Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.eso.org/sci/activities/thesis-topics/abstrValenti_VVV_1.pdf Äàòà èçìåíåíèÿ: Tue Nov 4 13:20:07 2014 Äàòà èíäåêñèðîâàíèÿ: Sun Apr 10 23:33:38 2016 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: galactic center

New Astronomy 15 (2010) 433­443

Contents lists available at ScienceDirect

New Astronomy
journal homepage: www.elsevier.com/locate/newast

VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way
D. Minniti a,b, P.W. Lucas c, J.P. Emerson d, R.K. Saito a,*, M. Hempel a, P. Pietrukowicz a,e, A.V. Ahumada f,g,h, M.V. Alonso f, J. Alonso-Garcia i, J.I. Arias j, R.M. Bandyopadhyay k, R.H. BarbÀ j, B. Barbuy l, L.R. Bedin m, E. Bica n, J. Borissova o, L. Bronfman p, G. Carraro g, M. Catelan a, J.J. ClariÀ f, N. Cross q, R. de Grijs r,s, I. DÈkÀny t, J.E. Drew c,u, C. Farißa v, C. Feinstein v, E. FernÀndez LajÇs v, R.C. Gamen j, D. Geisler w, W. Gieren w, B. Goldman x, O.A. Gonzalez y, G. Gunthardt j, S. Gurovich f, N.C. Hambly q, M.J. Irwin z, V.D. Ivanov g, A. JordÀn a, E. Kerins aa, K. Kinemuchi j,w, R. Kurtev o, M. LÑpez-Corredoira ab, T. Maccarone ac, N. Masetti ad, D. Merlo f, M. Messineo ae,af, I.F. Mirabel ag,ah, L. Monaco g, L. Morelli ai, N. Padilla a, T. Palma f, M.C. Parisi f, G. Pignata aj, M. Rejkuba y, A. Roman-Lopes j, S.E. Sale u, M.R. Schreiber o, A.C. SchrÆder ak,al, M. Smith am, L. SodrÈ Jr. l, M. Soto j, M. Tamura an, C. Tappert a, M.A. Thompson c, I. Toledo a, M. Zoccali a, G. Pietrzynski w
Departamento de AstronomÌa y Astrof´sica, Pontificia Universidad CatÑlica de Chile, Av. Vicußa Mackenna 4860, Casilla 306, Santiago 22, Chile i Vatican Observatory, Vatican City State V-00120, Italy c Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK d Astronomy Unit, School of Mathematical Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK e Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warszawa, Poland f Observatorio AstronÑmico de CÑrdoba, Universidad Nacional de CÑrdoba, Laprida 854, 5000 CÑrdoba, Argentina g European Southern Observatory, Av. Alonso de CÑrdova 3107, Casilla 19, Santiago 19001, Chile h Consejo Nacional de Investigaciones CientÌficas y TÈcnicas, Av. Rivadavia 1917, CP C1033AAJ, Buenos Aires, Argentina i Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090, USA j Departamento de FÌsica, Universidad de La Serena, Benavente 980, La Serena, Chile k Department of Astronomy, University of Florida, 211 Bryant Space Science Center, P.O. Box 112055, Gainesville, FL 32611-2055, USA l Universidade de SÖo Paulo, IAG, Rua do MatÖo 1226, Cidade UniversitÀria, SÖo Paulo 05508-900, Brazil m Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA n Universidade Federal do Rio Grande do Sul, IF, CP 15051, Porto Alegre 91501-970, RS, Brazil o Departamento de FÌsica y AstronomÌa, Facultad de Ciencias, Universidad de ValparaÌso, Ave. Gran Bretaßa 1111, Playa Ancha, Casilla 5030, ValparaÌso, Chile p Departamento de AstronomÌa, Universidad de Chile, Casilla 36-D, Santiago, Chile q Institute for Astronomy, The University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK r The Department of Physics and Astronomy, University of Sheffield, Hick Building, Hounsfield Road, Sheffield S3 7RH, UK s National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100021, China t Konkoly Observatory of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 67, Hungary u Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK v Facultad de Ciencias AstronÑmicas y GeofÌsicas, Universidad Nacional de La Plata, and Instituto de AstrofÌsica La Plata, Paseo del Bosque S/N, B1900FWA, La Plata, Argentina w Departmento de AstronomÌa, Universidad de ConcepciÑn, Casilla 160-C, ConcepciÑn, Chile x Max Planck Institute for Astronomy, KÆnigstuhl 17, 69117 Heidelberg, Germany y European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany z Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK aa Jodrell Bank Centre for Astrophysics, The University of Manchester, Oxford Road, Manchester M13 9PL, UK ab Instituto de AstrofÌsica de Canarias, VÌa LÀctea s/n, E38205 - La Laguna (Tenerife), Spain ac School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK ad Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, via Gobetti 101, 40129 Bologna, Italy ae Chester F. Carlson Centre for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA af The Astrophysics and Fundamental Physics Missions Division, Research and Scientific Support Department, Directorate of Science and Robotic Exploration, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands ag Service d'Astrophysique ­ IRFU, CEA-Saclay, 91191 Gif sur Yvette, France ah Instituto de AstronomÌa y FÌsica del Espacio, Casilla de Correo 67, Sucursal 28, Buenos Aires, Argentina ai Dipartimento di Astronomia, UniversitÀ di Padova, vicolo dell'Osservatorio 3, 35122 Padova, Italy aj Departamento de Ciencias Fisicas, Universidad Andres Bello, Av. RepÇblica, 252, Santiago, Chile ak Department of Physics & Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK al Hartebeesthoek Radio Astronomy Observatory, PO Box 443, Krugersdorp 1740, South Africa am The University of Kent, Canterbury, Kent CT2 7NZ, UK an Division of Optical and Infrared Astronomy, National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
a b

* Corresponding author. E-mail addresses: dante@astro.puc.cl (D. Minniti), P.W.Lucas@herts.ac.uk (P.W. Lucas), j.p.emerson@qmul.ac.uk (J.P. Emerson), rsaito@astro.puc.cl (R.K. Saito). 1384-1076/$ - see front matter ñ 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.newast.2009.12.002


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article
Article his Received Received Accepted Available

info

abstract
We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 h of observations with the 4-m VISTA telescope during 5 years (2010­2014), covering $109 point sources across an area of 520 deg2, including 33 known globular clusters and $350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9­2.5 lm) and a catalogue of more than 106 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars. ñ 2009 Elsevier B.V. All rights reserved.

tory: 28 September 2009 in revised form 2 December 2009 4 December 2009 online 14 December 2009

Communicated by G.F. Gilmore Keywords: Surveys Stars: variables: general Galaxy: bulge Galaxy: disk

1. Introduction The bulk of the stars, gas and dust in the Milky Way are confined to its bulge and plane. As a result, in these directions, the extinction and crowding are high, making any study of the inner structure of the Galaxy difficult. Knowing how the stellar populations are distributed within the Galaxy is essential for such studies and hence the main goal of the described survey. Traditional distance indicators have been used with various success in the past. The approach was to concentrate on clear ``windows", where optical surveys can be carried out (e.g., MACHO, OGLE, EROS). In this paper, we describe the VISTA Variables in the Via Lactea (VVV) survey,1 an ESO (European Southern Observatory) public near-IR variability survey. Its area includes the Milky Way bulge and an adjacent section of the mid-plane where star-formation activity is high. This survey will be conducted in the period 2010­2014 and will map the whole bulge systematically for multiple epochs. We plan to cover a 520 deg2 area (Fig. 1) containing $109 point sources. Our survey will give the most complete catalogue of variable objects in the bulge, with more than $106 variables. Chief among them are the RR Lyrae, which are accurate primary distance indicators, and well-understood regarding their chemical, pulsational and evolutionary properties. For the sake of space and coherence we concentrate on the RR Lyrae and the star clusters, noting that similar studies can be done for many of the other populations of variable objects. Earlier single-epoch near-IR surveys (e.g., COBE, 2MASS, GLIMPSE) have proven that the Galactic bulge is triaxial and boxy, and contains a bar (Dwek et al., 1995; LÑpez-Corredoira et al., 2005; Benjamin et al., 2005). Presently, the only model we have for the formation of boxy/barred bulges is through secular evolution of a pre-existing disk. This scenario is believed to be the dominant channel of formation of bulges in late-type spirals (Sbc), whereas early-type spiral bulges (S0/Sa) show structural and kinematic evidence for an early, rapid collapse, which seems to be confirmed by the old age of their stellar populations (e.g., Kormendy and Kennicutt, 2004). However, the best-studied spiral bulge, that of the Milky Way, is precisely the most problematic one to understand in this context. While its surface brightness shows a barred structure, its stellar

population is predominantely old (Kuijken and Rich, 2002; Zoccali et al., 2003) and has a-element enhancement, characteristic of rapid formation. Nevertheless, the high mean age of the Bulge still leaves space for a small fraction of young stellar objects (YSO) which have been found in the inner Bulge (e.g., Schuller et al., 2006; Yusef-Zadeh et al., 2009). This is in agreement with the results of Zoccali et al. (2006) which indicate that the chemical composition of the bulge stars is different from that of both thin and thick-disk stars. Thus, the predictions from the formation of the Milky Way bulge through secular evolution of the disk seem to be in conflict with some key properties of its stellar population. However, MelÈndez et al. (2008) recently published results that are in contradiction to Zoccali et al. (2006) and show that bulge and disk stars are indistinguishable in their chemical composition. Given that the near-IR colours depend strongly on metallicity, the VVV survey will help us to investigate the metallicity distribution in the survey region. Spectroscopic data (e.g., future APOGEE; Majewski et al., 2007) will provide additional a-element abundances. Our survey of the RR Lyrae in the Galactic bulge will allow us to map its 3-D structure (as shown by Carney et al., 1995) and will provide key information on the age of its population, given that RR Lyrae stars are tracers of the old population (e.g., Catelan, 2004b, 2009, and references therein). This will enable us to combine the ages of the stellar populations with their spatial distribu-

1

Detailed information about the VVV survey can be found in http://vvvsurvey.org/.

Fig. 1. 2MASS map of the inner Milky Way showing the VVV bulge (solid box, þ10° < l < + 10° and þ10° < b < + 5°) and plane survey areas (dotted box, þ65° < l < þ10° and þ2° < b < + 2°).


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tions. We note that most single-epoch surveys only provide 2-D maps. With the present survey, the peak and width of the RR Lyrae distribution is expected to be measured with an accuracy of better than 0.01 mag, which is the required precision to determine the 3D structure not only of the bulge, but also of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) located behind the Milky Way (e.g., Alard, 1996) and included in our survey. At the same time, a comparison between the RR Lyrae (and type II Cepheids) in the field and in globular clusters may hold precious information about the formation of the bulge (e.g., Feast et al., 2008). Modern KCDM cosmology predicts that large galaxies such as the Milky Way formed by accretion of hundreds of smaller ``protogalactic fragments", perhaps not unlike the progenitors of the present-day dwarf spheroidal satellites (e.g., Abadi et al., 2003). Interestingly, two very massive globular clusters in the Galactic bulge, NGC 6388 and NGC 6441, have recently been suggested to be the remnants of dwarf galaxies that were accreted in the course of the Galaxys history (Ree et al., 2002). These clusters might prove similar to the cases of M54 (NGC 6715), in the center of the Sgr dSph, which is currently being cannibalised by the Milky Way (Ibata et al., 1995), and of x Cen (NGC 5139), which has long been suspected to be the remnant nucleus of a dwarf galaxy (e.g., Altmann et al., 2005, and references therein). Our proposed search for RR Lyrae and type II Cepheids in the Galactic bulge will reveal the presence of debris related to the accretion events that might have left behind NGC 6441 as remnant object. The latter is part of our survey. In order to understand the Milky Way's populations globally, it is necessary to survey the inner Galactic plane as well. Therefore, we will survey an adjacent region of the mid-plane and provide a Legacy Database and 3-D atlas of a large Population I (i.e., young and luminous stars) region. We have selected the region þ65° < l < þ10° and |b| < 2° (see Fig. 1), where star-formation activity is high and for which there will be complementary optical, mid-IR, and far-IR data from VPHAS+, the Spitzer, GLIMPSE and MIPSGAL surveys, and from the all-sky AKARI and WISE survey. The addition of this region will also permit us to discriminate between various models of the inner Galactic structure which, besides the triaxial bulge, contain a long bar and a ring (e.g., LÑpez-Corredoira et al., 2007, and references therein) or not (e.g., Merrifield, 2004, and references therein). Indeed, the selected region includes the putative negative-longitude tip of the long bar (at l % þ14°, |b| < 1°), which has not yet been observed. The large survey area will allow several remaining astrophysical problems to be addressed. For example, the effect of the environment on star formation and in particular the initial mass function (IMF) at low masses is presently poorly known. This issue will be addressed statistically by observing hundreds of star-forming regions and cross-correlating the shapes of their luminosity functions with cluster density, the presence of high-mass stars, and galactocentric distance. For comparison, VVV survey will reach 1 mag deeper than UKIDSS Galactic Plane Survey (GPS), which overlaps with VVV in the region of þ2° < l < + 10°, |b| < 2°. Other important parameters, such as velocity dispersion and metallicity, will be determined by spectroscopic follow-up observations. In addition, the luminosity function of the clusters themselves will be measured, for both star-forming clusters and more evolved open clusters. These issues cannot be addressed with optical surveys, owing to the high extinction in the plane. The Spitzer data will be invaluable for detecting the most obscured high-mass protostars within starforming regions. A near-IR survey will be more sensitive to all but the reddest objects, and the superior spatial resolution in these wavebands will be essential for resolving distant clusters and the crowded field populations.

2. Technical description 2.1. Telescope and instrument design The Visible and Infrared Survey Telescope for Astronomy (VISTA) is a 4 m-class ``wide-field" telescope located at ESO's Cerro Paranal Observatory in Chile, designed to conduct large-scale surveys of the southern sky at near-IR wavelengths (0.9­2.5 lm). The telescope has an altitude-azimuth mount, and quasi Ritchey-ChrÈtien optics. An f/1 primary mirror was designed together with Cassegrain-focus instrumentation to offer the best solution to the difficult problem of combining a wide-field with good image quality, and results in a physically large focal plane with an f/3.25 focus (McPherson et al., 2006). VISTA's active optics uses two low-order curvature sensors, which operate concurrently with science exposures, and a high-order curvature sensor. The telescope is equipped with a near-IR camera containing 67 million pixels (an array of 16 á 2048 á 2048 Raytheon VIRGO IR detectors) of mean size 000 .34 and available broad-band filters at ZYJHKs and a narrow-band filter at 1.18 lm. Given VISTA's nominal pixel size, the diameter of the field of view is 1.65°. The pointspread function (PSF) of the telescope + camera system (including pixels) is designed to have a full width at half maximum (FWHM) of 000 .51, not including the contribution of atmospheric turbulence. Seeing, and other weather-related statistics for Cerro Paranal, are given at ESO's ``Astroclimatology of Paranal" web pages.2 The VISTA site is expected to have similar conditions, which are well suited to our survey requirements. The 16 detectors in the camera are not buttable and are arranged as shown in Fig. 2. Each individual exposure produces a sparsely sampled image of the sky known as a ``pawprint", covering an area of 0.599 deg2. To `fill in' the gaps between the detectors to produce a single filled ``tile" with reasonably uniform sky coverage, the minimum number of pointed observations (with fixed offsets) required is six (three offsets in Y and two offsets in X). After six steps an area of 1.501 deg2 on the sky, corresponding to one tile, is (almost) uniformly covered.

2.2. Data reduction We will use the enhanced VISTA Data Flow System3 (VDFS: Emerson et al., 2004; Irwin et al., 2004; Hambly et al., 2004). It includes all basic data reduction steps: (i) removing instrumental signature (bias and dark frames, twilight, and dome flatfields, linearity, bad pixel maps, crosstalk, gain calibrations), merging pawprints into tiles and calibrating photometrically and astrometrically; (ii) extracting source catalogues on a tile-by-tile basis; (iii) constructing survey-level products ­ stacked pixel mosaics, difference images, and merged catalogues; (iv) providing the team with both data access and methods for querying and analyzing the data; and (v) producing virtual observatory (VO)-compliant data products for delivery to the ESO archive. Fig. 3 shows a flow chart of the data processing. The pipeline products are: astrometrically corrected and photometrically calibrated tiles in each filter used, confidence maps, and homogeneous
http://www.eso.org/gen-fac/pubs/astclim/paranal/. The VISTA Data Flow System (VDFS) is a collaboration between the UK Wide Field Astronomy Unit at Edinburgh (WFAU) and Cambridge Astronomy Survey Unit (CASU), coordinated by the VISTA PI and funded for VISTA by the Science and Technology Facilities Council.
3 2


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2.3. Combination/image subtraction (archive) The ``second"-order data processing requires access to larger sets of data to produce survey products. It is carried out by the Wide Field Astronomy Unit's (WFAU) VISTA Science Archive (VSA) in Edinburgh. The Science Archive contains only calibrated data and catalogues, and no raw data. The Science Archive is responsible for: (i) image stacking to produce combined and differenced tiles and source merging; (ii) quality control: assessment of the data quality and filtering of the data that do not meet the established criteria for photometric and astrometric accuracy; (iii) light-curve extraction: this will be done by implementing an image-subtraction algorithm (Alard and Lupton, 1998; Alard, 2000), which will allow us to create the catalogue of variable sources. This method provides excellent results for crowded fields in which the traditional aperture or PSF-fitting photometry fails (e.g., Kaluzny et al., 2004; Pietrukowicz et al., 2005).

2.4. Photometric calibration During the first period tions and transformations and UKIDSS for bootstrapp The calibration scheme
Fig. 2. Diagram showing the array of the sixteen detectors on the VISTA camera and the axis orientation used to shift the camera in each exposure to obtain the tiles. For comparison we show the crescent Moon over the VISTA camera and the fields of view of UKIRT/WFCAM, HST/NICMOS, VLT/ISAAC, and VLT/HAWK-I.

we to ing for

.4 a given filter is as follow:
std

will carry out the external calibrathe standard system using 2MASS

mcal ¼ mi

nst

× ZP þ kÏX þ 1÷ ¼ mstd × clr

Ï1÷

where mcal is the calibrated magnitude, minst the measured instrumental magnitude, ZP the zeropoint, k the extinction coefficient, and X the airmass of the object. On the right-hand side of this equation, mstd and clrstd are the corresponding standard magnitude and colour. Calibration and quality control is done using 2MASS stars in the frames themselves, applying colour equations to convert 2MASS photometry to the VISTA photometric system (Skrutskie et al., 2006; Hodgkin et al., 2009). There are thousands of unsaturated 2MASS stars in JHKs with photometric errors <0.1 mag in every VISTA tile field. A large fraction of these can be sufficiently isolated even in the crowded fields. To calibrate the Y- and Z-band data (both filters are not available from 2MASS) we will use observations of the standard VISTA calibration fields as required by the ESO Public Survey Panel. Details will be published in a forthcoming paper describing the science verification. The internal gain correction applied through flat-fielding will place the detectors on a common zero-point system. After deriving this ZP in each tile, a double check using the overlap regions will be made to estimate the internal photometric accuracy. 3. Observing strategy The VISTA tile field of view is 1.501 deg2, hence 196 tiles are needed to map the bulge area and 152 tiles for the disk.5 Adding some X and Y overlap between tiles for a smooth match, the area of our unit tile covered twice is 1.458 deg2. Fig. 4 provides a sche4 The filter transmission curves for each instrument can be found at http:// www.vista.ac.uk/index.html (VISTA), http://web.ipac.caltech.edu/staff/waw/2mass/ opt_cal/index.html (2MASS) and http://www.ukidss.org/technical/instrument/filters.html (UKIDSS). 5 The tiles' spacing and orientation were calculated with the Survey Area Definition Tool (SADT) software to maximize the efficiency of the sky coverage. See http:// www.vista.ac.uk/observing/sadt/.

Fig. 3. Flow chart of the VVV data processing (QC: Quality Control; ZP: Zeropoint).

object catalogues (Cross et al., 2009). The pipeline records the processing history and calibration information of each file, including calibration files and quality control parameters. The Cambridge Astronomy Survey Unit (CASU) component of the VDFS will be responsible for the basic pipeline processing and the first calibration, all done on a daily basis.


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Msc img

18:40

18:20

18:00

17:40

17:20

-20:00

-25:00

-30:00

-35:00

-40:00

N E

5´ Powered by Aladin

28.73´ x 27.99´

Fig. 4. Coverage of the Galactic center region overlaid on a mid-IR map. Boxes mark the tiles needed to cover the bulge area (see Fig. 1), whereas the crosses mark the box centers.

Table 1 Ks-band magnitudes at the distance of the bulge. The absorption values are based on the standard extinction law as derived by Rieke and Lebofsky (1985). Population E(B þ V) = 0 AV = 0 AJ = 0 AK = 0 Ks Ks Ks Ks Ks Ks Ks = = = = = = = 8 1 1 1 1 1 1 .0a 0.5 2.9 4.3 5.4 6.8 7.0 E(B þ V) = 0.5 AV = 1.5 AJ = 0.4 AK = 0.2 K K K K K K K
s s s s s s s

E(B þ V) = 1.5 AV = 5.0 A J = 1. 4 AK = 0.6 K K K K K K K
s s s s s s s

E(B þ V) = 3.2 AV = 10.0 AJ = 2.8 AK = 1.1 K K K K K K K
s s s s s s s

E(B þ V) = 4.8 AV = 15.0 A J = 4. 2 A K = 1. 7 K K K K K K K
s s s s s s s

E(B þ V) = 8.4 A V = 26. 3 A J = 7. 4 A K = 3. 0 K K K K K K K
s s s s s s s

Bulge RGB tip Sgr dSph RGB tip Bulge RGB clump Bulge RR Lyrae Sgr dSph RGB clump Sgr dSph RR Lyrae Bulge MS turn-off
a b

= = = = = = =

8 1 1 1 1 1 1

.2a 0.7 3.1 4.5 5.6 7.0 7.2

= = = = = = =

8 1 1 1 1 1 1

.6a 1.1 3.5 4.9 6.0 7.4 7.6

= = = = = = =

9 1 1 1 1 1 1

.1 a 1.6 4.0 5.4 6.5 7.9 8.1

= = = = = = =

9 1 1 1 1 1 1

.7 2.2 4.6 6.0 7.1 8.5 8.7

b b

= = = = = = =

1 1 1 1 1 1 2

1. 0 3. 5 5. 9 7. 3 8. 4 b 9. 8 b 0 .0 b

Saturated. Beyond detection.

matic representation of the tiling scheme for the Galactic center region. The variability study in the bulge will be carried out in the Ks band down to $18 mag (signal-to-noise %3). The total exposure time for a VISTA tile field is 162 s. Our strategy yields about 30 deg2 per h or 300 deg2 per night. The combined epochs will reach Ks = 20 mag, which is three magnitudes fainter than the unreddened bulge main-sequence turn-off (MS turn-off), although the densest fields will be confusion-limited. However, applying both PSF fitting and image subtraction, we will recover the light curves of most objects down to Ks = 18 mag, even in moderately crowded fields. This is more than 3 mag fainter than the unreddened known RR Lyrae in the Galactic bulge. We expect to find RR Lyrae even in fields with AV = 10 mag.

Table 1 lists some reference Ks-band magnitudes at the distance of the bulge for a range of extinction and reddening values. These typical magnitudes were obtained from Carney et al. (1995), Alard (1996), Alcock et al. (1998), and Zoccali et al. (2003). As a reference point, for Baade's window E(B þ V) = 0.5 mag, so that AV = 1.5, AJ = 0.4, and AK = 0.2 mag (Rieke and Lebofsky, 1985). This table shows that the tip of the bulge red-giant branch (RGB) will saturate (Ks < 9.5), but for the RGB clump giants, and even for the tip of the RGB of the Sgr dSph galaxy, the VVV survey will be able to see giants throughout the bulge, even in the most obscured regions. The bulge RR Lyrae and the Sgr dSph galaxy red-clump giants will also be detected, even for the regions with the highest extinction (AV > 30 mag) at low Galactic latitudes. Finally, the RR Lyrae of the Sgr dSph galaxy and the bulge MS turn-off stars will be


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detected only in the regions with low absorption (AV < 10 mag) at higher latitudes. We are aware that the use of an `universal' extinction law AV = 3.1 E(B þ V) is problematic in the inner region of the bulge. According to Nishiyama et al. (2006) and Gosling et al. (2009) a single extinction law is not consistent with the observations of the Galactic center along different lines of sight. For the plane survey, the Ks-band observations require a total time of 80 s on target, and an elapsed time of 366 s per tile. Bright point sources with Ks < 9.5 mag will be saturated in the individual images. This will therefore include most unreddened bulge Mira variables, but Miras in the Sgr dSph galaxy can be monitored, as well as Miras located in regions with very high extinction (e.g., next to the Galactic center). The Mira population in the Galactic center has been studied by Matsunaga et al. (2009). In addition, bright-star saturation may be an issue, but we estimate that even in the worst cases only a small portion of the field will be rendered useless. Hence we do not expect that the saturation of the brightest stars to effect our conclusions about three-dimensional structure of the inner Milky Way. For example, in the optical microlensing surveys where CCD bleeding is comparatively worse, less than 5% of the most crowded bulge fields are lost. To illustrate the precision of crowded field IR photometry we include Fig. 5, showing photometry of the planetary transit OGLE-TR-113 obtained with NTT/SOFI (top panel), and photometric accuracy of those observations as a function of magnitude (bottom panel). In order to evaluate the amplitude threshold for our detections, we have carried out Monte Carlo simulations using the RR Lyrae light curve templates from Jones et al. (1996) and Del Principe et al. (2005). As a result, we find that, at a typical magnitude of Ks % 15­16, and taking into account the expected photometric errors, we should be able to detect RR Lyrae stars with amplitudes down to AK = 0.05­0.07 mag using 80 datapoints from the first 3 years of VVV operation, and further down to AK = 0.03­ 0.05 mag if the dataset is extended to cover 180 phase points over a time frame of 5 years. The total estimated time per observing period is given in Table 2, which also shows the requirements for Moon, seeing, and transparency conditions. The times include overheads (both for readout and for changing to a new tile) and time possibly spent on standard stars for the Z and Y observations (not used in the 2MASS and UKIDSS survey). This strategy allows us to provide various data

to the community, enabling follow-up throughout the survey. The full survey will require a total of 192 nights of observations over 5 years. A schematic schedule of the survey is shown in Fig. 6. During the first year, the whole bulge area will be observed in the Ks band for six consecutive epochs, for a total of 65 h. A further 86 h will be devoted to complete imaging of each bulge tile in ZYJH. This will provide reliable near-simultaneous fluxes and colours for each tile area. The same strategy will be applied to the 152 tiles covering the disk area for the single-epoch and the quasi-simultaneous multicolour disk survey. The total time spent on the disk for the first year is thus 141 h. Added to the 151 h for the bulge, we will thus spend 292 h in total on the survey during the first year. The multi-colour observations, in combination with datasets from UKIDSS GPS (near-IR), VST/VPHAS+ (optical) and GLIMPSE and GLIMPSEII (mid-IR), will be used to build improved extinction maps for the survey region. Note that the individual, single-epoch observation blocks (OBs) in Ks all have the same limiting depth (under the same conditions), whether forming part of the VVV's bulge or disk components. Being fully aware of the confusion and background limits, the observing plan would cycle alternately through fields of varying density for optimal sky subtraction. The filter order in the OBs will be optimized to minimize overheads. During the second year, we will acquire another 20 epo