Mem. S.A.It. Suppl. Vol. 5, 65c© SAIt 2004

Memorie della

Supplementi

On the occurrence of tidal tails in the CarinadSph

M. Monelli1,2, A.R. Walker3, G. Bono2, R. Buonanno1,2, F. Caputo2, C.E. Corsi2,

M. Dall’Ora1,2 L. Pulone2, H. Smith4, P.B. Stetson4

1 Dipartimento di Fisica, Universita di Roma Tor Vergata, via della Ricerca Scientifica 1,Roma e-mail: monelli@mporzio.astro.it

2 INAF, Sezione di Roma, via Frascati 33 00040 Monteporzio Catone, Rome, Italy3 Cerro Tololo Inter-American Observatory, NOAO, Casilla 603, La Serena, Chile4 Dept. of Physics and Astronomy, Michigan State University, East Lansing, MI 48824,

USA5 Dominion Astrophysical Observatory, Herzberg Institute of Astrophysics, NRC, 5071

West Saanich Road, Victoria, BC V9E 2E7, Canada

Abstract. Wide field B,V photometry of the outer regions of the Carina dwarf spheroidalgalaxy is presented. Six fields were observed to a distance from the centre up to 4.5 degrees.We detected a sizable sample of faint blue objects, with V≥ 23 and B-V≤ 0.4. These objectspopulate the same region of the color magnitude diagram of the Carina old Main Sequencestars. However, it is not clear whether they are tracing a tidal stream connecting Carina tothe Galaxy. We also present some plausible working hypothesis concerning the nature ofthe objects.

Key words. Local Group – Stellar populations – Tidal tails

1. Introduction

The Local Group (LG) is a small sample ofgalaxies, dominated by two giant spirals, theGalaxy and M31. They are surrounded by ≈40 dwarf galaxies of different morphologicaltypes. These dwarf galaxies present quite dif-ferent star formation histories (SFH), gas con-tent, and chemical enrichments (Mateo 1998;Smecker-Hane et al. 1994; Gallart et al.2004). Dwarf galaxies in the LG are a uniquelaboratory to study the formation and evolution

Send offprint requests to: M. MonelliCorrespondence to: Via Frascati 33 00040 MontePorzio Catone, Roma

of galaxies, since their stellar content can be in-vestigated at star-by-star level. This means thatthe evolution of single galaxies, as well as theinteractions with bigger ones, can be studied ona quantitative basis. Amongst dwarf galaxies,the class of dwarf spheroidals (dSphs) is im-portant, because they are stellar systems withlow-mass (M <≈ 107M�), low-surface bright-ness (m >≈ 23 mag arcsec−2), and low-internalvelocity dispersion (σv ≤ 10km/s). Moreover,neither dust nor cool gas has been detected inLG dSphs yet. Their dynamical properties ap-pear to be dominated by dark matter (DM), andindeed the M/L ratio estimated for these sys-tems ranges from 5 (Fornax) to ≈ 300 (Draco,

66 M. Monelli et al.: Tidal tails in Carina dSph

Fig. 1. 2◦ × 2◦ DSS images with overplotted the position of central, C, D, and E fields. The ellipse showsthe tidal radius of Carina. North is up, east is left.

Kleyna et al. 2001). For these reasons dSphsare key systems to test predictions based onΛ Cold Dark Matter (ΛCDM) models. In thisscenario dSph galaxies are the first structuresformed in the Universe, and from which biggergalaxies originate via merging and/or accretionprocesses. Moreover, (ΛCDM) models predictthat stellar systems with masses and internalvelocity dispersion similar to those of dSphsshould have been able to form stars only in an-cient epochs. This means that they should nothave been able to retain enough gas to expe-rience any star formation event in the last fewGyr. Such prediction supports the lack of gasand dust in dSphs, but it is in striking con-strast with the complex star formation historyof many LG dSph, in which a young popula-tion (≤ 1Gy) has been detected. Current es-timates concerning the M/L ratio rely on theassumption that dSphs are relaxed systems,and high values of the M/L follow from thevirial theorem. However, the firm detection ofextra-tidal stars would be a very interestingempirical evidence at odds with this assump-tion. If these systems are tidally interactingwith the Galactic potential well, the M/L ratiomay be significantly overestimated, and in turnthe DM content. It is worth mentioning that

recent theoretical predictions (Hayashi et al.2003; Moore et al. 1999), based on detailedN-body simulations, suggest that the tidal ra-dius of dSphs may be larger by a factor rangingfrom 3 to 20 when compared with current esti-mates. In this context, Carina is a very interest-ing dSph, since it is the protoype of complexSFHs, with a well-defined evidence of a recentepisode (≤ 1Gy, Monelli et al. 2003a). The ex-istence of extra-tidal stars has been discussedin the literature (Kuhn et al. 1996; Majewski etal. 2000; Walcher et al. 2003), but no firm de-tection is available yet. We present deep widefield B.V photometry of six selected areas inthe outskirts of the Carina dSph, and discussthe possible detection of extra-tidal stars.

2. Data set and reduction

Carina is characterized by low central den-sity and large tidal radius (≈ 30′, Irwin& Hatzidimitriou 1995). For these reasonswide field cameras are mandatory to samplea significant fraction of the stellar content ofthis galaxy. Our data were collected with theBlanco 4m CTIO telescope, equipped with theMosaicII camera. The field of view is 36′×36′,with a pixel scale of 0.27”/pix.

M. Monelli et al.: Tidal tails in Carina dSph 67

Fig. 2. CMD of the central field. Solid lines showtheoretical isochrone at fixed chemical composition(Z=0.0004) and ages ranging from 0.6 to 11 Gyr.The dashed line shows the Zero-Age-Horizontal-Branch for the same chemical composition.

To identify extra-tidal stars in Carina weselected four regions mapping both the majorand the minor axis up to 2◦ from the centre.Two more fields were chosen at intermediatedirections and distances of 1 to 4.5◦ from thecentre. For more details see Table 1 and Fig. 1.

field RA DEC exp timename (h) (deg) B V

C 06:44:30 -50:44:00 9600 7200D 06:39:30 -51:18:00 8800 6000E 06:41:38 -50:58:00 5000 3000

Standard IRAF routines were applied forbias subtraction and flat field correction. Thephotometric reductions were performed withDAOPHOTII/ALLFRAME (Stetson 1995) re-duction package. Fig. 2 and Fig. 3 show theCMD obtained from fields C and D. Individual

panels show data collected with a single chipof the CCD camera (note that chip 3 was blindduring the observations). These two fields arelocated approximately 25′ from the Carinacentre and cross the tidal radius of Carina. Thenumber of stars clearly decresases when mov-ing from the centre to the outskirts, but there isa blue spur of faint objects (B − V ≤ 0.4,V >≈23) that is present in all the panels. These ob-jects have the same colours and magnitudes ofthe old Main Sequence (MS) of Carina, and arelocated at distances from the centre of the or-der of ≈ 40′, i.e. well beyond the Carina tidalradius (Irwin & Hatzidimitriou 1995). Notethat the stars plotted in this CMD have beendetected in all the images. Fig. 2 shows the cu-mulative CMD of field E, located at one degreefrom the Carina centre. It is noteworthy that thesame blue spur still appears.

Preliminary reduction of other three exter-nal regions support the detection of the bluespur sequence until 4.5◦ from the Carina cen-tre. Moreover, the blue spur seems to be lesspopulated at larger distances from the centre(see Table 1). However, integration time for theexternal fields are quite different, and thereforea quantitative analysis has not been performedyet.

3. Blue spur stars

Before we can assess the detection of such astrong component of extra-tidal stars, we haveto analyze any possible source that can con-taminate our sample. There are two main possi-ble culprits: galactic field stars and backgroundgalaxies.

4. Conclusions

• We computed a synthetic CMD(Degl’Innocenti, (priv. comm.)) of theGalactic component for all our fields. Figure5 shows an example comparing the real datawith the simulation. All the features of theGalactic field are well represented by themodel, but it predicts only a few tens of thickdisk white dwarfs in the region correspondingto the blue spur. This supports the evidence

68 M. Monelli et al.: Tidal tails in Carina dSph

Fig. 3. CMD of field C. The eight panels clearly show the decreasing number of Carina stars, and theoccurrence of a blue spur of faint objects with B − V < 0.4 and V > 23.

Fig. 4. Same as Fig. 3, but for field D.

that the blue spur is marginally populated byforeground stars.• On the basis of current photometric

data it is not possible separate backgrounggalaxies from stars. To solve this problem we

decided to use color-color planes based onnear-ultraviolet and optical data. Interestiglyenough, we found that the U − V vs B−I col-ors allow a good separation between stars and

M. Monelli et al.: Tidal tails in Carina dSph 69

Fig. 5. Cumulative CMD of field E. This field is lo-cated 1◦ from the Carina centre in the southern di-rection.

Fig. 6. Theoretical predictions based on Galacticmodels for the field E. Note that in the CMD re-gion covered by blue spur stars are only present afew tens of disk white dwarfs.

galaxies. Figure 6 shows galactic evolution-ary tracks for different morphological typesand redshift smaller than 2 (Fioc & Rocca-

Volmerange 1997), together with several stel-lar samples, namely field stars (Fontana etal. 2000), Reticulum (an old LMC globularcluster, Monelli et al. 2003b, middle), andNGC 3201 (Bedin et al. 2000, bottom). Dataplotted in this figure show that the U − V vsB − I plane might allow us to split stars frombackground galaxies, since they are distributedin different regions.Note that the metal con-tent marginally affects this approach, becausethe metallicity of the selected stellar samplesranges from solar (field stars) to [Fe/H]=-1.7(Reticulum).

We have presented deep, wide-field B,Vphotometry of several regions in the outskirtsof the Carina dSph galaxy. We detected a bluespur of faint objects that present the same col-ors and magnitudes of the old MS stars ofCarina. These objects are present in all the ob-served fields, up to a distance of 4.5◦ fromthe centre. Comparisons with synthetic CMDsbased on Galactic models strongly suggest that

Fig. 7. Color-color U − V vs B − I plane, compar-ing three samples of stars of different metallicitieswith evolutionary tracks for galaxies. These tracksaccount for different morphological types for red-shift smaller than 2. Deep and accurate photometryin four bands will allow us to split stars from galax-ies, if any.

70 M. Monelli et al.: Tidal tails in Carina dSph

the contamination by Galactic field stars ismarginal. However, on the basis of current datait is not possible to supply a quantitative esti-mate of the number of background galaxies ineach field. We plan to address this open prob-lem with new multiwavelength observations.This will allow us to put firm constraints onthe detection of either extra-tidal stars or an ex-tended halo in the Carina dSph.

Acknowledgements. Part of this work was sup-ported by the Italian MIUR through the grantCOFIN-2002028935 assigned to the project Stellarpopulations in the Local Group Galaxies

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