J/MNRAS/491/3767 Automatic detection of ring galaxies in SDSS (Shamir+, 2020)
Automatic detection of full ring galaxy candidates in SDSS.
Shamir L.
<Mon. Not. R. Astron. Soc., 491, 3767-3777 (2020)>
=2020MNRAS.491.3767S 2020MNRAS.491.3767S (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, optical ; Galaxy catalogs
Keywords: methods: data analysis - techniques: image processing - catalogues -
galaxies: peculiar
Abstract:
A full ring is a form of galaxy morphology that is not associated with
a specific stage on the Hubble sequence. Digital sky surveys can
collect many millions of galaxy images, and therefore even rare forms
of galaxies are expected to be present in relatively large numbers in
image data bases created by digital sky surveys. Sloan Digital Sky
Survey (SDSS) data release (DR) 14 contains ∼2.6x106 objects with
spectra identified as galaxies. The method described in this paper
applied automatic detection to identify a set of 443 ring galaxy
candidates, 104 of them were already included in the Buta
(2017MNRAS.471.4027B 2017MNRAS.471.4027B, Cat. J/MNRAS/471/4027) catalogue of ring
galaxies in SDSS, but the majority of the galaxies are not included in
previous catalogues. Machine analysis cannot yet match the superior
pattern recognition abilities of the human brain, and even a small
false positive rate makes automatic analysis impractical when scanning
through millions of galaxies. Reducing the false positive rate also
increases the true negative rate, and therefore the catalogue of ring
galaxy candidates is not exhaustive. However, due to its clear
advantage in speed, it can provide a large collection of galaxies that
can be used for follow-up observations of objects with ring
morphology.
Description:
The data source used in this study is the set of galaxies with spectra
in SDSS DR14. The image of each galaxy was obtained by using the
cut-out service of SDSS as was done in Kuminski & Shamir
(2016ApJS..223...20K 2016ApJS..223...20K, Cat. J/ApJS/223/20). In summary, the images are
downloaded as 120x120 JPG colour images. Since galaxies have different
sizes, each galaxy was downloaded several times until 25 per cent or
less of the pixels on the edges of the image have grey value of less
than 125. The initial scale was set to 0.25arcsec per pixel, and it
increased by 0.05arcsec until 25 per cent or less of the pixels on the
edges are not bright, which means that the galaxy fits inside the
image (Kuminski & Shamir 2016ApJS..223...20K 2016ApJS..223...20K, Cat. J/ApJS/223/20).
Downloading that large data set of galaxy images required ∼16d. The
image analysis method is similar to the method used in Timmis & Shamir
(2017ApJS..231....2T 2017ApJS..231....2T). Each image was converted to a binary map such
that all pixels above the threshold were set to one, and the pixels
below the threshold were set to zero. The initial threshold was set to
50, and increased by five until it reached 200.
For each threshold, the image is inverted, and a four-connected
labelling algorithm is applied to label all objects in the inverted
image. If more than one object is detected, it means that the image
contained background areas that are inside foreground objects, and
therefore could be rings. Since a galaxy can contain many small areas
inside the arms, if the size of the background area is less than 10
per cent of the foreground galaxy the algorithm ignores that
background area and does not consider it as a ring candidate. The
algorithm is implemented as part of the G analyser galaxy image
analysis tool (Shamir 2011ApJ...736..141S 2011ApJ...736..141S, 2011ascl.soft05011S).
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 22 443 Ring galaxy candidates identified automatically
table2.dat 56 55 Galaxies that are part of previous catalogues
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See also:
J/MNRAS/471/4027 : CVRHS classifications for the GZ2 Ring Sample (Buta, 2017)
J/ApJS/223/20 : SDSS-DR8 galaxies classified by WND-CHARM (Kuminski+, 2016)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID [1/443] Internal object identifier
5- 13 F9.5 deg RAdeg Right ascension (J2000)
15- 22 F8.4 deg DEdeg Declination (J2000)
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 3 I3 --- ID [4/440] Internal object identifier
5- 18 A14 --- Name Galaxy name
20- 56 A37 --- ref Reference of previous studies
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 20-Feb-2023