GALWORKS Results for the COMA Core: Algorithm 2

This work represents our first efforts to verify the functionality and design of the Low Central Surface Brightness Galaxy Detector (or algorithm 2) component of GALWORKS. Since the GALWORKS package was ready for full scale tests sans real 3-channel data (as of late 1996), we 'created' a 3-channel dataset using a set of scans (056 to 058) from the prototype camera 1995 (950503) observing run. This memo describes some of the key algorithm 2 results from these "3-channel" tests.

The major goals of this study were to build the sort of analysis tools that will be used wth real 3-channel data, and to shed light on some of the problems that are unique to low surface brightness extended objects. Finally, the reader has a chance to become familiar with key algorithm 2 features .

GALWORKS Results for the COMA Core: Algorithm 1

• Algorithm 1 Results

Construction of 3-Channel Data Set from Prototype Camera Data

Algorithm 2

Detection and parameterization of low central surface brightness objects are described in the GALWORKS SDS section describing algorithm 2:

• LCSB Galaxy Detection

LCSB Postage Stamp Images

The following set of images comprise the LCSB candidate galaxies found in the COMA central core scan(s). The candidates include both real galaxies and unknown objects, and a few false galaxy detections.

The first column is the J band image, second column the H band image, third column the K band image and the fourth column is the DSS optical image. The light blue circular contour represents the radius = 10 area, and the dark blue contour the "flux growth" circular area. Sources that had been "subtracted" from the object fields are circled in red with the size of the circle given by the subtraction radius. Sources circled with a green circle/ellipse represent sources that were previously processed and subsequently blanked from the object field.

• LCSB Detections , scans 056-058

• LCSB Detections , scans 056-058

• LCSB Detections , scans 056-058

• LCSB Detections , scans 056-058

• LCSB Galaxies , scans 056-058

Detection of LCSB Objects

Parameter Definitions

The following plots/figures contain information with regard to the various parameters computed for LCSB objects. Here is a brief glossary of the nomenclature and terms.

J10, H10 and K10

Fixed circular JHK magnitudes corresponding to radius=10 pixels.

Jc, Hc, Kc

JHK mags as derived using the "flux-growth" algorithm. The "flux-growth" algorithem works in general as follows:


The object flux is computed by integrating either circular or elliptical apertures that are sequentially increased in size until the flux growth "converges" as measured by either of two indicators:


(1) the change in flux between adjacent apertures is less than 0.5%; this criterion is design to maximize the integrated flux of the object;


(2) the change in mean surface brightness between adjacent aperture-annuli is less than (-3 * sigma), where sigma is the standard deviation of the background counts in the annulus corresponding to a radius = 15 pixels; this criterion is designed to minimized confusion from stars and from background gradients (e.g., nearby extended sources).

JHK: CSB and C5SB

Central surface brightness as measured from the peak pixel and central surface brightness as measured using a circular aperture radius=5.


JHK: BSNR

The peak pixel SNR as measured in the blocked/smoothed JHK images. The noise corresponds to the background noise of the blocked/smoothed images. LCSB candidates that have BSNR = 0 correspond to non-detections in at least one of the bands.


JHK: TSNR

The integrated SNR using circular aperture radius = 10 with the inner radius=2 pixel blanked. This parameter is designed to minimize the SNR from stars as compared to extended objects (which should have emission beyond a radius=2 from center).


"SUPER" SNR: S2SNR, S4SNR, S8SNR, SSNRMAX

From the JHK images, a "super" coadd is generated. The "super" is block averaged, 2 X 2, 4 X 4, and 8 X 8, and boxcar convolved similarly to the individual JHK images. For each LCSB candidate, the SNR is computed for each of these blocked/smoothed images: S2SNR, S4SNR, and S8SNR. The noise corresponds to the background noise from these blocked/smoothed images. Finally, the maximum of the three SNR measures is SSNRMAX. These SNR measures are designed to take advantage of the increased signal-to-noise ratio in the "super" coadd. Low surface brightness objects should have increasing SNR as you go to higher blocked images (depending on the size of the galaxy).

Algorithm 2 Results

Central Surface Brightness

Blocked JHK-SNR

The dashed blue line represents the threshold that was imposed upon detection: Objects with BSNR < 3 were considered non-detections. This threshold is imposed to minimize contamination from faint stars. For the objects that survive this cut, the confirmed galaxies and unknown objects are degenerate in this phase space, but the false detections may not be degenerate, the small sample precludes any judgement here.

Sh Score Parameter

Only the brightest LCSB candidates have "sh" scores, which excludes nearly all of the candidates. This value of this parameter as a star/galaxy discriminant is unclear.

Integrated JHK-SNR

Most candidates have integrated SNR between 1 and 3, not very good -- which highlights the difficulty in finding these objects.

"Super" Blocked SNR vs. Jc

"Super" Blocked SNR vs. Hc

"Super" Blocked SNR vs. Kc

Most of the confirmed galaxies have SSNRMAX values greater than 7, while the false detections and some unknowns have values less than this limit, particulary at K. For the individual SSNR values, S2SNR, S4SNR and S8SNR, it is not clear if galaxies separate from stars. SSNRMAX looks the most promising of the SNR parameters to distinguish stars from galaxies. Only further testing can confirm this conclusion.

Summary

(1) We detect about 7 LCSB galaxies within 1 scan (or one square degree) toward the COMA core. Most of these objects are very faint, from 1 to 4 sigma detections.

(2) Another 1 to 2 LCSB candidates do not appear to be real galaxies; they are either faint stars, or they are noise bumps ("blobs", actually) that do not have a counterpart on the DSS. Most false detections are very faint, < 3 sigma. The remaining 4 to 5 objects are unknown.

(3) The reliability of this sample is approximately 70% or 80%, for SNR > 2.

(4) The initial detection cut using the BSNR parameter appears to filter out most stars and false detections. The remaining LCSB candidates appear to be for the most part degenerate in the parameter phase space used in GALWORKS. Of the parameters discussed in this memo, CSB, CS5B, BSNR, TSNR, "sh", S2SNR, S4SNR, S8SNR, SSNRMAX, the most promising phase space to separate LCSB galaxies from stars (and otherwise) appears to be through TSNR and SSNR. Further testing is required.