In astronomy, the color of a star is defined as the difference between the star's total magnitude in two different bandpasses.
Galaxies are extended objects and computing their total magnitude is more difficult. The simplest way to examine the color of a
galaxy is by making a color map. A color map is constructed bt taking the ratio of two images of the same
galaxy from different filters. If the image is displayed on a logarithmic scale, then each pixel value is proportional to the
difference in magnitude or the color.
The dimensions of the two images from different filters after reduction is frequently not the same. This comes about because
the images are created by combining two dithered images and it is impossible to insure that the telescope is offset by exactly
the same amount during each dither. We cannot divide two images that have different dimensions so we use IMCOPY to crop
the images by specifying the regions we want in the input line.
Using IMCOPY to crop the image will usually perturb the alignment of the images. The images can be realigned using
IMSHIFT. We use one image as a reference, compute the pixel offset of the second image, and enter these pixel offsets
into IMSHIFT to align the second image with the reference image. There are numerous ways to compute the pixel offsets
but we chose to identify the centroid pixel coordinates of a set of stars in common between the two images, take the difference in
x and y pixel coordinates for each star, and use the average difference as the pixel offset.
Before we can divide the different bandpass images, we must match their point spread functions (PSFs). The two dimensional light
distribution of a typical star is well described by a two-dimensional gaussian
function. Gaussian functions are described by their
value, which is related to the full width at half maximum (FWHM). This quantity is a measure of how spread out the distribution of a star's
light is. We want the spread to match in the different filters when making a color map. We can use IMEXAMINE with the 'a'
to check the gaussian FWHM for a set of unsaturated stars common to the images in both filters and use the IRAF task GAUSS to modify
values. When the values match reasonably well, we can divide the two images to make a color map.
For UGC 7576, the R image has larger sigma values than the B image. We run GAUSS on the B image with
a sigma value of 1.3. The logarithmically scaled output image is proportional to the B-R color for UGC 7576. This color map is
displayed below in Figure 10. The lighter regions correspond to bluer regions and the darker regions correspond to redder regions. We
can see that the polar ring is very blue and the central host is very red. This reflects the nature of the stellar population of the two
components. The polar ring is rich in young, hot blue stars
while the central host is populated by older, cooler red stars.
This is not surprising. What we would like to do is quantify the
color difference between the polar ring and the central host.
Unfortunately, our observations did not include standard stars
and it is difficult to calibrate our magnitudes.
|B-R Color Map
Figure 10. This figure shows the B-R color map for UGC 7576.
Figure 11 shows a U-B color map for NGC 2685. Again, we see a clear color difference between the two componenents of this polar ring galaxy.
The ring shows up as being bluer and the central host and the diffuse envelope as redder.
|U-B Color Map
Figure 11. This figure shows the U-B color map for NGC 2685.
We are able to make multiple color maps for NGC 3718 because we have images in more than two filters. If we were studying
star clusters, having multi-color data would allow use to make color-color diagrams and color-magnitude diagrams, the both
of which allow astronomers to obtain very interesting information about star clusters most notably their age. Figure 12 shows
U-V and V-R color maps for NGC 3718. We see that the polar ring galaxy looks different in the two colors. The
U-V color map reveals the presence of blue star clusters around NGC 3718, which show up as white spots in the images.
If we were to perform photometry on these star
clusters, we could obtain an age for NGC 3718. However, infrared observations would be more appropriate for this purpose since
there is less extinction in the IR and there is clearly much dust surrounding NGC 3718.
Figure 12. This figure shows the U-V (a) and V-R (b) color map for NGC 3718.