Super Hot Cores in NGC 253: Witnessing the formation and early evolution of Super Star Clusters

Abstract

Using $0.2^{\prime \prime}$ ($\sim3$ pc) ALMA images of vibrationally excited HC$_3$N emission (HC$_3$N$^*$) we reveal the presence of $8$ unresolved Super Hot Cores (SHCs) in the inner $160$ pc of NGC\,253. Our LTE and non-LTE modelling of the HC$_3$N$^*$ emission indicate that SHCs have dust temperatures of $200-375$ K, relatively high H$_2$ densities of $1-6\times 10^{6}$ cm$^{-3}$ and high IR luminosities of $0.1-1\times 10^8$ L$_\odot$. As expected from their short lived phase ($\sim 10^4$ yr), all SHCs are associated with young Super Star Clusters (SSCs). We use the ratio of luminosities form the SHCs (protostar phase) and from the free-free emission (ZAMS star phase), to establish the evolutionary stage of the SSCs. The youngest SSCs, with the larges ratios, have ages of a few $10^4$ yr (proto-SSCs) and the more evolved SSCs are likely between $10^5$ and $10^6$ yr (ZAMS-SSCs). The different evolutionary stages of the SSCs are also supported by the radiative feedback from the UV radiation as traced by the HNCO/CS ratio, with this ratio being systematically higher in the young proto-SSCs than in the older ZAMS-SSCs. We also estimate the SFR and the SFE of the SSCs. The trend found in the estimated SFE ($\sim40\%$ for proto-SSCs and $>85\%$ for ZAMS-SSCs) and in the gas mass reservoir available for star formation, one order of magnitude higher for proto-SSCs, suggests that star formation is still going on in proto-SSCs. We also find that the most evolved SSCs are located, in projection, closer to the center of the galaxy than the younger proto-SSCs, indicating an inside-out SSC formation scenario.