J/ApJ/781/12 Morphological parameters of galaxies from Spitzer (Holwerda+, 2014)
Morphological parameters of a Spitzer survey of stellar structure in galaxies.
Holwerda B.W., Munoz-Mateos J.-C., Comeron S., Meidt S., Sheth K.,
Laine S., Hinz J.L., Regan M.W., Gil de Paz A., Menendez-Delmestre K.,
Seibert M., Kim T., Mizusawa T., Laurikainen E., Salo H., Laine J.,
Gadotti D.A., Zaritsky D., Erroz-Ferrer S., Ho L.C., Knapen J.H.,
Athanassoula E., Bosma A., Pirzkal N.
<Astrophys. J., 781, 12 (2014)>
=2014ApJ...781...12H 2014ApJ...781...12H (SIMBAD/NED BibCode)
ADC_Keywords: Galaxy catalogs ; Morphology
Keywords: galaxies: elliptical and lenticular, cD - galaxies: general -
galaxies: irregular - galaxies: spiral - galaxies: statistics -
galaxies: stellar content - galaxies: structure
Abstract:
The morphology of galaxies can be quantified to some degree using a
set of scale-invariant parameters. Concentration (C), asymmetry (A),
smoothness (S), the Gini index (G), the relative contribution of the
brightest pixels to the second-order moment of the flux (M20),
ellipticity (E), and the Gini index of the second-order moment (GM)
have all been applied to morphologically classify galaxies at various
wavelengths. Here, we present a catalog of these parameters for the
Spitzer Survey of stellar structure in Galaxies, a volume-limited,
near-infrared (NIR) imaging survey of nearby galaxies using the 3.6
and 4.5µm channels of the Infrared Array Camera on board the
Spitzer Space Telescope. Our goal is to provide a reference catalog of
NIR quantified morphology for high-redshift studies and galaxy
evolution models with enough detail to resolve stellar mass
morphology. We explore where normal, non-interacting galaxies--those
typically found on the Hubble tuning fork--lie in this parameter space
and show that there is a tight relation between concentration (C82)
and M20 for normal galaxies. M20 can be used to classify galaxies
into earlier and later types (i.e., to separate spirals from
irregulars). Several criteria using these parameters exist to select
systems with a disturbed morphology, i.e., those that appear to be
undergoing a tidal interaction. We examine the applicability of these
criteria to Spitzer NIR imaging. We find that four relations, based on
the parameters A and S, G and M20, GM, C, and M20, respectively,
select outliers in morphological parameter space, but each selects
different subsets of galaxies. Two criteria
(GM>0.6,G>-0.115xM20+0.384) seem most appropriate to identify
possible mergers and the merger fraction in NIR surveys. We find no
strong relation between lopsidedness and most of these morphological
parameters, except for a weak dependence of lopsidedness on
concentration and M20.
Description:
The Spitzer Survey of stellar structure in Galaxies (S4G; Sheth et
al. 2010, cat. J/PASP/122/1397; http://www.cv.nrao.edu/~ksheth/s4g) is
a volume-, magnitude-, and size-limited
(D<40Mpc,|b|>30°,mBcorr<15.5,D25>1') survey of 2349 nearby
galaxies in 3.6µm and 4.5µm (IRAC channels 1 and 2) of the IRAC
of the Spitzer Space Telescope, using both archival cryogenic and
ongoing warm-mission observations (for a full description and
selection criteria, see Sheth et al. 2010, cat. J/PASP/122/1397). All
images have been reprocessed by the S4G pipeline. The reprocessed
pixel scale is 0.75''; the resolution is 1.7'' for 3.6µm and 1.6''
for 4.5µm. The data have been made public
(http://irsa.ipac.caltech.edu/data/SPITZER/S4G/).
For this paper, we use the first and second pipeline products (P1 and
P2) of S4G (M. W. Regan et al., in preparation) available from DR1
(2013 January) for 2349 galaxies: the photometry images (phot) from P1
and foreground and background object masks from P2 for both the 3.6
and 4.5µm images (see for more details Sheth et al. 2010, cat.
J/PASP/122/1397). Our morphological parameters are in concert with the
final S4G data products (J.-C. Munoz-Mateos et al., in preparation).
The Tables A1 and A2 present the full catalog of morphological
parameters for the S4G sample of galaxies for the 3.6 and 4.5µm.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 81 2345 The morphological parameters at 3.6µm for
the 2349 S4G galaxies
tablea2.dat 81 2345 The morphological parameters at 4.5µm for
the 2349 S4G galaxies
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See also:
VII/237 : HYPERLEDA. I. Catalog of galaxies (Paturel+, 2003)
J/MNRAS/416/2415 : Morphological parameters of WHISP galaxies (Holwerda+, 2011)
J/PASP/122/1397 : Spitzer Survey of Galaxies Stellar Structure (Sheth+, 2010)
J/AJ/128/163 : Galaxy morphological classification (Lotz+, 2004)
J/ApJS/147/1 : Classification of nearby galaxies (Conselice+, 2003)
J/ApJ/588/218 : i*g* photometry of SDSS EDR galaxies (Abraham+, 2003)
http://www.cv.nrao.edu/~ksheth/s4g : S4G survey
Byte-by-byte Description of file: tablea[12].dat
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Bytes Format Units Label Explanations
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1- 10 A10 --- Name Galaxy identifier (1)
12- 15 F4.2 --- Gini [0/1] The Gini index (indicator of equality:
1=all the flux is in one pixel, 0=all the
pixels in the object have equal values) (2)
17- 20 F4.2 --- e_Gini [0/10]? The uncertainty in Gini
22- 26 F5.2 --- M20 [-4.7/-0.06] Relative contribution of brightest
pixels to 2nd order moment of flux (M20) (3)
28- 31 F4.2 --- e_M20 [0/10] The uncertainty in M20
33- 36 F4.2 --- C82 [0/9.2] The concentration index (C82) (4)
38- 41 F4.2 --- e_C82 [0/1.9] The uncertainty in C82
43- 46 F4.2 --- A [0.07/1] The asymmetry parameter (5)
48- 51 F4.2 --- e_A [0/5]? The uncertainty in A
53- 56 F4.2 --- S [0.04/1.7] The smoothness parameter (6)
58- 61 F4.2 --- e_S [0/7]? The uncertainty in S
63- 66 F4.2 --- Ell [0/1] The ellipticity parameter (7)
68- 71 F4.2 --- e_Ell [0/0.25]? The uncertainty in Ell
73- 76 F4.2 --- GM [0.2/1] The Gini index of 2nd order moment
(GM) (8)
78- 81 F4.2 --- e_GM [0/10]? The uncertainty in GM
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Note (1): There are only 2345 objects in the tables because the code crashed
when calculating the parameters for 4 objects. We use the
concentration-asymmetry-smoothness (CAS) system from Bershady et al.
(2000AJ....119.2645B 2000AJ....119.2645B), Conselice et al. (2000ApJ...529..886C 2000ApJ...529..886C), and
Conselice 2003 (cat. J/ApJS/147/1), the Gini and M20 system from Lotz et
al. 2004 (cat. J/AJ/128/163), and a hybrid parameter GM, the Gini
parameter of the second-order moment (Holwerda et al.,
2011MNRAS.416.2426H 2011MNRAS.416.2426H).
Note (2): The Gini parameter is an economic indicator of equality (G=1 if all
the flux is in one pixel and G=0 if all the pixels in the object have equal
values). We use the implementation from Abraham et al. 2003 (cat.
J/ApJ/588/218) and Lotz et al. 2004 (cat. J/AJ/128/163):
G = [1/n(n-1)]∑i(2i-n-1)|Ii| (Eq.(4) in the paper), where
Ii is the intensity of pixel i in an increasing flux-ordered list of the
n pixels in the object and is the mean pixel intensity. B. W. Holwerda
et al. (in preparation) find a weak link between Gini and current star
formation.
Note (3): The relative second-order moment of the brightest 20% of the flux:
M20 = log(∑kiMi/Mtot), for which ∑kiIi<0.2Itot is
true (Eq.(6) in the paper), where pixel K marks the top 20% point in the
flux-ordered pixel list. The M20 parameter is a parameter that is
sensitive to bright structure away from the center of the galaxy; the flux
is weighted in favor of the outer parts. It therefore is relatively
sensitive to tidal structures (provided of course that these are included
in the calculation), specifically star-forming regions formed in the outer
spiral or tidal arms. If no such structures are in the image, the 20%
brightest pixels will most likely be concentrated in the center of the
galaxy, which is weighted lower. Thus, one can expect low values of M20
for smooth galaxies with bright nuclei (ellipticals, S0, or Sa) but much
higher values (less negative) for galaxies with extended arms featuring
bright HII regions.
Note (4): The log of the ratio of the radii including 80 over 20% of the flux.
Concentration is defined as Kent (1985ApJS...59..115K 1985ApJS...59..115K):
C82 = 5log(r80/r20) (Eq.(1) in the paper), where r% is the radius
of the circular aperture that includes that percentage of the total light
of the object.
Note (5): In an image with n pixels with intensities I(i,j) at pixel positions
(i,j), in which the value of the pixel is I180(i,j) in the image rotated
by 180°, asymmetry is defined as (Schade et al., 1995ApJ...451L...1S 1995ApJ...451L...1S;
Conselice 2003, cat. J/ApJS/147/1):
A = ∑i,j|I(i,j)-I180(i,j)|/2∑i,j|I(i,j)| (Eq.(2) in the
paper).
Note (6): Smoothness (also called clumpiness in the original Conselice 2003,
cat. J/ApJS/147/1) is defined as:
S = ∑i,j|I(i,j)-IS(i,j)|/∑i,j|I(i,j)| (Eq.(3) in the paper),
where IS(i,j) is the same pixel in the image after smoothing with a
choice of kernel.
Note (7): Scarlata et al. (2007ApJS..172..406S 2007ApJS..172..406S) added the ellipticity of a
galaxy's image to the mix of parameters in order to classify galaxies
according to type in the COSMOS field. Ellipticity is defined as:
E = 1-b/a (Eq.(8) in the paper), where a and b are the major and minor axes
of the galaxy, respectively, computed from the spatial second-order moments
of the light along the x- and y-axes of the image in the same manner as
SExtractor. We include this definition for completeness.
Note (8): Instead of the intensity of the pixel (Ii), one can use the
second-order moment of the pixel (Mi=Ii[(xi-xc)2+(yi-yc)2])
in Eq.(4). This is the GM parameter (Holwerda et al.,
2011MNRAS.416.2426H 2011MNRAS.416.2426H): GM = [1/n(n-1)]∑i(2i-n-1)|Mi| (Eq.(7) in
the paper), which is an indication of the spread of pixel values weighted
with the projected radial distance to the galaxy center. In essence, this
is the Gini parameter with a different weighting scheme than unity for each
pixel. Similar to the M20 parameter, it emphasizes the flux from the
outer regions of the galaxy. If there is significant flux in the outer
parts, this will boost the value of GM. Contrary to M20, it does not
depend on a somewhat arbitrary delineation of the brightest 20% flux for
the denominator but relies on all pixel values. Unlike the Gini parameter,
however, it does rely on a supplied center of the galaxy (to compute Mi).
For concentrated galaxies, the GM and Gini values will be close together,
but as relatively more flux is evident in the outer parts of the galaxy,
GM will be higher. Holwerda et al. 2011 (cat. J/MNRAS/416/2415) found
GM to be a good single parameter to identify active mergers (sweeping
tidal tails, etc.) from atomic hydrogen maps (HI).
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History:
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(End) Prepared by [AAS]; Sylvain Guehenneux [CDS] 12-Nov-2015