Astr 421 Fall 2004

Galaxies Sylvain Veilleux

September 24, 2004 DUE: October 6, 2004

PROBLEM SET # 2

Problems 1 - 4.

Sparke & Gallagher, Problems 2.4, 2.5, 2.8, and 2.15

Problem 5.

The following question is about interstellar reddening in external galaxies. Let's consider two different situations.

Case 1: Stars are distributed uniformly throughout a thin layer of thickness D with a layer of absorbing material of optical depth between the stars and the observer ( is the optical depth in the visual band).

Case 2: Same as Case 1, but now the absorbing medium is distributed uniformly with the stars.

Assume D is negligible compared to the distance to the observer, and the absorbing material obeys the so-called Whitford law with R = 3 where is called the color excess''.

(a)
The observer has measured integrated colors in U, B, and V for the slab of stars (seen face on), but these colors are naturally affected by reddening. Calculate the apparent value of R and S for Cases 1 and 2 in the limits 0 and .

(b)
Mention a couple of realistic situations in which this variation in R and S might actually occur.

Hint: Take a look at Section 3-11 of Mihalas & Binney (1981), attached with this problem sheet.

Note: The calculation is only academic since real dust grains have an albedo of about 50%, so they scatter much of the light rather than absorb it.