Simulations of star formation in a gaseous disc around Sgr A* - a failed active galactic nucleus

S Nayakshin,J Cuadra,V Springel

doi:10.1111/j.1365-2966.2007.11938.x

Abstract

We numerically model fragmentation of a gravitationally unstable gaseous disc under conditions that may be appropriate for the formation of the young massive stars observed in the central parsec of our Galaxy. In this study, we adopt a simple prescription with a locally constant cooling time. We find that, for cooling times just short enough to induce disc fragmentation, stars form with a top-heavy initial mass function (IMF), as observed in the Galactic Centre (GC). For shorter cooling times, the disc fragments much more vigorously, leading to lower average stellar masses. Thermal feedback associated with gas accretion on to protostars slows down disc fragmentation, as predicted by some analytical models. We also simulate the fragmentation of a gas stream on an eccentric orbit in a combined Sgr A* plus stellar cusp gravitational potential. The stream processes, self-collides and forms stars with a top-heavy IMF. None of our models produces large enough comoving groups of stars that could account for the observed 'ministar cluster' IRS 13E in the GC. In all of the gravitationally unstable disc models that we explored, star formation takes place too fast to allow any gas accretion on to the central supermassive black hole. While this can help to explain the quiescence of 'failed active galactic nucleus' such as Sgr A*, it poses a challenge for understanding the high gas accretion rates inferred for many quasars.

Highlights

There is no detailed understanding of how supermassive black holes (SMBHs) gain their mass, except that it must be mainly through gaseous disc accretion (Yu & Tremaine 2002)
Within our formalism with a locally constant cooling time, we find that (i) circular and eccentric discs alike can gravitationally fragment and form stars; (ii) the initial mass function (IMF) of formed stars is a strong function of cooling time, becoming top-heavy for marginally starforming discs; (iii) star formation feedback is able to slow down disc fragmentation, as suggested by several earlier analytical papers, but it is not yet clear if it can alleviate the fuelling problem of the SMBHs; (iv) our simulations do form some tightly bound binary stars but more populous systems do not survive long
The units of length and mass used in the simulations are RU = 1.2 × 1017cm ≈ 0.04 pc, which is equal to 1 arcsec at the 8.0 kpc distance to the Galactic Centre (GC), and MU = 3.5 × 106 M, the mass of Sgr A* (e.g. Schodel et al 2002), respectively

Summary

Introduction

There is no detailed understanding of how supermassive black holes (SMBHs) gain their mass, except that it must be mainly through gaseous disc accretion (Yu & Tremaine 2002). Paczynski 1978; Kolykhalov & Sunyaev 1980; Shlosman & Begelman 1989; Bertin & Lodato 1999; Collin & Zahn 1999; Gammie 2001). Many theorists pointed out that these discs should be unstable to self-gravity, and must form stars or giant planets by gravitational fragmentation This creates a dilemma for the field, as star formation might be a dynamical, very fast process which may result in a complete transformation of the gas into stars. The SMBH would be starved of fuel. Goodman (2003), Sirko & Goodman (2003) demonstrated that star formation cannot be quenched by stellar feedback, unless one is prepared to grossly violate constraints that we have from ac-

Methods

Results

Conclusion