Bullet shooting cloud-cloud collision in MIR bubble N65

Abstract: We report that the formation of the twin-bubble system N65 and N65bis may be caused by the cloud-cloud collision (CCC) from the Bullet Nebula. The blue-shifted $^{13}$CO gas component (N65a [47, 55] km s$^{-1}$) is associated with the twin-bubble system, while the red-shifted $^{13}$CO gas component (N65b [55, 62] km s$^{-1}$) is linked to the Bullet Nebula. The distinct signatures of CCC, such as the bridge feature, the U-shape cavity and the complementary distribution with displacement, are found between N65a and N65b. The collision timescale is estimated to be 1.15 to 2.0 Myr, which is consistent with the dynamical ages of the two \HII regions in N65a (0.73 Myr for N65bis and 1.19 Myr for N65, respectively), indicating their CCC-related origin. A total of 354 young stellar objects (YSOs) are founded, which are clustered into eight MST (Minimum Spinning Tree) groups. The distribution of M1 (at the post-frontal edge) and M2, M3, M4 (at the pre-frontal edge) suggests that the CCC triggers star formation along the collision path of $b=0^{\circ}.35$, with younger YSOs present at the pre-frontal edge. Therefore, the bipolar morphology of the twin-bubble system can be interpreted by the collision of N65a and N65b along $b=0^{\circ}.35$ about 2 Myr ago.