The Bubbling Galactic Disk II: The Inner 20 Degrees

We report 269 mid-infrared bubbles within 10 • of the Galactic center from visual inspection of the Spitzer GLIMPSE II Legacy Science program images. The main results of this study are as follows. The surface density of bubbles is ∼5 per square degree or about three times that detected in longitudes |l| =10 • to 65 • because the inner 10 • of longitude were more thoroughly searched for small bubbles. There is a gradient in the number of bubbles with longitude with an increase of about a factor of two from 2 • to 10 • ; this is probably the result of several factors including decreasing diffuse background brightness, confusion, and opacity with longitude. Bubble eccentricities are large, typically between 0.6 to 0.8, and >50% show evidence for blow-outs, which we suggest result from local density fluctuations of the ISM and/or anisotropic stellar winds and radiation fields. The average shell thickness of the bubbles scales with the average radius with a slope of ∼0.24; this is shallower than the theoretically predicted asymptotic slope of ∼0.32 because most of the bubbles have not had time to reach the asymptotic value. The fraction of bubbles identified with H II regions and clusters is only about half that found at |l| > 10 •. This is largely a result of the much – 2 – smaller angular diameter of bubbles cataloged in the inner Galaxy than in the outer Galaxy. Finally, at least 12% of the bubbles have morphologies suggestive of triggered star formation. The main difference between the bubbles at |l| > 10 • and those at |l| < 10 • is that those cataloged in the inner Galaxy are on average smaller than those in the outer Galaxy; this is a selection effect not a physical reality. Most of the bubbles that show evidence for triggered star formation in the inner Galaxy have not formed secondary bubbles; it is postulated that this may be because they are too young to have had time for this to occur. More than 10% of the bubbles have apparent YSOs in projection on their outer boundaries indicative of, but not proof of, triggered star formation.