# Astronomy Physics - Apparent Magnitudes

1. Nov 10, 2008

### Rahzizzle

1. The problem statement, all variables and given/known data
The WIYN 3.5m telescope has a multi-object spectrograph called “Hydra.” The instrument has 100 independently deployable fibers that can be placed anywhere over a 1 degree field of view (FoV). Assume the FoV is a circle one degree in diameter. You can get a spectrum through each fiber, but the spectrograph will only allow you to look at 100nm at a time (e.g. 400nm-500nm). Assume you have detected a cluster of galaxies at a redshift of z~0.01. Your initial estimate is that the cluster spans a bit less than one degree on the sky.

Question : In order to carry out some observations you’ll first want to predict what the apparent magnitudes of the three brightest galaxies to be. You also need to predict how many galaxies you expect to see that are 3 magnitudes fainter than the brightest galaxy in the cluster.

I have absolutely no idea where to start, this is a weird question.

2. Nov 10, 2008

### AstroRoyale

Its a bit ambiguously worded, you really don't know what the 3 brightest members of cluster are until you look at it, but. . .

Part A: Well, what is the relationship between the apparent and absolute magnitudes of objects. What types of galaxies are usually the brightest members of a cluster? Aside from the brightest members of a cluster, what are the next brightest types of galaxies in general? I

Part B: What is the distribution of galaxy luminosities? I.E. google "galaxy luminosity function" or look it up in your book. Its not a gaussian at all, but for example, if it were, then how could you calculate the number of galaxies between say m=18 and m=20 if it was a gaussian distribution centered at m=17 and $$\sigma$$=1 mag?