Photoelectric photometry of the Andromeda nebula in the UBV system

Abstract

An isophotic map of M31 in blue light was established by means of direct photoelectric scans spaced at 10' intervals in declination from + ' to ' The color distribution at selected declinations and the mean color indices in the U, B, V system were determined The luminosity distribution in the central region was derived from earlier data reduced to the same system The integrated magnitude and colors of M31 (exclusive of companions) within an area of 4 0 square degrees limited by the isophote B0 = 26 8 mag/sec2 are mo(B) = 4 39, B - V = +091, U - B = +050; the probable errors are +0 02 mag The position angle of the major axis is 37'7 + 0'2 (p.e) The axial ratio reaches a minimum (b/a),,, 0 25 near 2a = 100', where B 22.5 mag/sec2, corresponding to the outline of the main spiral arms Allowing for the tilt of the equatorial plane on the line of sight, the true axial ratio of the flat component of M31 is q = 0.042, and the corresponding "thickness" of the spiral structure is about 0 8 kpc The luminosity profile along the major axis can be analyzed, after removal of the flat component (B - V = +085, U - B = +04) in terms of a spheroidal component (B - V = +10, U - B = +06) closely obeying the luminosity law typical of elliptical galaxies (de Vaucouleurs 1953); this spheroidal component contributes 24 per cent of the total luminosity in blue light; it dominates the luminosity distribution in the central regions up to a 30' and may become important again beyond a 3'. The effective dimensions of the isophote within which is emitted half the total luminosity are 2a, = 71', b,/a, = 0.30 with B, = 22 62 mag/sec2 corresponding figures for the spheroidal component alone are 2a,' = 35', b/a', = 0 6, B,' = 22 86 mag/sec2 Comparison with the mass derived by M Schmidt (1957b) leads to a mean apparent mass/ ratio J(B) = / B = 24 The mass of the spheroidal component derived from Minkowski's estimate of the velocity dispersion leads to an apparent mass/ luminosity ratio 7' (B) = 70. The mass of the flat component obtained by difference leads to an apparent mass/luminosity ratio J"(B) = 6