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

Abstract

Using 0.2 (∼ 3 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 × 10 6 cm −3 and high IR luminosities of 0.1−1×10 8 L . As expected from their short lived phase (∼ 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 (∼ 40% 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.

Clustered Star Formation in the Center of NGC 253 Contributes to Driving the Ionized Nuclear Wind

“…The oldest sources would be 6, 9, 11, and 12, which all have stellar masses 4-20 times larger than the gas masses. This is consistent with the analysis of Rico-Villas et al (2020), who find that sources 5, 6, 7, 9, 10, and 12 are more evolved than the embedded clusters 1, 2, 3, 4, 8, 13, and 14.…”

Starburst Energy Feedback Seen through HCO+/HOC+ Emission in NGC 253 from ALCHEMI

“…Strong radio/submillimeter continuum sources suggest multiple star clusters in the CMZ (Turner & Ho 1985;Ulvestad & Antonucci 1997;Leroy et al 2018). Super hot cores, early stages of super star clusters, are reported by Rico-Villas et al (2020). There are multiple signs of feedback in the CMZ of NGC 253.…”

Outflows from Super Star Clusters in the Central Starburst of NGC253

“…In general the chemistry-based age sequences presented by Krieger et al (2020) lead to different relative cluster ages than the dynamical progression presented here, which are also different from the ZAMS age sequence of Rico-Villas et al (2020). It is important to keep in mind, however, that the oldest clusters are not necessarily the most evolved, and vice versa.…”

“…An age sequence using the chemistry of sulfur bearing molecules suggests instead that SSCs 5 and 14 are younger whereas SSC 4 is older (Krieger et al 2020). This is in disagreement with the age progression suggested by Rico-Villas et al (2020), who suggest an inside out formation with SSCs 4−12 being the oldest and SSCs 1−3, 13, and 14 being the youngest. The detections of outflows towards SSCs 4a, 5a, and 14 would suggest that they are the most evolved clusters in the young burst, in the simplest model where the clusters completely and finally clear their gas at the end of their formation periods.…”

“…In the absence of gas cooling, stellar winds can impart momentum into the surrounding material typically p r /M * ≈ 50− 65 km s −1 (Weaver et al 1977), assuming a wind luminosity of 10 34 erg s −1 (Starburst99; Leitherer et al 1999), the ages of these clusters to be ≈ 10 5 yrs (Rico-Villas et al 2020), and n H = 10 5 cm −3 . These estimates are on the high side of our observed range of p r /M * ≈ 5 − 126, 1 − 3, and 5 − 40 km s −1 for SSCs 4a, 5a, and 14 respectively (Table 2), and are fairly insensitive to the assumed average gas density (an order of magnitude in n H results in a factor of ≈ 1.5 change in the expected momentum).…”