Positive feedback, quenching and sequential super star cluster (SSC) formation in NGC 4945

TL;DR

This study uses ALMA imaging to identify and analyze the evolutionary stages of super star clusters in NGC 4945, revealing sequential formation influenced by feedback and outflow dynamics.

Contribution

It provides the first detailed observational evidence of proto-SSC phases and their evolution in NGC 4945 using vibrational molecular lines.

Findings

8 out of 14 SSCs are in proto-SSC phase with vibrational HCN emission.

Estimated proto-SSC ages range from 5 to 9.7×10^4 years.

Sequential formation of SSCs is suggested by their spatial distribution and ages.

Abstract

We have used ALMA imaging (resolutions 0.1\arcsec-0.4\arcsec) of ground and vibrationally excited lines of HCN and HC$_3$N toward the nucleus of NGC 4945 to trace the protostellar phase in Super Star Clusters (proto-SSC). Out of the 14 identified SSCs, we find that 8 are in the proto-SSC phase showing vibrational HCN emission with 5 of them also showing vibrational HC$_3$N emission. We estimate proto-SSC ages of 5-9.7$\times$10$^4$ yr. The more evolved ones, with only HCN emission, are close to reach the Zero Age Main Sequence (ZAMS; ages $\gtrsim$10$^5$ yr). The excitation of the parental cloud seems to be related to the SSC evolutionary stage, with high ($\sim$65 K) and low ($\sim$25 K) rotational temperatures for the youngest proto and ZAMS SSCs, respectively. Heating by the HII regions in the SSC ZAMS phase seems to be rather local. The youngest proto-SSCs are located at the edges of the molecular outflow, indicating SSC formation by positive feedback in the shocked regions. The proto-SSCs in NGC 4945 seem to be more evolved than in the starburst galaxy NGC 253. We propose that sequential SSC formation can explain the spatial distribution and different ages of the SSCs in both galaxies.