Recovery of the star formation history of the LMC from the VISTA survey of the Magellanic system

Abstract

The VISTA near infrared survey of the Magellanic System (VMC) will provide deep Y JK s photometry reaching stars in the oldest turn-off point throughout the Magellanic Clouds (MCs). As part of the preparation for the survey, we aim to access the accuracy in the star formation history (SFH) that can be expected from VMC data, in particular for the Large Magellanic Cloud (LMC). To this aim, we first simulate VMC images containing not only the LMC stellar populations but also the foreground Milky Way (MW) stars and background galaxies. The simulations cover the whole range of density of LMC field stars. We then perform aperture photometry over these simulated images, access the expected levels of photometric errors and incompleteness, and apply the classical technique of SFH-recovery based on the reconstruction of colour-magnitude diagrams (CMD) via the minimisation of a chi-squared-like statistics. We verify that the foreground MW stars are accurately recovered by the minimisation algorithms, whereas the background galaxies can be largely eliminated from the CMD analysis due to their particular colours and morphologies. We then evaluate the expected errors in the recovered star formation rate as a function of stellar age, SFR(t), starting from models with a known age-metallicity relation (AMR). It turns out that, for a given sky area, the random errors for ages older than ∼0.4 Gyr seem to be independent of the crowding. This can be explained by a counterbalancing effect between the loss of stars from a decrease in the completeness and the gain of stars from an increase in the stellar density. For a spatial resolution of ∼0.1 deg 2 , the random errors in SFR(t) will be below 20% for this wide range of ages. On the other hand, due to the lower stellar statistics for stars younger than ∼0.4 Gyr, the outer LMC regions will require larger areas to achieve the same level of accuracy in the SFR(t). If we consider the AMR as unknown, the SFHrecovery algorithm is able to accurately recover the input AMR, at the price of an increase of random errors in the SFR(t) by a factor of about 2.5. Experiments of SFH-recovery performed for varying distance modulus and reddening indicate that these parameters can be determined with (relative) accuracies of Δ(m− M) 0 ∼ 0.02 mag and ΔE B−V ∼ 0.01 mag, for each individual field over the LMC. The propagation of these errors in the SFR(t) implies systematic errors below 30%. This level of accuracy in the SFR(t) can reveal significant imprints in the dynamical evolution of this unique and nearby stellar system, as well as possible signatures of the past interaction between the MCs and the MW.

The VMC survey

“…There is a noticeable amount of excess sources beyond (J − K s ) = 1.5 mag and K s = 16.5−19 mag. These are typical magnitudes and colours of background galaxies (Kerber et al 2009).…”

The VMC Survey

“…The (J − K s ) vs. J or K s CMDs offer instead the best separation between QSOs and stars of the MW or LMC. In the (J − K s ) vs. K s CMD, galaxy-like QSOs overlap with the cone of galaxies (with base 1 < (J − K s ) < 2 mag and vertex at (J − K s ) ∼ 1.5 mag); see Kerber et al (2009), while star-like QSOs seem well separated from it.…”

The VMC Survey. XXII. Hierarchical Star Formation in the 30 Doradus-N158–N159–N160 Star-forming Complex

“…The detectors usually have saturation limits of Y = 12.9 mag, J = 12.7 mag, and K s = 11.4 mag, and the stacked images from all epochs can provide sources with typical 5σ limiting magnitudes of Y = 21.9 mag, J = 22.0 mag, and K s = 21.5 mag; but the saturation limits and photometric depths also depend on sky conditions and crowding (Cioni et al 2011;Tatton et al 2013). This depth reaches the oldest main-sequence (MS) turn-offs in both the LMC and SMC (Kerber et al 2009). …”