Sagittarius A East: A supernova remnant or a star torn apart?

The galactic center’s a busy place — and a dangerous one.

The supermassive black hole at the center of the galaxy continuously captivates people’s imaginations. At over 4 million solar masses, Sagittarius A* is the most massive single object in the Milky Way. Though not as luminous as the supermassive black holes in galaxies with active nuclei, it is nonetheless a strong radio source, at least from the point of view of telescopes here on Earth. However, Sagittarius A* is embedded in a larger radio-emitting region, called Sagittarius A — and Sagittarius A is rich and complex. It’s only within the last few decades that we’ve begun to put together a picture of the entire region, which includes fast-moving gas clouds, Sagittarius A*, high-velocity stars, and, perhaps most mysteriously, a shell-like structure whose origin is, as of today, still unknown.

Maps, gas, and early observations

The first hint that the galactic center might be special came in the 1970s. Balick & Brown (1974) used the interferometer at the Green Bank Observatory and discovered that there was a small but luminous structure in the center. They compared it to the previously-observed active galactic nuclei (AGNs), although the true identities of AGNs were as yet unknown. The point source the pair discovered was later dubbed Sagittarius A*, the supermassive black hole; around the same time, others were discovering that the area was much more complicated than had been previously imagined.

Figure 4, Ekers et al. (1975). The Ekers map of Sagittarius A, using data from Westerbork and Owens Valley. Notice the multiple emission regions.

A number of groups attempted to map the galactic center via radio waves. Ekers et al. (1975) studied radio emission at 5 GHz using the Westerbork and Owens Valley radio telescopes, and confirmed the existence of two separate component sources, Sagittarius A West and Sagittarius A East. The two had already been observed, by Downes & Martin (1971), but little was known about them. Ekers et al. were able to make comparatively high-resolution measurements. They concluded that Sagittarius A was likely different from AGNs, not least because it was dim compared to those extragalactic sources.

In addition, the group concluded that there were several different types of radio emission occurring. In addition to the compact source, there was thermal emission from other parts of Sagittarius A West, meaning that part of the system was in some sort of temperature-related equilibrium. It fit the signature of ionized gas, like you might find in an H II region, where ionized hydrogen is abundant. This emission could not be coming from a compact source, and so gas clouds were a likely candidate. Modern-day observations have borne out this hypothesis.

A Hubble Space Telescope image of the Orion Nebula, an H II region. Image courtesy of NASA and the ESA, in the public domain.

Another interesting conclusion Ekers et al. came to was that the emission from Sagittarius A East was distinctly non-thermal. In terms of luminosity and surface brightness, this section resembled supernova remnants. The authors stopped short of explicitly claiming that the source was such a remnant, but the idea had already been proposed in Jones (1974), and was slowly gaining some traction. If it was a supernova remnant, it might be an extraordinary one, given its surroundings.

A year after the detailed observations from the group were published, Gopal-Krishna & Swarup (1976) analyzed the selection of maps that had recently been constructed, and, like Jones, noted that the shell-like shape in Sagittarius A East distinctly appeared like a supernova remnant. This idea continued to enjoy some popularity throughout the decade, and more detailed measurements of morphology and connection with the rest of the structure provided additional circumstantial evidence (see Ekers et al. (1983)).

A new challenge: a close explosion

Toward the end of the 1980s, however, a pair of papers challenged the supernova interpretation. Yusuf-Zadeh & Morris (1987) and Mezger et al. (1989) discovered more complex features of dust and gas. The latter group made multi-wavelength maps at 0.35, 1.3, and 2.8 cm using telescopes on Mauna Kea and Spain. The results were surprising. In addition to the circumnuclear disk (CND) surrounding Sagittarius A West, they found a dust ring around Sagittarius A East, associated with a high-density region in surrounding molecular clouds. They interpreted this as a shell moving into the clouds. In addition, the geometric alignment suggested that the CND and the shell were related and could possibly both once have been part of the same feature.

Figure 7, Mezger et al. (1989). Some models for the structure of the region, including the Sagittarius A East shell.

After considering the emission from the region and the structure of the clouds, the authors came up with four possible scenarios:

1. The shell around Sagittarius A East is the result of a supernova exploding into the cloud, creating a new bubble.
2. The shell around Sagittarius A East was formed from strong stellar winds of O-type stars, and has since been filled by a supernova remnant.
3. Several tens of thousands of years ago, a catastrophic explosion occurred while Sagittarius A* was embedded in a molecular cloud.
4. Some sort of continuous wind from the galactic center, maybe originating from AGN-like activity, carved out the bubble in Sagittarius A East.

The group discarded the first and fourth interpretations. If a supernova had formed the bubble, it would have to have been an order of magnitude more powerful than a normal one. Additional, a galactic wind could not have explained the synchrotron emission observed in the region. Now, there were also problems with the second and third scenarios (namely, that O stars would have needed tens of thousands of years to form the bubble, and the explosive event might not have fit well with the dust and synchrotron emission found), but they were still thought to be more plausible.

A possible explanation: tidal disruption

Supermassive black holes have a tendency to severely disrupt objects around them. For example, Gillessen et al. (2012) showed that Sagittarius A* is undergoing close interactions with a gas cloud, G2, and a number of stars move extraordinarily quickly around the galactic center. In 1996, Khoklov & Melia (1996) suggested that an extreme close encounter was responsible for the emission from Sagittarius A East, as the previous two groups believed.

The pair suggested that the supermassive black hole had caused a tidal disruption event, a scenario in which a star passing close to Sagittarius A* was essentially torn apart. Some of the matter would have been accreted onto the black hole, while some of it would have rebounded outwards, forming a shell-like structure resembling a supernova remnant. Several characteristics of the shell supported this hypothesis, including the fact that Sagittarius A East is not spherical, but elliptical, and that a tidal disruption event could have released that energy needed to form such a bubble, while a standard supernova could not.

Recent activity from Sagittarius A*. Images from NASA’s Chandra X-ray Observatory, in the public domain.

As of today, neither interpretation has been fully accepted. Some observations at other wavelengths (e.g. X-ray observations by Maeda et al. (2002)) continue to support the supernova hypothesis, and it seems more and more likely that the original hypotheses were correct. As interest has been drawn to the high-velocity stars in the region, such as S2, the dynamics and mass of Sagittarius A* have been studied in more detail. The tidal disruption hypothesis still retains some support, however, and hopefully closer observations of the region will either conclusively support or refute it.