Candidates of Halpha emitting regions in Magellanic Stream IV cloud

Abstract

From Hαnarrow band observations, we identified three Hαemitting regions in the direction of Magellanic Stream IV (MS IV). They consist of three parallel filaments with 2 arcmin width and 6 -- 30 arcmin length at 12 arcmin intervals. The mean surface brightness of them is $\sim 2 \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. Because of their low surface brightness, the regions were not detected in previous Hαsurveys. In HI map, the position of the filaments overlap MS, suggesting that they are parts of MS, but there also exists a local HI structure. If the filaments associate with MS, the sizes are 30 pc $\times$ 100 -- 500 pc. The filaments lie at the leading edge of a downstream cloud, which supports a shock heating and its propagation (shock cascade) model for the ionizing source. If they are local objects, on the other hand, Fossil Strömgren Trails of more than two stars is a possible interpretation, and the sizes would be 0.1 pc $\times$ 0.3 -- 1.5 pc at 180 pc distance. The positional information of the Hαfilaments presented in this letter enables us future spectroscopic observations to clarify their nature.

5 pages, 5 figures. Published in ApJL. References are corrected