Discovery of spiral arms in M821

Y.D. Mayya, L. Carrasco & A. Luna (To appear in ApJL) astro-ph/0506275

M82 is the prototype of the Irr II class of galaxies, which are characterized by the absence of spiral arms. In a recent study, we have discovered two symmetric spiral arms in this nearly edge-on starburst galaxy. The arms are recovered when an axi-symmetric disk is subtracted from the near infrared images. The winding of the arms suggest a morphological classification of SBc to this galaxy. The arms emerge from the tips of the bar, and are most probably formed during the tidal interaction of M82 with M81 and other members of the M81 group. The nearly edge-on orientation, high disk surface brightness, and the presence of a complex network of dusty filaments in the optical images, are responsible for the non-detection of the arms in previous studies.

(Left Top) A color composite Subaru image of M82 showing the disk and vertical structures. No spiral arms are seen in this image.

(Left Bottom) A color composite image of the near infrared residual images, obtained by subtracting an exponential disk from the observed J,H, and K-band images. The red, green and blue colors in the image represent K,H, and J-band residual images, respectively. The central bright part (yellow) corresponds to the bar, with the outer structures suggesting the presence of a two-armed spiral mode.

K-band residual image contours are superposed on the best-matched model of a two-armed spiral mode. Near and far sides of the galaxy are noted. The ellipse with arrows indicates the sense of rotation of stars expected for trailing spiral arms. For this configuration, the arms are trailing in nature.
1First refereed paper using data obtained with the recently acquired NIR camera CANICA.

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