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
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
calculate a virial 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.
- Describe the properties of radio galaxies,
including the different sources of radio emission and an
explanation of what's going on at the galaxy's center that
powers all this emission.
- 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.
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
- 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 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
- How do we measure the motions of gas and stars
inside a spiral galaxy? How can we do it for 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 vs 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 in an elliptical galaxy halo.
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 LV = 1.2x1011 Lsun,
what is its apparent V magnitude?
- If you had a telescope that could detect
objects down to an apparent magnitude limit of mB,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 LB ~ 6x105 Lsun)?
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)
- and remember that
same relationship holds for absolute magnitudes and
luminosity: M - Msun = -2.5*log10(L/Lsun)
- the magnitude
distance relationship: m - M = 5*log10(d) -
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)
- Don't mix up the "2.5" in the magnitude
definition equation with the "5" in the magnitude-distance
- 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!
- 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
- 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.