Properties and environment of the molecular complex near Holmberg IX
Résumé
This paper is aimed at providing new insight into the nature and origin of the molecular complex situated near the line of sight toward Holmberg IX in the M81 group of galaxies. The first high resolution CO maps of the complex as well as single dish 13CO(1-0), 12CO(3-2) and millimeter continuum observations and the results of a survey of 12CO in the region are presented. These data together with the available HI, optical and X-ray observations are analyzed to study the properties and environment of the complex. We confirm there is no unobscured massive star formation inside the complex and from the millimeter constraint on the extinction it must have a low star formation rate or be forming only low mass stars. According to the CO line ratios the abundances and physical conditions could be similar to that of cold gas in spirals. We find from its dynamics (no rotation) and its mass (2--6 million solar masses) that it resembles a massive GMC. Also, re-inspecting N-body simulations of the M81 group and the HI data we find that it might be located inside the extreme outer disk of M81 and be cospatial with the HI feature known as Concentration I. The negative result of the CO survey suggests that the complex is unique in this region and calls for a peculiar local formation process. We find that the distribution of the CO emission in the data cube is asymmetrical in a way similar to a cometary object. The optical observations of the nearby supershell MH9/10 suggest the existence of an outflow toward the complex. We consider the possibility that the molecular complex is related to this hypothetical outflow.
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