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The Very Large Array (VLA)
Photo courtesy AUI/NRAO |
VLA FIRST Radio Survey
The Very Large Array (VLA), a network of 27 radio telescopes in the New
Mexico desert, is one of the most sophisticated radio telescope arrays in
the world. Right now, the VLA is conducting a sky survey called FIRST (Faint Images of
the Radio Sky at Twenty-one centimeters). The FIRST Survey covers
exactly the same area of the sky as SDSS - so we can use FIRST to find radio sources that
correspond to our data.
| Exercise 1.
The table below shows the sky coordinates (right ascension and declination) of several radio
sources detected by VLA FIRST. Get the VLA FIRST image of
the source from the
VLA FIRST archive server.
Type in the ra and dec, separated by a space (note: be sure to
include a + or - before the dec, or you will get an error message).
For the VLA FIRST image, type the ra in hours (column 1). Set the
Image Size to 2 arcmin, then click Extract the Cutout. You will see a radio
image of the object in the center of the picture. The scale on the right
shows the intensity of radio emissions from each point in the image.
Now, get SDSS's visible-light image of the same object
using the
Navigation tool. In the Navigation tool window, click the link
for RA and Dec in the left-hand frame. Enter the ra in degrees (column 2), and the dec.
Click OK. A new field will open up in the Zoom window with the object
marked by a green circle. You may have to use the scrollbars to find
the circle.
Get a radio and a visible-light image (in separate windows) for each object,
then answer questions 1 and 2.
| Ra (hours) |
Ra (degrees) |
Dec |
| 0.2519432 |
3.7791838 |
-0.13355811 |
| 11.70095 |
175.51423 |
+0.85996027 |
| 11.75959 |
176.39381 |
-0.72744359 |
| 2.178655 |
32.679825
|
-0.30508195 |
|
Question 1.
How do the radio images compare to the visual images? Would
you notice anything unusual about the objects if you just looked at
the visual images? Do you see any unusual features in the radio
images? |
Question 2.
SDSS scientists used a computer program called Photo to automatically
classify all the objects they found as stars, galaxies,
etc. What did Photo call the objects you sawn in Exercise 1? |
Photo does not have enough information from an image to classify all
types of objects correctly. Sometimes, to tell the difference between a star or
galaxy and a potentially more interesting object, you
need to look at the object's spectrum. In the next section, you will look at the
spectrum of one of the objects you saw and compare it to a typical star spectrum
and galaxy spectrum.
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