Magnesium gradients in elliptical galaxies. III - Results and interpretation

Abstract

view Abstract Citations (35) References (91) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Magnesium Gradients in Elliptical Galaxies. III. Results and Interpretation Thomsen, Bjarne ; Baum, William A. Abstract In Papers I and II of this series, we reported magnesium-index gradients and luminosity profiles of two elliptical galaxies in the Coma Cluster, namely, the E1 cD galaxy NGC 4874 near the center of the cluster and the much-studied E0 galaxy NGC 4881 18' north of the center. In Paper III we now add similar results for the bright E4 galaxy NGC 4839 situated 41' southwest of the cluster center, and we review the combined results in terms of galaxy formation models. NGC 4839 was observed with the same CCD camera and filters used for NGC 4874 and NGC 4881, and the data reduction methods were basically similar except that the elongated form of NGC 4839 required special procedures for the "seeing" correction. As before, the data reach very faint levels with high accuracy. In addition to the magnesium index gradient and the luminosity profile for NGC 4839, we investigated isophotal ellipticity and asymmetry. The relative merits of several robust regressions for determining surface brightness distributions in elliptical galaxies are also discussed and compared. In dimensionless form, the magnesium-index gradient in NGC 4839 is found to be δMg_2_/δμ_v_ = -0.055 +/- 0.005 in the surface brightness range 18.4 < μ_v_ < 20.0 mag arcsec^-2^, and -0.020 +/- 0.003 in the range 20.2 < μ_v_ < 23.8 mag arcsec^-2^. Those two values bracket the gradients found in NGC 4874 and NGC 4881. Unless the relationship of the Mg_2_ index to the metallicity [Fe/H] differs greatly from that given in 1981 by Terievich et al., the metallicity near the center of NGC 4839 is (as in the bulge of our own Galaxy) greater than the solar value, and it drops to only a little less than the solar value in the NGC 4839 halo. It is much greater there than the metallicities typical of halo globular clusters. The luminosity profile of NGC 4839 departs from a r^1/4^ law in the sense of having a slope that very gradually decreases with increasing radius. No pronounced bulge is apparent. Out to a level equivalent to 2% of sky brightness (where our accuracy is ~ 0.2% of sky), no "cD" feature is seen in the profile. Combining our results for the three Coma ellipticals with various findings of others, we call attention to the following properties that any successful theory of elliptical galaxy formation needs to address: (1) If magnesium indices are tracers of metallicity, giant elliptical galaxies are inferred to have high metallicities but low metallicity gradients, resulting in comparatively metal-rich halos. (2) Giant ellipticals are inferred to have triaxial figures supported by anisotropic velocity dispersion rather than by rotation. (3) Globular clusters in giant ellipticals have lower metallicities than halo stars in the same regions. (4) The distribution of globular clusters in a giant elliptical is less centrally concentrated than the main body is. (5) Halo stars of the Milky Way (a disk galaxy) are not so metal-rich as those in giant ellipticals. No single existing model can explain all these observed properties. In general, some form of clumpy inhomogeneous collapse appears to be indicated. As to metallicity gradients, we use a simple straw man example based on a closed-box model to show that low gradients in ellipticals should not be surprising. Publication: The Astrophysical Journal Pub Date: December 1989 DOI: 10.1086/168111 Bibcode: 1989ApJ...347..214T Keywords: Abundance; Elliptical Galaxies; Galactic Evolution; Magnesium; Metallicity; Astronomical Photometry; Brightness Distribution; Charge Coupled Devices; Globular Clusters; Luminosity; Astrophysics; GALAXIES: ABUNDANCES; CLUSTERS: GLOBULAR; GALAXIES: PHOTOMETRY; GALAXIES: STRUCTURE full text sources ADS | data products SIMBAD (7) NED (4) Related Materials (2) Part 1: 1986ApJ...301...83B Part 2: 1987ApJ...315..460T