Environment, Ram Pressure, and Shell Formation in Holmberg II

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Abstract
Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII, a prototype galaxy for studies of shell formation, are presented. These were extracted from the multiconfiguration data set of Puche and colleagues. H I is detected to radii over 16' or 4R25, almost a factor of 2 better than previous studies. The total H I mass M = 6.44 × 108 M⊙. The integrated H I map has a comet-like appearance, with a large but faint component extending to the northwest and the H I appearing compressed on the opposite side. This suggests that HoII is affected by ram pressure from an intragroup medium (IGM). The velocity field shows a clear rotating disk pattern, and a rotation curve corrected for asymmetric drift was derived. However, the gas at large radii may not be in equilibrium. Puche and colleagues' multiconfiguration data were also reanalyzed, and it is shown that they overestimated their fluxes by over 20%. The rotation curve derived for HoII is well defined for r 10 kpc. For 10 r 18 kpc, however, velocities are only defined on the approaching side, such that this part of the rotation curve should be used with caution. An analysis of the mass distribution, using the whole extent of this rotation curve, yields a total mass of 6.3 × 109 M⊙, of which ≈80% is dark. Similarly to what is seen in many dwarfs, there is more luminous mass in H I than in stars. One peculiarity, however, is that luminous matter dominates within the optical body of the galaxy and dark matter only in the outer parts, analogous to what is seen in massive spirals rather than dwarfs. HoII lies northeast of the M81 Group's core, along with Kar 52 (M81 dwarf A) and UGC 4483. No signs of interaction are observed, however, and it is argued that HoII is part of the NGC 2403 subgroup, infalling toward M81. A case is made for ram pressure stripping and an IGM in the M81 Group. Stripping of the outer parts of the disk would require an IGM density nIGM 4.0 × 10-6 atoms cm-3 at the location of HoII. This corresponds to ~1% of the virial mass of the group uniformly distributed over a volume just enclosing HoII and is consistent with the known X-ray properties of small groups. The H I tail is consistent with additional turbulent viscous stripping and evaporation, at least for low IGM temperatures. It is argued that existing observations of HoII do not support self-propagating star formation scenarios, whereby the H I holes and shells are created by supernova explosions and stellar winds. Many H I holes are located in low surface density regions of the disk, where no star formation is expected or observed. Alternative mechanisms are discussed, and it is suggested that ram pressure can help. Ram pressure has the capacity to enlarge preexisting holes and lower their creation energies, helping to bridge the gap between the observed star formation rate and that required to create the holes.
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