Andromeda’s double nucleus
Most galaxies only have one nucleus at their center. Why does Andromeda have two?
The centers of galaxies are often extremely turbulent places. They usually contain supermassive black holes — weighing in at millions or billions of solar masses — that gravitationally dominate the area around them. A couple of weeks ago, I wrote about the nucleus of the Milky Way, focusing on a structure called Sagittarius A East that might be the result of a star being torn apart by the supermassive black hole, Sagittarius A*. This week, I’ll discuss the center of a well-known neighboring galaxy: Andromeda.
Andromeda is an interesting counterpart to the Milky Way in a number of ways. Both are spiral galaxies, and the most massive objects in our galaxy cluster, the Local group. But Andromeda has twice as many stars, and could be up to twice as massive. An even more exciting difference? Andromeda may not have just one nucleus at its center, but two.
Early images from Hubble
In 1991, astronomers using the then newly-launched Hubble Space Telescope imaged Andromeda’s nucleus using the short-lived Wide Field and Planetary Camera (WFPC) (Lauer et al. (1993)). They took five exposures in the V-band — a section of the electromagnetic spectrum visible to human eyes — and two longer exposures in the I-band, in the near-infrared. The image analysis was difficult; not only was Hubble not fully calibrated, but an imperfection on its primary mirror caused an effect called spherical aberration, which required a better understanding of how images spread out on the detectors.
It became evidence that the center of Andromeda had two bright peaks, designated P1 and P2 (the brighter and dimmer sources, respectively). The group showed that P2 was at the center of the nucleus, and P1 was slightly farther away. This was strange; it would be expected that the brighter source would be at the center, note vice versa. Several explanations were suggested for the existence of the double nucleus:
- P1 and P2 could be two regions of one larger structure, and the dark line in between could be a band of dust obscuring some of the light. However, dust would be expected to re-radiate in the infrared, and the same double nucleus was observed at those wavelengths. It was suggested that a strange shape for the region of dust could explain the absorption.
- P1 could be a cluster of stars orbiting the true nucleus P2. The problem with this hypothesis was that modelling showed that such a cluster could not survive for very long — perhaps only tens of thousands of years. P1 and P2 should be separated by only a parsec or so, which isn’t enough to avoid substantial tidal effects.
- A modification to the second theory is that P1 is the remains of the nucleus of a separate galaxy, absorbed by Andromeda. It could also hold a supermassive black hole. Galaxies often cannibalize nearby companions; the Milky Way has likely torn apart dwarf galaxies, creating structures like the Virgo Stellar Stream.
All three explanations suggested that P2 is a supermassive black hole just like Sagittarius A* — something that would be expected to lie in Andromeda’s nucleus. The main open question, then, was the nature of P1.
Scott Tremaine’s eccentric disk
An enduring theory of the structure of P1 was first proposed by Tremaine (1995). He hypothesized that it was a highly eccentric disk of stars orbiting P2, kept in alignment by their own gravity. This would account for observations suggesting that P2 was just off from the center of the bulge, as this space should be occupied by the center of the mass of the disk-black hole system.
The main problem, Tremaine noted, was the idea of how such an eccentric disk could have formed. One existing possibility was that a molecular cloud could have been tidally disrupted by the supermassive black hole, similarly to the possible previously-mentioned progenitor of Sagittarius A East. Two other scenarios involved a circular disk experiencing an instability or experiencing dynamical friction from the rest of the nucleus.
Eight years later, Tremaine elaborated on the idea with the help of Hiranya Peiris (Peiris & Tremaine (2003)). Their models found that the disk could either be aligned with the disk of Andromeda itself, or could be slightly misaligned. Both cases reproduced the Hubble data quite well, lending more support to the eccentric disk model — a theory that had begun to be widely accepted.
P3: A third component?
The story of Andromeda’s nucleus took a turn in 2005, when a third component, P3, was discovered. Bender et al. (2005) showed that a patch of blue in images of P2 was in fact its own cluster of blue stars, distinct from the largely red stellar population in the nucleus. They used Hubble’s Space Telescope Imaging Spectrograph (STIS) at shorter wavelengths than Lauer et al., and found that P3 fit the expectations of a cluster of A-type stars. In fact, its spectrum was very close to the spectrum of a single A-type star, indicating that the stars were all quite similar and had likely formed together.
There must be around 200 stars in P3, according to the STIS observations, showing that a starburst event of some sort must have taken place. A primary explanation was that the necessary gas could have come from the eccentric disk itself, which would be unexpected this close to a supermassive black hole. The astronomers also suggested that many of the stars in P3 could have been formed by lower-mass red stars colliding. A similar phenomenon is thought to occur in globular clusters, forming younger stars called blue stragglers. The necessary instabilities could have come from periodic mass loss from P1 and P2 (see e.g. Chang et al. (2007)).
The observations showed that the stars lay in a circular disk. This wasn’t an implausible idea; given their spectral type, they were probably young, and there wouldn’t have been time for the small disk (0.8 parsecs in diameter) to deform. It could remain much more circular than the larger, extended disk, especially if it had only existed for a short amount of time.
The existence of P3 and its disk-like structure provided more evidence for Tremaine’s eccentric disk models, and the disk hypothesis is still regarded as the most likely explanation for Andromeda’s double nucleus. More detailed observations of the kinematics of P3, as well as better constraints on the mass of the central supermassive black hole in P2, will hopefully give us a better understanding of the structure and dynamics of the system, and more insight into the double nucleus of Andromeda.
