De Vaucouleurs Atlas Type: (R')SB(rs)m
Telescope: Pal 1.5-m
North up , East left
Field Dimensions: 5.7 x 4.9 arcminutes
RC3 Type: SB(rs)m
RSA Type: SBbc(rs)II.2
Surface Brightness Range Displayed: 17.0-27.0 mag per square arcsec
Absolute Blue Magnitude: -18.2
Elmegreen Spiral Arm Class: AC 4
De Vaucouleurs Atlas Description:
NGC 4618 is a classic late-type Magellanic spiral with a fairly regular outer disk. As noted by Odewahn (1989), many of the characteristics that define the SBm class are present in this object (de Vaucouleurs and Freeman 1973). There is a well-defined outer disk whose center is significantly offset from the center of the apparent bar. There is an embryonic arm on the northeast side of the bar that is typical of the class, with weaker arms on the southwest side. The color index map shows how star formation defines a single prominent arm that winds outward from the east side of the bar. On early photographs, the faint disk was not detected well, and this characteristic asymmetry was emphasized over the more symmetric background disk.
The bar is defined to a great extent by star formation and dust. The significant dust patch seen in the bar is actually a characteristic feature of the SBm class as much as is the asymmetry .
The (rs) variety classification is based on the weak trace of an inner pseudoring around the bar, defined in part by the embryonic arms around the ends of the bar. In the outer disk, the deep image clearly shows two diffuse spiral arms that fade near the disk edge. The galaxy is relatively symmetric only in these outer arm zones, which seem to have little recent star formation. The color index map suggests that the two outer arms are mainly defined by dust. The Carnegie Atlas classification of NGC 4618 , SBbc(s), is very different from the de Vaucouleurs classification. NGC 4618 has no clear bulge, and cannot be of type SBbc.
No completely satisfactory model for the origin of the SBm morphology has been established. The characteristic mass asymmetry and offset bar potential could be results of a gravitational encounter with a companion (Odewahn 1989 and references therein). NGC 4618 has a smaller companion, NGC 4625 , that is also an SBm type, and so this scenario may work for NGC 4618 . However, as noted by Odewahn, this model cannot account for all properties of SBm systems.