Title: The Milky Way Supermassive Black Hole: Dynamical Feeding from the Circumnuclear Environment
Authors: Hauyu Baobab Liu, Pei-Ying Hsieh, Paul T. P. Ho, Yu-Nung Su, Melvyn Wright, Ai-Lei Sun, Young Chol Minh
First Author’s Institution: Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan
Recent studies have revealed a surprising amount of activity happening in the heart of our own Milky Way. Several young, massive stars orbit within a mere 0.1 parsecs of our supermassive black hole, Sagittarius A* (Sgr A*, or “Sag. A-star”, for short), which weighs in at about 4 million times the mass of our Sun. As discussed in this astrobite, their surprisingly young age implies that they formed extremely recently, most likely from a molecular cloud (or two) falling towards the supermassive black hole. In another astrobite, we saw that a gas cloud that was likely ejected at the death of one of these massive, nearby stars is plunging directly towards Sgr A*, promising a spectacular show when it is accreted in 2013. On the less massive end, some astronomers suggest that Sgr A* is constantly munching on asteroids and planets that wander too close to the Galactic center (also an astrobite!).
Why should we care about these accretion events? For starters, all galaxies, not just ours, harbor supermassive black holes (SMBHs) at their centers. Moreover, when these SMBHs accrete enough material to release significant amounts of energy compared to the galaxy itself, we classify the central regions as active galactic nuclei (AGN). (See this astrobite, e.g., for a discussion of AGN.) While the energy output from the events in our own galaxy may seem puny in comparison to what we see from its more luminous cousins, any accretion events that we can see with such fine detail can give valuable insight into the workings of distant galaxies. It is even possible that our galactic center was even more active not so long ago. Enormous bubbles of gamma-ray emitting gas, the “Fermi bubbles”, extend an impressive 25,000 light years above and below the disk of the Milky Way, and may have been energized by jets from Sgr A* just as we see in “radio-loud” AGN. In order to make better comparisons to other galaxies, and to predict the likelihood of future activity, however, we need to understand where this accreted material is coming from. In this paper, Liu et al. explore the kinematics of the gas outside the most central regions of our galaxy, and reveal that the Galactic center is not sitting in isolation, but instead is being fed even more material from the main structure of the Milky Way.
The authors of this paper examine material including and just outside of the circumnuclear disk (CND) – a ring of molecular gas enclosing the innermost 1.5-4 parsecs of the Galactic center. Part of the motivation for looking here is that the CND itself is quite clumpy and irregular, suggesting that it may be stirred up by an inflow or outflow of material. In order to observe the gas, Liu et al. go after atomic transitions of various molecules. While the gas, like most of the Universe, is predominately composed of hydrogen, molecular hydrogen is a nice, symmetric molecule that doesn’t lend itself to atomic transitions that would emit photons for us to see. Instead, the authors choose carbon monosulfide (CS), hydrogen cyanide (HCN), and carbon monoxide (CO) as tracers. The fact that two of these compounds happen to be toxic to humans is coincidental; instead, these are logical choices because they are composed of some of the most abundant elements in the Universe next to hydrogen and helium (carbon and oxygen rank 3rd and 4th by mass, while nitrogen and sulfer come in 7th and 8th). Additionally, each transition between the electron energy levels for each molecule can give astronomers information about the temperature of the gas. At higher temperatures, the electrons are more likely to hang out at higher energy levels, and thus we should expect to see high-excitation transitions, while at lower temperatures, the electrons are more likely to be near the ground state and we should expect to see low-excitation transitions. In this way, the authors can actually use the CS molecule twice: once for the CS levels 1-0 low-excitation transition and once for the CS levels 7-6 high-excitation transition. As such, they use the Greenbank Telescope (GBT) to observe the CS(1-0) transition tracing the lowest temperature gas at ~10k, and they use the Submillimeter Array (SMA) to observe the CO(3-2), HCN(4-3), and CS(7-6) transitions that trace gas at progressively higher temperatures from ~30- 60K.
Their main results are shown in the figures above. In the warm gas, they see four distinct, spiral-like arms that feed directly into the CND, seen in Figure 1. In Figure 2, the kinematic structure of the coldest gas reveals large streams also approaching and receding from the CND, where they use the Doppler effect to get the relative velocities of the gas. All of these features are stretched out by tidal forces from the Galactic center; nonetheless, they are rotating in a manner consistent with the general motion of the Milky Way’s disk. A complete interpretation of the data – large, cool streams of gas coming in to and leaving the Galactic center, and warm, close-in spiral arms that attach to the warm CND – is summarized by the cartoon in Figure 3.
This simple yet intriguing study adds a missing piece to the puzzle of our Galactic center, adding a link between the inner few parsecs and the rest of the galaxy. It seems that the energetic displays of Sgr A* may not be over, as new material is constantly being brought closer to its domain. This paper also suggests that the CND itself, which seems quasi-stationary, could change dramatically over time as material is added to or taken away from it. Our ability to see this constant flurry of activity around our own, modest supermassive black hole can give us pause to wonder what other seemingly quiescent supermassive black holes may be up to, and what secrets of theirs may be revealed in the future.