Systematics of Bulge-to-Disk Ratios

Abstract

The main results of a new analysis of the spheroidal (I) and disk (II) components of 98 lenticular and spiral galaxies are :

1. (i) on the average, the magnitude difference between spheroid and total luminosity, Δm_I = B_T(I) - B_T, varies smoothly along the Hubble sequence from early lenticulars to late-type spirals (Fig. 1);

2. (ii) the trend of Δm_I confirms the concept of the lenticular class as intermediate between E and S classes, not as a parallel sequence;

3. (iii) the large scatter at any given type, σ(Δm_I) ≃ 0.7 mag, is still dominated by measuring and decomposition errors.

4. (iv) The effective surface brightness of the spheroid, μ^c_e(I), corrected for galactic extinction A_B, decreases by ~2 mag from early to late types, but with a large range (~ 3 mag) at T = const. (Fig. 2a).

5. (v) The effective surface brightness of the disks, corrected for galactic extinction and inclination, μ^c_e(II) = μ_e(II)- A_B + 3 log R_2s, is almost independent of type, with <μ^c_e(II)> ≃ 23.5 for spirals. This implies a corrected central surface brightness μ^c(0) = μ^c_e −1.82 ≃ 21.7, in good agreement with the Freeman rule, but with a large scatter. However, the disks of lenticulars (T< 0) tend to be ~ 0.5 mag fainter than the disks of spirals (Fig. 2b).

6. (vi) The linear effective radii of the spheroidal components are largest, <r_e (I)> ≳ 1 kpc, among the early type spirals, in agreement with the Hubble classification criterion. The spheroid of lenticulars and late-type spirals tend to be smaller, <r_e(I)> ≃ 0.5 kpc, but with a large scatter (Fig. 3a). There is no indication of systematic difference between ordinary (SA) and barred (SB) spirals.

7. (vii) The linear effective radii of the disk components are largest <r_e (I)> ≳ 5 kpc, among intermediate type spirals. The disks of lenticular and late type spirals tend to be smaller (Fig. 3b).

8. (viii) The mean absolute magnitudes of the disk and spheroidal components depend on type (Fig. 4). On the average the disks are brighter (M_II ≃ −19.5) among types Sb-Sbc, spheroids (M_I ⋍ −19) among types L⁺ to Sa, but, again, with a large scatter. Disks and spheroids are about equally bright (M_I ⋍ M_II ⋍ − 19) at stage SO/a (T = 0).