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. =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 (M_20_), ellipticity (E), and the Gini index of the second-order moment (G_M_) 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 4.5{mu}m 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 (C_82_) and M_20_ for normal galaxies. M_20_ 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 M_20_, G_M_, C, and M_20_, respectively, select outliers in morphological parameter space, but each selects different subsets of galaxies. Two criteria (G_M_>0.6,G>-0.115xM_20_+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 M_20_. Description: The Spitzer Survey of stellar structure in Galaxies (S^4^G; 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{deg},m_Bcorr_<15.5,D_25_>1') survey of 2349 nearby galaxies in 3.6{mu}m and 4.5{mu}m (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 S^4^G pipeline. The reprocessed pixel scale is 0.75''; the resolution is 1.7'' for 3.6{mu}m and 1.6'' for 4.5{mu}m. 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 S^4^G (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 4.5{mu}m images (see for more details Sheth et al. 2010, cat. J/PASP/122/1397). Our morphological parameters are in concert with the final S^4^G data products (J.-C. Munoz-Mateos et al., in preparation). The Tables A1 and A2 present the full catalog of morphological parameters for the S^4^G sample of galaxies for the 3.6 and 4.5{mu}m. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file tablea1.dat 81 2345 The morphological parameters at 3.6{mu}m for the 2349 S^4^G galaxies tablea2.dat 81 2345 The morphological parameters at 4.5{mu}m for the 2349 S^4^G galaxies -------------------------------------------------------------------------------- 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 : S^4^G survey Byte-by-byte Description of file: tablea[12].dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 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 (M_20_) (3) 28- 31 F4.2 --- e_M20 [0/10] The uncertainty in M20 33- 36 F4.2 --- C82 [0/9.2] The concentration index (C_82_) (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 (G_M_) (8) 78- 81 F4.2 --- e_GM [0/10]? The uncertainty in GM -------------------------------------------------------------------------------- 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), Conselice et al. (2000ApJ...529..886C), and Conselice 2003 (cat. J/ApJS/147/1), the Gini and M_20_ system from Lotz et al. 2004 (cat. J/AJ/128/163), and a hybrid parameter G_M_, the Gini parameter of the second-order moment (Holwerda et al., 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)]{sum}_i_(2i-n-1)|I_i_| (Eq.(4) in the paper), where I_i_ 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: M_20_ = log({sum}^k^_i_M_i_/M_tot_), for which {sum}^k^_i_I_i_<0.2I_tot_ is true (Eq.(6) in the paper), where pixel K marks the top 20% point in the flux-ordered pixel list. The M_20_ 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 M_20_ 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): C_82_ = 5log(r_80_/r_20_) (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 I_180_(i,j) in the image rotated by 180{deg}, asymmetry is defined as (Schade et al., 1995ApJ...451L...1S; Conselice 2003, cat. J/ApJS/147/1): A = {sum}_i,j_|I(i,j)-I_180_(i,j)|/2{sum}_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 = {sum}_i,j_|I(i,j)-I_S_(i,j)|/{sum}_i,j_|I(i,j)| (Eq.(3) in the paper), where I_S_(i,j) is the same pixel in the image after smoothing with a choice of kernel. Note (7): Scarlata et al. (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 (I_i_), one can use the second-order moment of the pixel (M_i_=I_i_[(x_i_-x_c_)^2^+(y_i_-y_c_)^2^]) in Eq.(4). This is the G_M_ parameter (Holwerda et al., 2011MNRAS.416.2426H): G_M_ = [1/n(n-1)]{sum}_i_(2i-n-1)|M_i_| (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 M_20_ 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 G_M_. Contrary to M_20_, 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 M_i_). For concentrated galaxies, the G_M_ and Gini values will be close together, but as relatively more flux is evident in the outer parts of the galaxy, G_M_ will be higher. Holwerda et al. 2011 (cat. J/MNRAS/416/2415) found G_M_ to be a good single parameter to identify active mergers (sweeping tidal tails, etc.) from atomic hydrogen maps (HI). -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Prepared by [AAS]; Sylvain Guehenneux [CDS] 12-Nov-2015