Vibrationally excited HC3N emission in NGC 1068: tracing the recent star formation in the starburst ring

Using the ALMA data, we have studied the HC₃N and continuum emission in the starburst pseudo-ring (SB pseudo-ring) and the circumnuclear disc (CND) of the SB/active galactic nucleus (AGN) composite galaxy NGC 1068. We have detected emission from vibrationally excited HC₃N (HC₃N*) only towards one star-forming region of the SB pseudo-ring. Remarkably, HC₃N* was not detected towards the CND despite its large HC₃N v = 0 column density. From local thermodynamic equilibrium (LTE) and non-LTE modelling of HC₃N*, we obtained a dust temperature (T_dust) of ∼250 K and a density $(n_{\text{H}_2}) \text{ of }6\times 10^5$ cm^-3 for this star-forming region. The estimated infrared (IR) luminosity of 5.8 × 10⁸ L_⊙ is typical of proto-superstar clusters (proto-SSCs) observed in the SB galaxy NGC 253. We use the continuum emissions at 147 and 350 GHz, along with CO and Pa α, to estimate the ages of other 14 SSCs in the SB pseudo-ring. We find the SSCs to be associated with the region connecting the nuclear bar with the SB pseudo-ring, supporting the inflow scenario. For the CND, our analysis yields T_dust ≤ 100 K and $n_{\text{H}_2}\sim (3\!-\!6)\times 10^5$ cm^-3. The very different dust temperatures found for the CND and the proto-SSC indicate that, while the dust in the proto-SSC is being efficiently heated from the inside by the radiation from massive protostars, the CND is being heated externally by the AGN, which in the IR optically thin case can only heat the dust to 56 K. We discuss the implications of the non-detection of HC₃N* near the luminous AGN in NGC 1068 on the interpretation of the HC₃N* emission observed in the SB/AGN composite galaxies NGC 4418 and Arp 220.