Radio Galaxy Zoo Talk

A Classical Morphological Analysis of Galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S4G)

  • JeanTate by JeanTate

    Buta+ (2015) may not, at first glance, seem all that relevant to RGZ:

    The Spitzer Survey of Stellar Structure in Galaxies (S4G) is the largest available database of deep, homogeneous middle-infrared (mid-IR) images of galaxies of all types. The survey, which includes 2352 nearby galaxies, reveals galaxy morphology only minimally affected by interstellar extinction. This paper presents an atlas and classifications of S4G galaxies in the Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. The CVRHS system follows the precepts of classical de Vaucouleurs (1959) morphology, modified to include recognition of other features such as inner, outer, and nuclear lenses, nuclear rings, bars, and disks, spheroidal galaxies, X patterns and box/peanut structures, OLR subclass outer rings and pseudorings, bar ansae and barlenses, parallel sequence late-types, thick disks, and embedded disks in 3D early-type systems. We show that our CVRHS classifications are internally consistent, and that nearly half of the S4G sample consists of extreme late-type systems (mostly bulgeless, pure disk galaxies) in the range Scd-Im. The most common family classification for mid-IR types S0/a to Sc is SA while that for types Scd to Sm is SB. The bars in these two type domains are very different in mid-IR structure and morphology. This paper examines the bar, ring, and type classification fractions in the sample, and also includes several montages of images highlighting the various kinds of "stellar structures" seen in mid-IR galaxy morphology.

    However, buried in Section 3.3.4. (Other Comparisons), is this gem:

    Our general conclusion from all of the comparisons described in this section is that CVRHS classifications have both good internal and external consistency. This consistency could, however, still mask resolution and especially inclination biases in the classifications, even if there were perfect agreement between phases or between different sources. Campbell et al. (2014) describe “imaging classification bias” in the context of the GalaxyZoo citizen
    morphological catalogue, which they argue has excessive numbers of distant early-type galaxies that are likely to be misclassified spirals. As the authors correctly note, the ability to make reliable distinctions between galaxy types demands “the most stringent imaging requirements.” Although S4G images are of much greater depth than groundbased near-IR images, the limited resolution coupled with high inclination could introduce some “imaging classification bias.” This is explored further in the next sections.

    Campbell+ (2014) is "The 6dF Galaxy Survey: Fundamental Plane Data":

    We report the 6dFGS Fundamental Plane (6dFGSv) catalogue that is used to estimate distances and peculiar velocities for nearly 9 000 early-type galaxies in the local (z<0.055) universe. Velocity dispersions are derived by cross-correlation from 6dF V-band spectra with typical S/N of 12.9 A˚−1 for a sample of 11 315 galaxies; the median velocity dispersion is 163 km s−1 and the median measurement error is 12.9%. The photometric Fundamental Plane (FP) parameters (effective radii and surface brightnesses) are determined from the JHK 2MASS images for 11 102 galaxies. Comparison of the independent J- and K-band measurements implies that the average uncertainty in XF P , the combined photometric parameter that enters the FP, is 0.013 dex (3%) for each band. Visual classification of morphologies was used to select a sample of nearly 9 000 early-type galaxies that form 6dFGSv. This catalogue
    has been used to study the effects of stellar populations on galaxy scaling relations, to investigate the variation of the FP with environment and galaxy morphology, to explore trends in stellar populations through, along and across the FP, and to map and analyse the local peculiar velocity field.

    Here's an interesting para, in Section 4.2 (Results):

    However, Figure 22 also shows a tendency towards early typing by Galaxy Zoo relative to the 6dFGSv classes, in that a greater number of galaxies lie below the 1:1 line. We contend that this was due to the redder optical passbands used by Galaxy Zoo, and their reliance on composite images rather than examination
    of separate images in each passband. Morphological typing of 6dFGSv galaxies was done using multiple independent optical images (bJrF), with supporting near-infrared JHK frames, and a colour near-infrared composite. In contrast, Galaxy Zoo classifications used only a single-colour composite image made from optical gri passbands. Since none of these sample the galaxy light shortward of λ ~< 4000 A˚, the blue light of the OB stars that dominate the arms of spiral galaxies is under-represented in the composite frames. Therefore, galaxies with fainter spiral features tend to be classified as S0s (predominantly on the basis of their bulge).

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  • ivywong by ivywong scientist, admin

    Thanks for the highlights!

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