A Three-dimensional Study Using Light Echoes of the Structure of the Interstellar Medium in Front of SN 1987A
A thorough study of the interstellar light echo images from SN 1987A over more than 5 years reveals a complicated three-dimensional structure in front of the supernova, ranging from 310 to 3240 lt-yr. A three- dimensional map was constructed based on a long procedure of image processing and an artificial intelligence analysis. The three-dimensional properties of these structures are carefully measured and are presented in this paper. The map shows that several dust clouds form a bubble shape of 700-800 lt-yr in diameter. By comparing it with an Hα image and by computing its geometric configuration, we identified this structure as superbubble N157C, centered on star cluster LH 90. Based on our fitting result of the curvature center of N157C, we propose that LH 90 is located about 1600 lt-yr in front of SN 1987A and hence that the supernova is a runaway star behind the disk of the Large Magellanic Cloud. This map also suggests that N157C and two dense dust clouds about 3000 lt-yr in the supernova foreground embrace a giant superbubble of 2000 lt-yr in diameter. The southeastern one of the two faraway objects correlates with a prominent Hα filament reaching out to the southwest from its origin over N157C. We suspect that this giant bubble is the merger of two or more smaller superbubbles and is filled with hot X-ray-emitting gas. The possibility that this region is a remnant of ancient, active star formation is advocated. Our ongoing light echo monitoring program will further reveal this structure.