Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics,and stellar population properties in the extended, low-surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers,velocity dispersion and rotation profiles have been followed to ~ 4 and ~ 5-8 effective radii, respectively, and in M87 to the outer edge at ~ 150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicate that non-thermal components to the pressure may be important in the inner ~ 10 kpc and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [α/Fe] abundances indicate longer star formation histories pointing toward late accretion of the halo. This is consistent with independent evidence for ongoing accretion and suggests a connection to the observed size evolution of elliptical galaxies with redshift.
Galaxies and their Masks
- Pub Date:
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- 8 pages. Invited review to appear in the proceedings of "Galaxies and their Masks" eds. Block, D.L., Freeman, K.C. &