snow5379 wrote:Why does the galaxy density seem to drop off so quickly the farther you go? Is it because far away galaxies are harder to detect? I'm trying to do a statistical analysis of the universe using star and galaxy catalogs and I'm having a lot of trouble.
Well, your first problem is that you're using Sloan, which is a magnitude-limited search (~22 in all bands, for reference). This isn't a knock against Sloan, really, since any sky survey we do is going to be magnitude-limited just due to our instruments, but you need to keep it in mind - as you go further out, you naturally expect to see fewer galaxies, since they're getting (apparently) fainter and smaller. Plus, even at z=1, you're not exactly in the local universe - you're looking at something that is presently some 13 billion light years away.
It's also worth noting that there should be several hundred billion galaxies in the observable universe, based on surveys of the local Gpc or so plus the assumption that the universe is homogenous and isotropic on large scales, but SDSS, by their own admission, catalog only 930,000 of them. We have a very nice representative
sample of galaxies that we have observed and cataloged, but we're nowhere close to bagging them all, even out to z=0.1.
Also I've thought about it more... star brightness would have a bigger impact than star size. A bright star would block much more from view than a large star... so I don't think size estimates are useful here.
Twistar wrote:What about the probability if you travel towards the center of the milky way? I feel like realistically if you travel a random direction in space the properties of the galaxy will dominate the probability of hitting something (and then you could compare this to the probability of hitting something if you end up missing everything in the galaxy)
Still horrifically low; the average density of stars in the Galaxy is something like ~10 pc-3
(it's about 1 near the Sun, and about 100 in the Bulge, so split the difference), and we need to go ~20 kpc to go through the center of the Galaxy and out the other side, so the probability of hitting a star is about 3E-10, or something like a one in a billion chance of actually hitting anything.