Mon. Not. R. Astron. Soc. 000, 1–?? (2008) Printed 1 November 2018 (MN LATEX style file v2.2) Stellar populations in the Canis Major over-density Giovanni Carraro1⋆, Andr´eMoitinho2, and Ruben A. V´azquez3† 1ESO, Casilla 19001, Santiago 19, Chile 2SIM/IDL, Faculdade de Ciˆencias de Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016, Lisboa, Portugal 3Facultad de Ciencias Astron´omicas y Geof´ısicas de la UNLP, IALP-CONICET, Paseo del Bosque s/n 1900, La Plata, Argentina Accepted 1988 December 15. Received 1988 December 14; in original form 1988 October 11 ABSTRACT We performed a photometric multicolor survey of the core of the Canis Major over- density at l ≈ 244o, b ≈−8.0o, reaching V ∼ 22 and covering 0o.3×1o.0. The main aim is to unravel the complex mixture of stellar populations toward this Galactic direction, where in the recent past important signatures of an accretion event have been claimed to be detected. While our previous investigations were based on disjointed pointings aimed at revealing the large scale structure of the third Galactic Quadrant, we now focus on a complete coverage of a smaller field centered on the Canis Major over- density. A large wave-length baseline, in the UBV RI bands, allows us to build up a suite of colour colour and colour magnitude diagrams, providing a much better diagnostic tool to disentangle the stellar populations of the region. In fact, the simple use of one colour magnitude diagram, widely employed in all the previous studies defending the existence of the Canis Major galaxy, does not allow one to separate the effects of the different parameters (reddening, age, metallicity, and distance) involved in the interpretation of data, forcing to rely on heavy modeling. In agreement with our previous studies in the same general region of the Milky Way, we recognize a young stellar population compatible with the expected structure and extension of the Local (Orion) and Outer (Norma-Cygnus) spiral arms in the Third Galactic Quadrant. Moreover we interpret the conspicuous intermediate-age metal poor population as belonging to the Galactic thick disk, distorted by the effect of strong disk warping at this latitude, and to the Galactic halo. Key words: Milky Way – structure: stars. arXiv:0801.2084v1 [astro-ph] 14 Jan 2008 1 INTRODUCTION characterized by significant absorption windows as the Pup- pis (l ∼ 243o) window (Fitzgerald 1968, Moffat et al. 1979, In the last years, we have used photometric observations Janes 1991, Moitinho 2001), which allows one to detect very of young open cluster fields to probe the spiral structure distant star clusters (Baume et al. 2006). Besides, and in- in the third Galactic Quadrant (TGQ, 180o 6 l 6 270o; terestingly, young star clusters are found at low Galactic Carraro et al. 2005a, Moitinho et al. 2006, V´azquez et al. latitudes, underlining the fact that the young Galactic disk 2008), motivated by the very poor knowledge of this por- is significantly warped in these directions (May et al. 1997, tion of the Galaxy’s periphery. Interestingly, important low Momany et al. 2004, Moitinho et al. 2006, Momany et al. latitude accretion phenomena have been recently claimed 2006, L´opez-Corredoira et al. 2007). to be ongoing in this part of the Galaxy, such as the Ca- nis Major over-density (CMa, Bellazzini et al. 2004), and In V´azquez et al (2008), by combining optical and CO the Monoceros Ring (MRi, Newberg et al. 2002). Clearly, observations, we have provided a fresh and very detailed a detailed description of the structure and stellar popula- picture of the spiral structure in the TGQ, showing that tions of the Galactic disc (thin plus thick) is mandatory to this region is characterized by a complicated spiral pattern. discriminate between Galactic and extragalactic material. The outer (Norma-Cygnus) arm is found to be a grand de- The TGQ is a special region of the Milky Way’s outskirts, sign spiral feature defined by young stars, whereas the re- gion closer to the Sun (d⊙ less than 9 kpc) is dominated by a conspicuous inter-arm structure, at l ∼ 245o, the Lo- ⋆ On leave from Dipartimento di Astronomia, Universit´a di cal spiral arm. In this region, Perseus is apparently defined Padova, Italy by gas and dust, and does not appear to be traced by an † E-mail: [email protected] (GC); an- evident optical young stellar population, similarly to what [email protected](AM);[email protected] (RAV); can be found in other galaxies such as M 74 (V´azquez et c 2008 RAS 2 G. Carraro et al. Figure 1. 20 arcmin on a side field in the CMa over-density (Field Figure 3. 20 arcmin on a side field in the CMa over-density (Field 1). This field is centered at RA = 07:22:51, DEC = -30:59:20. 3). This field is centered at RA = 07:20:21, DEC= -31:15:43. North is up, East to the left North is up, East to the left. many et al. 2006), the lines of sight are expected to cross both the thin and thick disk population in front of a partic- ular target, in a way that the analysis of the CMD becomes very challenging (see for instance the analysis of the field toward the star cluster Auner 1, Carraro et al. 2007). In this paper we present a photometric analysis in the UBV RI filters of the stellar populations in 3 wide field pointings toward the CMa over-density. Sect. 2 describes the observation and data reduction strategies. In Sect. 3 we discuss various colour combination CMDs, while Sects. 4 and 5 are dedicated to illustrate and analyze the TCD as a function of magnitude. Finally, Sect. 6 summarizes our findings. 2 OBSERVATIONS AND DATA REDUCTION 2.1 Observational details UBV RI images of 3 overlapping fields (see Figs. 1 to 3) in the Third Quadrant of the Milky Way toward the Figure 2. 20 arcmin on a side field in the CMa over-density (Field CMa over-density were obtained at the Cerro Tololo Inter- 2). This field is centered at RA = 07:20:46, DEC = -31:09:36. American Observatory 1.0m telescope, which is operated by North is up, East to the left. the SMARTS1 consortium. The telescope is equipped with a new 4k×4k CCD camera having a pixel scale of 0′′.289/pixel which allows to cover a field of 20′ × 20′ on the sky. Obser- al. 2008). The analysis carried out on a substantial frac- vations were carried out on the nights of November 28 and tion of the stellar fields we observed revealed a complicated December 3, 2005. The two nights were part of a 6 night mixture of young and old populations. Although centered run. In the first night we observed the fields #1 and #2 (see on catalogued star clusters (Dias et al. 2002), a few colour Table 1) while field #3 was observed in the last night of the magnitude diagrams (CMD) do not reveal star clusters but, run. and more interestingly, show hints of a young, diffuse, and The CCD is read out through 4 amplifiers, each one distant stellar populations, which has become recently re- with slightly different bias levels and gains. Pre-processing ferred to as Blue Plume ( Bellazzini et al. 2004, Dinescu et al. 2005, Mart´ınez-Delgado et al. 2005,Carraro et al. 2005 ). Since the disk is warped and flared in these directions (Mo- 1 http://www.astro.yale.edu/smarts/ c 2008 RAS, MNRAS 000, 1–?? Stellar populations in the Canis Major over-density 3 Designation α(2000.0) δ(2000.0) l b U B V R I Airmass Seeing [deg] [deg] secs secs secs secs secs arcsec Field 1 07:22:51 -30:59:20 244.00 -07.50 20,180,1800 10,150,1500 5,60,900 5,60,900 5,60,900 1.00-1.30 0.83-1.02 Field 2 07:20:46 -31:09:36 244.00 -08.00 20,180,1800 10,150,1500 5,60,900 5,60,900 5,60,900 1.00-1.30 0.90-1.12 Field 33 07:20:21 -31:15:43 244.00 -08.10 20,180,1800 10,150,1500 5,60,900 5,60,900 5,60,900 1.20-1.80 1.28-2.11 Table 1. List of pointings discussed in this paper. For each pointing, equatorial and galactic coordinates are reported together with the set of filters used, and the range of exposure time, air-mass and typical seeing. The fields are shown in Fig. 1 to 3 Photometry from this last night was tied to the other nights through the comparison of stars in common. Since the photometric solutions were identical, all the stan- dard star measurements were used together in obtaining (V-I) a single photometric solution for the entire run. This re- sulted in calibration coefficients derived using about 200 standard stars. Photometric solutions have been calculated following Patat & Carraro (2001). Fig. 4 shows the run of (V-R) magnitude differences (standard versus instrumental) for the whole standard set. Notice that the colour baseline is suffi- ciently broad. On the right, the rms of the fit is shown for (B-V) each colour. The calibration equations read: u = U + u1 + u2(U − B)+ u3X (1) b = B + b1 + b2(B − V )+ b3X (2) v V v v B − V v X (B-V) = + 1 + 2( )+ 3 (3) r = R + r1 + v2(V − R)+ r3X (4) i = I + i1 + i2(V − I)+ i3X (5), where UBV RI are standard magnitudes, ubvri are the in- (U-B) strumental magnitudes, X is the airmass, and the derived coefficients are presented in Table 2.