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# Midterm Questions &amp; Answers - NS 209 What is there...

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NS 209 What is there in the universe, beyond the Milky Way Midterm Exam 10 April 2007 --- 14:40-15:30 Instructor: Emrah Kalemci Surname: Name: Student No: Problem (54 pts) Multiple choice (56 pts) Total (110 pts): Instructions: 28 points multiple choice questions (2 pts each), and 1 problem (54 pts), over 110. i)You MUST use the key below for multiple choice questions. ii)Wrong answers do not cancel right answers, feel free to guestimate the answers iii) All double answers erased. iv) No calculators!

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Some useful information: Ho = 70 km/s/Mpc c=3 x 10 5 km/s pc = 3 x 10 13 km 1 AU = 1.5 x 10 8 km ly = 10 13 km radian = 57.3 o . 1. Using the Hβ line in the figure above, a) find the redshift z of the quasar X (ignore all relativistic effects) 5pts λ obs ref = 1+z = 1.2. Then z=0.2 b) find the recession velocity of the quasar 5pts v/c=z, v=60000 km/s (no relativistic effects) c) find the distance to the quasar (for this problem only, use Ho=60 km/s/Mpc) 5pts v=Ho x d, 60000 km/s = 60 km/s/Mpc x d, d = 1000 Mpc. d) Assume the host galaxy is an elliptical with an extent ( radius ) of 100 kpc. Can we resolve the host galaxy with a telescope with a resolution 0.1'' ? (calculate, a yes or no answer is not enough) 15 pts 1 AU is 1'' at 1 pc 10 9 AU is 1'' at 1000 Mpc 5 kpc is 1'' at 1000 Mpc 0.5 kpc is 0.1'' at 1000 Mpc. The telescope can resolve distances of 0ç5 kpc, therefore easily resolves the galaxy of size 100 kpc.
e) The spectral measurements indicate that the radial velocity of Hydrogen gas at the edge of the galaxy is 60 au/yr. Calculate the inner mass of the galaxy. (Take π=3) 15 pts (5 pts bonus) 100 kpc = 2 x 10 10 au T = 2 x 3 x 2 x 10 10 au/60au/yr = 2 x 10 9 yr M in = R 3 /T 2 = 8 x 10 30 / 4 x 10 18 = 2 x 10 12 Msun. f) At the center of this galaxy, an AGN exists. The flux measured from the AGN is 1/π 10 -10 W/m 2 . What is the luminosity of the AGN in Watts? (10 pts) d = 1000 Mpc, L = 4 x π (1000 Mpc)2 x 1/π 10 -10 W/m 2 1000 Mpc = 3 10 25 km, L = 4 x 9 x 10 50 x 10 -10 W = 3.6 10 41 W Multiple choice 1) Which of the following structures is not a part of our own Milky Way Galaxy? A) halo B) nucleus C) disk D) ring E) spiral arms Answer: D 2) Star formation ceased first in the: A) nucleus. B) spiral arms. C) bulge. D) halo. E) galactic disk. Answer: D 3) Of what use are the Doppler shifts of the 21-cm radio signals? A) They let astronomers map the large-scale motion and distribution of gas in the Milky Way's spiral arms.

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