Black Hole Binaries
The X-ray point source populations of nearby galaxies consist almost entirely of accreting X-ray binary stars and supernova remnants (SNRs). Thanks to the superb spatial resolution and sensitivity of the Chandra X-ray Observatory, we can study these objects in detail in galaxies beyond our own Galactic neighborhood (the Milky Way and its neighbors, called the Local Group). X-ray binaries are classified as either high-mass or low-mass (HMXBs and LMXBs), depending on the mass of the donor star, not the compact object. They are further classified by the type of compact object as either black holes, neutron stars, or white dwarfs.
Since the mass-donors in HMXB systems are short-lived high-mass stars, they are accurate tracers of active star formation within a galaxy. Conversely, LMXBs are associated with longer lived progenitor stars, and are thus an accurate tracer of the total stellar mass in a galaxy. The X-ray bright SNRs are an independent tracer of star formation.
However, the X-ray data alone are not enough to classify the majority of sources that we see. We must therefore rely on other wavelengths: optical to identify stellar counterparts, radio to identify SNRs, IR as a tracer of dust and of star-formation, UV as a tracer of mass, etc. Combining all these data provides the complete picture of X-ray sources in galaxies and what they can tell us about the star-formation histories of those galaxies.
The binaries that contain both black holes and high-mass stars are interesting for two important reasons: first, they trace the most massive recent star formation in a galaxy; and second, they are excellent laboratories for studying black hole growth and evolution. These sources are the progenitors of the merging black holes that have now been detected by the LIGO facility.
Intermediate-Mass Black Holes
Most if not all galaxies contain supermassive black holes at their centers. No matter how far back in the universe we look, we see such black holes, with masses that are millions to billions of times the mass of our Sun. However, we still do not know how these black holes formed. We understand the process for forming stellar-mass black holes fairly well, but there is not sufficient time between the Big Bang and the emergence of the first supermassive black holes to have built up the supermassive black holes hierarchically from stellar-mass black holes.
Another class of black holes may help answer the question of how supermassive black holes formed. Intermediate-mass black holes have masses from hundreds to tens of thousands of times the mass of the Sun, bridging the gap between the stellar-mass black holes and the supermassive ones. In the last decade, we have discovered a few candidate objects in this new class, which may be the missing link in supermassive black hole formation. These objects are also interesting for physical reasons: are there processes that behave the same way across eight orders of magnitude in mass?
Other Research Interests
My primary research interest beyond X-ray populations is in deep X-ray surveys and, in particular, the normal galaxy contribution to the X-ray background. As we probe to ever fainter fluxes with X-ray telescopes, we are rapidly approaching the point where most of the objects detected will be normal (non-AGN) galaxies. It is important to understand these galaxies not only nearby, but at higher redshift, in order to completely understand the evolution of galaxies and their X-ray populations.
I am also interested in statistical challenges in X-ray astronomy. The high quality of modern X-ray telescopes pushes the limit of our understanding of small number statistics. Cooperative work between statisticians and astronomers is beginning to introduce new techniques in source detection, spectral fitting, and other more robust methods of data analysis.
For students who may be interested in getting involved in research, here are some projects I’m currently working on:
- Ultraluminous X-ray Sources: or ULXs, are X-ray sources whose luminosities exceed the Eddington limit for mass accretion onto a typical stellar-mass black hole. That is to say, they get too bright to be explained by comparison with typical sources in the Milky Way. It is becoming clear that some of these sources are probably extremely massive stellar-mass black holes: the most massive black holes that can be created from a single parent star. However, some of these sources are too luminous even for that explanation. Those sources may be intermediate-mass black holes, with masses between 100 and 10,000 times that of the Sun. They may represent the “missing link” between stellar-mass and supermassive black holes.
- Tidal tails and low surface brightness features: Spiral galaxies appear to be fairly regular in shape, but look a little deeper and you’ll discover that they have complex extended structures that paint a picture of the galaxies’ interaction histories. Using the 24-inch Perkin telescope here at Wesleyan, it is possible to resolve these structures by doing deep (multi-hundred hour) observations of individual galaxies.
- Diffuse X-ray emission in galaxies: Though much of the X-ray emission from nearby spiral galaxies comes from the discrete point sources, there is also diffuse X-ray emission from hot gas. This gas is largely the result of supernovae. Studying the elemental abundances in this gas can teach us about the progenitor stars that produced the supernovae, and studying the distribution and intensity of the diffuse X-ray emission provides an independent measure of star-formation distribution in the galaxies.
- X-ray source stacking: In X-ray observations, just because a source isn’t detected, that doesn’t mean it isn’t really there. By stacking the signal at the positions of lots of sources detected in another wavelength (for example, starburst galaxies detected in the infrared), it is possible to detect some flux from these sources and even determine some properties of the sources.