Observational
Cosmology - Year 4 Assessed Exercise:
Probing
the dark halos of elliptical galaxies with Chandra
Astronomers generally acquire their data by submitting scientific
proposals to bid for observing time on international facilities. In the
case of X-ray data, which generally probes very hot (>106K)
gas, the two major observatories operating at the present time are the
US Chandra Observatory, and
the European XMM-Newton
Observatory. Both of these issue annual "Announcements of Opportunity"
(AOs), inviting interested astronomers from any country to submit
proposals. These proposals are reviewed by panels of experts, and those
judged the best are awarded time. However, since the available time is
typically oversubscribed by a factor ~7, it is necessary to put a very
convincing case, in order to be successful.
A proposal typically consists of a number of elements:
- Abstract - concise
summary of the case in no more than 800 characters.
- Introduction and scientific
motivation - this provides background to the proposal, and
explains the problem which is it is intended to address, and why it is
important.
- Science aims - the
specific goals of the proposed observations.
- Target list - this
introduces and justifies the selection of targets to be observed.
- Feasibility - justifies
the observing time requested, and demonstrates that the science aims
can be successfully addressed with the proposed observations.
- References - list of
papers cited in the text.
An example of a proposal, which I wrote myself, is available here. This particular proposal was
unsuccessful, but I have chosen it as an example since it is reasonably
straightforward and illustrates the main features of a proposal fairly
well (though a slightly unusual feature of this particular proposal is
the introduction of the target before the discussion of the science
goals).
Your task is to write such a proposal to acquire Chandra data in order
to study the dark halos of elliptical galaxies. Details of the Chandra Observatory can be found
via the main Chandra Science website.
You should use the ACIS instrument on Chandra
(this is by far the most popular of the Chandra instruments), and so
can completely ignore the high resolution camera (HRC) and the
HETG/LETG transmission grating instruments.
Completing this exercise will involve a number of steps:
- Find out about Chandra
- great detail is not required at this stage, a brief introduction can
be found here,
whilst
the POG
contains a much more detailed description. You do not have time to read through the
POG - take a look at the introductory sections, and scan through
section 6, on the ACIS instrument. If you want a pdf file of the entire
POG, you can find it here. (1 hour)
- Research the issue of the dark
halos of elliptical galaxies -
why is this important? what are some of the open questions? how could
X-ray observations help to resolve one of them? Your main tools here
will be ADS and astro-ph, which are both linked from the Obs Cos home
page. Since you need to be up-to-date (no panel will award you
time to investigate a problem which has already been solved) you should
concentrate on recent work,
referring back to earlier papers only where necessary. You do not have
time to read more than a few entire papers, so concentrate on
identifying a small number of really useful ones, which review the
topic, or perhaps focus directly on the role of X-ray observations. You
are encouraged to work collaboratively on this aspect of the exercise. (3-4 hours)
- Trawl the recent literature
to see what has already been
done in
this area with Chandra and XMM, this may also give you some ideas. (1-2 hours)
- Familiarise yourself with the main
steps required to make
feasibility
calculations for Chandra observations. I am not expecting a
sophisticated analysis, merely some count rate estimates for your
targets, together with some justification that the number of counts you
expect should permit you to address your science goals. You can find
advice on this in section 6 of the last Chandra Call for
Proposals, although this provides much more than you will need to
use. The only software I expect you to use is PIMMS
and possibly Colden
(don't worry about "visibility", for example). In terms of how many
photons you actually need to detect, I do not expect you to carry out
simulations. You can estimate flux errors using simple Poisson
statistics (remembering that background may be important), whilst for
spectral errors, a good rule of thumb is that 1000 source counts gives
T to ~5% if background is negligible, and this error is increased
by roughly sqrt(2) if the background contributes as many counts as the
source. You may also find the simple feasibility section in my sample proposal useful. (1
hour)
- Find an attractive angle, and a
suitable target list of one to
ten galaxies - check whether these have already been observed by
Chandra using the WebChaser
tool, or consulting the full list of
Chandra observations. (2 hours)
- Check feasibility for
these, and modify the list if necessary.
You may find the catalogue of O'Sullivan useful
when assessing the likely count rates from different elliptical
galaxies. (1
hour)
- Write the case. (4-6 hours)
- Proof read case
carefully, and revise. (2 hours)
You are expected to spend 15-20 hours on this assignment,
which
accounts for 30% of the credit for the course. You will need to
exercise considerable self-discipline to stop this time from inflating.
To help you, a guide to the time each stage might take you is included
in the list above. Try to keep to these times.
Proposals must not exceed 4 sides at a
font size no smaller than 11pt. Any excess will be disregarded, as in the case of a real Chandra
proposal. Don't forget to include your name as P.I. on the
first page. Your completed proposal must be submitted to the TSO by 4pm on
Friday December 12th.
Assessment
I have been a member of many time allocation committees, and I will
assess your effort using the criteria I would use for a real proposal:
- Is the scientific problem to be addressed important and
well-explained? [25%]
- Do you have clear and convincing goals for your specific
observations? Would these observations add usefully to what is already
known? [25%]
- Is your choice of targets sensible and clearly explained? Is the
case that you would achieve
your goals well-made? [25%]
- Is the whole proposal clear, concise, logical and well-presented?
[25%]
N.B. Please note that scripts
cannot be returned to you after
marking,
though of course the marks will be.
A word of warning
You are encouraged to discuss this assigment with others on the course
- this is an excellent way of testing and sharpening
your understanding. However, it is
essential
that the work you submit is your own. I
will be marking all the scripts myself,
and
I do not want to see near-identical chunks in different people's work.
It is also unacceptable to lift material from published (or web)
sources without
referencing. All these things are plagiarism, and the School and
University
have very strict rules about this, as you will know. See http://www.sr.bham.ac.uk/skills/Referencing.html
for guidance on referencing web-based (and other) material.
Send comments or suggestions on these pages to Trevor Ponman.
Last updated 24 Oct. 2008