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4.1 Recent Observational Techniques and Results

The early velocity dispersion results were based on only a handful of stars and on observations with appreciable measurement errors. More recent programs have concentrated on obtaining velocities for larger numbers of stars in each galaxy and in reducing the errors in the velocity measurements. In fact, two distinct strategies have been adopted. The first is to obtain velocities for a large number of stars using spectrographic observations, typically of the Ca II triplet. Velocity dispersions are obtained to an accuracy of around 5 km s-1, which is comparable to the expected velocity dispersions of these systems. However, the use of fibers allows not only a large number of stars to be obtained with a single exposure, but also makes repeat observations a practical possibility. This is important to reject high apparent velocity dispersions caused by binary stars and pulsating variables. So far, this technique has been applied to Sculptor and Sextans, but all the local dwarf spheroidals could be studied in this way.

The second strategy is to obtain much higher resolution velocities. Echelle observations have achieved velocities accurate to 1-2 km s-1 in Carina, Ursa Minor, Fornax and Draco. These observations can go somewhat fainter than the fiber technique, so that potentially more stars are available for observation.

The results of such studies suggest large amounts of DM in most of the nearby dwarf spheroidals, as well as extremely high central DM densities. Mateo et al. (1991) used Echelle spectroscopy to obtain velocities for 44 stars and 4 globular clusters around Fornax. Importantly, Mateo et al. (1991) studied two different fields, one near the center of Fornax and another displaced appreciably along the major axis. In both fields, large velocity dispersions were obtained, suggesting the presence of DM with a more extended spatial distribution than the stellar component of the galaxy. Moreover, the central DM density is high with a value of 0.07 ± 0.03 Msun pc-3. This is an order of magnitude higher than typically found in bright spirals (see Section 6 below). The repeat observations that Mateo et al. (1991) have for some stars indicate that there are few binaries or pulsating variables contained in the sample.

Recent results have been obtained for Carina (Mateo et al. 1992a) and Sextans (Suntzeff et al. 1992). Large central mass-to-light ratios are obtained in both cases, implying dark-to-luminous mass ratios around 10. Central DM densities are also high, with values around 0.1 Msun pc-3. Since few repeat measurements are available for these galaxies, it is conceivable that binary stars or pulsating variables may be artificially inflating the velocity dispersions. However, if Fornax is typical, this possibility seems unlikely.

Da Costa and Armandroff (1992) have made two or more observations of some of their sample of stars in Sculptor. They have observed stars both inside and beyond the core radius and find high velocity dispersions in both fields, indicative of an extended dark halo. The central mass-to-light ratio is not extreme, although the central DM density is once again high.

The two faintest local dwarf spheroidals, Ursa Minor and Draco, appear to have the highest DM fractions. Aaranson and Olszewski (1987; 1988) and Olszewski and Aaronson (1992) report a series of observations of stars in both galaxies, in which a number of repeat measurements have been made. This study has revealed extremely high central M / LV with values approaching 100 and central densities around 1 Msun pc-3 (see however Section 4.2 below). If the dark halos in Draco and Ursa Minor are more extended than the light, as is typically the case in other galaxies, the total dark-to-luminous mass ratio in these objects is at least 100. Similar observations of Carina and Sextans give central M / LV values around 50 (Olszewski and Aaronson 1992).

There is some indication from the results summarized above that the dwarf spheroidals closer to the Galactic center have the highest mass-to-light ratio and lowest luminosities. The observations are also consistent with the view that the masses of all the dwarf spheroidals are similar, despite an appreciable range in luminosity. However, observational and modelling uncertainties make these conclusions tentative at this stage.

Further details of the observational techniques and strategies in this field are given in the excellent review by Pryor (1992).

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