's Sextet. If the universe is expanding, that means that the farther an object is from us, the faster it is receding. That doesn't really mean that those galaxies are flying away through space - it's space ITSELF that's expanding. Think of raw bread dough. You want raisin bread, so you throw in a handful and knead them around a bit. Some are stuck in little clumps. Some are solo. Measure the distance between each raisin/raisin group you can see. Now bake the loaf until it's done. Measure the distances between all the raisins again. AHA! The farther apart they WERE, the MORE they moved apart while the loaf expanded! And remember, the universe isn't sitting in a cosmic loaf pan, so it has no boundaries on its expansion (except gravity maybe, but that's a different story).
OKAY, back to the problem with Seyfert's Sextet. Remember the redshift? The other members are, by redshift calculations, around 190 million light-years distant. The redshift calculation on the face-on spiral shows a distance of around 800 million light-years. It doesn't LOOK 4 times the distance of the others. It would have to be extremely huge to look in proportion to the others in the group.
So we have a paradox here. Do we simply choose to believe that the one spiral is really way in the background, or that the redshift numbers are somehow skewed (gravitational lensing, maybe?), or that the redshift really doesn't measure distance accurately, and that it's caused wholly or partially by another (unknown) mechanism.
The redshift calculation on the face-on spiral shows a distance of around 800 million light-years. It doesn't LOOK 4 times the distance of the others. It would have to be extremely huge to look in proportion to the others in the group.
I like to think that the face-on spiral is just extremely huge.
Thanks for the astronomy lesson! I enjoy starting the day with this thread.