In order to reliably detect low surface brightness galaxies (LCSB) it is imperitive that the seeing remain stable -- or more specifically, that GALWORKS accurately track the seeing -- and that the atmospheric airglow emission be at a minimum. LCSBs by definition are at the sensitivity limit of the survey and thus are at the mercy of changing environmental conditions.
The SA 57 data has both poor seeing (untracked for J and H in many cases) and in spots poor photometric conditions. We see several cases of false LCSB objects appearing for no apparent reason (other than being airglow blobs; see images below). This study is severely hampered by the poor quality in the data (plus the fact that the JHK scans were acquired at different times during the night of 95-04-24). Nevertheless, the results do give us some idea what Algor 2 will achieve with real quality 3-channel data.
Algorithm 2
Detection and parameterization of low central surface brightness objects are described in the GALWORKS SDS section describing algorithm 2:
LCSB Postage Stamp Images
The following set of images comprise the LCSB galaxies found
in the SA57 scans. The objects have been checked against the
DSS or POSS images and /or NED to confirm their extended
extragalactic nature.
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 Galaxies , scan 039
- LCSB Galaxies , scan 039
- LCSB Galaxies , scan 040
- LCSB Galaxies , scan 041
- LCSB Galaxies , scan 041
- LCSB Galaxies , scan 041
- LCSB Galaxies , scan 042
- LCSB Galaxies , scan 042
- LCSB Galaxies , scan 043
- LCSB Galaxies , scan 043
- LCSB Galaxies , scan 044
False LCSB Postage Stamp Images
The following set of images comprise the falsely classified LCSB galaxies found in the SA57 scans. Notice several instances of K-band detections (with K mag < 14) where no J or H or DSS equivalents are seen. Either these are cases of airglow blobs (localized to a few pixels) or a set of 3-sigma proximal noise bumps conspire to form a source (statistically we estimate that this event is probable given the 6 square degree coverage).
Other false LCSB galaxies are faint stars that have
gone slightly "fuzzy" probably due to a severe change in seeing conditions
(particulary at J and H).
- False LCSB Candidates , scan 039
- False LCSB Candidates , scan 039
- False LCSB Candidates , scan 040
- False LCSB Candidates , scan 040
- False LCSB Candidates , scan 041
- False LCSB Candidates , scan 041
- False LCSB Candidates , scan 042
- False LCSB Candidates , scan 042
- False LCSB Candidates , scan 043
- False LCSB Candidates , scan 044
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
It can be seen from the K band contribution, there is a population of
false LCSB galaxies that have a very low central surface brightness, as compared
to the confirmed galaxies and "unknown" objects. These false sources are low
surface brightness blobs with hardly any discernable peak brightness. Other than
these objects, the galaxies and false detections are more or less degenerate
in this phase space.
Blocked JHK-SNR
The dashed blue line represents the threshold that was imposed upon
detection: Objects with BSNR < 4 were considered non-detections. This
threshold is imposed to minimize contamination from faint stars.
For the objects that survive this cut,
the confirmed galaxies, false detections and unknown objects are
degenerate in this phase space.
Integrated JHK-SNR
Again the population of K-band blobs (false LCSB galaxies) stand
apart from real galaxies and the unknown objects. The
these blobs are not centrally condenced and they integrate up to something
rather bright (K < 14). There is some suggestion at J and H that the
false detections have lower SNRs (systematically) from the confirmed galaxies.
Only better quality data can confirm this trend.
"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 or 8, while many false detections have values less than this limit,
particulary at K (the K-band blobs have very low SSNR values).
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 better
quality data and further testing can confirm this conclusion.
Summary
The quality of the data is not good enough to draw many firm conclusions.
If we take the results at face value, we claim the following:
(1) We detect about 32 LCSB galaxies within 6 scans (or six square degrees)
toward the galactic pole. Most of these objects are very faint, from 3 to 5
sigma detections.
(2) Another 20 or so 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, < 5 sigma. The K-band blobs are, however, fairly bright with integrated mags between 13.5 and 14.0 (with very little central condensation).
(3) The reliability of this sample is approximately 50%, for SNR > 3.
(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 with higher quality data is required.