VARIABLE STARS AND STELLAR POPULATIONS IN ANDROMEDA XXI. II. ANOTHER MERGED GALAXY SATELLITE OF M31?

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.

[1]  A. Deason,et al.  SATELLITE DWARF GALAXIES IN A HIERARCHICAL UNIVERSE: THE PREVALENCE OF DWARF–DWARF MAJOR MERGERS , 2014, 1406.3344.

[2]  D. Merritt,et al.  Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies , 2014, 1406.1799.

[3]  F. Fraternali,et al.  The extended structure of the dwarf irregular galaxies Sextans A and Sextans B. Signatures of tidal distortion in the outskirts of the Local Group , 2014, 1404.1697.

[4]  G. Ven,et al.  The remnant of a merger between two dwarf galaxies in Andromeda II , 2014, Nature.

[5]  M. Moretti,et al.  The VMC survey - VIII : First results for anomalous Cepheids , 2013, 1310.5967.

[6]  K. Boutsia,et al.  DWARF SPHEROIDAL SATELLITES OF M31. I. VARIABLE STARS AND STELLAR POPULATIONS IN ANDROMEDA XIX , 2013, 1310.1809.

[7]  S. McGaugh,et al.  ANDROMEDA DWARFS IN LIGHT OF MOND. II. TESTING PRIOR PREDICTIONS , 2013, 1308.5894.

[8]  Alan W. McConnachie,et al.  THE PAndAS VIEW OF THE ANDROMEDA SATELLITE SYSTEM. I. A BAYESIAN SEARCH FOR DWARF GALAXIES USING SPATIAL AND COLOR–MAGNITUDE INFORMATION , 2013, 1307.7626.

[9]  P. Kroupa,et al.  Local Group timing in Milgromian dynamics : a past Milky Way-Andromeda encounter at z>0.8 , 2013, 1306.6628.

[10]  Bonn,et al.  The vast thin plane of M31 corotating dwarfs: an additional fossil signature of the M31 merger and of its considerable impact in the whole Local Group , 2013, 1303.1817.

[11]  M. Irwin,et al.  A KINEMATIC STUDY OF THE ANDROMEDA DWARF SPHEROIDAL SYSTEM , 2013, 1302.6590.

[12]  M. Irwin,et al.  A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy , 2013, Nature.

[13]  K. Bekki,et al.  THE FORNAX DWARF GALAXY AS A REMNANT OF RECENT DWARF–DWARF MERGING IN THE LOCAL GROUP , 2012, 1207.5223.

[14]  G. Clementini,et al.  VARIABILITY AND STAR FORMATION IN LEO T, THE LOWEST LUMINOSITY STAR-FORMING GALAXY KNOWN TODAY , 2012, 1207.2764.

[15]  M. Catelán,et al.  STELLAR ARCHEOLOGY IN THE GALACTIC HALO WITH ULTRA-FAINT DWARFS. VII. HERCULES , 2012, 1206.4031.

[16]  G. Clementini,et al.  HIDE AND SEEK BETWEEN ANDROMEDA'S HALO, DISK, AND GIANT STREAM , 2011, 1111.5301.

[17]  N. Martin,et al.  A BAYESIAN APPROACH TO LOCATING THE RED GIANT BRANCH TIP MAGNITUDE. I. , 2011, 1107.3206.

[18]  M. Irwin,et al.  PAndAS’ CUBS: DISCOVERY OF TWO NEW DWARF GALAXIES IN THE SURROUNDINGS OF THE ANDROMEDA AND TRIANGULUM GALAXIES , 2009, 0909.0399.

[19]  C. Brook,et al.  THE DUAL ORIGIN OF STELLAR HALOS , 2009, 0904.3333.

[20]  C. Mancone,et al.  STELLAR POPULATIONS IN THE ANDROMEDA V DWARF SPHEROIDAL GALAXY , 2008, 0808.3285.

[21]  F. Meissner,et al.  Global fitting of globular cluster age indicators , 2006 .

[22]  D. Astronomia,et al.  The metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy , 2005, astro-ph/0506206.

[23]  G. Jacoby,et al.  The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda I and Andromeda III , 2005, astro-ph/0501083.

[24]  Vanessa Hill,et al.  Two Distinct Ancient Components in the Sculptor Dwarf Spheroidal Galaxy: First Results from the Dwarf Abundances and Radial Velocities Team , 2004 .

[25]  A. Helmi,et al.  Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART , 2004, astro-ph/0411029.

[26]  B. Carney,et al.  A Multicolor and Fourier Study of RR Lyrae Variables in the Globular Cluster NGC 5272 (M3) , 2004, astro-ph/0409567.

[27]  Telescopio Nazionale Galileo,et al.  Metal abundances of RR Lyrae stars in the bar of the Large Magellanic Cloud , 2004, astro-ph/0405412.

[28]  M. Marconi,et al.  Updated pulsation models for anomalous Cepheids , 2004, astro-ph/0401332.

[29]  K. Freeman,et al.  Shell Structure in the Fornax Dwarf Spheroidal Galaxy , 2003, astro-ph/0311241.

[30]  V. Ripepi,et al.  BVI Time-Series Data of the Galactic Globular Cluster NGC 3201. I. RR Lyrae Stars , 2002, astro-ph/0205219.

[31]  G. Clementini,et al.  Distance to the Large Magellanic Cloud: The RR Lyrae Stars , 2000, astro-ph/0007471.

[32]  I. Ivans,et al.  CU Comae: A New Field Double-Mode RR Lyrae Variable, the Most Metal-poor Discovered to Date , 2000, astro-ph/0006174.

[33]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[34]  W. Krzemiński,et al.  CCD PHOTOMETRY OF VARIABLE STARS IN THE GLOBULAR CLUSTER RU 106 , 1995, astro-ph/9505129.

[35]  Peter B. Stetson,et al.  THE CENTER OF THE CORE-CUSP GLOBULAR CLUSTER M15: CFHT AND HST OBSERVATIONS, ALLFRAME REDUCTIONS , 1994 .

[36]  P. Stetson.  DAOPHOT: A COMPUTER PROGRAM FOR CROWDED-FIELD STELLAR PHOTOMETRY , 1987 .

[37]  M. Milgrom.  A modification of the newtonian dynamics as a possible alternative to the hidden mass hypothesis , 1983 .

[38]  B. F. Madore,et al.  The period-luminosity relation. IV. Intrinsic relations and reddenings for the Large Magellanic Cloud Cepheids. , 1982 .

[39]  P. Th. Oosterhoff,et al.  Some remarks on the variable stars in globular clusters , 1939 .

[40]  N. Martin,et al.  A BAYESIAN APPROACH TO LOCATING THE RED GIANT BRANCH TIP MAGNITUDE. II. DISTANCES TO THE SATELLITES OF M31 , 2012 .

[41]  Thanu Padmanabhan,et al.  Stars and stellar systems , 2001 .