Mosaic Construction of NGC 2903

T.H. Jarrett

The late-type barred spiral, NGC 2903, is a nearby large angular-sized galaxy, D25 ~ 10 arcmin. Its near-infrared diameter is ~8 arcmin (see below), spanning several 2MASS Atlas images (including adjacent scans). Hence, cross-scan background removal is critical to successful mosaic construction -- as is the case for most nearby galaxies. The following images show the intermediate steps in the H-band background removal (the H-band is the most difficult case owing to the strong "airglow" emission features in the images).

Step 1. Retrieve Atlas images from database. Retrieve table of "stars" local to the target, also from the database.

Step 2. Place Atlas images on a common grid, subtract the mean background sky value from each Atlas image.

Step 3. Apply offset adjustment to coadd-to-coadd boundaries (common to each scan) when needed. This step is necessary to eliminate artificial "steps" between coadds within a scan. There is typically little if any adjustment needed.

Step 4. Mask stars and target galaxy. For NGC 2903, an elliptical mask with an 8 arcmin major axis is sufficient.
Step 5. Determine scale length of background removal. The in-scan and cross-scan lengths are dependent on the masked target galaxy.

Step 6. For each pixel of each column, derive the background solution using a cubic polynomial applied with the corresponding in-scan scale-length.

Step 7. Subtract column solution from initial construction.
Step 8. Using the residual image, determine the row (cross-scan) solution for each pixel using a cubic polynomial applied with the corresponding cross-scan scale-length.
Step 9. Build background image from column and row solutions.