A blue stellar population in the HI bridge between the two Magellanic Clouds

Abstract

Radio observations have shown¹ that a common HI cloud surrounds the Magellanic Cloud system. However, no stellar link between the two galaxies has previously been found. The furthest known stellar extension eastward of the Small Magellanic Cloud (SMC) is the blue association of stars found by Kunkel², which lie close to the extreme tip of the ‘wing’ of the SMC. The SMC wing was discovered by Shapley³, who described it as “a large cloud of faint stars extending eastward from the SMC toward the LMC [Large Magellanic Cloud]”. More recently, Westerlund⁴ described the brighter (V<20.0) stellar population of the wing as being a mixture of extreme Population I, OB associations, HII regions and both blue and yellow supergrants. The rest of the Magellanic system shows a strong correlation between HI density and the presence of stellar condensations, and this has led to speculation that stellar associations may be present in the HI extension of the optical wing (the HI bridge indicated in Fig. 1) and also independently in regions of high HI density in the Magellanic stream⁵. Deep Hα photography⁶ shows weak diffuse emission between the Magellanic Clouds., supporting the possible existence of an optical counterpart to the HI bridge, but star counts near the eastern tip of the wing⁷ failed to reveal any link and indeed corroborated earlier work on the structure of the wing⁸. We now report the discovery of several hundred blue main-sequence stars in an area reaching 9° east of the SMC, coinciding with the HI ridge extending from the wing. These stare lie midway between the two Clouds and are more than 2 kpc further east than what was believed to be the eastern tip of the SMC wing. The apparent magnitudes of the brightest main-sequence stars suggest an age of ∼100 Myr, similar to ages of associations in the wing.