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The infrared observations were made on two nights in April, 1976 and one night in June, 1977 with the Mt. Hopkins 60-inch (1.5 m) Tillinghast reflector; and on one night in June, 1-977 with the Kitt Peak National observatory #1 36-inch (0.9m) telescope. The Harvard College Observatory InSb photometer was used, with filters and focal plane apertures cooled to solid nitrogen temperatures. The effective wavelengths and bandwidths of the filters employed are: J - 1.24 µm (0.28 µm), H - 1.65 µm (0.30 µm), K - 2.22 µm (0.41 µm), and L - 3.49 µm (0.54 µm). The photometric system (called the HCO system) and set of standard stars to which the measurements are referred are described fully by Frogel et al. (1977, hereafter Paper I) and in Paper IV. In this system, which is close to the original photometric system of Johnson (1966), alpha Lyr has magnitude 0.00 at all wavelengths.

The photometric results are presented in Table 1. The data consists of 21", 41", and 107" aperture JHK measurements, and a 21" L measurement, all centered on the position of peak infrared intensity, and also a 41" JHK measurement centered on the brightest optical patch, which has been designated point A, following the notation of G. and A. de Vaucouleurs (1963). The position and sizes of the measuring beams are superimposed on an optical photograph of the galaxy in Figure 1.

Table 1. Infrared Photometry of NGC 4631

  1950   Aperture          
  alpha delta Telescope (") log A / D0 K J-H H-K K-L

Infrared Peak 12h39m40.s9 32° 49'19" MH60 21 -1.48 9.51 1.03 0.51 0.83 ± 0.15  
      MH60 41 -1.18 8.45 0.96 0.41 -
      KP36 107 -0.77 7.34 0.87 0.34 -
Point A 12h39m44.s7 32° 49'16" MH60 41 - 9.25 0.78 0.37 -  
Mean spiral** - - - - - - 0.72 ± 0.03 0.18 ± 0.01 0.22 ± 0.1

* Positions were determined by offsetting to nearby field stars, and have an accuracy of ± 5" in each coordinate.

Nominal errors for K, J-H, and H-K are ± 0.03,± 0.04, and ± 0.03 mag, respectively.

** Quoted values for J-H and H-K are the mean and dispersion of colors for 8 Scd and Sd galaxies (Aaronson 1977). The K-L color is an unpublished measurement of M31 with a 27" aperture.

The chopping throws used were 82" at Mt. Hopkins and 252" at Kitt Peak. Because all chopping was in the north-south direction, and the galaxian spindle is aligned almost exactly east west, corrections for extended flux in the reference beam are negligible. However, following the precepts in Papers I and IV, corrections were applied for beam profile and aperture effects, although the largest such correction was only 0.04 mag. Since the galactic latitude of NGC 4631 is bII = 84° , no correction has been applied for interstellar reddening. With the formulae in Paper I, and a redshift of z = 0.0021 (de Vaucouleurs, de Vaucouleurs, and Corwin 1976), the redshift corrections to the stellar component of the flux are all < 0.01 mag.

The statistical accuracy of the JHK measurements was always < 0.02 mag. The adopted nominal errors given in Table 1 for the JHKL data are generally equal to or larger than the nightly variations, after application of all instrumental corrections.

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