Astronomy 616 - Spring 2009
Supplementary notes to go along with Jon Holtzman's notes. Note that
some of my notes may be discussed or handed out in class as
handwritten/white board written discussions, to I encourage you to take
notes during our discussions in class.
January 15.
We discussed syllabus, grading, homework and paper, and an overview of
the lecture notes of Jon.
Additional comments:
Under ISM, we should also include magnetic fields and cosmic rays
(relativistic electrons, protons, and heavier nuclei) as integral
constituents of galaxies. The evidence for this is synchrotron
radiation in disk galaxies and measurements of magnetic field strength
and order through Faraday rotation and polarization in the MW and other
galaxies.
January 20.
Regarding "What limits the
size of galaxies?"
The "cooling curve" shows the rate at which a gas (in this case an
ionized plasma over most of the temperature range shown) cools. The
quantity on the left axis is the cooling function, Lambda, which is
expressed in erg cm3/sec. Note that it is normalized by the number
density of particles (hydrogen atoms) squared; hence the unit of cm3
instead of per cm3. You get a cooling rate by multiplying this cooling
function by the density of hydrogen atoms squared at the particular
temperature and problem you are considering.
In-class discussion questions:
1. Why would it be normalized this way, with density squared and e.g.
not linearly with density?
2. In the limit for high temperature, beyond 10^7K the curve will
approximate the pure bremsstrahlung (equals "free-free thermal)
continuum. What will the slope of the curve be in that case?
3. What would the effect be to the cooling curve of adding metals to
the gas, so it is no longer a primordial composition gas?
It is important to keep in mind that the argument given for this limit
to the size of galaxies is not the whole story. Arguments based on the
Jean's instability and Jean's length have also been used to advocate
why objects collapse on the scale of globular masses, a factor 10^6 or
more smaller than this galaxy scale. We know that the actual
hierarchical picture of galaxy formation is far more complex, and it
really does not seem to answer the question why galaxies haven't grown
beyond the 10^12 mass. Still, it is interesting to ponder if this view
has any bearing on the rapid initial collapse of the most massive
objects or not.
Observations of galaxies
Find out yourself how far galaxies can be away for HST to still be able
to resolve individual stars in galaxies. Be aware of the distinction
between "all the galaxies can be resolved" versus "few galaxies can be
resolved into individual stars".
Consider the following issues in your considerations:
a. diffraction limit of the telescope, collecting area, and sensitivity
to unresolved sources for a diffraction limited telescope (i.e.
limiting magnitude that HST can detect in certain amount of time).
b. distance modulus (in other words, the weakening of flux with
distance).
c. crowding (how close would those stars appear to be together as a
function of surface brightness?).
d. are there any background noise sources we'd have to worry about,
e.g. sky emission (zodiacal and galactic), read noise on the detectors?
Photometric measurements.
e. Why does cosmological dimming of the surface brightness go as
(1+z)^-4?
f. The Sloan database uses the "Petrosian radius" to characterize the
size of galaxies. Look up what this is and why they use that.