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We present radio and optical observations of Holmberg I (HoI), a member ofthe M81 group of galaxies (distance 3.6Mpc). HoI is a low-mass, low surface-brightness dwarf galaxy. High-resolution multi-array VLA HI observations reveala supergiant shell (diameter: 1.7 kpc) which covers about half the opticalextent of HoI and which comprises 75% of the total HI content (total HI mass:1.1 10^8 M_o). We set a tentative upper limit to the dark matter content of <3.1 10^8 M_o. The HI data are complemented by deep, optical UBV(RI)_c andHalpha observations obtained at the Calar Alto 2.2m telescope. We find M_HI/L_B= 1.1 M_o/L_B_o. The total visible (stars plus gas) mass of HoI adds up to 2.410^8 M_o. This leads to a total mass of < 5.5 10^8 M_o. The origin of HoI'speculiar HI morphology is discussed in terms of a supergiant shell created bystrong stellar winds and supernova explosions (energy: equiv. 20-260 type IISN; age: 80+-20 Myr). The morphological center of HoI is offset by 0.75 kpcwith respect to the dynamical center. Within the interior of the shell thelight distribution is exponential with a rather shallow gradient and blueoptical colors. Beyond a radius corresponding to an HI column density of 10^21cm^-2, the putative star formation (SF) threshold, the disk becomesconsiderably redder and the slope for the exponential light distributionsteepens. It is speculated that a generation of stars (age 15-30 Myr) is theresult of secondary SF on the rim of the shell. Based on the global morphologyand velocity dispersion as well as the location of the HII regions we findevidence for ram pressure within the M81 group. Finally, we discuss thelikelihood of HoI having lost some of its interstellar material to theintergalactic medium (''blow-out'' scenario).Comment: 36 pages, 20 figures, 1 color figure (Fig. 2). Accepted for  publication in the Astronomical Journal (AJ). For full resolution see  ftp://ftp.astro.uni-bonn.de/pub/jott/down/ott_HoI_AJ2001.ps.g

Evidence for BlowOut in the Low-Mass Dwarf Galaxy Holmberg I