Optically Selected Superthin Galaxies Remain Thin in the Near-infrared

Abstract

Abstract We investigate whether galaxies identified as superthin in optical images remain superthin in the near-infrared (NIR), and how their extreme disk morphology is related to environment. From a nearby volume-limited sample, we select 210 superthin galaxies using two-dimensional bulge/disk decomposition of SDSS r-band images, requiring the disk component to have a minor-to-major axis ratio b/a < 1/9. We measure disk shapes from SDSS griz to UKIDSS JHK bands. Both the major- and minor-axis scales decrease from the optical to the NIR, reaching ∼0.6 of their r-band values in the K band, but the disk axis ratio remains nearly unchanged. Thus, optically selected superthin galaxies remain superthin in the NIR, suggesting that any NIR-dominant thick component, if present, is not prominent enough to alter the fitted disk shape. Reanalysis of our sample and a previous superthin sample shows that earlier reported NIR thickening is mainly due to a magnitude- and band-dependent bias in one-dimensional fitting. We further compare their environments with matched control samples using projected cross-correlations, reconstructed local overdensities, and large-scale-structure classifications. Superthin galaxies show lower clustering on ∼0.1–1 h−1 Mpc scales and lower overdensities at 1 h−1 Mpc, but similar large-scale clustering and no clear residual dependence on large-scale-structure type. These results are consistent with a higher central-galaxy fraction and lower satellite fraction in the superthin sample, while the similar large-scale clustering points to comparable typical halo masses relative to the controls. This halo-occupation interpretation is qualitatively consistent with simulation-based scenarios in which high host-halo spin helps build extended thin disks, although the present data do not directly measure halo spin.