J/A+A/709/A21 Morphology in DECaLS galaxies (Sampaio+, 2026)
Morphologies for DECaLS galaxies through a combination of nonparametric indices
and machine learning methods. A comprehensive catalog using the
Galaxy Morphology Extractor (galmex) code.
Sampaio V.M., Jaffe Y., Lima-Dias C., Veliz Astudillo S., Martinez-Marin M.,
Mendez-Hernandez H., Herrera-Camus R., Monachesi A.
<Astron. Astrophys. 709, A21 (2026)>
=2026A&A...709A..21S 2026A&A...709A..21S (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, optical; Galaxy catalogs; Morphology; Photometry
Keywords: galaxies: elliptical and lenticular, cD - galaxies: general -
galaxies: spiral - galaxies: structure
Abstract:
Galaxy morphology encodes key information about formation and
evolution. Large imaging surveys require automated, reproducible
methods beyond visual inspection. Nonparametric indices provide a
useful framework, but their performance must be quantitatively
assessed. We present a homogeneous catalog of nonparametric
morphological indices for DECaLS galaxies with effective radii larger
than 2 arcsec. Our goal is to evaluate the reliability of indices in
separating spirals and ellipticals, test their consistency with
existing classification schemes, and establish their applicability for
the upcoming surveys focused on the southern hemisphere. We developed
galmex, a modular Python package for preprocessing images and
measuring a variety of nonparametric indices. Using bona fide spirals
and ellipticals as control samples, we assessed the discriminatory
power of each index, and compared them with CNN-based T-Types and
Galaxy Zoo DECaLS labels. We used the indices as input for a light
gradient boosting machine (LightGBM) to obtain probabilistic
classifications. Concentration is the most reliable parameter from the
concentration and asymmetry and smoothness system (CAS), while
asymmetry-based indices (A and S) are limited to detecting disturbed
morphologies. MEGG indices (M20, Entropy, Gini, G2) provide stronger
separation and trace a gradient with T-Type. By using a simple binary
(0 or 1) label for ellipticals and spirals, classifiers trained on
nonparametric indices achieve high accuracy and well-calibrated
probabilities, dominated by entropy, concentration, and Gini. We
release the first public catalog of CA[AS]S+MEGG indices for DECaLS,
together with galmex. We combine the nonparametric indices with
machine learning framework to derive spiral and elliptical separation
for galaxies below z∼0.15 through a probabilistic approach.
Description:
Measurement of non-parametric indexes for three subset of galaxies:
(1) Galaxy Zoo 1 Spiral galaxies that are also present in the Galaxy
Zoo DECaLS;
(2) GZ 1 Elliptical galaxies that are also present in the Galaxy Zoo
DECaLS;
(3) 1,7 million galaxies from the Legacy Survey; All galaxies are in
the local universe (z < 0.15), and with r-band magnitude brighter than
18.5. For each galaxy, RA, DEC, Concentration, Asymmetry, Smoothness,
M20, Shannon Entropy, Gini index, and Gradient Pattern Asymmetry is
given. All the provided measurements are done using the galmex
package.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
elliptic.dat 248 9395 Galaxy Zoo 1 Spiral galaxies that are also
present in the Galaxy Zoo DECaLS
spirals.dat 248 59439 GZ 1 Elliptical galaxies that are also present
in the Galaxy Zoo DECaLS
full.dat 235 1744454 1.7 million galaxies from the Legacy Survey
--------------------------------------------------------------------------------
Byte-by-byte Description of file: elliptic.dat spirals.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 A19 --- IAUName IAU designation of the galaxy (iauname)
21- 35 F15.11 deg RAdeg Right ascension (J2000) of the
galaxy (ra)
37- 51 F15.11 deg DEdeg Declination (J2000) of the galaxy (dec)
53- 61 F9.5 pix a Semi-major axis of the object (a)
63- 70 F8.5 pix b Semi-minor axis of the object (b)
72- 79 F8.5 rad theta Position angle of the object (theta)
81- 87 F7.5 --- C Concentration index (C)
89- 96 F8.5 arcsec r20 Radius containing 20% of the flux,
from elliptical apertures (r20)
98-106 F9.5 arcsec r80 Radius containing 80% of the flux,
from elliptical apertures (r80)
108-113 I6 --- Npix Number of pixels in the segmentation
mask (Npix_segmmask)
115-124 F10.8 --- A ?=- Asymmetry index (A)
126-136 F11.8 --- S ?=- Smoothness index (S)
138-148 F11.8 --- M20 ?=- M20 index (M20)
150-159 F10.8 --- E ?=- Shannon entropy index (E)
161-171 F11.8 --- Gini ?=- Gini index (Gini)
173-183 F11.8 --- G2 ?=- Gradient pattern asymmetry
index ( G2)
185 I1 --- MainGalFlag [0] Flag set to 1 if no object is
detected at the image center
(maingalaxy_flag)
187-195 F9.5 pix rFlagPix Radius of the circular aperture used in
flagging (1.5 Rp) (rflag_pixels)
197 I1 --- EdgeFlag [0/1] Flag set to 1 if zero- or
NaN-valued pixels are present within the
flagging area (edge_flag)
199-201 I3 --- Nrcheck Number of secondary objects within the
flagging area (N_rcheck)
203 I1 --- NsecFlag [0/1] Flag set to 1 if more than 4
objects are found within the flagging
area (Nsec_flag)
205-206 I2 --- NdeltaMAG Number of objects with magnitude
difference smaller than 1 (N_deltaMAG)
208-218 F11.9 mag minMagDiff ?=- Magnitude difference to the brightest
secondary object (minMAG_diff)
220-229 F10.5 pix distminMagdiff ?=- Distance to the brightest secondary
object (distminMAGdiff)
231 I1 --- BrightObjFlag [0/1] Flag set to 1 if a bright object is
present within the flagging radius
(BrightObj_flag)
233-241 F9.7 --- nDistClosest ?=- Distance to the closest secondary
object, normalized by Rp
(normDist_closest)
243 I1 --- Status [0/2] Processing status flag (status) (1)
245 I1 --- ErrCode [0] Error code indicating why processing
failed (err_code)
247-248 I2 --- WarnMask Warning bitmask encoding non-fatal
processing issues (warn_mask) (2)
--------------------------------------------------------------------------------
Note (1): Status as follows:
0 = full sucess
1 = not sucess
2 = sucess with warnings
Note (2): Warning bitmask:
0 = No warnings (special value)
4 = Conselice asymmetry computation failed
8 = Ferrari asymmetry computation failed
12 = Conselice and Ferrari asymmetry computation failed
--------------------------------------------------------------------------------
Byte-by-byte Description of file: full.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 15 F15.11 deg RAdeg ?=- Right ascension (J2000) of the
galaxy (ra)
17- 31 F15.11 deg DEdeg ?=- Declination (J2000) of the
galaxy (dec)
33- 41 F9.5 pix a ?=- Semi-major axis of the object (a)
43- 51 F9.5 pix b ?=- Semi-minor axis of the object (b)
53- 60 F8.5 rad theta ?=- Position angle of the object (theta)
62- 69 F8.5 --- C ?=- Concentration index (C)
71- 79 F9.5 arcsec r20 ?=- Radius containing 20% of the flux,
from elliptical apertures (r20)
81- 89 F9.5 arcsec r80 ?=- Radius containing 80% of the flux,
from elliptical apertures (r80)
91- 96 I6 --- Npix ?=- Number of pixels in the segmentation
mask (Npix_segmmask)
98-107 F10.8 --- A ?=- Asymmetry index (A)
109-119 F11.8 --- S ?=- Smoothness index (S)
121-131 F11.8 --- M20 ?=- M20 index (M20)
133-142 F10.8 --- E ?=- Shannon entropy index (E)
144-154 F11.8 --- Gini ?=- Gini index (Gini)
156-166 F11.8 --- G2 ?=- Gradient pattern asymmetry
index ( G2)
168 I1 --- MainGalFlag [0]? Flag set to 1 if no object is
detected at the image center
(maingalaxy_flag)
170-179 F10.5 pix rFlagPix ?=- Radius of the circular aperture used
in flagging (1.5 Rp) (rflag_pixels)
181 I1 --- EdgeFlag [0/1]?=- Flag set to 1 if zero- or
NaN-valued pixels are present within the
flagging area (edge_flag)
183-185 I3 --- Nrcheck ?=- Number of secondary objects within
the flagging area (N_rcheck)
187 I1 --- NsecFlag [0/1]?=- Flag set to 1 if more than 4
objects are found within the flagging
area (Nsec_flag)
189-191 I3 --- NdeltaMAG ?=- Number of objects with magnitude
difference smaller than 1 (N_deltaMAG)
193-203 F11.9 mag minMagDiff ?=- Magnitude difference to the brightest
secondary object (minMAG_diff)
205-214 F10.5 pix distminMagdiff ?=- Distance to the brightest secondary
object (distminMAGdiff)
216 I1 --- BrightObjFlag [0/1]?=- Flag set to 1 if a bright object
is present within the flagging radius
(BrightObj_flag)
218-226 F9.7 --- nDistClosest ?=- Distance to the closest secondary
object, normalized by Rp
(normDist_closest)
228 I1 --- Status [0/2]?=- Processing status flag
(status) (1)
230-231 I2 --- ErrCode ?=- Error code indicating why processing
failed (err_code)
233-235 I3 --- WarnMask ?=- Warning bitmask encoding non-fatal
processing issues (warn_mask) (2)
--------------------------------------------------------------------------------
Note (1): Status as follows:
0 = full sucess
1 = not sucess
2 = sucess with warnings
Note (2): Warning bitmask:
0 = No warnings (special value)
1 = Flagging step failed (FlaggingHandler.flag_objects(...) raised)
4 = Conselice asymmetry computation failed
8 = Ferrari asymmetry computation failed
--------------------------------------------------------------------------------
Acknowledgements:
Vitor Sampaio, vitorms999(at)gmail.com
License: CC-BY-4.0 [see https://spdx.org/licenses/]
(End) Patricia Vannier [CDS] 16-Apr-2026