NGC 4631 is an edge-on spiral galaxy classified as "probably late Sc" by Sandage (1961), and as a barred-Sd by G. and A. de Vaucouleurs (1963), with the bar oriented end-on along the line of sight. Optical photographs of this object show a patchy distribution of stars, H II regions, and dust (Figure 1), but reveal no obvious nuclear feature. The absence of a well-defined nucleus hinders understanding of the rotation curve measured by G. and A. de Vaucouleurs (1963), and of the radio continuum maps of Pooley (1969).
Figure 1. The circles superimposed on an optical photograph of NGC 4631 indicate the positions and sizes of the beams used to collect the photometric data in Table 1. The inset shows a section of an east-west 2.2 µm scan with a 41" beam across the galaxy at the declination of the infrared peak. Both the scan and the photograph are shown at the same scale.
NGC 4631 was first observed by the author in the course of surveying the infrared colors of a large number of spiral galaxies (Aaronson 1977, hereafter Paper IV). 2.2 µm scans along the major axis revealed that the peak infrared intensity lay not on the most optically prominent part of the galaxy, but in the nearby area of strongest dust absorption (Figure 1) The observations are presented in Section II, and the location of the infrared peak is discussed in Section III in relation to the aforementioned work of the de Vaucouleurs' and of Pooley.