Astr 500: Seminar


Instructor:  Rene Walterbos

Class times: (mostly) We 12:30 - 1:30 pm, AY 119

Department: Astronomy

Schedule: Seminar schedule

The purpose of seminar is to familiarize you with topics in current astronomical research, learn to critically analyze and summarize a paper, and hone your preparation and presentation skills for presenting talks at professional meetings. Everyone needs to read the paper for every class period and come prepared to class. Prepared in this sense means that you have thoroughly read the paper, identified any important issues you didn't understand, looked up some basic concepts that you ran into and realized that perhaps you should have known, etc. Class grades will be based on your presentation and demonstrated level of preparation for your seminar (60%), and also on your class participation when you are not giving the talk (40%). Each student will present one paper.

For the student presenting the paper, here are some specific tips and instructions:

For the other students:

To help everyone come prepared (i.e. read the assigned papers!) I will randomly draw some names at the beginning of each seminar and ask questions about the paper. The intent of seminars is to stimulate discussion and exchange of ideas and questions. Thus, class participation is important and this can only happen if you read the paper.

We will likely touch on the following areas: Galaxy dynamics, Halo Gas and Outflows, Molecular Gas and Star Formation (with an eye towards ALMA), Radio Galaxies, HI Properties of Galaxies, etc. Each paper will generally be based on a particular radio telescope (single dish or interferometer), wavelength regime, and observational technique.

Tips for your presentation:


Radio Astronomy:

I. Wavelength regime and emission mechanisms

Sub-regimes: "mm", "cm", "low frequency" (longer than ~ 1m, so <=327MHz). You may also run into terms referring to frequency bands (e.g. P-band, L-band, etc at the VLA). Convert frequency to wavelength and vice versa if the paper doesn't do it for you.

II. Radiation mechanisms and sources of radio emission

HII regions

cosmic microwave background

galaxy disks (synchrotron + bremsstrahlung)

compact cores, radio jets and lobes in AGN (mostly non-thermal)

Note: every type of radiation can be optically thin or optically thick. We will not go in the details here can come back to this for the papers being discussed.

The radiation can also appear in emission or absorption, depending on the physical circumstances in the source and between us and the source (Kirchoff's laws!).

Note: Radio astronomers use "Jansky" or "mJy" or even "micro-Jy" as a unit of flux.

They also use "brightness temperature" as a measure of intensity. The relation is established through the Rayleigh Jeans approximation to the Planck black-body emission law, which allows determination of the conversion between "Jansky/beam" to "K" (in reality, we measure "antenna temperature" so there is a telescope efficiency correction factor in there too). We are always in the RJ regime given the long wavelengths in radio astronomy, even at short wavelengths (e.g. microwave background peaks where?).

Look up what a Jansky is. The Planck function is given in intensity units, erg/(cm2 s Hz sterad). Can you see how Jansky/beam is a unit of intensity?

III. Observational techniques

Single dish, Multiple Beam systems

A single dish is much like an optical telescope, except that in its simplest form it, at the receiving end only 1 signal per pointing is recorded. To get an image you must scan or make a set of pointings over a raster. The size of the telescope beam is given by the usual formula for the point spread function of a telescope, FWHM ~ 1.2 lambda/D. The beam can also have "sidelobes" (cf Airy pattern in optical telescopes). All the radiation collected is detected as one flux measurement.

Only fairly recently have multiple beam systems become the norm, so that now a small "array" of receivers can detect radiation over a larger area simultaneously. Still, single dish radio telescopes are quite primitive in this sense compared to relatively enormously large field sizes of optical telescopes. On the plus side, they are big so scanning can be quick.


Because radio dishes operate at long wavelengths, generally their angular resolving power is low even if they are big. E.g. 25-m dish at 21-cm would have 36' beam. Very early on the power of interferometry was realized in radio astronomy, where it is much easier than at shorter wavelengths. The earth's rotation can be used to mimic a large 2-D dish even with sparsely populated arrays. This is called Aperture Synthesis. Examples: Westerbork Synthesis Radio Telescope (used in first paper), Very Large Array near Socorro, CARMA (mm-array in California), and the future ALMA.

An interferometer "observes" or rather provides the Fourier Transform of the brightness distribution on the sky. Interferometry required the advent of fast computers and the FFT routine to produce images of the sky.

Great advantages of interferometry:

Great disadvantage of interferometers:

Generally you do not observe a "zero-m baseline", nor "short spacings", nor do you get full coverage of your simulated dish (which we call "the UV plane). The first means you have no measurement of the total flux (in fact you measure a total flux of zero), the second produces big negative "bowls" surrounding your sources, the third introduces worse grating rings that need to be "cleaned". We will encounter some of these issues in the papers and can go in a bit more detail at that point.

Very Long Baseline Interferometry: A special application of aperture synthesis where baselines can span intercontinental distances. The main differences with "normal" interferometers: better angular resolution, very sparse sampling of the UV plane, so large sidelobes and grating patterns. Since the dishes are so far apart, signals are combined not through hard-wired connections, but after the fact. The need for very accurate time signatures to preserve phase is critical.

Good primer on radio astronomy:NRAO "Essentials of Radio Astronomy" lecture notes

IV. New developments: EVLA, ALMA, LOFAR, SKA

We may not discuss (m)any papers on these, since there are no science results yet, obviously, but it is good to keep these projects in mind.

EVLA = extended VLA. In phase I (the only one approved thus far) this involves new correlators and receivers allowing for much faster imaging over wide bandwidths. Especially relevant to increasing sensitivity in the continuum. It won't do that much for lines since there is not more collecting area.

ALMA = Atacama Large Millimeter Array (a name only astronomers can comprehend). This will be a spectular instrument for interferometry in the mm regime, allowing unprecedented studies of star forming regions and galaxies (far and near). A giant leap forward, much like the VLA and Westerbork were in the cm-regime.

LOFAR = Low Frequency Array. Project developed in the Netherlands to open up the low-frequency regime (below 300MHz or so), a hithertoo largely ignored part of the EM spectrum. Weeding out interference is extremely critical, the design is unique, no dishes but large areas covered with individual little antennas, providing sensivitity over the entire sky at once. Long baselines to get good angular resolution. At least one other array is being considered for NM.

SKA = Square Km Array. A hundred fold increase in collecting area over WSRT and VLA, the next big thing for e.g. HI studies at high redshift where current capabilities run out of sensitivity due to limited collecting area. Likely to be built in Australia, as a large international partnership. Not yet funded.