T. Jarrett, IPAC
Note from Jarrett (970428): This work represents our initial efforts at analysis of very large galaxies, in this case M51. A more up to date memo can be found in GALWORKS Production of M51 & Companion. However, the reader should look at this memo first to become aquainted with some of the issues facing GALWORKS per large galaxies.
The whirlpool galaxy, M51, is a large nearby spiral galaxy with a barred companion. It fills about half of one coadd. In the GALWORKS known-galaxy catalog, it has a recorded major axis size of 11.2 arcmin, while its companion has size 5.8 arc min.
The data were acquired with the 2MASS 3-channel camera and 2MASS telescope atop Mt. Hopkins. Thus, this data represents our first efforts at galaxy analysis with real data. M51 is the largest galaxy (to date) to pass through the GALWORKS end of the pipeline. Since M51 is so large, it surely affects the coadd background, blanking and source extraction, so it behooves us to check what actually happens during production. This memo provides some details into this matter.
Search Galaxy Database for Large Galaxies
The first step that GALWORKS performs is to check the database catalog for any large galaxies that may be within the coadds or partially covered by the coadds (very large galaxies). For scan 006 of the night of 970418, it finds two galaxies, M51 and its companion, within coadd "184". The galaxies have a recorded size of 11.2 arcmin and 5.8 arcmin, respectively. The current setting for GALWORKS (parameter set in the galworks namelist "nl.galworks") is to process only galaxies smaller than 5 arcmin. This value was chosen to avoid working on very large galaxies that are not likely to fit on one coadd. It's value can, of course, be tuned to a different value if need be. In any case, M51 and its companion were considered too large to process. Therefore, the two galaxies were "cut" and extracted from the coadd, with the postage stamp images written to disk. This information was recorded in the file "bright006.gals" under the "extd" directory of scan 006.
Background Determination
Before the background can be properly determined, all detected sources (including galaxies) need to be blanked from the coadd. With sources blanked from the coadd, the resulting 'cleaned' image should contain only "background" light and whatever diffuse light from large galaxies and bright stars not blanked in the coadd. The background is computed by passing the cleaned image through a median filter and iteratively fitting the residuals with a cubic polynomial function (further details on this operation can be found in the GALWORKS SDS).
Since M51 and its companion have entries in the GALWORKS known-galaxy catalog, these two objects were blanked from the coadd accordingly (with sizes roughly equal to their known optical size) before background determination. A large fraction of the coadd is therefore blanked away (~50%, nearly all of it in the lower half of the coadd), so this information is not available in the background determination operation.
Notice from the image that even after the galaxies are blanked, there does remain a slight diffuse component (due to M51 and companion) in the cleaned image. The background solution image directly reflects this diffuse image; a higher background solution is found for the area encompassing the large galaxies. Since we fit a cubic polynomial to a median-filtered cleaned image, any diffuse component will be easily detected if it has a size scale greater than 1-2 arcmin. In this case, the diffuse component is amplified because there is little or no information in the southern half of the coadd (because of source blanking) to offset this emission. This case only occurs when a large object (e.g., nearby galaxy) fills the coadd thus rendering background determination a more hazardous operation. Fortunately this case will happen very infrequently.
What happens once we subtract the background solution from the raw image? The image below shows again the raw image, the cleaned image with the background subtracted, and finally the background subtracted image of the M51 coadd. It can be seen that the background was well fit to the raw image, particularly where it counts most -- in the northern half of the coadd where some information still remains (remember that M51 and its companion are forever blanked from the images; no sources can be examined or extracted in the near confines of M51. The background is not well determined near M51 because of the reasons outlined above, namely the diffuse component, but this is not relevant anyway since this region is blanked and no further processing is performed by GALWORKS; the right panel image showing M51 and companion with the background subtracted are displayed only as a demonstration of the effect of M51 on the background solution).
With the galaxy pair blanked and the background subtracted, the resulting
image is then processed by GALWORKS, including extended source
detection, star-galaxy discrimination, extraction, and finally,
LCSB detection and
extraction (Algorithm II). A future memo will summarize the results
from these processing steps.
Conclusion
We have demonstrated that GALWORKS suitably computes the background solution for the 2MASS coadd image that contains M51 and its companion galaxy. Although M51 and companion comprise over one-half of the coadd itself, the background is well determined for the region of the coadd where the galaxy pair was not subtracted (in this case, the upper 50% of the coadd). So we conclude that for galaxies as big as the M51 system (~10 arcmin) and smaller, GALWORKS is able to determine the background with minimal effect from the galaxy(s) itself. For larger galaxies, this conclusion obviously may not hold. Further testing of larger galaxies is required (e.g., M31).
