In this contribution we summarize results on the search for variable stars and the study of the resolved stellar populations in four dwarf spheroidal satellites of the Andromeda galaxy that we have observed with the Large Binocular Cameras (LBC) at the Large Binocular Telescope (LBT)

CLEMENTINI, Gisella

CUSANO, FELICE

Garofalo, A.

OA@INAF - Istituto Nazionale di Astrofisica

2016Publication Year2020-04-28T13:18:18ZAcceptance in OA@INAFRR Lyrae stars in the Andromeda satellite galaxiesTitleCUSANO, FELICE; Garofalo, A.; CLEMENTINI, GisellaAuthorshttp://hdl.handle.net/20.500.12386/24279HandleCOMMUNICATIONS FROM THE KONKOLY OBSERVATORY OF THE HUNGARIAN ACADEMY OF SCIENCESSeries105NumberRRL2015 – High-Precision Studies of RR Lyrae StarsL. Szabados, R. Szabó, K. Kinemuchi (Eds.) 2016RR Lyrae stars in the Andromeda satellite galaxiesFelice Cusano1, Alessia Garofalo1,2, & Gisella Clementini11INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127,Bologna, Italy2Dipartimento di Fisica e Astronomia-Universitá di Bologna, VialeBerti Pichat 6/2, I-40127 Bologna, ItalyAbstract. In this contribution we summarize results on the search for vari-able stars and the study of the resolved stellar populations in four dwarfspheroidal satellites of the Andromeda galaxy that we have observed with theLarge Binocular Cameras (LBC) at the Large Binocular Telescope (LBT).1. IntroductionIn the context of the Λ-CMD theory massive galaxies like the Milky Way (MW)and Andromeda (M31) form by accretion and merging of smaller structures(e.g. Bullock & Johnston 2005). Candidates to be the possible residual buildingblocks of this accretion process are the satellite galaxies observed nowadaysaround the MW and M31. However, some issues challenge this scenario, like,for instance, the missing satellite problem (Moore et al. 1999) or the observedalignment of a large fraction of the satellite galaxies in planar structures (see e.g.Ibata et al. 2013). In this framework, we have obtained multi-band photometricobservations with the Large Binocular Cameras (LBC) at the Large BinocularTelescope (LBT) of a sample of M31 satellites (see Cusano et al. 2013) tocharacterize both their variable and constant star populations. The final aimof our survey is to derive hints on the formation history of the M31’s satellitesand relate it to the global context of merging and accretion episodes occurringin M31.2. Observations and data reductionTime series photometry in the B and V bands was obtained with the LBC atthe foci of the LBT for four M31 satellites, namely, And XIX (Cusano et al.2013), And XXI (Cusano et al. 2015), And XXV (Cusano et al. 2016) andAnd XXVII (Cusano et al., in preparation). Observations in the B band wereobtained with the Blue camera of the LBC, whereas the V images were acquiredwith the Red camera. PSF photometry of the pre-reduced images was performedusing the DAOPHOT-ALLSTAR-ALLFRAME package (Stetson 1987, 1994). Variablestars were identified using the variability index computed in DAOMASTER (Stetson1994), then the light curves of the candidate variables were analyzed with theGraphical Analyzer of Time Series (GRATIS), custom software developed at theBologna Observatory (see e.g. Clementini et al. 2000). Examples of light curvesfor two RR Lyrae stars (a fundamental-mode − RRab and a first-overtone −163164 Cusano, Garofalo, & ClementiniRRc) and an Anomalous Cepheid (AC) that we detected in And XXV are shownin Figure 1.Table 1. Summary of number and properties of the variable stars identifiedin M31 satellite galaxiesName N (RRab+RRc) 〈Pab〉 N(AC) E(B − V ) (m − M)0 GpoS RefmemberAnd I 72+26 0.57 1? 0.05 ± 0.01 24.49 ± 0.06 yes 1And II 64+8 0.57 1 0.06 ± 0.01 24.11 ± 0.06 no 2And III 39+12 0.66 4 0.05 ± 0.01 24.38 ± 0.06 yes 1And VI 91+20 0.59 6 0.06 ± 0.01 24.55 ± 0.07 no 3And XI 10+5 0.62 0 0.11 24.33 ± 0.05 yes 4And XIII 12+5 0.66 0 0.15 24.62 ± 0.05 yes 4And XIX 23+8 0.62 8 0.11 ± 0.06 24.52 ± 0.23 no 5And XXI 37+4 0.63 9 0.15 ± 0.04 24.40 ± 0.17 no 6And XXV 41+11 0.60 1 0.06 ± 0.04 24.62 ± 0.16 yes 7And XXVII 55+36 0.59 1 0.09 ± 0.05 24.57 ± 0.25 yes 81 Pritzl et al. (2005); 2 Pritzl et al. (2004); 3 Pritzl et al. (2002);4 Yang & Sarajedini (2012); 5 Cusano et al. (2013); 6 Cusano et al. (2015);7 Cusano et al. (2016); 8 Cusano et al., in preparationFigure 1. Light curves of a fundamental-mode (upper panel) and first-overtone (lower panel) RR Lyrae stars and of an AC (middle panel) inAnd XXV; left: in B band, right: in V band.RR Lyrae stars in the Andromeda satellite galaxies 165Figure 2. CMD and stellar populations in And XIX. Panel A: CMD of theobjects in half light radius of the galaxy. Panel B: same as panel A, but thered filled circles are RR Lyrae stars, while the green filled squares are ACs.The blue dashed line is the isochrone at 13 Gyr and Z = 0.0003. Panel C:isochrones at 6 and 10 Gyr with Z = 0.0004. Panel D: isochrones at 0.75 and1.25 Gyr with Z = 0.0004. The isochrones are from Bressan et al. (2012).3. Results and conclusionsWe discovered a total of 215 RR Lyrae stars and 19 Anomalous Cepheids (ACs)in the four M31 satellite galaxies observed with our program. The number andtype of variable stars found in each galaxy are summarized in the last fourrows of Table 1, whereas the six upper rows report the census and propertiesof the variable stars detected in other M31 satellite galaxies analysed so far forvariability. In particular, the average period of the fundamental-mode RR Lyraestars (〈Pab〉) is given in column 3.The M31 satellite galaxies that we have observed with the LBT can be classifiedas Oosterhoff Intermediate (Oo-Int) systems (see Oosterhoff 1939; Catelan 2009,for a description of the different Oosterhoff types). A comparison of the 〈Pab〉values reported in Table 1, shows that the majority of the M31 satellites haveOo-Int properties, with only And III and And XIII being Oosterhoff type II(Oo II) systems. In column 7 of Table 1 we report also the membership ofeach galaxy to the Great plane of satellites (GpoS) discovered by Ibata et al.166 Cusano, Garofalo, & Clementini(2013). The average period of the RRab stars for galaxies on and off the plane is〈Pab〉 = 0.62 ± 0.07 d and 〈Pab〉 = 0.60 ± 0.06 d, respectively. Both populationsare thus compatible with an Oo-Int classification. The number of ACs insteaddiffers significantly among satellites and galaxies on the GpoS have very fewACs when compared to the galaxies off the plane.Using the RR Lyrae stars we also derived independent estimates of the redden-ing and distance to the galaxies investigated by us. These values are summarizedin columns 5 and 6 of Table 1, respectively.Besides identifying the variable stars we also built colour-magnitude diagrams(CMDs) to characterize the stellar populations of these four satellite galax-ies. We found evidence for three different stellar generations in And XIX andAnd XXI (see Fig. 2) and of a single old star formation burst in And XXV andAnd XXVII. We note that And XIX and And XXI do not belong to Ibata et al.’sGpoS, while And XXV and And XXVII lie on it. This and the different numberof ACs led us to speculate on whether there might be a connection between starformation complexity/simplicity and GpoS membership.The large field of view of the LBC allowed to accommodate in just one pointingthe half light radius of each of the four galaxies we have observed with the LBTand thus study the spatial distribution of the different stellar components in eachgalaxy. The location of the RR Lyrae stars along with the isodensity contoursof stars in selected regions of the CMD helped us to trace signatures of pastand/or ongoing gravitational interactions with M31 or other dwarf galaxies. Inparticular, we found evidence for And XXI to be the result of a past minormerger between two dwarf galaxies.ReferencesBressan, A., Marigo, P., Girardi, L., et al. 2012, MNRAS, 427, 127Bullock, J. S., Johnston, K. V. 2005, ApJ, 635, 931Catelan, M. 2009, Ap&SS, 320, 261Clementini, G., Di Tomaso, S., Di Fabrizio, L., et al. 2000, AJ, 120, 2054Cusano, F., Clementini, G., Garofalo, A., et al. 2013, ApJ, 779, 7Cusano, F., Garofalo, A., Clementini, G., et al. 2015, ApJ, 806, 200Cusano, F., Garofalo, A., Clementini, G., et al. 2016, submitted to ApJIbata, R. A., Lewis, G. F., Conn, A. R., et al. 2013, Nature, 493, 62Moore, B., Ghigna, S., Governato, F., et al. 1999, ApJ, 524, L19Oosterhoff, P. T. 1939, The Observatory, 62, 104Pritzl, B. J., Armandroff, T. E., Jacoby, G. H., et al. 2002, AJ, 124, 1464Pritzl, B. J., Armandroff, T. E., Jacoby, G. H., et al. 2004, AJ, 127, 318Pritzl, B. J., Armandroff, T. E., Jacoby, G. H., et al. 2005, AJ, 129, 2232Stetson, P. B. 1987, PASP, 99, 191Stetson, P. B. 1994, PASP, 106, 250Yang, S.-C., Sarajedini, A. 2012, MNRAS, 419, 1362

RR Lyrae stars in the Andromeda satellite galaxies

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RR Lyrae stars in the Andromeda satellite galaxies

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