LSB Galaxies Detection Using Markovian Segmentation on
Astronomical ImagesMireille Louys1, Benjamin Perret 1
Bernd Vollmer 2, François Bonnarel 2 Sebastien Lefèvre1, Christophe Collet1
1 Laboratoire des Sciences de l’Informatique , de l’Image et de la Télédétection
2 Centre de donnees astronomiques de Strasbourg,
Rationale
• Surface brightness is highly variable from one galaxy to another (Disney, 1976)
• Low surface brightness galaxies are close to the image background level and satisfy:
SB > 22.5 mag.arcsec-2
• Found inside and outside galaxy clusters – How complete are our detections?
• More understanding in the baryonic fraction of the Dark Matter in the universe
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Noise and background
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Easy case More difficult
Overview
Markovian Quadtree
SegmentationObjects Profiles
Fitting Selection
DetectLSBEllipse parameters Fit error Magnitude profile LinLog, LinLin Measured Surface brightnessEstimated Surface brightnessScale length
Visual Inspection and
Validation
3D mathematical morphology
Segmentation Map
Original Image
Objects list
VO Objects list
VOTable XML
Markovian segmentation
• The goal is to classify pixel values and discriminate bright objects, sky background and faint sources to be considered later as LSB candidates .
• It works as an inverse method :
– Try to find the Bayesian estimate of the most likely class partition that maps the observation
– We make 2 assumptions : • on the noise and inner class statistics (here gaussian),
• on the statistical phenomenon: Markovian quadtree.
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Markovian quadtree•The succesive tree levels correspond to increasing scales of the image. •We want to compute the a posteriori probability of having site s in one class c, knowing the observation Ys and the class number of the parent node within the Markov tree.
•The Quadtree provides an in-scale regularisation framework, providing exact and non iterative solution
•It is fully applicable to multiband observations
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Steps achieved with DetectLSB • For each detection map:
– Identify pixel connected components – Remove very extended components : > 300 pixels– Remove components at the edges of the image
• Ellipse fitting • Luminosity profile analysis• Selection criteria:
– Sort out stars according to their profile (steep slope, etc.)– Remove bad centered and/or overlaping objects (crowded
regions) – Check for central surface brightness
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Luminosity profile analysis• For each object in the detection map:
– Average profile along each ellipse radius
f(r)=U0*exp(-r/R0)
At least 3 points aligned abovebackground level
Source Measurements SurfaceBrightness
Profile Fit Error
RadiusFitted SB
Data set• B-images of a region of the Virgo cluster
obtained with the Isaac Newton Telescope WFCS (Wild Field Camera Survey)
• Analysed by Sabina Sabatini et coll. inSABATINI S.; DAVIES J.; SCARAMELLA R.; SMITH R.; BAES M.; LINDER S.M.; ROBERTS S.; TESTA V. Mon. Not. R. Astron. Soc., 341, 981-992 (2003) .and provided to us as a test set
• Full data set = 80 images of 4096x2048 pixels • Analysed: 18 images with X-match with the
detection lists provided by the authors
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Results on the INT data set
• Found 79 % of our LSB candidates are confirmed by Sabatini et al.23 (Markov+detectLSB) compared to 29 (Sabatini)
• Found new LSB candidates
• The validation on the full data set is still an on-going collaboration effort with S. Sabatini and W. van Driel.
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Conclusion
• The Markovian segmentation approach allows to study LSB candidates even in a noisy environment
• Multiband image analysis is beeing currently applied to the same data set in B band completed with I band
• First tests are promising
• Such a procedure has been implemented in the AIDA image processing workflow project at CDS
(Cf Schaaff et al, this conference)
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2 detections for the same object
