What Controls The Star Formation Rate In Nuclear Rings? (Astronomy)

Nuclear rings are the sites of compact and extremely vigorous star formation, powered by bar-driven inflows from the larger-scale Interstellar Medium (ISM) . Despite many observational and theoretical studies of nuclear rings, it has remained unclear what determines the gas ring’s properties and star formation rate (SFR) , and how star formation in nuclear rings proceeds with time. To address these issues, Moon and colleagues in their recent paper, constructed a semiglobal model of a nuclear ring where the bar-driven mass inflows are treated by the boundary conditions. An advantage of their framework over fully global simulations is that it enables them to directly control the mass inflow rate and ring radius.

They have adapted and modified the 3–phase Interstellar medium in Galaxies Resolving Evolution with Star Formation and Supernova feedback (TIGRESS) framework to handle radiative heating and cooling, star formation, and related supernova (SN) feedback.

They found that the SN feedback is never strong enough to destroy the ring or quench star formation everywhere in the ring. Under the constant mass inflow rate, the ring star formation is very steady and persistent, with the SFR exhibiting only mild temporal fluctuations.

They also found that the ring SFR is tightly correlated with the inflow rate as: M˙_SF ≈ 0.8M˙in, for a range of M˙in = 0.125 − 8 M yr¯1. In addition, within the ring, vertical dynamical equilibrium is maintained, with the midplane pressure (powered by SN feedback) balancing the weight of the overlying gas.

Figure 1. Dependence of the SFR surface density ΣSFR on (a) the ring surface density Σring, (b) the midplane pressure Pmid, and (c) the total gas weight Wtot. © Moon et al.

Moreover, by using scaling relations for pressure and star formation density, they showed that SFR surface density is correlated nearly linearly with the midplane pressure, as predicted by the pressure-regulated, feedback-modulated star formation theory.

Based on above results, they argued that the star fomation rate in the ring is controlled primarily by the mass inflow rate (rather than the ring mass). While, the ring gas mass adapts to the SFR to maintain the vertical dynamical equilibrium under the gravitational field arising from both gas and stars.

Featured image: NGC 4314. The nuclear ring appears in violet. © NASA, ESA, D. Maoz, G. F. Benedict et al.

Reference: Sanghyuk Moon, Woong-Tae Kim, Chang-Goo Kim, Eve C. Ostriker, “Star Formation in Nuclear Rings with the TIGRESS Framework”, Arxiv, pp. 1-31, 2021. https://arxiv.org/abs/2104.10349

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