Star formation history
Information about the SFH of the LMC has been derived from the study of many relatively small regions located in the outer and inner disk as well as along the bar. In the SMC, apart from the comprehensive study by Harris & Zaritsky (2004), which does not probe the outer structure, there have been considerably fewer (and less detailed) observations of the field and cluster stellar populations than in the LMC. Using VMC data the distribution of field stars in different phases of evolution will be traced out to distances never yet explored. In particular we aim to sample the population of RGB stars because their behaviour is better understood and, because they are likely more metal poor and, they trace the tidally stripped parts of the galaxies and the extended halo component. Densities of different stellar objects are strongly correlated with the SFH.
The most powerful tool for quantitatively measuring the SFH in nearby galaxies is the analysis of colour-magnitude diagrams via objectives algorithms that search for the composite model that best fits the observations. A primary target of the VMC survey is to allow the SFH measurement in the Magellanic system with unprecedented accuracy and detail, via this kind of analysis. Simulations show that the nominal VMC sensitivity is required to determine the age of the stellar populations with a resolution of 0.2 dex with 20% errors.
VMC data represent a unique, currently missing, counterpart for optical sources of similar depth, this will provide us with the ultimate understanding of the SFH across the system.
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Fig. 1: Simulated colour-magnitude diagram for a 0.8 deg sq. LMC area (~2 VISTA detectors). The physical inputs to the simulations were: a constant star formation rate, an age-metallicity relation consistent with the LMC clusters, crowding, photometric errors and completeness typical for the LMC disk, foreground Milky Way stars. The total number of stars is ~55000 where the colours correspond to the density of stars in a logarithmic scale. |
To derive the 3D structure of the Magellanic system different density and distance indicators will be used, the most relevant are: the red clump's apparent magnitude, the period-luminosity (PL) relations for RR Lyrae stars and Cepheids, and standard candles in clusters. Their combination will produce a consistent measurement of the structure of the system free from individual problems. In particular, stellar clusters are, usually, single stellar populations often without internal extinction, thus they are easier to interpret compared to field stars but offer a statistically limited sample. The red clump's magnitude related to any other quantity depends on the SFH. Recovering the SFH with a resolution of 0.2 dex in the age will provide us with an accuracy of 0.05 mag on the determination of distance moduli.
RR Lyrae stars trace the old (t>10 Gyr) stellar component and follow a PL only in the K band (Longmore et al. 1986). Although it is weakly affected by evolutionary effects, spreads in stellar mass inside the instability strip, and uncertainties in the reddening correction, it does depend on metallicity. The theoretical calibration of this relation relies strongly on the (V-K) colour. RR Lyrae stars in the Clouds have Ks=18.0-19.0 mag and optical data of comparable sensitivity covering most of the LMC and the SMC from which to derive the period of the variation, are or will be available from microlensing surveys, across the entire area that VMC will cover. Thus, it is of prime importance to measure the mean Ks-band magnitude of RR Lyrae stars with the VMC survey. The observed properties of RR Lyrae and Cepheid stars will be compared with updated theoretical work based on nonlinear convective models of pulsating stars.
Cepheids are young or intermediate-age stars (100 Myr) which follow a much narrower PL in the Ks band than the corresponding optical relations and less affected by systematic uncertainties related to our knowledge of the reddening and metal content; the intrinsic accuracy of the PL-metallicity relation is ~0.05 mag. The application of theoretical PL and PL-colour relations to both near-IR and optical data will allow us to evaluate self consistently distances, reddenings and metal abundances, while information on the SFH could be inferred from the application of theoretical period-age and period-age-colour relations.
Stellar clusters
Despite a wealth of detailed studies, it has not yet been firmly established whether the field star population has experienced the same, or a similar, SFH as the star cluster system. By combining integrated photometry with resolved stellar population studies, the cluster system of the Magellanic Clouds offers unique chance to independently check the accuracy of age (and corresponding mass) determinations based on broad-band spectral energy distributions. The addition of near-infrared photometry to the existing optical photometry will beat the scatter that currently exists in the age-metallicity distribution, also constraining extinction variations more precisely. The sensitivity and resolution of VMC will make it possible to look for the fraction binaries in many clusters. Wide-field VMC data will produce a complete census of the cluster population that will be used to draw statistically robust conclusions on spatial differences between the Clouds. In addition, the search for new clusters based on the stellar surface density will work better than in the Galaxy because of the lower confusion along the line of sight.
Magellanic System's simulations
The Magellanic system has in total four distinct components: the LMC, the SMC, the Bridge and the Stream attached to the SMC. The latter two are predominantly formed of gas and are of tidal origin. The Bridge holds important clues about the most recent (~108 Gyr ago) interaction between the Clouds, its own formation history and possible origin. What is the age difference between the western and eastern parts of the Bridge? If it formed by LMC-SMC collision, and if the SMC is a rotating disk system, we should observe the late-type stars, PNe, etc. (not just HI). The VMC multi-passband survey will answer these questions and will also test the formation scenario predicted by simulations of a Magellanic collision. By comparing the observed amount of old stellar populations with the simulated one, we can constrain the gas/stellar structure of the SMC before interaction. It is a known mystery that only HI gas, not stars, shows rotation, thus 3D structures dependent on ages of stellar populations will provide hints to solve this problem.
Galaxy interactions play a major role in controlling the SFH of field stars and clusters but also in transforming galactic morphologies. The SFH of the Magellanic Clouds depends strongly on the interaction history of the triple system (LMC-SMC-Milky Way), thus on the orbits and masses of the Magellanic Clouds. Constraints on these fundamental parameters can be set by comparison with simulations: chemodynamical (Bekki & Chiba 2005), hydrodynamical (Mastropietro et al. 2005) and pure N-body (Connors, Kawata & Gibson 2006) while the self-consistency of the predicted orbits will be assessed using precise proper motion data. Although current models explain reasonably well the formation of the bar, the disk and the halo of the LMC as well as the gap in the age distribution of clusters, there are still significant uncertainties. For example, because the formation histories of field stars and globular clusters as well as the structural and kinematic properties in the LMC are different from those in the SMC, it is not yet clear whether these two galaxies were born as a pair. Moreover, did the stellar halo form via a different mechanism for Magellanic-type dwarf galaxies than for large spirals like the Milky Way? (I.e. by tidal galaxy interaction rather than by merging/accretion of subgalactic clumps?)
Planetary Nebulae
Previous shallow near-infrared surveys have been unable to reach the Ks magnitudes required to reveal large numbers of obscured, reddened stars, late-type and post asymptotic-giant branch stars as well as faint Planetary Nebulae (PNe). The current number of PNe in the Magellanic Clouds is incomplete and biased Shaw et al. (2006). We are missing the majority of the population which is faint and distributed over the 'halo'. VMC data combined with deep optical imaging and spectroscopy will not only contribute to the selection and identification of new PNe but will resolve the ambiguity with HII regions, young stellar objects, SN remnants and background galaxies. PNe will be bright in Ks because of Brackett Gamma emission and much fainter in Y and J compared to other emission line objects. The study of PNe and HII regions will complement the information from the oldest (red giant branch and horizontal branch stars) and youngest (O and B supergiants) stellar population, contributing to the evolutionary scenarios and to an age-dependent 3D structure.
Proper motion
A highly accurate astrometry across the Magellanic system will be provided by GAIA. However, VMC data alone could pre-empt the determination of the proper motion of the Magellanic Clouds with a stable as well as accurate astrometric solution. Assuming that projected distances to reference objects (quasars, distant galaxies or foreground stars of known motion) and their centroids can be determined equally accurately. The VMC survey should contain a sufficient number of stars of different types and locations to disentangle streaming motions from the bulk proper motion. According to King (1983) 0.08 arcsec seeing, S/N=10, 12 epochs and 5 years we obtain a precision of 5 mas/yr (0.05 for 104 stars). The combination with 2MASS gives a time baseline of ~15 years that could further improve the proper motion determination providing stability is conserved. For comparison HST measured 0.05 and 0.18 mas/yr for the LMC and SMC (Kallivayalil et al. 2006a, 2006b), respectively.
Star formation
With the benefit of near-infrared VMC observations we will study embedded star formation finding the counterpart for many Spitzer sources. We may be able to answer what is the effect of metallicity on star formation opening the doors to detailed follow-up studies as well as to a robust comparison with Milky Way stars for the study of environmental influence. Wide-area VMC observations will also provide an unbiased survey of pre-main sequence stars.
Distance scale
The distance modulus of the LMC, a key corner stone of the extragalactic distance ladder, estimated by several independent methods and indicators is at present 18.5 mag with a residual uncertainty of ~0.1 mag. The homogeneous VMC dataset will allow us to cut down by a factor of two the present uncertainty since many independent indicators (RR Lyrae stars, Cepheids, Miras, red clump stars, etc.) which appear more powerful and reliable in the near-infrared will be used.