Variable Stars and Stellar Populations in Andromeda XXI. II. Another Merged Galaxy Satellite of M31?

Journal
Date Issued
2015
Author(s)
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Cignoni, Michele
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Sani, Eleonora
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DOI
Description
We warmly thank P. Montegriffo for the development and maintenance of the GRATIS software, G. Battaglia for providing the software to compute And XXI’s density maps, and A. Veropalumbo for software assistance. Financial support for this research was provided by PRIN INAF 2010 (PI: G. Clementini) and by Premiale LBT 2013. The LBT is an international collaboration among institutions in the United States, Italy, and Germany. LBT Corporation partners are The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota, and University of Virginia. We acknowledge the support from the LBT-Italian Coordination Facility for the execution of observations, data distribution, and reduction. Facility: LBT
Abstract
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 (< {P}{ab}> =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 = 24.40 ± 0.17 mag, which is smaller than previous literature estimates, although still consistent with them within 1σ. The galaxy color-magnitude diagram shows evidence for the presence of three different stellar generations in And XXI: (1) an old (̃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) an intermediate age (̃1 Gyr) component with the 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.

Based on data collected with the LBC at the LBT.

Volume
806
Issue
2
Start page
200
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2015ApJ...806..200C
Rights
open.access
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