Cluster-forming site AFGL 5157: colliding filamentary clouds and star
  formation
  We observationally investigate star formation (SF) process occurring in AFGL
5157 (area ~13.5 pc X 13.5 pc) using a multi-wavelength approach. Embedded
filaments are seen in the {\it Herschel} column density map, and one of them is
identified as an elongated filamentary feature (FF) (length ~8.3 pc; mass ~1170
Msun). Five Herschel clumps (Mclump ~45-300 Msun) are traced in the central
part of FF, where an extended temperature structure (Td ~13.5-26.5 K) is
observed. In the direction of the central part of FF, the warmer region at Td
~20-26.5 K spatially coincides with a mid-infrared (MIR) shell surrounding a
previously known evolved infrared cluster. Diffuse H-alpha emission is traced
inside the infrared shell, suggesting the presence of massive stars in the
evolved cluster. Based on the surface density analysis of young stellar objects
(YSOs), embedded clusters of YSOs are traced toward the central part of FF, and
are distributed around the infrared shell. Previously detected H2O masers, H2
knots, massive protostar candidates, and HII region are also seen toward the
embedded clusters. Using the 12CO and 13CO line data, the central part of FF is
observed at the overlapping zones of two filamentary molecular clouds (length
~12.5 pc) around -20 and -17 km/s, which are also connected in velocity. Our
observational results suggest that the formation of massive stars appears to be
triggered by a collision of two filamentary molecular clouds, which might have
also influenced the birth of YSOs in AFGL 5157.
