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Colliding interstellar bubbles in the direction of l=54°

Interstellar bubbles are structures in the interstellar medium with diameters of a few to tens of parsecs. Their progenitors are stellar winds, intense radiation of massive stars, or supernova explosions. Star formation and young stellar objects are commonly associated with these structures. We compare IR observations of bubbles N115, N116 and N117 with atomic, molecular and ionized gas in this region. While determining the dynamical properties of the bubbles, we also look into their ambient environment to understand their formation in a wider context. For finding bubbles in HI (VLA Galactic Plane Survey) and CO data (Galactic Ring Survey), we used their images from Galactic Legacy Infrared Mid-Plane Survey. We manually constructed masks based on the appearance of the bubbles in the IR images and applied it to the HI and CO data. We determined their kinematic distance, size, expansion velocity, mass, original density of the maternal cloud, age and energy input. We identified two systems of bubbles: the first, background system, is formed by large structures G053.9+0.2 and SNR G054.4-0.3 and the infrared bubble N116+117. The second, foreground system, includes the infrared bubble N115 and two large HI bubbles, which we discovered in the HI data. Both systems are independent, lying at different distances, but look similar. They are both formed by two large colliding bubbles with radii around 20-30 pc and ages of a few million years. A younger and smaller (~4 pc, less than a million years) infrared bubble lies at the position of the collision. We found that both infrared bubbles N115 and N116+117 are associated with the collisions of larger and older bubbles. We propose, that such collisions increase the probability of further star formation, probably by squeezing the interstellar material, suggesting that it is an important mechanism for star formation.