Super Hot Cores in NGC 253: witnessing the formation and early evolution of super star clusters

Using 0.2 arcsec (∼3 pc) ALMA images of vibrationally excited HC₃N emission (HC₃N*) we reveal the presence of eight unresolved Super Hot Cores (SHCs) in the inner 160 pc of NGC 253. Our LTE and non-LTE modelling of the HC₃N* emission indicate that SHCs have dust temperatures of 200-375 K, relatively high H₂ densities of (1-6) × 10⁶ cm^-3 and high IR luminosities of (0.1-1) × 10⁸ L_⊙. As expected from their short-lived phase (∼10⁴ yr), all SHCs are associated with young super star clusters (SSCs). We use the ratio of luminosities from 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 10⁴ yr (proto-SSCs) and the more evolved SSCs are likely between 10⁵ and 10⁶ 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 (∼ 40 per cent for proto-SSCs and > 85 per cent 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 centre of the galaxy than the younger proto-SSCs, indicating an inside-out SSC formation scenario.