Differential population synthesis of 17 early-type galaxy nuclei in the Fornax cluster

Abstract

Flux calibrated spectra have been obtained of the nuclei of 12 Elliptical and 5 Lenticular galaxies in the FORNAX cluster, at a resolution of 10 - 20 A and in the wavelength range 0.36 to 0.72 micron. The observations have been extended to a long wavelength limit of 0.88 or 1.0 micron for 12 of the galaxies. The galaxies observed cover a 6 magnitude range of absolute magnitude and define the form and intrinsic width of the cluster colour-magnitude relation well. The galaxy spectra have been synthesised with a flux library of 48 synthesis standard spectra, compiled from spectrophotometry of ~200 stars in the wavelength range 0.36 to 1.0 micron at a resolution of 15 A. The stars observed cover a complete solar abundance sequence of all spectral types and luminosity classes, with additional metal-weak and metal-rich G-K giant branch sequences. The results presented here confirm that population synthesis of several galaxies covering a large range of absolute magnitude in one cluster permits good differential comparison of the mean galactic metallicities and ages associated with the main sequence turnoff group. The major conclusions are : 1) The metallicity results confirm the well known trend of decreasing mean metallicity with decreasing galactic luminosity. A metallicity gradient in [Fe/H] of at least 0.16 dex per magnitude is indicated over the 6 magnitude range of absolute magnitudes studied here. The brightest ellipticals NGC1399 and NGC1404 are 2 to 3 times more metal-rich than solar. The trend of mean metallicity with luminosity is the principle factor determining the slope of the cluster CM relation. 2) The ages associated with the main sequence turnoff groups are generally quite young (6 to 10 Gyr). This implies that substantial star formation has occurred in all early-type galaxies for about 8-10 Gyr after the epoch of globular cluster formation. The only possible trend of turnoff age with luminosity is towards younger ages (more extended star formation) in brighter ellipticals. 3) The turnoff ages of the lenticulars are significantly older (> 3 Gyr) than those of ellipticals of the same absolute magnitude, implying that star formation terminated earlier in lenticular systems. The mean metallicity of the lenticulars is also lower than that of ellipticals of the same absolute magnitude. 4) Three bright ellipticals (NGC’s 1404, 1399 and 1379) show strong evidence for a significant blue light contribution due to late 0 dwarfs, implying some ongoing star formation in at least these systems. Syntheses of other early-type galaxies also favour some 0-B dwarf contribution, but these galaxies can be equally well fitted with an enhanced metal-weak G-K giant contribution and some horizontal branch starlight. A significant 0 dwarf contribution (10 % at U) is consistent with a strong turnup in the uv flux below 1800 A, whereas a purely horizontal branch contribution would provide a ~flat uv continuum. Hot dying stars can not be present in sufficient numbers to contribute significantly to the uv flux in any early-type galaxy. UV bright stars could provide a significant fraction of the uv flux in the fainter, metal-weak galaxies. 5) There is a weak correlation between the percentage light contribution due to OB dwarf stellar groups, and the colour residuals from the mean (U36-V) vs. V nuclear colour-magnitude relation. Variations in the contribution due to recently formed OB dwarfs could account for a significant fraction of the intrinsic width in (U-V) of the cluster CM relation. 6) The faintest ellipticals studied here have absolute magnitudes similar to that of M32. The faintest elliptical 0333-36 is isolated from other galaxies and is clearly very metal-weak with [Fe/H] < -0.6. Star formation ceased at least 8 Gyr ago in 0333-36. The faint elliptical JJ79 is situated in the cluster core close to several other early-type galaxies. The metallicity of JJ79 is solar or greater, and this galaxy shows strong indications that substantial star formation activity occurred as recently as 3 Gyr ago. Relatively high (solar) metallicity and indications of recent star formation in low luminosity ellipticals such as M32 and JJ79 are probably consequences of tidal interactions with massive companions. The population synthesis technique can also give galactic mass to light ratios which are independent of distance. The results show that detailed population syntheses of near infra-red spectra from 0.7 to 1.0 micron enable only upper limits to be reliably calculated for the mass to light ratios of early-type galaxies. The conclusions regarding galactic mass to light ratios are : 7) The measured near infra-red spectra of galaxies can be well fitted by several combinations of M dwarf to M giant contribution ratios, resulting in M/Lv ratios in the range 1 to 13 for most early-type galaxies. The Na I doublet feature at 8190 A does not provide sufficient discrimination between M dwarfs and M giants to determine more precise values of M/Lv, although the fairly low measured equivalent widths clearly excludes M/Lv values greater than 20. Good quality spectra of the Wing-Ford (FeH) band at 9910 A may enable more reliable estimates of mass to light ratios to be derived from synthesis techniques. 8) Upper limits to M/Lv ratios calculated from synthesis fits conform well with M/Lv ratios calculated from nuclear velocity dispersion measurements. There is no reliable evidence for hidden mass in the nuclei of early-type galaxies of any luminosity. The dynamical estimates are consistent with galaxy M/L^ values of about 5 (M/LB ~ 7) , independent of absolute magnitude. Show more