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تسجيل مستخدم جديدVariable stars and stellar populations in Andromeda XXI: II. Another merged galaxy satellite of M31?
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B and V time-series photometry of the M31 dwarf spheroidal satellite Andromeda XXI (And XXI) was obtained with the Large Binocular Cameras at the Large Binocular Telescope. We have identified 50 variables in And XXI, of which 41 are RR Lyrae stars (37 fundamental-mode RRab, and 4 first-overtone RRc, pulsators) and 9 are Anomalous Cepheids (ACs). The average period of the RRab stars (<Pab> = 0.64 days) and the period-amplitude diagram place And~XXI in the class of Oosterhoff II - Oosterhoff-Intermediate objects. From the average luminosity of the RR Lyrae stars we derived the galaxy distance modulus of (m-M)$_0$=$24.40pm0.17$ mag, which is smaller than previous literature estimates, although still consistent with them within 1 $sigma$. The galaxy color-magnitude diagram shows evidence for the presence of three different stellar generations in And~XXI: 1) an old ($sim$ 12 Gyr) and metal poor ([Fe/H]=$-$1.7 dex) component traced by the RR Lyrae stars; 2) a slightly younger (10-6 Gyr) and more metal rich ([Fe/H]=$-$1.5 dex) component populating the red horizontal branch, and 3) a young age ($sim$ 1 Gyr) component with same metallicity, that produced the ACs. Finally, we provide hints that And~XXI could be the result of a minor merging event between two dwarf galaxies.
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We present B and V time series photometry of the M31 satellite galaxy Andromeda XXVII (And XXVII) that we observed with the Large Binocular Cameras of the Large Binocular Telescope. In the field of And XXVII we have discovered a total of 90 variables
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or 77 member stars in this system, more than doubling previous studies to determine whether this galaxy is truly a low density outlier. We measure a systemic velocity of $v_r=-363.4pm1.0,{rm kms}^{-1}$ and a velocity dispersion of $sigma_v=6.1^{+1.0}_{-0.9},{rm kms}^{-1}$, consistent with previous work and within $1sigma$ of predictions made within the modified Newtonian dynamics framework. We also measure the metallicity of our member stars from their spectra, finding a mean value of ${rm [Fe/H]}=-1.7pm0.1$~dex. We model the dark matter density profile of And~XXI using an improved version of GravSphere, finding a central density of $rho_{rm DM}({rm 150 pc})=2.7_{-1.7}^{+2.7} times 10^7 ,{rm M_odot,kpc^{-3}}$ at 68% confidence, and a density at two half light radii of $rho_{rm DM}({rm 1.75 kpc})=0.9_{-0.2}^{+0.3} times 10^5 ,{rm M_odot,kpc^{-3}}$ at 68% confidence. These are both a factor ${sim}3-5$ lower than the densities expected from abundance matching in $Lambda$CDM. We show that this cannot be explained by `dark matter heating since And~XXI had too little star formation to significantly lower its inner dark matter density, while dark matter heating only acts on the profile inside the half light radius. However, And~XXIs low density can be accommodated within $Lambda$CDM if it experienced extreme tidal stripping (losing $>95%$ of its mass), or if it inhabits a low concentration halo on a plunging orbit that experienced repeated tidal shocks.
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