M33 combined 6cm+20cm catalog description (2015 February 03)

Text catalog: catalog_2015Feb03.txt.gz
FITS catalog: catalog_2015Feb03.fits
Islands (segmentation) image: m33-islands-m.fits.gz
Detection image: m33-combined-filter.fits.gz

The 6cm+20cm detection image was created by averaging the entire 6cm and 20cm bandpasses. The 20cm high and low frequency images were used, and 14 of the 16 125-MHz channels from the 6cm observations were used. (6cm channels 1 and 2 were contaminated by interference.) The 6cm and 20cm bands are equally weighted in the combined image. That's not optimal for a typical extragalactic source (which has a power law spectrum nu**alpha with alpha=-0.7) but is a reasonably "spectrum-neutral" weighting for the source detection. The resulting source lists do not appear to be a strong function of the weighting.

A multi-resolution (median pyramid) image processing algorithm was used both for source detection and to subtract the background from the image to make the source detection threshold more uniform. The images without background subtraction can be compared to the original summed image to see the effect of the background subtraction.

The island map (segmentation map) shows where sources where detected. The numbers in the island map correspond to the "Island" field in the catalog. Islands were detected by thresholding the multi-resolution image. Islands detected at more than one resolution are merged. It is possible for several sources detected separately at high resolution to be embedded in one larger source at low resolution; in that case the morphology and sizes of the high-resolution islands are used to determine how to break the large island into pieces. Padding is added around the islands to give enough context for an accurate fit of the fluxes, positions and sizes. Rarely the padding causes convergence problems, in which case it is reduced to get a better fit. The FITS file with the island numbers is available for download above.

The pixels in each island are fitted with elliptical Gaussian source models to determine sizes, positions, and peak and integrated flux densities. For most islands a constant background offset is also fitted (although that is omitted when it causes convergence problems). The data are fitted simultaneously in 16 frequency bands (2 20cm bands and 14 6cm bands). The Gaussian is constrained to have the same position and source size in all bands, with the flux and background varying between bands. The effect of the resolution varying with frequency is included in the fits. The fitted size quoted in the catalog is for the highest frequency (= highest resolution) 6cm band, which has a FWHM resolution of 4.08 arcsec. For the fitting, the image PSF is treated as a round Gaussian (ellipticity is ignored); the actual elliptical beam has major and minor axes that differ from the round beam by at most 2%. For high SNR sources that could limit the accuracy of the modeling, but for the vast majority of sources a round PSF is a good approximation. For an unresolved source the fitted size will be approximately equal to 4.08 arcsec. Note that the fitted size is not constrained to be larger than this value; sources with sizes smaller than the beam PSF will have integrated fluxes smaller than their peak fluxes. Here is a table of the frequencies and sizes:

In addition to the Gaussian fits, the catalog also includes integrated and peak fluxes for the island. The peak is just the brightest pixel in the island, and the integrated flux is the sum of all the flux within the island with a PSF-dependent correction factor (close to unity) for flux that spills outside the island. Usually the island fluxes are are noisier than the Gaussian fits but they work better when the source is not well modeled as a Gaussian. The image with the Gaussian source models from the catalog subtracted can be used to identify sources where the Gaussian fits are poor.

The images with islands overlaid show the positions of the sources. Red islands have source entries in the catalog; green islands either are too faint (less than 4-sigma) or failed in fitting, usually because the region was crowded.

Only a single Gaussian component was fitted to each island. This is sometimes not very good when islands touch or overlap. I can eventually improve this by doing simultaneous fitting to neighboring islands. It is not hard (although getting convergence is harder), but I have not had time to do it yet. That is one of the major limitations of the current catalog.

Catalog column descriptions

The 2015Feb03 M33 catalog from 6cm+20cm data includes 2024 sources detected at 4-sigma or greater in island or in spectral index fit. It is available for download in text or FITS format. The text catalog has comments at the top that describe the contents. Here is a description of the columns: