The VMC survey – XXXI: The spatially resolved star formation history of the main body of the Small Magellanic Cloud

Abstract

We recover the spatially resolved star formation history across the entire main body and Wing of the Small Magellanic Cloud (SMC), using fourteen deep tile images from the VISTA survey of the Magellanic Clouds (VMC), in the Y JK s filters. The analysis is performed on 168 subregions of size 0.143 deg 2 , covering a total contiguous area of 23.57 deg 2 . We apply a colour-magnitude diagram (CMD) reconstruction method that returns the best-fitting star formation rate SFR(t), age-metallicity relation, distance and mean reddening, together with their confidence intervals, for each subregion. With respect to previous analyses, we use a far larger set of VMC data, updated stellar models, and fit the two available CMDs (Y −K s versus K s and J −K s versus K s ) independently. The results allow us to derive a more complete and more reliable picture of how the mean distances, extinction values, star formation rate, and metallicities vary across the SMC, and provide a better description of the populations that form its Bar and Wing. We conclude that the SMC has formed a total mass of (5.31 ± 0.05) × 10 8 M in stars over its lifetime. About two thirds of this mass is expected to be still locked in stars and stellar remnants. 50 per cent of the mass was formed prior to an age of 6.3 Gyr, and 80 per cent was formed between 8 and 3.5 Gyr ago. We also illustrate the likely distribution of stellar ages and metallicities in different parts of the CMD, to aid the interpretation of data from future astrometric and spectroscopic surveys of the SMC.

Discrete star formation events in the central bar of the Small Magellanic Cloud

“…The fact that SF activity has been very intense over the past 100-200Myr is also corroborated by the burst of cluster formation observed over roughly the same period (e.g., Bitsakis et al 2018;Nayak et al 2018). It is interesting to compare our rough estimate of the SFH of the SMC Bar with the results of the full analysis of Rubele et al (2018) for the same fields. These authors derived the spatially resolved star formation history across the entire main body and Wing of the SMC, using the VISTA survey of the Magellanic Clouds (VMC), in the Y JK s filters and a CMD reconstruction method.…”

“…These assumptions are not intended to constitute an accurate representation of the age-metallicity relation of the SMC, however, they are in broad agreement with the age-metallicity relations derived for SMC clusters and field populations (e.g. Da Costa & Hatzidimitriou 1998, Kayser et al 2007, Rubele et al 2018, assuming Z =0.0152 (Bressan et al 2012). It is clear that the MSTO region is well bracketed by the 4 and 7 Gyr isochrones, suggesting enhanced star formation during this period.…”

“…Younger populations therefore, seem to be more abundant in the North-NorthEasern regions of the Bar (see also e.g. Rubele et al 2018).…”

“…We indicate on the diagrams the location of the 50% and 20% completeness limits with vertical dotted lines. We also show for comparison the SFH for the same regions from Harris & Zaritsky (2004), who used a Salpeter IMF and a metallicity of Z=0.001, and from Rubele et al (2018) who used a Kroupa IMF and a nonconstant metallicity. Despite the errors, it is clear that the SFR for the younger populations derived here are too high compared to these studies.…”

The VMC survey – XXXIV. Morphology of stellar populations in the Magellanic Clouds

“…Figure 1 outlines the regions that disentangle different stellar populations across the LMC and the SMC. Figure 2 (top panels) shows the same regions overplotted on CMDs produced from theoretical models (Rubele et al 2018). These models represent synthetic stellar populations covering a grid of age and metallicity bins shifted to an established distance modulus and extinction, adapted to the conditions of our observations by applying the photometric errors and completeness obtained from artificial star tests as well as to the VISTA system zero points.…”