Ogle-Ing the Magellanic System: Stellar Populations in the Magellanic Bridge

Abstract

We report the discovery of a young stellar bridge, that forms a continuous connection between the Magellanic Clouds. This finding is based on number density maps for stellar populations found in data gathered by OGLE-IV, that fully cover over 270 deg 2 of the sky in the Magellanic Bridge area. This is the most extensive optical survey of this region up to date. We find that the young population is present mainly in the western half of the MBR, which, together with the newly discovered young population in the eastern Bridge, form a continuous stream of stars connecting both galaxies along δ ∼ −73.5 deg. The young population distribution is clumped, with one of the major densities close to the SMC, and the other, fairly isolated and located approximately mid-way between the Clouds, which we call the OGLE island. These overdensities are well matched by HI surface density contours, although the newly found young population in the eastern Bridge is offset by ∼ 2 deg north from the highest HI density contour. We observe a continuity of red clump stars between the Magellanic Clouds, which represent an intermediate-age population. Red clump stars are present mainly in the southern and central parts of the Magellanic Bridge, below its gaseous part, and their presence is reflected by a strong deviation from the radial density profiles of the two galaxies. This may indicate either a tidal stream of stars, or that the stellar halos of the two galaxies overlap. On the other hand, we do not observe such an overlap within an intermediate-age population represented by the top of the red giant branch and the asymptotic giant branch stars. We also see only minor mixing of the old populations of the Clouds in the southern part of the Bridge, represented by the lowest part of the red giant branch.

The Properties of the Magellanic Bridge Based on OGLE IV RR Lyrae Variables

“…Bagheri, Cioni & Napiwotzki (2013);Skowron et al (2014);Noël et al (2015) all present evidence of an older population of stars throughout the Bridge based on CMD analyses. The OGLE observations support this as well, with the presence of RR Lyrae variables in the Bridge confirming the residence of an old (at least 10 Gyr) stellar population.…”

“…By examining the Bridge between the LMC and SMC, we gain insight into the structure of the system and their history of interactions. Studies have shown that the young and intermediate age stars in the MB have a clumpy, non-uniform distribution (Noël et al 2013(Noël et al , 2015Skowron et al 2014). The asymmetric locations of these stellar populations suggest that either they formed in situ or were stripped from the LMC or the SMC during an interaction.…”

“…Such an interaction is thought to have triggered star formation in the MB, as pointed out by Grondin, Demers & Kunkel (1992); Mizuno et al (2006); Skowron et al (2014); Chen et al (2014). However, based on their ages, the young stars could have either formed in situ in the MB, or formed in the SMC and been tidally stripped from it in the past ∼ 200 Myr (Noël et al 2015).…”

Bridge over troubled gas: clusters and associations under the SMC and LMC tidal stresses

“…Concerning the age, metallicity and reddening, we searched for solutions in the following ranges: 6.60 < log(τ /yr)< 10.00 (steps of 0.05); 0.0001 < Z < 0.0020 (steps of ∼ 0.002); 0.00 < E(B − V ) < 0.30 (steps of 0.01). Once again, these ranges are sufficiently wide to cover any result concerning the SMC or the Bridge, like the ones present in the SMC age-metallicity determinations for stellar clusters (Piatti 2011;Parisi et al 2014) and field stellar populations (Carrera et al 2008;Rubele et al 2015), in the analysis of Bridge field stars (Nidever et al 2013;Skowron et al 2014), and reddening maps (Burstein & Heiles 1982;Schlegel, Finkbeiner & Davis 1998).…”

The VMC Survey - XXIV. Signatures of tidally stripped stellar populations from the inner Small Magellanic Cloud

“…Earlier studies by Demers & Battinelli (1998) and Harris (2007) did not find the presence of intermediate-age/old stellar populations in fields centred on the H i ridge line of the MB. However, recent studies (Bagheri, Cioni & Napiwotzki 2013;Noël et al 2013Noël et al , 2015Skowron et al 2014) found evidence of the presence of intermediate-age/old stellar populations in the central and western regions of the MB.…”