De Vaucouleurs Atlas Type: SA(r)a
Telescope: UH 2.2-m
North left , East down
Field Dimensions: 2.3 x 1.7 arcminutes
RC3 Type: SA(r)0+
RSA Type: Sa(r)
Surface Brightness Range Displayed: 17.5-26.0 mag per square arcsec
Absolute Blue Magnitude: -18.6
De Vaucouleurs Atlas Description:
The RC3 classification of SA(r)0+ is too early; the galaxy is clearly an early-type spiral in the Atlas image. The inner ring is a weak but well-defined feature. It is unusual in that dust appears to affect one half of its major axis, something which will not necessarily be an artifact of tilt. Scattered dust patches lie inside the ring region and at least one cuts into the center. Outside the ring, the disk is dominated by flocculent spiral structure.
NGC 4138 was discovered by Jore et al. (1996) to have a disk of ionized and atomic gas, as well as stars, rotating counter to a primary disk comprised of 80\% of the galaxy's stars. Jore et al. consider that the origin of the counter-rotating material is either a dwarf galaxy merger or gas accretion from an outside reservoir. They state that counter-rotating material is found throughout the galaxy, but peaks around the radius of the inner ring .