ﻻ يوجد ملخص باللغة العربية
Aims. The method of deriving photometric metallicities using red giant branch stars is applied to resolved stellar populations under the common assumption that they mainly consist of single-age old stellar populations. We explore the effect of the presence of mixed-age stellar populations on deriving photometric metallicities. Methods. We use photometric data sets for the five Galactic dwarf spheroidals Sculptor, Sextans, Carina, Fornax, and Leo II in order to derive their photometric metallicity distribution functions from their resolved red giant branches using isochrones of the Dartmouth Stellar Evolutionary Database. We compare the photometric metallicities with published spectroscopic metallicities based on the analysis of the near-infrared Ca triplet (Ca T), both on the metallicity scale of Carretta & Gratton and on the scale defined by the Dartmouth isochrones. In addition, we compare the photometric metallicities with published spectroscopic metallicities based on spectral synthesis and medium-resolution spectroscopy, and on high resolution spectra where available. Results. The mean properties of the spectroscopic and photometric metallicity samples are comparable within the intrinsic scatter of each method although the mean metallicities of dSphs with pronounced intermediate-age population fractions may be underestimated by the photometric method by up to a few tenths of dex in [Fe/H]. The star-by-star differences of the spectroscopic minus the photometric metallicities show a wide range of values along the fiducial spectroscopic metallicity range, with the tendency to have systematically lower photometric metallicities for those dwarf spheroidals with a higher fraction of intermediate-age populations. Such discrepancies persist even in the case of the purely old Sculptor dSph, where one would naively expect a very good match when comparing with medium or low resolution metallicity measurements. Overall, the agreement between Ca T metallicities and photometric metallicities is very good in the metallicity range from ~ -2 dex to ~ -1.5 dex. We find that the photometric method is reliable in galaxies that contain small (less than 15%) intermediate-age stellar fractions. Therefore, in the presence of mixed-age stellar populations, one needs to quantify the fraction of the intermediate-age stars in order to assess their effect on determining metallicities from photometry alone. Finally, we note that the comparison of spectroscopic metallicities of the same stars obtained with different methods reveals similarly large discrepancies as the comparison with photometric metallicities.
We have found that the high velocity dispersions of dwarf spheroidal galaxies (dSphs) can be well explained by Milky Way (MW) tidal shocks, which reproduce precisely the gravitational acceleration previously attributed to dark matter (DM). Here we su
We calculate the effective $J$-factors, which determine the strength of indirect detection signals from dark matter annihilation, for 25 dwarf spheroidal galaxies (dSphs). We consider several well-motivated assumptions for the relative velocity depen
The nature of Milky Way dwarf spheroidals (MW dSphs) has been questioned, in particular whether they are dominated by dark matter (DM). Here we investigate an alternative scenario, for which tidal shocks are exerted by the MW to DM-free dSphs after a
We discuss the detection limits and current status of a uniform survey of SDSS I for ultra-faint Milky Way dwarf galaxies. We present the properties of two new, low surface brightness Milky Way companions discovered as a result of this survey. One of
We present an analysis of the kinematics of 14 satellites of the Milky Way (MW). We use proper motions (PMs) from the $Gaia$ Early Data Release 3 (EDR3) and line-of-sight velocities ($v_{mathrm{los}}$) available in the literature to derive the system