Star formation and ionized regions in the Inner Galactic Plane

We present a comprehensive statistical study to understand the impact of galactic bubble structures detected in the Spitzer observations throughout the galactic plane on the star formation process. We analysed 1 360 galactic bubbles and ∼70 000 star-forming sources, from both Hi-GAL and GLIMPSE surveys, located in their vicinity. The spatial distribution of the star-forming sources seen in surface density maps reveal a clear evolutionary gradient, were more evolved Young Stellar Objects (YSOs) are typically found in the center, while recent star-forming sources (prestellar and protostellar) can be seen at the edges of the bubbles. Based on the dynamic ages derived for the bubbles and numerical simulations we find that the timescale for star-formation are better describe advocating for the pre-existence of density structures in the medium prior to the creation of the ionizing source(s). Identical pattern of age distribution of star-forming sources has been found in a recent detailed study of the nearby λ Ori bubble using APOGEE-2 and GAIA DR2 observations, which provides compelling evidence of what we obtained in our statistical result. In light of these results we propose a scenario for the star formation process in expanding ionizing bubbles