GALFIT EXAMPLES: Nearby Galaxies

USING GALFIT to FIT NEARBY GALAXIES

One of the science applications of GALFIT is to fit big nearby galaxies, to probe the detailed structures of galaxies. Even though many big galaxies look smooth and featuresless to the eye, when one removes the bright features, residuals may remain that reveal evidence of fine structures, like shells, double nuclei, dust, nuclear rings, stellar nuclei, etc.. These structures may indicate past episodes of galaxy mergers and interactions, or faint emissions coming from around supermassive black holes.

The following images came from work done on nearby galaxies and published in Peng, Ho, Impey, & Rix (2002, AJ, 124, 266). The original data were obtained using the Wide Field and Planetary Camera 2 (WFPC2) onboard the Hubble Space Telescope (HST). The images below show the entire PC chip, or about 800×800 pixels. The number and type of components used vary from galaxy to galaxy depending on how complicated they are.

Left = original data, Middle = model fit, Right = residual image.

A beautiful edge-on S0 galaxy NGC 4111, with a nuclear dust ring. The galaxy is fitted by 4 Sérsic + an exponential disk components. The exponential disk component is the largest, with a scale length of 43 arcsec. The remaining 4 Sérsic components form various parts of the complicated inner galaxy bulge and a bright, nuclear, stellar disk (visible even before image subtraction). Two more strong dark features are evident in the residual image around an obvious nuclear dust ring. The lack of symmetry vertical to the disk plane indicates they are dust lanes.

This looks like an elliptical galaxy, but it's actually just the bulge of a barred spiral galaxy, NGC 2787. The bulge itself fills up the entire Planetary Camera chip, which is about 40 to 50 arcsec in diameter. Apparently, it has a nuclear dust ring that doesn't quite make it all the way around, and the ring has nice ripples! At the center there appears to be a stellar disk that's at a steep inclination with respect to the bulge. This bulge requires 3 components to fit cleanly: one elliptical component, one rounder component, and a nucleus. Two components leave behind a nucleus at the center. One component leaves behind large residual features.

This again looks like a boring elliptical galaxy. In fact it is (an elliptical), but it's anything but boring! The galaxy is NGC 4278, which is type E1, and it has an active nucleus (AGN). Even this simple looking elliptical needs 4 components to get a good subtraction. One of the components fitted away at the center is the point source, and the rest of the galaxy needs 3 Sérsic components. The residuals show a beautiful dust fan structure near the central AGN. The AGN is located dead center of the images.

Yet another big elliptical NGC 4589, with an enormous dust lane running through the center. This guy needs 3 Sérsic components to fit cleanly.

The inner-most 10 arcseconds of the well known M31 Andromeda galaxy (images as published in Peng (2002, AJ, 124, 294), showing the famous double nucleus (Lauer et al. 1993, AJ, 106: 1436). The entire bulge+nucleus takes 7 components to fit: 2 for each of the double nucleus, a round bulge component, an elliptical bulge component, and the exponential disk. Keep in mind that the entire galaxy itself spans over 10,800 arcseconds -- so we're really peering down into the core! The grainy appearance of the residual image is caused by the surface brightness fluctuation being resolved into individual stars by HST.

That's all for now. One can find many more examples in Peng, Ho, Impey, & Rix (2002, AJ, 124, 266), from which most of these images were taken.

Chien Peng (

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