A Normal Stellar Disk in the Galaxy Malin 1

Since its discovery, Malin 1 has been considered the prototype and mostextreme example of the class of giant low surface brightness disk galaxies.Examination of an archival Hubble Space Telescope I-band image reveals thatMalin 1 contains a normal stellar disk that was not previously recognized,having a central I-band surface brightness of mu_0 = 20.1 mag arcsec^-2 and ascale length of 4.8 kpc. Out to a radius of ~10 kpc, the structure of Malin 1is that of a typical SB0/a galaxy. The remarkably extended, faint outerstructure detected out to r~100 kpc appears to be a photometrically distinctcomponent and not a simple extension of the inner disk. In terms of its diskscale length and central surface brightness, Malin 1 was originally found to bea very remote outlier relative to all other known disk galaxies. The presenceof a disk of normal size and surface brightness in Malin 1 suggests that suchextreme outliers in disk properties probably do not exist, but underscores theimportance of the extended outer disk regions for a full understanding of thestructure and formation of spiral galaxies.Comment: 13 pages, 7 figures. To appear in AJ. Typographical error correcte