Major axis size using imsad and imfit

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Major axis size using imsad and imfit

Postby greggold » Sat Dec 22, 2012 12:22 pm

I am studying a sample of AGN at 20 cm (beam size 40 arcsec) and at times it is unclear whether a particular source is resolved or not. I am using imfit and imsad deconvolved major axis size (which give similar but not identical values for major axis), and also general inspection and the ratio of integrated/peak flux density as indicators of whether a source is extended.

How high may the value of deconvolved major axis be for a point source?
I get the impression weak sources (say under 10mJy flux density at 20 cm) are more likely to have a high major axis size (? a falsely high value).

Is the deconvolved position angle of use in determining whether as source is resolved or not, eg showing a value near 0 or 180 deg if not resolved?

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Re: Major axis size using imsad and imfit

Postby Mark.Wieringa » Tue Feb 05, 2013 10:05 am

Hi Greg,

I think your query came in just as everyone left for Xmas holidays...
So here's a very belated answer:

The accuracy of the major axis is mainly dependent on the signal to noise, for 5-10 sigma sources it will not be very reliable - another approach to take in this case is to try fitting a point source model and look at the residuals for evidence the source is resolved.
Calibration errors will also distort sources, but this should affect all point sources in the field similarly.
If the source is strong enough you can inspect an amplitude vs uv distance plot for evidence the source is resolved (you might need to subtract out other sources in the field).


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