Further observations of the elliptical galaxy NGC 5813

Abstract

We have obtained the rotation curve and velocity dispersion profile along the major axis of the elliptical galaxy NGC 5813. The measurements extend out to almost 90 arcsec (≈ 16 kpc, taking H0 = 50 km s−1 Mpc−1) on either side of the centre of the galaxy. Surface photometry has also been obtained which reveals significant departures from the standard King (1966) or de Vaucouleurs (1959) profiles interior to 12 arcsec. The rotation curve is quite unusual. The peak velocity of 89 ± 7 km s−1 is reached at ≈ 3 arcsec from the centre, the rotational velocity then falls rapidly and remains roughly constant at 8 ± 2km s−1 between 10 and 80 arcsec. The velocity dispersion peaks at ≈ 10 arcsec and decreases at large and small radii. We compare the observations with simple dynamical models using the equations of stellar hydrodynamics (Binney 1980a). We show that the shape of the rotation curve may be qualitatively understood if the galaxy has highly anisotropic velocity dispersions in the outer parts, but the anisotropy vanishes in the central regions. The models predict velocity dispersion profiles which fall too steeply at large radii when compared to the observations. The discrepancy may be resolved if there exists a small increase in the mass-to-light ratio at large radii (less than a factor of 2 out to ≈ 90 arcsec). The models predict a decrease in the velocity dispersion towards the centre of the galaxy in rough agreement with the observations.