An outsider's view of the Galaxy - Photometric parameters, scale lengths, and absolute magnitudes of the spheroidal and disk components of our Galaxy

Abstract

A two-component model of the Galaxy consisting of (I) an R 1/4 spheroid of effective radius r1 -267 kpc (for r0= 8.0 kpc) and peak brightness i BI (0) =15.11 mag , and (Il) an exponential disk of effective radius r IeI =5.98 kpc and (assumed) peak brightness ` (0- ) =21.65 (after Freeman) is shown to be consistent with the galactic disk brightness inferred from star counts and to give a good representation of new observations of the brightness distribution in the bulge along galactic meridian 0 . An ellipsoidal model of the spheroid with a el = 3.45 kpc and c /a =0.6 is also considered with little change in most results; averages from the two cases are adopted. For an outside observer the face-on total absolute magnitude of the Galaxy (with allowance for uniform internal extinction of total optical depth A B =0.4 mag) is M % (B ) = -20.08 or L T (B ) =1.6 x ., of which half is emitted within the effective radius r e = 5.1 kpc. The fraction K1 =L' /L T =0. .04 contributed by the spheroidal component is in close agreement with corresponding values for several Sbc (T =4) galaxies. The calculated standard face-on isophotal diameter of the Galaxy at the p =25.- 0 level, D o= kpc is in close agreement with three independent estimates (21. 23.4) from statistical relations for spiral galaxies. The luminosity index A=(T +L )/l0 of the Galaxy derived by three independent methods is A=0.7l 0.03, implying a luminosity class L =3 t 1 (LC =11), if T =4 0.5. The mean intrinsic color (corrected for extinction) of the spheroidal component derived from the observations is <B - V >1=0.65 0.05; that of the disk stellar population derived from local luminosity functions is <B - V >11=0.40 (after Holmberg, transformed to B , V system). In the face-on view of the Galaxy the absorbing layer with A B =0.4 mag produces a color excess E (B - V) =0.039 mag and the total color index of the Galaxy for an external observer is <B - V > T =0.53 0.05, in fair agreement with the average value (0.57) for Sbc galaxies. The hydrogen-luminosity ratio implied by the luminosity index and statistical relations for spiral galaxies is log / B =0.g3, and the corresponding total Hi mass is =2.3 x 10 of which 71% is observed in the disk within r = 1.5r0 (after Baker and Burton, corrected to r0 = 8.0 kpc), leaving 29% to be distributed in the disk at r >12 kpc and in the corona. If the total mass of the Galaxy is 2x , the Hi mass fraction is h= 1 / =0.012. Quantitative com- parison of photometric parameters with other galaxies indicates a close agreement with the mean of NGC 1073, 4303, 5921, and 6744 and is consistent with the proposed classification SAB (r s)bc II.