The VMC survey – XXV. The 3D structure of the Small Magellanic Cloud from Classical Cepheids

Abstract

The VISTA near-infrared Y JK s survey of the Magellanic System (VMC) is collecting deep K s -band time-series photometry of pulsating stars hosted by the two Magellanic Clouds and their connecting Bridge. Here we present Y, J, K s light curves for a sample of 717 Small Magellanic Cloud (SMC) Classical Cepheids (CCs). These data, complemented with our previous results and V magnitude from literature, allowed us to construct a variety of period-luminosity and period-Wesenheit relationships, valid for Fundamental, First and Second Overtone pulsators. These relations provide accurate individual distances to CCs in the SMC over an area of more than 40 deg 2 . Adopting literature relations, we estimated ages and metallicities for the majority of the investigated pulsators, finding that: i) the age distribution is bimodal, with two peaks at 120±10 and 220±10 Myr; ii) the more metal-rich CCs appear to be located closer to the centre of the galaxy. Our results show that the three-dimensional distribution of the CCs in the SMC, is not planar but heavily elongated for more than 25-30 kpc approximately in the east/north-east towards south-west direction. The young and old CCs in the SMC show a different geometric distribution. Our data support the current theoretical scenario predicting a close encounter or a direct collision between the Clouds some 200 Myr ago and confirm the presence of a Counter-Bridge predicted by some models. The high precision three-dimensional distribution of young stars presented in this paper provides a new testbed for future models exploring the formation and evolution of the Magellanic System.

Revealing the tidal scars of the Small Magellanic Cloud

“…Most observations point at a line of sight depth above 5 kpc with extremes of even 20 kpc (see de Grijs & Bono 2015, for a review). This observed great depth of the SMC along line of sight implies an extended bound structure (as concluded by Ripepi et al 2017 andMuraveva et al 2018) that is at odds with the highly perturbed kinematics we found here and the signs of interaction within ∼ 2−2.5 kpc from the SMC centre found by Subramanian et al (2017) in the Magellanic Bridge.…”

“…This was already hinted at by , briefly mentioned in Belokurov & Erkal (2019, fig. 3 where the gradient in the proper motions is evident) and implied by the smaller distance measured for the Eastern regions of the SMC (Subramanian & Subramaniam 2012;Deb et al 2015;Ripepi et al 2017;Muraveva et al 2018) as they are being pulled towards the LMC and thus closer to us. This last point is corroborated by the gradient seen in Fig.…”

“…The implication of this body of evidence is that it is not possible anymore to extract a clear rotational signal from the motion of stars and gas in the SMC. This in turn means that the H i kinematic centre, which is significantly displaced from both the H i 'maximum brightness' centre (Staveley-Smith et al 1997) and numerous photometric centres -old stellar populations (Gonidakis et al 2009), RR Lyrae (Subramanian & Subramaniam 2012) and Classical Cepheids (Ripepi et al 2017), all in good agreement with one another -is not indicative of the true centre of the galaxy. Recent work by Murray et al (2019) shows that the rotating disc model for the gas cannot reproduce the observed motions of young O and B stars, typically used as gas tracers.…”

“…These results are supported by numerical models of the Magellanic system that predict that the SMC is likely to show coherent rotation only at large radii (Besla et al 2012). A comprehensive study of the classical Cepheids in the SMC (Ripepi et al 2017) showed a complex geometric structure, with the near side forming a rough spheroidal shape before gradually shifting into a more linear shape, adding more detail to the SMC but presenting yet another potentially conflicting stellar structure.…”

“…Distance estimations using Classical Cepheids (e.g. Jacyszyn-Dobrzeniecka et al 2016 andRipepi et al 2017) report evidences for an ellipsoidal morphology of the SMC that appears to be very extended along the line of sight, with different depths for different regions and a mean depth of ∼5 kpc (rising to and above ∼20 kpc in some regions). Using RR Lyrae as distance tracers there are observations of similar morphology and depth (e.g.…”