10.1.1 Fitting a Single Galaxy

By single galaxy, we mean a galaxy, which has a large angular separation from other galaxies, i.e., no other galaxies overlap with the galaxy in question.

First, we run galfit on all the single galaxies using the default parameters. Most of the galaxies are so relatively small, that it is not necessary to use the entire 1024 pixel $\times$ 1024 pixel image to fit them. Usually, a 400 pixel $\times$ 400 pixel section will do. When possible, the section should be centered on the galaxy. A typical call to galfit is:

galfit d1293[096:495,517:916] infiles=r214_GR_1 output=r214_GR_1

where the name of the galaxy image in the example is d1293.

Besides the galaxy image, the input files to galfit are galaxy.gal, galaxy.star, and
galaxy.cosmic, and the output files are galaxy.har.tab, galaxy.resh.imh, galaxy.pos.del,
galaxy.prf.tab, and galaxy.res.imh.

The two images are examined to see whether the obtained fit is good. It is mostly the ellipfit residual image (i.e. galaxy.res.imh) that is interesting. Two typical types of problems are:

• $r_{\mbox{\scriptsize free}}$ is wrong and/or there are overlapping ellipses. Since the center and shape parameters of the galaxy actually change with r, we want to have $r_{\mbox{\scriptsize free}}$ as large as possible. However, when the signal to noise becomes too low, the fit is poorly constrained, and one can get overlapping ellipses, which is undesirable. In a number of cases, it is necessary to manually intervene to get the best fit:
  • If $r_{\mbox{\scriptsize free}}$ is too low, i.e. if one can see from the ellipfit residual image that e.g. the ellipticity is still rising beyond $r_{\mbox{\scriptsize free}}$, one can try to increase errcen and errshap, or set rcen and rshap to something suitable.
  • If $r_{\mbox{\scriptsize free}}$ is too high and one gets overlapping ellipses, one can try to set rcen and rshap to the radius just before the overlap occurs.
  • If a galaxy has a sharp isophote twist, and has been observed in good seeing, it is not possible to just decrease $r_{\mbox{\scriptsize free}}$ to avoid the overlapping ellipses, because then the outer parts beyond the sharp isophote twist are not fitted correctly. This can be solved by making sure $r_{\mbox{\scriptsize free}}$ is large enough to fit the entire galaxy, and then manually remove the radius/radii from galaxy.prf.tab at which the ellipses overlap (this is reported by galfit as e.g. `MODELINIT: WARNING: ellipses overlapping at r = 24.05'). One can construct a residual image corresponding to this new profile using model with residual=yes. From this image it should be checked, that the model is well behaved in the flagged pixels, where the model values will be used when calculating aperture magnitudes. One can do that by blinking the above cmodel residual image with an image that has the flagged pixels marked, e.g. the residual image from galfit. The above procedure was done for 1 of the 147 observations that I (BMJ) fitted (r253_JB_2).
• Problems with flagging other objects.
  • Even though classify is quite good at assigning radii to the extended objects, there will still be some objects where the radius is not correct, being either too large or too small. This is particularly a problem close to the center of the galaxy, both because the non-flat background confuses classify, and because the flagging is more critical here. Correction of the assigned radii is done by editing galaxy.star.
  • Some extended objects might not have been detected, typically faint galaxies, so they need to be added to galaxy.star.
  • Some cosmic-ray-events might not have been detected, so they need to be added to galaxy.cosmic.
  • In a few cases, there are cosmic-ray-events very close to the center. Flagging them using galaxy.cosmic or letting deviate do the job might not work, because the interpolation routine used by ellipfit cannot handle having such a large fraction of the ellipse flagged. One has to replace the affected pixels by the values in their symmetric counterparts, assuming symmetry around the center of the galaxy. This was done for 3 of the 147 observations which were fitted by me.