Astr 222 Midterm Exam 2 Study Questions

Short Answer Questions

There will be 7 questions similar in style to the ones below; you pick 5 of them to answer.
Each question is worth 5 points.
Your answer should be a a few sentences / short paragraph long for each.

Describe the general properties of starburst galaxies and explain how these starbursts might be triggered.
Describe what we mean by the gas depletion time for galaxies, and how it differs between spiral galaxies and starburst galaxies.

You are studying a galaxy cluster whose distance you have determined from its redshift, and you use the virial theorem to calculate a total mass of 10¹⁵ Msun for it. How and why does your answer depend on the Hubble constant? If the Hubble constant was actually 20% larger than you thought, would your estimated mass get bigger, get smaller, or stay the same? Explain why.

You are studying three galaxies: a massive elliptical, a massive disk galaxy with no central bulge, and spiral galaxy like the Milky Way with a bulge-to-disk ratio of about 1:4. Which one of these galaxies would probably have the most massive central black hole? Which one would have the least massive central black hole (or maybe no black hole at all)? Explain your reasoning.

Describe the mass-metallicity relationship for galaxies, and how it explains the color trends of galaxies on the red sequence. Give a physical explanation for why the mass-metallicity relationship might exist.
Why were the spectra of quasars so confusing to astronomers in the 1950s? How did Maarten Schmidt solve this confusion?

Essay Questions

There will be 1 question, which will be taken from the list below.
The question is worth 15 points.
Your answer should be ~ 2-3 blue book pages long

Describe the difference in physical properties between the inner and outer disks of spiral galaxies, and how this provides evidence for the idea of "inside-out galaxy formation." Make sure to explain what "inside-out galaxy formation" is!

Sketch what the "central engine" of an AGN looks like. Use this sketch to explain how the "unified model" for AGN explain the differences seen in the spectra of Type I and Type II Seyferts. Explain how AGN might power radio galaxies, along with a plausible reason why radio galaxies are preferentially associated with elliptical galaxies.

Describe the Tully-Fisher relationship and the Fundamental Plane. What quantities are involved, and what types of galaxies do they describe? How can they tell us about stellar populations and dark matter in galaxies, and how can they be used to obtain distances to galaxies?

Describe the properties of massive galaxy clusters: sizes, masses, galaxy types, gas content, etc. When we look at distant clusters, which we see as they were billions of years ago, the mix of galaxy types looks a bit different than in clusters today. Describe these differences, and explain the various processes which might have led to these changes in the galaxy populations over time.
Explain why interacting galaxies form tidal tails, and why different interactions can give so widely varied shapes and properties for the tails. Explain also why galaxies merge, and what happens to the orbital energy and angular momentum the galaxies originally had.

Describe the properties of spiral arms. Explain how density waves work, and how they solve the winding problem. Why would star formation occur preferentially in spiral arms?
Explain what we mean when we say that elliptical galaxies are "dynamically hot" while spiral galaxies are "dynamically cold". How do we measure the rotation velocity and velocity dispersion of elliptical galaxies? What do these measurements tell us about how V/sigma is different for spirals and for ellipticals (give characteristic values for V/sigma for each type of galaxy)? Finally, describe the difference between pressure support vs rotational support to explain the flattening seen in some ellipticals. Which types of ellipticals can be explained by rotational support and which kinds by pressure support?

Describe how the merger of two spiral galaxies could produce an elliptical. Think both about the shapes of galaxies and how stars orbit inside them. Explain how a galaxy merger could transform some of the cold gas we see in spiral galaxy disks into the hot gas we see in the halos of elliptical galaxies.

Calculations

There will be 2 questions similar in style to the ones below.
Each question is worth 5 points.
Make sure to bring a stand-alone scientific calculator! No smartphones allowed.

A distant galaxy cluster has a velocity dispersion of sigma=800 km/s and a half-light radius of 120 arcminutes. It has a average recession velocity of 8000 km/s. How far away is it (in megaparsecs)? Calculate the mass of the cluster (in solar masses). If the brightest galaxy has a V-band luminosity of L_V = 1.2x10¹¹ L_sun, what is its apparent V magnitude?

If you had a telescope that could detect objects down to an apparent magnitude limit of m_B,lim=23, what is the maximum distance a galaxy could be at if you wanted to study its globular clusters (which have a typical luminosity of L_B ~ 6x10⁵ L_B,sun)?

Equations

You will get a page of equations and constants with the test, so you do not need to memorize constants or equations except for these:

the definition of magnitudes: m₁ - m₂ = -2.5*log₁₀(f₁/f₂) (m=apparent mag, f=flux)
and remember that same relationship holds for absolute magnitudes and luminosity: M - M_sun = -2.5*log₁₀(L/L_sun)
the magnitude distance relationship: m - M = 5*log₁₀(d) - 5 (m=apparent mag, M=absolute mag, d=distance in parsecs)
the distance / angular size relationship: d = alpha * D / 206265 (d=physical size, D=distance, alpha=angular size in arcseconds)

Tips:

Don't mix up the "2.5" in the magnitude definition equation with the "5" in the magnitude-distance equation.

Remember the distance / angular size relationship is just derived from basic trigonometry: d = D*tan(alpha), but if you use that make sure that the angular size alpha is in the units your calculator expects it (most likely in degrees), not arcseconds!

Helpful Advice

Study using your notes, the online course notes, and the HW assignments and solutions. Arguments and explanations you make in your answers must be based on material covered in class or on HW sets.
When answering questions, try to address both "what" and "why" in your answer. That is, both describe/define whatever is being asked about (that's the "what") and also give an explanation for how or why something works (the "why").
Sketches are useful, but if you are sketching a plot you should always have axes labeled (i.e., "Time", "Apparent Magnitude", etc). It's also a good idea to indicate which direction the values increase (i.e., "the Y-axis is magnitude, with brighter objects to the top").
When calculating something, if your answer doesn't make sense, say so! ("I got a size for the Sun of 10 meters, which is crazy small -- I must have done something wrong!"). Otherwise I would worry that you don't really understand how big the Sun really is...
Also on calculations, SHOW YOUR WORK and EXPLAIN YOUR STEPS. Without that information, if you make a mistake, it is very difficult to see where you went wrong and award proper partial credit. Wrong answers with no explanations will get zero points.
If you are absolutely stuck on a calculation and don't know how to finish (or even start), if you can at least make a qualitative estimate for what you expect the answer to be, and why, that can help with partial points.
Make sure to always have units attached to your numbers.