Herein, we present the 12CO (J=1-0) and 13CO (J=1-0) emission line observations via the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) toward a Spitzer bubble N4. We observed clouds of three discrete velocities: 16, 19, and 25 km/s. Their masses were 0.1x10^4 Msun, 0.3x10^4 Msun, and 1.4x10^4 Msun, respectively. The distribution of the 25-km/s cloud likely traces the ring-like structure observed at mid-infrared wavelength. We could not find clear expanding motion of the molecular gas in N4. On the contrary, we found a bridge feature and a complementary distribution, which are discussed as observational signatures of a cloud-cloud collision, between the 16- and 25-km/s clouds. We proposed a possible scenario wherein the formation of a massive star in N4 was triggered by a collision between the two clouds; however whereas the 19-km/s cloud is possibly not a part of the interaction with N4. The time scale of collision is estimated to be 0.2-0.3 Myr, which is comparable to the estimated dynamical age of the HII region of ~0.4 Myr. In N4W, a star-forming clump located west of N4, we observed molecular outflows from young stellar objects and the observational signature of a cloud-cloud collision. Thus, we also proposed a possible scenario in which massive- or intermediate-mass star formation was triggered via a cloud-cloud collision in N4W.
@article{arxiv.1808.00709,
title = {Molecular gas in a Spitzer bubble N4: possible evidence for cloud-cloud collisions as a trigger of massive star formation},
author = {Shinji Fujita and Kazufumi Torii and Kengo Tachihara and Rei Enokiya and Katsuhiro Hayashi and Nario Kuno and Mikito Kohno and Tomoka Tosaki and Mitsuyoshi Yamagishi and Atsushi Nishimura and Tomofumi Umemoto and Tetsuhiro Minamidani and Mitsuhiro Matsuo and Yuya Tsuda and Hidetoshi Sano and Daichi Tsutsumi and Akio Ohama and Satoshi Yoshiike and Kazuki Okawa and Yasuo Fukui and Other Fugin Members},
journal= {arXiv preprint arXiv:1808.00709},
year = {2019}
}