Appendix D. Detection of Bright Extended Stars and Low Surface Brightness Galaxies
D.1 Bright Extended (Fuzzy) Stars
Bright fuzzy stars are identified as such using a separate algorithm within GALWORKS. The basic method is to look for emission in and around the source at levels elevated above that expected for a bright star characterized by the PSF. After nearby stars have been masked and the source itself has been subtracted on the shape of the PSF, the remaining or residual emission is measured with respect to the mean background level of the coadd. A more effective indicator of enhanced emission is to calculate the root mean square of the residual emission versus the mean background AND versus a zero background (i.e., assume the true background level is zero). The rms values are then normalized by the measured noise for the coadd (atlas image) as a whole. Stars with associated emission, like reflection nebulae, clearly stand out. This operation is referred to as the "bright extended source" processor. Extracted sources are stored in the 2MASS database and a special catalog to be released at some date in the future. There are no set requirements for these kinds of objects and the completeness and reliability of this supplemental catalog are unknown at this time. Examples of sources found with this technique are shown in Figure 5, from scans crossing the Orion trapezium and the large magellanic clouds. The top row shows J-band postage stamp images, middle row the H-band and bottom row the K-band images. The integrated flux for the example sources range from 5th to 7th mag. Examples of Galactic H II regions, molecular cloud nebulosity and young stellar objects are found in Figure 29b.
D.2 Low Central Surface Brightness Galaxies
Low surface brightness galaxies (e.g., dwarf ellipticals) present a different challenge to GALWORKS than the average ‘normal’ galaxy 2MASS encounters. They are typically very faint (as measured in a standard aperture for ‘normal’ galaxies) and they do not have well defined cores; see Figure 6 for examples of typical low central surface brightness galaxies found within 2MASS. The integrated flux of the example sources range from J=15 to 15.6 and K=13.8 to 15.2. The LSB galaxy nature of these sources was confirmed with deep optical images.
The galaxy core is an important component for star-galaxy separation since many of the parametric measurements for star-galaxy separation are anchored to the core or vertex of the galaxy. The low central surface brightness detector (referred to as the LCSB processor) of GALWORKS is executed last in the chain of operations that comprise GALWORKS (see flowchart). The input to the LCSB processor is a fully cleaned coadd image (per band), where stars (brighter than some limit, typically K = 14.5) and galaxies have been entirely masked. The image is then blocked up (using three independent kernel sizes: 2´ 2, 4´ 4 and 8´ 8) and ‘boxcar’ smoothed to increase the signal to noise ratio for large (but faint) object normally hidden in the 1" pixel noise. A block average is not the optimum method (as compared to a gaussian convolution, for example) but with pipeline runtime constraints it is the best option.
The detection step consists of 3-sigma threshold isolation of local in the blocked-up cleaned images. Source detections are then parameterized (using the blocked and smoothed image) with the primary measurements being: signal to noise ratio of the peak pixel, radial extent ("sh" score), integrated signal to noise, surface brightness, integrated flux, and SNR measurements using a J+H+Ks combined "super" image. The "super" image, in principle, provides the best median from which to find faint LSB galaxies given the effective increase in the signal to noise ratio. In practice, the "super" image only increases the SNR by approximately sqrt (2) due to the significant background levels (which directly maps into noise) at H and K and assuming normal (i.e., J-K = 1) galaxy colors. Remaining faint stars in the cleaned image will have a relatively low SNR due to most of the light being confined to a few pixels that are averaged with blank sky the blocking and boxcar-smoothing step. Galaxies, on the other hand, will add up since their light is distributed over a large area.
The preliminary results for the LCSB processor reveal a reliability rate of about ~70 to 80% using a threshold on the ‘maximum’ SNR (between 2´ 2, 4´ 4 and 8´ 8 blockings) of the "super" coadd. The major contaminants are faint stars and diffuse emission associated with bright stars. However, if a meteor streak (or other transient phenomenon) occurs in the coadd, then numerous false sources are picked up as LSB galaxies. It still remains a future task to learn how to improve the reliability of sources coming from the LCSB detector. It is important to note that these sources are nearly always fainter than the level-1 specifications (K > 13.5, J > 15) which means that there are currently no requirements on the completeness and reliability. Also note that the faintness level of the LCSB detector implies that most LSB galaxies are being detected and processed with the ‘normal’ or standard 2MASS/GALWORKS operation (see below). Thus, we do not anticipate any completeness failure for LSB galaxies brighter than K ~ 13.5. The fainter LSBs, however, will have to be detected and processed with the LCSB processor described here. A special catalog of faint LSB galaxies will be released at some date in the future. The reliability of this catalog is to be determined. Further information and some early science results with 2MASS LSB galaxies can be found in Jarrett (1997) and Schneider et al. (1998).