Astronomical Science

VISTA Variables in the Vía Láctea (VVV): Current Status and First Results

Roberto Saito1 Maria Messineo 33 13 Universidade de São Paulo, Brazil Maren Hempel1 Félix Mirabel 34, 35 14 Facultad de Ciencias Astronómicas y Javier Alonso-García1 Lorenzo Monaco 3 Geofísicas, Universidad Nacional de Ignacio Toledo1 Christian Moni Bidin 22 La Plata, Argentina Jura Borissova 2 Lorenzo Morelli 36 15 Space Telescope Science Institute, Oscar González 3 Nelson Padilla1 Baltimore, USA Juan Carlos Beamin1 Tali Palma7 16 Universidade Federal do Rio Grande do Dante Minniti1, 4 Maria Celeste Parisi7 Sul, Porto Alegre, Brazil Philip Lucas 5 Quentin Parker 37,38 17 Departamento de Astronomía, Univer­ Jim Emerson 6 Daniela Pavani16 sidad de Chile, Santiago, Chile Andrea Ahumada7, 3, 8 Pawel Pietrukowicz 39 18 Institute for Astronomy, The University Suzanne Aigrain 9, 24 Grzegorz Pietrzynski 22, 40 of Edinburgh, UK Maria Victoria Alonso7 Giuliano Pignata 41 19 Joint Astronomy Centre, Hilo, USA Eduardo Amôres10 Marina Rejkuba 3 20 Kavli Institute for Astronomy and Astro­ Rodolfo Angeloni1 Alejandra Rojas1 physics, Peking University, China Julia Arias11 Alexandre Roman-Lopes11 21 Astrophysics Group, Imperial College Reba Bandyopadhyay12 Maria Teresa Ruiz17 London, UK Rodolfo Barbá11 Stuart Sale1, 2 22 Departmento de Astronomía, Universi­ Beatriz Barbuy13 Ivo Saviane 3 dad de Concepción, Chile Gustavo Baume 14 Matthias Schreiber 2 23 Max-Planck Institute for Astronomy, Luigi Bedin15 Anja Schröder 42, 43 Heidelberg, Germany Eduardo Bica16 Saurabh Sharma 2 24 Department of Astrophysics, University Leonardo Bronfman17 Michael Smith44 of Oxford, UK Giovanni Carraro 3 Laerte Sodré Jr.13 25 Department of Physics, University of Márcio Catelan1 Mario Soto11 Cincinnati, USA Juan Clariá7 Andrew Stephens45 26 School of Physics & Astronomy, Univer­ Carlos Contreras1 Motohide Tamura46 sity of Leeds, UK Nicholas Cross18 Claus Tappert 2 27 Institute of Astronomy, University of Christopher Davis19 Mark Thompson 5 Cambridge, UK Richard de Grijs 20 Elena Valenti 3 28 Jodrell Bank Centre for Astrophysics, István Dékány1 Leonardo Vanzi47 The University of Manchester, UK Janet Drew 5, 21 Walter Weidmann7 29 NASA–Ames Research Center, Moffett Cecilia Fariña14 Manuela Zoccali1 Field, USA Carlos Feinstein14 30 Instituto de Astrofísica de Canarias, Eduardo Fernández Lajús14 Tenerife, Spain Stuart Folkes 2 1 Departamento de Astronomía y 31 School of Physics and Astronomy, Roberto Gamen14 Astrofísica, Pontificia Universidad University of Southampton, UK Doug Geisler 22 Católica de Chile, Santiago, Chile 32 Istituto di Astrofisica Spaziale e Fisica Wolfgang Gieren 22 2 Departamento de Física y Astronomía, Cosmica di Bologna, Italy Bertrand Goldman 23 Universidad de Valparaíso, Chile 33 ESTEC, Noordwijk, the Netherlands Andrew Gosling 24 3 ESO 34 Service d’Astrophysique — IRFU, Guillermo Gunthardt11 4 Vatican Observatory, Italy CEA–Saclay, France Sebastian Gurovich7 5 Centre for Astrophysics Research, 35 Instituto de Astronomía y Física del Nigel Hambly18 University of Hertfordshire, Hatfield, UK Espacio, Buenos Aires, Argentina Margaret Hanson 25 6 School of Mathematical Sciences, 36 Dipartimento di Astronomia, Universitá Melvin Hoare 26 Queen Mary, University of London, UK di Padova, Italy Mike Irwin 27 7 Observatorio Astronómico de Córdoba, 37 Department of Physics, Macquarie Valentin Ivanov 3 Argentina University, Sydney, Australia Andrés Jordán1 8 Consejo Nacional de Investigaciones 38 Australian Astronomical Observatory, Eamonn Kerins 28 Científicas y Técnicas, Buenos Aires, Epping, Australia Karen Kinemuchi 29 Argentina 39 Nicolaus Copernicus Astronomical Radostin Kurtev 2 9 School of Physics, University of Exeter, Center, Warsaw, Poland Andy Longmore18 UK 40 Warsaw University Observatory, Poland Martin López-Corredoira 30 10 SIM, Faculdade de Ciências da Univer­ 41 Departamento de Ciencias Físicas, Tom Maccarone 31 sidade de Lisboa, Portugal Universidad Andres Bello, Santiago, Eduardo Martín 30 11 Departamento de Física, Universidad Chile Nicola Masetti 32 de La Serena, Chile 42 SKA/KAT, Cape Town, South Africa Ronald Mennickent 22 12 Department of Astronomy, University of 43 Hartebeesthoek Radio Astronomy David Merlo7 Florida, USA Observatory, South Africa

24 The Messenger 141 – September 2010 44 The University of Kent, Canterbury, UK Figure 1. Illustration 45 National Astronomical Observatory of showing the Milky Way galaxy and the area Japan, Tokyo, Japan being observed by the 46 Gemini Observatory, Hawaii, USA VVV Survey (based on 47 Departamento de Ingeniería Eléctrica, Figure 16 of Churchwell Pontificia Universidad Católica de Chile, et al., 2009). Santiago, Chile

VISTA Variables in the Vía Láctea (VVV) is a public ESO near-IR variability ­survey aimed at scanning the Milky Way Bulge and an adjacent section of the mid-plane. VVV observations started in October 2009 during ESO science veri- (1.64 μm) and Ks- (2.14 μm) bands of a Pipeline processing and calibration of fication. Regular observations for the field at Galactic coordinates l = 2.2º, the VVV data are performed by the UK first year of the survey have been con- b = –3.1º. An additional thirteen epochs Cambridge Astronomy Survey Unit ducted since February 2010 and will in Ks-band were also taken, since the (CASU) using the VISTA Data Flow Sys­ cover a total area of 520 square degrees variability study will be done only in this tem (VDFS) pipeline3. The VISTA Science in five passbands and five epochs. Here band. This is one of the most crowded Archive (VSA) at the Wide Field Astron­ we address the first results obtained fields in the Galactic Bulge, allowing us omy Unit (WFAU) in Edinburgh 4 performs: from the VVV Survey as well as the cur- to test whether there are too many satu­ (i) image stacking to produce stacked rent status of the observations. rated stars. It is also known to contain and subtracted tiles; (ii) photometric and numerous variable stars, allowing us to astrometric calibration; (iii) source merg­ test their detectability. In addition to this ing; (iv) quality control; and (v) identifica­ Introduction field we also observed three other tiles tion of variable sources. ­Photometric cali­ (each tile corresponds to a sky field of bration on the VISTA system is done via VISTA Variables in the Vía Láctea is one 1 × 1.5 deg2) in the Bulge in Ks-band to thousands of unsaturated Two Micron All of the six ESO Public Surveys selected test different sky subtraction strategies. Sky Survey (2MASS) stars present in to operate with the new 4-metre Visible every VVV tile, including for Z and Y filters and Infrared Survey Telescope for As­ (not observed by 2MASS) where colour tronomy VISTA1 (Emerson & Sutherland, Regular observations equations are used. The method is similar 2010). VVV is scanning the Milky Way to that used for WFCAM (Hodgkin et al., (the Vía Láctea) Bulge and an adjacent Regular observations for the first year 2009). Our project takes advantage both section of the mid-plane, where star for­ started in February 2010 and will cover of the VDFS team’s experience in han­ mation activity is high. The survey will the total survey area in the five pass­ dling the WFCAM/UKIRT and VISTA data, take 1929 hours of observations during bands. An additional set of five Ks-band and the experience of the VVV team five years (2010–14), covering ~ 109 point exposures will also be taken. Table 1 members, who are leading participants in sources across an area of 520 square shows the number of Observational other surveys such as OGLE and in rou­ degrees, including 33 known globular Blocks (OBs), each of which corresponds tine data processing and delivery to ESO. and ~ 350 open clusters; the survey area to one tile, scheduled for observation is shown in Figure 1. The final product during the first period, as well as the cur­ Figure 2 shows a J-, H- and Ks-band will be a deep near-infrared (NIR) atlas in rent status of the observations. The OBs ­colour-composite image centred on the five passbands and a catalogue of more named “colour” contain observations in Galactic H ii region M8 (NGC 6523). than 106 variable point sources (Minniti more than one filter. ­Figure 3 (top panels) shows a small por­ et al., 2010). Detailed information about tion of a VVV image centred on the the VVV Survey can be found at the sur­ Table 1. Overview of VVV observations: first year. ­globular cluster Palomar 6, compared vey web page2. with 2MASS (Skrutskie et al., 2006). VVV Planned Executed by Fraction images are usually a combination of J-, for 2010 1 July 2010 % H- and Ks-band observations. However, Science verification Bulge some early images (including this one Colour ZY 196 40 20 of Palomar 6) were created using Z-, H- Some VVV data were obtained on 19–30 Colour JHKs 196 170 87 and Ks-band colours. The higher quality October 2009, during ESO science veri­ Variability Ks 1004 92 9 of the VVV data stands out in this com­ fication (SV) of VISTA (Arnaboldi et al., Disc parison. As another example of the supe­ 2010), following the same observing Colour ZY 152 121 80 rior quality of the VVV data, the lower strategy as the VVV survey. The observa­ Colour JHKs 152 152 100 panels of Figure 3 show colour images of tions consisted of multi-colour imaging, Variability Ks 760 507 67 the Bulge planetary nebula NGC 6629, Z- (0.87 μm), Y- (1.02 μm), J- (1.25 μm), H- Total 2460 1082 44 located in a field centred at l = 9.8º,

The Messenger 141 – September 2010 25 Astronomical Science Saito R. et al., VISTA Variables in the Vía Láctea (VVV)

Figure 2. VVV image of the Lagoon Nebula (NGC 6523), a giant interstellar cloud in the constel­ lation of Sagittarius, towards the Bulge of our Gal­ axy. This colour image was made combining J- (blue), H- (green) and Ks-band (red) observations. Saturated objects show a black dot in the centre. Credit: Ignacio Toledo and Dante Minniti.

Colour–magnitude diagrams

The left-hand panel of Figure 4 shows a colour–magnitude diagram (CMD) for the SV field in the Bulge located at l = 2.2º, b = –3.1º, in one of the most crowded regions of our Galaxy. In spite of the high stellar density and large number of giants, high precision photometry is pos­ sible over most of the field. This figure shows the dominant Bulge red giant branch along with the main sequence track of the foreground Disc, and it is evi­ dent that the VVV photometry is about 4 magnitudes deeper than 2MASS in this field, almost reaching the Bulge main sequence turn-off. About a million stars are measured in total in this 1.5 deg2 field. CMDs like this can be used to study the stellar populations across the Bulge, as well as the 3D structure of the inner Milky Way.

The right-hand panel of Figure 4 illustrates the CMD of the first Galactic Plane field observed. Located at l = 295.4º, b = –1.7º (in the outskirts of Carina), this field suf­ fers from large and very inhomogeneous extinction. The Disc main sequence domi­ nates and numerous reddened giants are also seen. We stress that 2MASS is very complementary to our survey, ­providing the external photometric and astrometric calibration, and photometry for the brighter sources (Ks < 10), which ­saturate even in the short (4 s) VVV expo­ sures. The depth of the VVV allows us in many cases to see all the way through the Plane of the Milky Way. In fact, very often our CMDs show numerous back­ ground galaxies. CMDs like these can also be used to study the 3D structure of the Milky Way Plane, as well as the spiral arms, the edge of the Disc and the outer b = –5.3º. Clearly, even though the ­survey lines present in our filters, particularly warp. About half a million stars are meas­ is not designed to detect or map emis­ Paschen β in the J-band and Brackett γ ured in this 1.5 deg2 field. Taking into sion line objects, it turns out that plane­ in the Ks-band. We expect to identify a account that the VVV Survey covers over tary nebulae (PNe) are also prominent few hundred PNe in our fields in the inner 520 deg2 in total, we will provide photo­ in the VVV images. The colour is typical Disc and Bulge. metry for ~ 5 × 108 sources in the Bulge of most PNe, due to the intense emission and Disc of our Galaxy.

26 The Messenger 141 – September 2010 Figure 3. Upper panels: in I-band, as expected. The photometric Comparison between accuracy of the data points in the light VVV (left) and 2MASS (right) images of the curve is ~ 0.05 mag. Our simulations globular cluster Palomar show that, at a typical magnitude of 6. Lower panels: VVV Ks ~ 15−16 mag, we should be able to image (left) of the plane­ detect RR Lyrae stars with amplitudes tary nebula NGC 6629 compared with 2MASS down to 0.03−0.05 mag, using 100 (right). phase points over the time frame of five N years of the VVV Survey.

In parallel with the main VVV Survey, 1o E we are also obtaining a large database of high quality (“template”) light curves in the Ks-band for different variability classes, using a variety of other tele­ scopes. This will allow us to automati- cally classify an important fraction of the ~ 106 VVV Ks-band light curves. While automated classification is routinely accomplished in other variability surveys (e.g., Debosscher et al., 2007; 2009), the VVV Survey is the first of its type to N be carried out in the NIR, where the required light-curve templates are, for the 15ǥ E most part, not available in the literature.

Figure 4. Colour-magni­ First moving object tude diagrams compar­ VVV data 8 ing VVV data (black) and The VVV Survey will allow the detection 2MASS 2MASS (red) for two extreme examples. The of Solar System objects down to left-hand panel shows Ks ~ 18 mag. We are searching for Trans- one of the most Neptunian objects (TNOs), Jupiter Trojan 10 crowded Bulge fields (SV field), while the right- asteroids (L5Js), Neptune Trojan aster­ hand panel shows one oids (N5Js), Main Belt asteroids (MBAs), of the Disc fields that are and near-Earth objects (NEOs). Since the 12 most heavily and inho­ VVV Bulge fields lie just on the Ecliptic, mogeneously reddened (see text for further our search is limited to this region. Satellite Ks details). tracks are also common and readily iden­ tified, even in the short VVV exposures. 14 As an example, Figure 6 shows MBA 199 Byblis, the first moving object detected 16 by the VVV Survey while making the col­ our tiles. The sequence of observations (in this case taken on 23 October 2009) started with the H-band (green), 3 min­ 18 utes later the Ks-band (red) was exposed, and finally 13 minutes later the Z-band

0 1 2 0 1 2 (blue). The object was also recovered in J–Ks Ks-band images acquired on 22 and 24 October 2009. 199 Byblis shows a mag­ First VVV RR Lyrae light curve the VVV SV data. The object, OGLE nitude of Ks = 12.12 mag and a motion 189770, is an ab-type RR Lyrae star with of about 0.22 arcminutes/day. The excel­ The detection of variable stars and the a period P = 0.72949 days and an am­ lent VISTA image quality (typically full monitoring of their variability is the main plitude of 0.33 mag (Collinge et al., 2006). width at half maximum < 1 arcsecond) goal of the VVV Survey. In Figure 5 we In Ks-band the minimum magnitude is and scale (0.34 arcseconds/pixel), also present the first, and preliminary, light Ks ~ 14.2 mag, with an amplitude of allows us to identify high proper motion curve of an RR Lyrae star obtained from ~ 0.20 mag, smaller than that observed stars with the survey baseline of 4–5 years

The Messenger 141 – September 2010 27 Astronomical Science Saito R. et al., VISTA Variables in the Vía Láctea (VVV)

Figure 6. Main Belt asteroid 199 Byblis, the OGLE 189770 first moving object 13.6 comparison star detected by the VVV Survey. It is the central object seen from left to right in filters Z (blue), Ks 13.8 (red), and H (green). Bright saturated stars Ks have Ks < 10 mag, and 14 show a black dot in the centre.

N 14.2

30ǥ E 0 0.5 1 1.5 2 Phase

Figure 5. (above) First VVV light curve for a Bulge RR Lyrae, obtained from the SV data. This ab-type RR Lyrae was identified from the OGLE sample, and has a period P = 0.72949 days (Collinge et al., 2006). The data are repeated in phase for better visualisation.

for faint objects, and with a baseline of more than ten years for bright objects with 2MASS. As a complementary pro­ ject, we will also search for background QSOs in some selected fields, to provide an extragalactic frame for accurate N astrometry.

1o Searching for new clusters and streams E

Another goal of the VVV is to search for Figure 7. Star cluster candidate identified in the new star clusters of different ages. To VVV Survey. This cluster is located in a Disc field. The faintest stars in this picture have Ks ~ 17 mag. trace the early stages of star cluster for­ mation, we are carrying out a survey of infrared star clusters and stellar groups. Acknowledgements Collinge, M. J. et al. 2006, ApJ, 651, 197 These are found towards known massive Emerson, J. & Sutherland, W. 2010, The Messenger, The VVV Survey is supported by the European 139, 2 star formation regions associated with Southern Observatory, the BASAL Center for Astro­ Debosscher, J. et al. 2007, A&A, 475, 1159 methanol maser emission and hot molec­ physics and Associated Technologies (PFB-06), the Debosscher, J. et al. 2009, A&A, 506, 519 ular cores. Using the list of star-forming FONDAP Center for Astrophysics (15010003) and Hodgkin, S. T. et al. 2009, MNRAS, 394, 675 regions provided in Longmore et al. the MIDEPLAN Milky Way Millennium Nucleus (P07- Longmore, S. N. & Burton, M. G. 2009, PASA, 26, 021-F). We would like to thank the staff of CASU and 439 (2009), we have already identified 25 WFAU, who provide the pipeline processing, data Minniti, D. et al. 2010, New Astronomy, 15, 433 small star cluster candidates by visual calibration and archive. Some VVV tiles were made Skrutskie, M. F. et al. 2006, AJ, 131, 1163 inspection. Almost all of them seem using the Aladin sky atlas, SExtractor software and indeed very young, because most of the products from the TERAPIX pipeline (Bertin et al., 2002). This publication makes use of data products Links mass is still concentrated in the gas. A from the Two Micron All Sky Survey, a joint project typical example of a newly identified of the University of Massachusetts and IPAC/ 1 Visible and Infrared Survey Telescope for Astron­ ­cluster candidate is shown in Figure 7. CALTECH, funded by NASA and NSF. omy (VISTA): http://www.vista.ac.uk 2 We are also studying the old metal-poor VISTA Variables in the Vía Láctea project page: http://www.vvvsurvey.org stellar population histories of the Milky References 3 VISTA Surveys page at the Cambridge Astronomi­ Way Disc and Bulge, with the aim to cal Survey Unit (CASU): http://casu.ast.cam.ac.uk/ find and study disrupted stellar streams Arnaboldi, M. et al. 2010, The Messenger, 139, 6 surveys-projects/vista 4 produced during past accretion events. Bertin, E. et al. 2002, ADASS XI, 281, 228 Wide Field Astronomy Unit (WFAU) of the University Churchwell, E. et al. 2009, PASP, 121, 213 of Edinburgh: http://horus.roe.ac.uk/vsa/

28 The Messenger 141 – September 2010