De Vaucouleurs Atlas Type: SB(s)d sp or Sc sp
Telescope: WIYN 3.5-m
North up , East left
Field Dimensions: 6.7 x 6.3 arcminutes
RC3 Type: SB(s)d sp
RSA Type: Sc on edge
Surface Brightness Range Displayed: 19.0-26.0 mag per square arcsec
Absolute Blue Magnitude: -20.6
De Vaucouleurs Atlas Description:
This interesting late-type spiral once sparked a disagreement between de Vaucouleurs and Geoffrey Burbidge. On the basis of the asymmetry and the general appearance of the disk, de Vaucouleurs concluded that NGC 4631 is probably an edge-on SBd galaxy. Burbidge, Burbidge, and Prendergast (1964) challenged this conclusion, claiming that the best we could say about this galaxy is that it is an Sc. The main extinction feature appears projected against what could be a boxy bulge. If this is the case, then NGC 4631 could indeed be barred. The Sd classification could come from the asymmetry and the general patchiness of the disk. However, it is unlikely that an Sd galaxy could have a boxy bulge. The bars of face-on Sd's are usually defined by star formation. We take no side in the dispute since either interpretation has merit.
The asymmetry in NGC 4631 's morphology could be caused by an interaction with its peculiar small companion, NGC 4627 , or, more likely, its much larger neighbor NGC 4656 /7 (Weliachew 1969). The B-V color index map shows that the main asymmetry on the left side in the Atlas image involves a mass of star formation; this area is very blue. In the apparent bulge area, there appears to be a lot of extinction but not much reddening. In this area, there is considerable scattered young starlight, which makes the main dust patch bluish. The colors in NGC 4627 are fairly neutral.