Two-Dimensional Decomposition of a Disky Elliptical Galaxy, NGC 4621

Abstract

Abstract We decomposed two-dimensionally an elliptical galaxy, NGC 4621, which shows deviations from the brightness distribution of the r1/4-law. We have found that its brightness distribution can be reproduced by three components possessing constant ellipticities with an rms 0.085 Bs (the sky brightness) of the residuals in the circular region of radius r ≤ 200″, except for r < 1″. The component obeying the r1/4 -law has 62% of the total light, and, hence, is the main body of this elliptical galaxy. The component obeying the exponential law is considered to be a disk, because the rotational velocity, 120 km s(−1), is comparable to that of a disk galaxy. The light ratio of the disk component to the main body is 0.25, about half of which is in S0 galaxies. It is justified that NGC 4621 be classified not as a disk galaxy, but as elliptical. The envelope component is circular and has 22% of the total light. This circular component is considered to be spherical three-dimensionally and to be a halo. The inclination angle of the disk component is estimated to be 65° from its ellipticity 0.43. The main body is inferred in terms of the probabilities to be an oblate spheroid from the fact that the position angle of its major axis coincides with that of the disk, and its rotational velocity is comparably as large as that of a disk galaxy. If the main body is an oblate spheroid, the ratio of its intrinsic principal axes is derived to be 0.64 from its apparent axis ratio 0.72 and its inclination angle 65°.