The Discovery of Spiral Arms in the Starburst Galaxy M82

We report the discovery of two symmetric spiral arms in the near-infrared (NIR) images of the starburst galaxy M82. The spiral arms are recovered when an axisymmetric exponential disk is subtracted from the NIR images. The arms emerge from the ends of the NIR bar and can be traced up to 3 disk scale lengths. The winding of the arms is consistent with an m = 2 logarithmic spiral mode of pitch angle 14°. The arms are bluer than the disk in spite of their detection on the NIR images. If the northern side of the galaxy is nearer to us, as is normally assumed, the observed sense of rotation implies trailing arms. The nearly edge-on orientation, high disk surface brightness, and presence of a complex network of dusty filaments in the optical images are responsible for the lack of detection of the arms in previous studies.