2001AJ....122.3070O

We present radio and optical observations of Holmberg I (Ho I), a member of the M81 group of galaxies (distance ∼3.6 Mpc). Ho I is a low-mass, low surface brightness dwarf galaxy. High-resolution, multiarray, Very Large Array observations in the line of neutral hydrogen (H I) reveal a supergiant shell (diameter 1.7 kpc) that covers about half the optical extent of Ho I and that comprises 75% of the total H I content (total H I mass: 1.1x10⁸ M_☉). We estimate the scale height of the H I layer to be 250 pc≲h≲550 pc. We set a tentative upper limit to the dark matter content of ≲3.1x10⁸ M_☉. The H I data are complemented by deep, optical UBV(RI)_C observations and narrowband Hα imaging obtained at the Calar Alto 2.2 m telescope. We find M_H I _/LB_=1.1M☉_/LB,☉_. The total visible (stars plus gas) mass of Ho I adds up to 2.4x10⁸ M_☉. This leads to a total mass of ≲5.5x10⁸ M_☉ and an inclination for Ho I of 10°≲i≲14°. The origin of Ho I's peculiar H I morphology is discussed in terms of a supergiant shell created by strong stellar winds and supernova explosions. We estimate that the energy deposited falls in the range of 1.2x10⁵³ ergs≲E≲2.6x10⁵³ ergs (equivalent to 120-260 Type II SNe explosions). From a comparison with isochrones, as well as from dynamical modeling based on the H I data, we derive an age for the supergiant H I shell of ∼80±20 Myr. The morphological center of Ho I (i.e., the center of the ring) is offset by 0.75 kpc with respect to the dynamical center. Within the interior of the shell, the light distribution is exponential with a rather shallow gradient and blue optical colors. Beyond a radius corresponding to an H I column density of ∼10²¹ cm^–2, the putative star formation threshold, the disk becomes considerably redder and the slope for the exponential light distributions steepens. We attribute this to a uniform star formation activity in the recent past within the central 2 kpc of Ho I. Color-magnitude diagrams based on our CCD data show that young stars, with ages of 15-30 Myr, are situated along the inside of the rim of the giant H I shell. On the rim itself, we find even younger stars and some faint H II regions. It is speculated that these stars are the result of secondary star formation on the rim of the shell. Based on the global morphology and velocity dispersion, as well as the location of the H II regions, we find evidence for ram pressure within the M81 group. Finally, we discuss the likelihood of Ho I having lost some of its interstellar material to the intergalactic medium (``blow-out'' scenario).