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The Cool Stellar Populations of Early-Type Galaxies and the Galactic Bulge

Publications of the Astronomical Society of the Pacific1996Vol. 108, pp. 828–828

Abstract

The cool stellar populations of early-type galaxies have been examined through an analysis of red (6800-9200A) and near-infrared (K-band) spectra of the nuclear regions of 17 E/S0 galaxies in the Virgo cluster, ten Coma cluster members and seven field galaxies. The Virgo galaxy sample spans about 4.5 mag. in K-band luminosity. To characterize the red stellar populations of these galaxies, the strengths of the Ca II triplet at lambda-lambda 8498, 8542, 8662A, the Na I doublet at lambda-lambda 8183, 8195A, the Mg I lambda 8807A line, several molecular bands of TiO and VO, and the CO band with bandhead near 2.29 microns have been measured from the spectra. The variations in the colors and spectral features of E/S0 galaxies as a function of luminosity are described and compared to the radial changes seen in the integrated light of the Galactic bulge as a function of galactocentric distance. The Virgo galaxy observations were used to define the behavior of the colors and spectral indices of early-type galaxies as a function of luminosity. A comparison to the Coma and field galaxies was hindered by the low signal-to-noise ratio of the Coma galaxy data and the lack of published photometry for three of the field galaxies; however, the Coma and field zgalaxies did not exhibit any trends which were statistically different than those seen in the Virgo galaxies. In general, the spectral features which varied significantly with K-band luminosity also showed statistically significant correlations with V-K and J-K color. Specifically, (1) the CO band, the TiO bands and the Na I index (the doublet itself is unresolved in the spectra obtained here) strengthen with redder colors and higher luminosities, (2) the Mg I line and the lines of the Ca II triplet do not vary significantly in strength among galaxies of different color or brightness, and (3) the VO band was not detected in any galaxy. In addition, the redshift correction to the CIT photometric CO index was found to be well-fit by the relation KCO = +5.23z. Using previously published photometry and spectroscopy of stars in the Galactic Bulge, the radial variations in the bulge's stellar population have been estimated by constructing models of the integrated light of Baade's Window (BW), located at b}= -3.9 degrees, and a Galactic bulge field near b = -8 degrees. A close examination of the BW color-magnitude diagram (CMD) and the resulting BW model indicate that: (1) the isochrone which best traces the "average" distribution of stars in the BW CMD has [Fe/H] = -0.23 and an age of 8.0 Gyr; (2) the I-band fluctuation magnitude of the BW stellar population is consistent with current estimates of the distance to the Galactic center; (3) the stellar population in Baade's Window is quite complex, consisting of either a purely "bulge" population with a large spread in its properties (such as metallicity) or a mixture of separate populations, possibly halo and bulge; (4) most, if not all, of the observed characteristics of the BW population which have usually been attributed to an overall increased metallicity in BW stars can be better explained by selective enrichment of Ti and perhaps variations in the relative abundances of the CNO elements without a corresponding Fe enhancement; and (5) the integrated light of BW is very similar to the integrated light of the nucleus of a low-luminosity early-type galaxy and is giant-dominated for lambda > 5500A. Three major conclusions were drawn from the radial variations seen in the integrated light models of the Galactic bulge. First, extrapolating the derived radial gradients to the inner bulge, it appears that the differences between the BW model and a luminous E/S0 galaxy cannot be attributed solely to the fact that BW represents a region of the Galactic bulge much further from the center of the galaxy than the analogous regions which the spectra of the early-type galaxies sample. Second, the two indices which measure spectral features in the vicinity of the Na I lines increase steeply with redder colors in the E/S0 galaxies but appear to decrease with color in the Galactic bulge; this effect is not understood but may be caused by some deficiency in the modelling. Spectral synthesis of this region of the spectrum would be extemely useful in disentangling the Na I doublet from other contaminating species (e.g. TiO, nearby atomic lines and the CN bands prevalent in K giants). Third, differences in the slopes of the TiO/color trends in E/S0 galaxies and in the Galactic bulge may indicate that the [Ti/Fe] ratio is changing differently in these two instances. The overall trends exhibited by the Virgo E/S0 galaxies are most consistent with the following scenario: the integrated light of early-type galaxies is dominated by giant stars having an average [Fe/H] > -0.3, but the chemical composition of this population changes with the luminosity of the galaxy-- either the overall metallicity increases or specific elements are selectively enriched with redder colors and increasing luminosity. If it is assumed that the radial stellar population gradients in the Galactic bulge are similar to those in early-type galaxies, then the bulge models indicate that these stellar population variations are different than those which occur among the nuclei of E/S0 galaxies of differing luminosity.

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