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| Illustration of binary black holes. Image credit: P. Marenfield (NOAO) |
Last week, I attended a workshop on Binary Black Holes and Dual AGN sponsored by NOAO in Tucson, Arizona. This workshop was held in memory of David de Young, an NOAO astronomer who passed away last December. Dave received his PhD in 1967 from Cornell University and joined the NOAO staff in 1980. Dave's own research focused on Active Galactic Nuclei (AGN) and he had over 120 scientific publications over the course of his career. The focus of last week's workshop was on a topic that Dave's research related to in many ways.
In previous posts, we introduced super massive black holes and AGN. This meeting focused on the search for pairs of black holes in galaxies. Why are these interesting? There is evidence to suggest that all massive galaxies contain a supermassive black hole in their centers. When two galaxies merge together, each of the two likely had their own black hole. Eventually, the two black holes will merge together at the center of the coalesced galaxy, but before this happens there should be a time when the two black holes are observable separately. In some cases, both may be active, in which case a dual AGN might be observable. Finding and studying these systems can tell us a lot about what happens in the final stages of a merger and we can learn a lot about black hole physics.
The goal of the workshop was to discuss many aspects of binary black holes, including how to identify them and how they affect their host galaxy. One of the methods used to identify binary black holes is to simply look for pairs of AGN on the sky at the same redshift. This is usually done by selecting AGN identified in the X-ray, but can also be done using a variety of other AGN selection techniques. This method is straight-forward, but it requires that both black holes be active and detectable, and thus finding such systems is very rare. Another common method is to look for spectroscopic signatures of two black holes. Due to the motions of two black holes in orbit around each other, spectra of such systems often have emission lines with two peaks, one for each black hole. However, since other processes can cause these types of emission lines, detailed follow-up observations and analysis is required for these candidates.
We also heard a lot about adaptive optics (AO) observations of binary black holes. Since the two black holes in a merging system are so close together on the sky, it can be very difficult to separate them from each other in images. One technique for obtaining very high resolution images from the ground is called adaptive optics (or AO for short). With AO, images are corrected for the distortions that are caused by atmospheric turbulence so that the final image is much sharper than would have been obtained otherwise. With AO, astronomers are able to peer into the very center of nearby merging galaxies and separate double nuclei and identify the two black holes. In many cases, the surrounding area can be studied in great detail in order to determine how the black holes have affected their surroundings.
Since mergers are important for producing binary black holes, one of the topics of the meeting was trying to understand the connection between mergers and AGN activity. In this context, the recent CANDELS paper by Dale Kocevski came up as an example of the many studies that have tried to investigate this question at high redshift. There is currently a lot of debate about the role that galaxy mergers play in forming AGN in general. The relationship between AGN and their host galaxies is the topic of a meeting taking place right now in Germany and will be discussed more in a future post!
In previous posts, we introduced super massive black holes and AGN. This meeting focused on the search for pairs of black holes in galaxies. Why are these interesting? There is evidence to suggest that all massive galaxies contain a supermassive black hole in their centers. When two galaxies merge together, each of the two likely had their own black hole. Eventually, the two black holes will merge together at the center of the coalesced galaxy, but before this happens there should be a time when the two black holes are observable separately. In some cases, both may be active, in which case a dual AGN might be observable. Finding and studying these systems can tell us a lot about what happens in the final stages of a merger and we can learn a lot about black hole physics.
The goal of the workshop was to discuss many aspects of binary black holes, including how to identify them and how they affect their host galaxy. One of the methods used to identify binary black holes is to simply look for pairs of AGN on the sky at the same redshift. This is usually done by selecting AGN identified in the X-ray, but can also be done using a variety of other AGN selection techniques. This method is straight-forward, but it requires that both black holes be active and detectable, and thus finding such systems is very rare. Another common method is to look for spectroscopic signatures of two black holes. Due to the motions of two black holes in orbit around each other, spectra of such systems often have emission lines with two peaks, one for each black hole. However, since other processes can cause these types of emission lines, detailed follow-up observations and analysis is required for these candidates.
We also heard a lot about adaptive optics (AO) observations of binary black holes. Since the two black holes in a merging system are so close together on the sky, it can be very difficult to separate them from each other in images. One technique for obtaining very high resolution images from the ground is called adaptive optics (or AO for short). With AO, images are corrected for the distortions that are caused by atmospheric turbulence so that the final image is much sharper than would have been obtained otherwise. With AO, astronomers are able to peer into the very center of nearby merging galaxies and separate double nuclei and identify the two black holes. In many cases, the surrounding area can be studied in great detail in order to determine how the black holes have affected their surroundings.
Since mergers are important for producing binary black holes, one of the topics of the meeting was trying to understand the connection between mergers and AGN activity. In this context, the recent CANDELS paper by Dale Kocevski came up as an example of the many studies that have tried to investigate this question at high redshift. There is currently a lot of debate about the role that galaxy mergers play in forming AGN in general. The relationship between AGN and their host galaxies is the topic of a meeting taking place right now in Germany and will be discussed more in a future post!
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