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.
Chien Peng ()