De Vaucouleurs Atlas Type: (R')SA(s)a
Telescope: CTIO 1.5-m
North up , East left
Field Dimensions: 3.4 x 4.4 arcminutes
RC3 Type: (R)SA(s)a
RSA Type: Sa(s)
Surface Brightness Range Displayed: 18.0-26.0 mag per square arcsec
Absolute Blue Magnitude: -20.5
Elmegreen Spiral Arm Class: AC 6
De Vaucouleurs Atlas Description:
This galaxy is a very regular-looking, bulge-dominated nonbarred spiral. The spiral structure appears to be single-armed, a visual impression which is confirmed by Fourier analysis (Byrd, Freeman, and Howard 1994). This arm winds for more than 360o and defines an outer pseudoring that de Vaucouleurs originally recognized as a closed ring. The images shown here are much deeper than most previous images of NGC 4378 and we reclassify the outer ring as R'. The ring is well-defined in both B and I.
The approximate B-I color index map shows that the spiral structure is defined by enhanced blue colors. The surface brightness of the arms is fairly low, however, and the galaxy is classified as "anemic" in the van den Bergh (1976) classification system, meaning the arms have an unusually low level of star formation, as if the galaxy is gas deficient. There is a weak dust arc that rims the outer part of the bulge; this arc is slightly enhanced on the east side, suggesting that the east side is the near side. In conjunction with kinematic data (Rubin et al. 1978), which indicates that the north recedes, the single arm is found to be trailing. Byrd, Freeman, and Howard (1994) interpret the arm as impulsively created by the grazing passage of a small companion.