VLA FIRST Radio Survey

The Very Large Array (VLA) Photo courtesy AUI/NRAO

The Very Large Array (VLA)
Photo courtesy AUI/NRAO

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 almost exactly the same area of the sky as the 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 in the table below). 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. Enter the RA and Dec (in degrees) in the appropriate boxes, then click “Get Image.” An image of the object will appear in the main frame of the tool, and the object’s data will appear in the right-hand frame.

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
0.2519 3.7791 -0.1336
11.7010 175.5142 +0.8560
11.7596 176.3938 -0.7274
2.1786 32.6798 -0.3051
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 saw 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.