Extremely Large Telescope

Abstract:

We explore the origin of the color-magnitude relation (CMR) of early-type galaxies in the Virgo Cluster using spectra of very high signal-to-noise ratio for six elliptical galaxies selected along the CMR. The data are analyzed using a new evolutionary stellar population synthesis model to generate galaxy spectra at the resolution given by their velocity dispersions. In particular, we use a new age indicator that is virtually free of the effects of metallicity. We find that the luminosity-weighted mean ages of Virgo ellipticals are greater than ∼8 Gyr and show no clear trend with galaxy luminosity. We also find a positive correlation of metallicity with luminosity, color, and velocity dispersion. We conclude that the CMR is driven primarily by a luminosity-metallicity correlation. However, not all elements increase equally with the total metallicity, and we speculate that the CMR may be driven by both a total metallicity increase and a systematic departure from solar abundance ratios of some elements along the CMR. A full understanding of the role played by the total metallicity, abundance ratios, and age in generating the CMR requires the analysis of spectra of very high quality, such as those reported here, for a larger number of galaxies in Virgo and other clusters.

Abstract:

The nucleus of the bulge-dominated, multiply barred S0/a galaxy NGC 2681 is studied in detail using the high-resolution Hubble Space Telescope Faint Object Camera (FOC), Near-Infrared Camera and Multiobject Spectrometer (NICMOS) imaging, and the Faint Object Spectrograph (FOS). The ionized gas central velocity dispersion is found to increase by a factor ≈2 when narrowing the aperture from R ≈ 1″.5 (ground) to R ≈ 0″.1 (FOS). Dynamical modeling of these velocity dispersions suggests that NGC 2681 does host a supermassive black hole (BH) for which one can estimate a firm mass upper limit MBH ≲ 6 × 107 M⊙. This upper limit is consistent with the relation between the central BH mass and velocity dispersion MBH - σ known for other galaxies. The emission-line ratios place the nucleus of NGC 2681 among LINERs. It is likely that the emission-line region comes from a rather mild, but steady, feeding of gas to the central BH in this galaxy. The inner stellar population lacks any measurable color gradient (to a radius of 0.6 kpc) from the infrared to the ultraviolet, consistently with FOC, FOS, and IUE data, all indicating that this system underwent a starburst ≈1 Gyr ago that encompassed its whole interior, down to its very center. The most likely source of such a widely distributed starburst is the dumping of tidally extruded gas from a galaxy neighbor. If so, then NGC 2681 can be considered as the older brother of M82, seen face-on as opposed to the edge-on view we have for M82.