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We attempt to determine the nature of the bubbles observed by Spitzer in theGalactic plane, mainly to establish if possible their association with massivestars. We take advantage of the very simple morphology of these objects tosearch for star formation triggered by HII regions, and to estimate theimportance of this mode of star formation. We consider a sample of 102 bubblesdetected by Spitzer-GLIMPSE, and catalogued by Churchwell et al.(2006). We usemid-infrared and radio-continuum public data to discuss their nature. We usethe ATLASGAL survey at 870 micron to search for dense neutral materialcollected on their borders. Results: We find that 86% of the bubbles containionized gas detected by means of its radio-continuum emission at 20-cm. Thus,most of the bubbles observed at 8.0 micron enclose HII regions ionized by O-B2stars. Ninety-eight percent of the bubbles exhibit 24 micron emission in theircentral regions. The ionized regions at the center of the 8.0 micron bubblesseem to be devoid of PAHs but contain hot dust. Among the 65 regions for whichthe angular resolution of the observations is high enough to resolve thespatial distribution of cold dust at 870 micron, we find that 40% aresurrounded by cold dust, and that another 28% contain interactingcondensations. The former are good candidates for the collect and collapseprocess, as they display an accumulation of dense material at their borders.The latter are good candidates for the compression of pre-existingcondensations by the ionized gas. Eighteen bubbles exhibit associatedultracompact HII regions and/or methanol masers in the direction of dustcondensations adjacent to their ionization fronts. Our results suggest thatmore than a quarter of the bubbles may have triggered the formation of massiveobjects.

A gallery of bubbles - The nature of the bubbles observed by Spitzer and what ATLASGAL tells us about the surrounding neutral material