The VMC survey – XIV. First results on the look-back time star formation rate tomography of the Small Magellanic Cloud★

Abstract

We analyse deep images from the VISTA survey of the Magellanic Clouds in the Y JK s filters, covering 14 deg 2 (10 tiles), split into 120 subregions, and comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply a colour-magnitude diagram reconstruction method that returns their best-fitting star formation rate SFR(t), age-metallicity relation (AMR), distance and mean reddening, together with 68% confidence intervals. The distance data can be approximated by a plane tilted in the East-West direction with a mean inclination of 39 • , although deviations of up to ±3 kpc suggest a distorted and warped disk. After assigning to every observed star a probability of belonging to a given age-metallicity interval, we build high-resolution population maps. These dramatically reveal the flocculent nature of the young star-forming regions and the nearly smooth features traced by older stellar generations. They document the formation of the SMC Wing at ages < 0.2 Gyr and the peak of star formation in the SMC Bar at ∼ 40 Myr. We clearly detect periods of enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related to a new feature found in the AMR, which suggests ingestion of metal-poor gas at ages slightly larger than 1 Gyr. The latter constitutes a major period of stellar mass formation. We confirm that the SFR(t) was moderately low at even older ages.

The VMC survey

“…The foreground absorption given in the NASA/IPAC Extragalactic Database amounts to A V = 0.101 mag, A J = 0.026 mag and A K s = 0.011 mag assuming the Fitzpatrick (1999) reddening law. The total absorptions in the external SMC region and the total reddening in 47 Tuc are low, A V = 0.35 mag (Rubele et al 2015) and E(B − V) = 0.04 mag (Harris 1996) for the SMC and 47 Tuc, respectively.…”

“…Subramanian & Subramaniam 2012;Nidever et al 2013;de Grijs & Bono 2015, and references therein) and a morphology that clearly depends on stellar population age (e.g. Cioni et al 2000a;Zaritsky et al 2000;Dobbie et al 2014;Subramanian & Subramaniam 2015;Deb et al 2015;Rubele et al 2015). The SMC has been recognised as a complex object to study and this explains why there are fewer studies of this galaxy than of its companion galaxy, the Large Magellanic Cloud (LMC).…”

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

“…We use their local completeness estimates to assign weights to stars in our sample (see Sections 3 and 4). A detailed description of the PSF homogeneity and photometry can also be found in Rubele et al (2015).…”

“…In the right-hand panel, the blue solid box shows the selection criterion of UMS stars and dashed lines are the MS to RGB sections of PARSEC (version 1.2S) isochrones of metallicity Z = 0.008 and ages log(τ /yr) = 6.6, 7.0, 7.5, 8.0, and 9.0, shifted by a distance modulus of (m − M ) 0 = 18.49 mag. Offsets of 0.026 mag in J and 0.003 mag in Ks, given by Rubele et al (2015), have been subtracted from the isochrone magnitudes to correct for the small differences between the VISTA system and the model Vega system. The color scale in the right-hand panel is the same as that in the middle panel; we overplot the contours, sample selection criterion, and isochrones in the different panels simply for clarity.…”

Time-series surveys and pulsating stars: The near-infrared perspective

“…The sixteen detectors, 67 million pixels effectively covering ∼0.6 deg 2 , together with the relatively large telescope aperture have a very high survey ability ( [94]). Among the six public surveys, two surveys are targeted at surveying pulsating stars: VISTA Variables in the Via Lactea (VVV, [60]) and VISTA survey of the Magallanic Clouds (VMC, [12] [39,85,86]). In addition to time variations in brightness, both of the VVV and VMC surveys are providing proper motions ( [13,14,33,40]) which will be very useful to discuss the nature of variable stars and other populations.…”