Size and Shape of Galaxies as seen with 2MASS, IRAC and WISE
An ellipsoid model (representing an exponential-disk galaxy) is convolved with a symmetric gaussian that is matched to the beams of 2MASS, IRAC and WISE. The shape and size of the resulting ellipsoid is measured and compared between beams.

### Example: Relatively small, inclined disk galaxy

 Ellipsoid: alpha = 2.0 arcsec; axis ratio = 0.500; R95 = 9.50 arcsec w/ 2MASS FWHM = 2.5 arcsec; axis ratio = 0.562; R95 = 9.60 arcsec w/ IRAC FWHM = 2.2 arcsec; axis ratio = 0.549; R95 = 9.55 arcsec w/ WISE W1, W2, W3 FWHM = 6 arcsec; axis ratio = 0.717; R95 = 10.85 arcsec w/ WISE W4 FWHM = 12 arcsec; axis ratio = 0.874; R95 = 17.00 arcsec

Table notes:

• the ellipsoid is defined as: f = f0 * exp(-r / alpha), where r is the radius based on the ellipse with axis ratio and position angle parameters;
• R95 represents the semi-major axis at which 95% of the total flux is captured.

 Semi-major axis of the disk galaxy compared to the actual radius due to the beam resolution. Axis ratio departure from 0.5 due to the beam resolution.

Last update - [3 May 2009]
T.H. Jarrett (IPAC/SSC)