### ASTRONOMY 3303     HW#3     due Wed Sep 20, 2017

Print a copy of this page and add it to your portfolio. Answer the questions (preferably type them or write legibly) separately. Include the graded answers in your portfolio after they are handed back. In all cases, be sure that you give complete derivations, showing the equations you use, including the values of any constants you use and always giving the units involved.

For this homework, be sure to have read Chapters 2.1-2.4 and 3.1-3.7 of the textbook.

Part I: Quick calculation: Evolution of an HII Region.
Assume a massive star situated in a cloud of neutral hydrogen, which has just started to ionize its surroundings. The cloud has a density of nH = 100 cm-3 and a temperature of TH = 25 K.
a. What is the particle density inside the newly ionized HII region?
b. The temperature inside the HII region is THII = 104 K. What is the pressure ratio between inside the HII region and the surrounding neutral gas?
c. What kind of star appears to be in the center?
d. The ionized region will now expand until it reaches pressure equilibrium with the neutral medium. What is the size of the Strömgren sphere, relative to the initial size?
e. The described situation is idealized. What aspects of the problem would you expect to be missing from the situation as described here?

Part II: Interesting galaxies.

For class on Sep 20th, you are assigned to deliver a brief report on an interesting nearby galaxy as given below. We will provide an image of each galaxy (as linked below). By noon on the 20th, you should send to Martha (mph6_AT-cornell) 1 or 2 slides which illustrate some interesting (to you) aspect/phenomenon about your assigned galaxy. Please send the slides in PDF format (if you don't know how to make a PDF, please ask in advance). You should be prepared to talk for THREE (3) minutes about the fascinating aspect/phenomenon of your galaxy.

• Also print your slide(s) and include it/them with the rest of this assignment.

• Provide a list of all sources which you used to research your galaxy. You must include at least one scholarly reference (publication in a refereed journal or conference proceedings) that you actually used. You may use ADS, as in your previous assignment.

To get you started, here are some of the topics you might investigate for each object:
• What is its morphological classification? Is it "active"?
• How far away is it? How accurate is the distance measurement? How has the distance been measured? How does the "best" distance compare to the "redshift distance"?
• Has a supermassive black hole been observed in it? If so, what is the mass of the supermassive black hole?
• What is its mass? How is the mass determined? Does it appear to contain dark matter?
• Does it contain gas? What kind of gas? What telescope(s) are used to measure the gas?
• What is its environment like? Is it in a group or cluster? What other galaxies are nearby?
• What other characteristics/phenomena associated with it are interesting? Give details where possible.
Not all of these suggested topics will be equally applicable to all the galaxies, and there may be other interesting aspects that you uncover. Feel free to be creative! And, ask if you need help deciding on a focus.

Object IDPresenter
M82 Calvin
Messier 81 Christopher
NGC 3628 Clare
Arp 220 David
I Zwicky 18 Hortense
Messier 106 Isabel
NGC 5128 Julia
Messier 90 Peter
LMC Ruixin
NGC 6946 Sam
NGC 1365 Timothy

NOTE: In contrast to the item below, parts I and II are not team exercises. As such, you may discuss them with other students, but you do need to provide your own solutions and develop your own discussion!

Part III: Group discussion: star-forming gas outside the Milky Way.

In class, we had a detailed discussion about the interstellar medium properties of the Milky Way. It is difficult to carry out studies to similar levels of detail for other galaxies, but the ALMA telescope in particular is helping us to make significant progress in this field.

As an example, here is a recent press release on new results from ALMA.

Based on what you have learned in class, discuss with your group the relevance of these observations, and what you find most interesting about them. What is the ALMA telescope? At what wavelengths does it observe, and why? What made these observations challenging for previous observatories? How is this galaxy different from the Milky Way? What can we learn from studies like this one? Provide a brief (1-2 page) write-up of your discussion. Also, feel free to reference any part of the group discussion in your presentation for Part II if you find any comparisons to this study useful.
NOTE: Self-organize in groups of up to 3-4 students for this discussion. Clearly indicate and give credit to all members of the discussion group, and clearly reference all source materials you consider in your discussion (e.g., specific parts of the press release, newspaper articles based on this release, the textbook, class notes, etc.). You may hand in only one copy of the report for your group if preferred, but make sure that all names are provided!