NGC 3998 - SA(r)0o

NGC 3998

De Vaucouleurs Atlas Type: SA(r)0o

Filter: B

Telescope: UH 2.2-m
North up , East left
Field Dimensions: 4.6 x 3.5 arcminutes

RC3 Type: SA(r?)0o
RSA Type: S01(3)
Surface Brightness Range Displayed: 16.0-26.0 mag per square arcsec
Absolute Blue Magnitude: -19.3

De Vaucouleurs Atlas Description:

This galaxy shows the clear multi-zone structure of an S0. It has an inner lens -like region, and a very faint but definite ring within its disk. The ring was apparently also noticed by de Vaucouleurs, but it is very diffuse and has low contrast compared to what is seen in NGC 1553 . We are also uncertain as to whether this feature is really an ``inner ring " and not an ``outer ring ." We have adopted the S0o classification because the structure is subtle and the galaxy is clearly an S0.

The color index map reveals a peculiar dust distribution south of the center (core colors are not reliable). The disorganized nature of the dust suggests that it has been acquired from an external source. This is supported by the HI observations of Knapp, van Driel, and van Woerden (1985), who detected an inclined HI ring approximately in the position of these dusty features. These authors interpret the HI as being associated with a polar ring inclined 80o to the optical major axis. Polar rings are believed to represent small companion galaxies that have been disrupted into a polar orbit around an S0. Welch, Welch, and Dupuy (1991) attempted to detect the dust associated with the HI but were unsuccessful.