The VVV color-magnitude diagram of the Galactic bulge

Asociación Argentina de Astronomía BAAA, Vol. 54, 2011 J.J. Clariá, P. Benaglia, R. Barbá, A.E. Piatti & F.A. Bareilles, eds.

PRESENTACIÓN ORAL

The VVV color-magnitude diagram of the Galactic bulge R. K. Saito1 , D. Minniti1,2,3 & the VVV Collaboration (1) Pontificia Universidad Católica de Chile, Chile (2) Vatican Observatory, Vatican City State, Italy (3) Peyton Hall, Princeton University, NY, USA Abstract. We present the 100+ million star color-magnitude diagram for the Galactic bulge based on the VVV survey data. This is the hugest CMD ever published for a large homogeneous data set. We discuss the differences in the morphology and the effects caused by crowding and extinction. Resumen. Se presenta el diagrama color-magnitud de +100 millones de estrellas para el bulbo galactico basado en los datos del Survey VVV. Éste es el CMD más grande publicado usando un conjunto de datos homogéneos. Se discuten las diferencias en la morfología y los efectos causados por alta densidad y extinción.

1.

Introduction

The bulge of the Milky Way is a fundamental galactic component to understand the formation and evolution not only of our Galaxy, but of galaxies in general. However, observations of the Galactic center are difficult since they are affected by extinction and crowding. Past near-IR surveys are limited to bright sources (e.g., 2MASS; Skrutskie et al. 2006) while optical surveys (e.g., OGLE; Udalski et al. 1993) are highly affected by extinction, and do not allows us a complete view of the Galactic bulge. The new VISTA Variables in the Vía Láctea (VVV) Survey maps 562 sq. deg. in the southern disk and bulge of the Galaxy. VVV observes in five near-IR passbands (ZY JHKs ) reaching Ks ∼ 18, 5 mag, as well as performs a variability campaign spanning over 5 years (Minniti et al. 2010). Particularly for the bulge area, VVV covers ∼ 314 deg2 within −10.0◦ . l . +10.4◦ and −10.3◦ . b . +5.1◦ . Here we present the +100 million star color-magnitude diagram (CMD) for the Galactic bulge based on the VVV data. This is the hugest CMD ever published for a large homogeneous data set.

2.

Observational data and catalogues

The VVV bulge area is covered by 196 pointings called “tiles”, each one covering 1.64 deg2 in the sky. The number of point sources found in each catalogue varies from ∼ 500, 000 at the outermost region and up to 1, 500, 000 sources per tile at the innermost Galactic Center. The single-band J, H and KS catalogues were matched resulting in a multiband, JHKs catalogue for each bulge tile. 285

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Figure 1. Density plot showing the VVV bulge area. The map is made using the point sources brighter than Ks = 16.5 mag present in the JHKs multiband catalogues. High Crowded areas appear in yellow, while less populated regions as well as high extincted areas are shown in blue. The overlapping regions between tiles are highlighted since the point sources are accounted twice.

Figure 1 shows the VVV bulge area as a density map corresponding to sum of all bulge multiband catalogues. In order to equalize the increasing sky brightness towards the Galactic Center due to the contribution of underlying/unresolved faint stars, a cut in Ks = 16.5 mag was applied. Highly density areas appear in yellow, while less populated regions as well as high extinction areas are shown in blue. The overlapping regions between tiles are highlighted since the sources are accounted twice. The density changes abruptly between few adjacent tiles, due to the rejection of some sources in lower quality images. 3.

The 100+ million star CMD

Figure 2 shows in the left-hand panel the CMD of the Galactic Bulge based on the VVV multiband catalogues, with a total of 147 million point sources. The CMD is shown as a density plot, with contour curves marking same density levels. The data include also point sources fainter than Ks = 16.5 mag, in contrast with the cut in magnitude applied in Fig. 1. Sources on the overlapping regions between tiles are present and accounted twice. These comprise ∼ 10% of the total point sources. Since the overlapping areas are equally spaced along the bulge they do not produce any bias or special trend in the CMDs. Point sources in the VVV catalogues receive different flags according with the most probable morphological classification (see Saito et al. 2011b). The right-hand panel of Fig. 2 shows the CMD built only with sources flagged as stellar in all J, H, and Ks catalogues, in a total of 77.4 million sources. Structures are clearer seen in the stellar CMD, since it contains only the best photometry data points.

The 100M star VVV CMD of the Galactic bulge

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Figure 2. The left-hand panel shows the CMD for all point sources found in the VVV bulge area while the right-hand panel shows stellar sources only. Contour lines mark the density levels. The total number of sources in each panel is shown in the top-left of the diagrams.

The CMD has an elaborated shape, mostly due to the complexity of the stellar population, seen at different ranges of magnitudes and colors, and the effects of extinction and reddening towards the Galactic Center. In particular, the red giant brunch (RGB) is seen very broad, with the main peak of the red giant clump (RGC) at (J − Ks ), Ks ∼ 1.05, 13.20. The RGC has structures ranging in color and magnitude, with a secondary peak at (J − Ks ), Ks ∼ 0.85, 13.20, and an elongated structure in magnitude with ∆Ks ∼ 1 mag. These are caused mostly due to the X-shaped bulge of the MW (e.g., Saito et al. 2011a), producing a double clump in magnitude; and differential extinction, producing multiple structures in color. The RGC extends along the direction of the reddening vector for more than ∆(J − Ks ) > 4. Fig. 3 presents CMDs for smaller regions in the bulge, each one containing 8 to 12 tiles (see dashed contours in Fig.1). The regions are on the minor axis, starting at the outermost bulge area (left-most panel, b = −9.02◦ ), and reaching the Galactic Center at the right-most panel (l, b = +0.15, +0.26). The outermost region is the the poorly affected by reddening, and the CMD shows a prominent bulge main-sequence turn-off reaching Ks ∼ 18.5 mag. The sequence with disk main sequence (MS) stars is seen at the left side and the bulge red giant branch (RGB) at the right, with red clump giants (RCG) at Ks ∼ 13 mag. At the right-most side is also possible to see the a sequence of late dwarfs. The affects of extinction and reddening increase towards the Galactic Center as well as the unresolved stars contribute to increase the the underground brightness limit. The CMD at the middle-left panel (l, b = +0.21, −6.28) shows structures similar than that appearing in the outermost region, but the RCG is more prominent and presents a double-peaked structure, with the farther RC at Ks ∼ 13.4, and the closest one at Ks ∼ 12.9. The CMD at l, b = +0.16, −3.01 (middle-right panel) is affected by reddening and extinction at different levels and the RCG spreads along the reddening vector and, together with red giant branch, seems to be double in color. That can be caused either by reddening effects or for distinct stellar content in the same line of sight.

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Figure 3. CMD for 4 different areas along the bulge. The areas have 8-12 tiles (see dashed contours in Fig. 1). The central (l, b) coordinates and the mean density (sources/deg 2 ) for each area are shown in the panels.

The Galactic Center is sampled right-hand panel (l, b = +0.15, +0.26). This region is strongly affected by reddening and extinction. The sky brightness is even higher, with the faintest objects seen at Ks ∼ 16.5 mag, reason to the cut in magnitude applied in Fig. 1. 4.

Discussion and Conclusions

We have presented the hugest CMD ever published for a large homogeneous dataset, based on the VVV survey data. The VVV bulge CMD provides valuable information about the stellar contents and structure of the MW which will be the subject of forthcoming papers. Acknowledgments. We acknowledge use of data from the ESO Public Survey programme ID 179.B-2002 taken with the VISTA telescope, data products from the CASU, and funding from the FONDAP Center for Astrophysics 15010003, the BASAL CATA Center for Astrophysics and Associated Technologies PFB-06, the FONDECYT from CONICYT, and the Ministry for the Economy, Development, and Tourism’s Programa Iniciativa Científica Milenio through grant P07-021-F, awarded to The Milky Way Millennium Nucleus, and from CONICYT through Gemini Project No. 32080016. DM acknowledge support by Proyecto FONDECYT Regular No. 1090213. References Minniti, D., Lucas, P. W., Emerson, J. P., et al. 2010, New Astronomy, 15, 433 Saito, R. K., Zoccali, M., McWilliam, A., et al. 2011, AJ, 142, 76 Saito, R. K., Hempel, M., Minniti, D., et al. 2011, arXiv:1111.5511 Skrutskie, M. F., Cutri, R. M., Stiening, R., et al. 2006, AJ, 131, 1163 Udalski, A., Szymanski, M., Kaluzny, J., Kubiak, M., & Mateo, M. 1993, Acta Astron., 43, 69