Measuring shapes of galaxy images - II. Morphology of 2MASS galaxies

We study a sample of 112 galaxies of various Hubble types imaged in the Two Micron All Sky Survey (2MASS) in the near-infrared (NIR; 1-2 mum) J, H and K-s bands. The sample contains (optically classified) 32 ellipticals, 16 lenticulars and 64 spirals acquired from the 2MASS Extended Source Catalogue (XSC). We use a set of non-parametric shape measures constructed from the Minkowski functionals (MFs) for galaxy shape analysis. We use ellipticity (epsilon) and orientation angle (Phi) as shape diagnostics. With these parameters as functions of area within the isophotal contour, we note that the NIR elliptical galaxies with epsilon > 0.2 show a trend of being centrally spherical and increasingly flattened towards the edge, a trend similar to images in optical wavelengths. The highly flattened elliptical galaxies show strong change in ellipticity between the centre and the edge. The lenticular galaxies show morphological properties resembling either ellipticals or disc galaxies. Our analysis shows that almost half of the spiral galaxies appear to have bar-like features while the rest are likely to be non-barred. Our results also indicate that almost one-third of spiral galaxies have optically hidden bars. The isophotal twist noted in the orientations of elliptical galaxies decreases with the flattening of these galaxies, indicating that twist and flattening are also anticorrelated in the NIR, as found in optical wavelengths. The orientations of NIR lenticular and spiral galaxies show a wide range of twists.