#### ASTR 620 Galaxies Problem Set 3Due in class Wednesday 22 October 2003

1. Binney & Merrifield 9.4

2. Elliptical Galaxies

We have used the exponential disk formula to describe the surface brightness profiles of spiral galaxies. For giant elliptical galaxies (and the bulge components of many spirals), the most commonly employed formula is the de Vaucouleurs R1/4 Law:

I(r) = Iee-7.67[(r/Re)1/4-1]
Give the total luminosity and the central surface brightess in terms of Ie and Re.

3. Proper Motion of Sgr A*

The radio source Sgr A* associated with the black hole at the center of the Galaxy is observed with the VLBI technique to have a proper motion of 6 milliarcseconds per year.

• A. Explain why this observation provides a constraint on the Galactic constants.
• B. Identify the combination of these constants which is constrained.
• C. Assuming we are 8 kpc from the galactic center, what numerical value for one of the Galactic constants can you extract?

4. The Milky Way

Refer to Majewski (1993) ARA&A, 31, 575 to answer the following questions. Restrict yourself to no more than 10 lines per answer. Be sure to include relevant references. (References do not count against the line limit.)

• A. Compare and contrast the ELS and Searle & Zinn galaxy formation scenarios.
• B. Give two distinct arguments against stochastic heating of the thin disk as the sole origin of the thick disk.
• C. What is the main argument which suggests that the formation of the halo and thick disk are disconnected?
• D. Describe two observations which support an accretion origin for at least part of the halo.
• E. What is the single most important results which suggests a dissipative origin for the thick disk?