UKIRT Imaging and Keck Interferometry Combine to Understand the Accreting Material Around Supermassive Black Holes in Galactic Nuclei

An international research team led by Makoto Kishimoto from the Max Planck Institute for Radio Astronomy in Bonn has combined infrared imaging from the United Kingdom Infrared Telescope (UKIRT) and some of the first ever infrared long-baseline interferometric measurements from the Keck Observatory to observe nearby Active Galactic Nuclei (AGN). The measurements show a ring-like emission from sublimating dust grains, and its radius yields insights into the shape and geometry of the accreting material around the black hole in these galactic centers.

In May 2009, Makoto Kishimoto and his team successfully observed four such AGN with the Keck Interferometer, and they followed the observations up with infrared imaging from the UK Infrared Telescope (UKIRT). Both observatories are on Mauna Kea in Hawaii. The target sources included NGC 4151, a relatively nearby galaxy only 50 million light-years away, and a distant quasar at redshift 0.108 (corresponding to a distance of more than a billion light-years).

“The Keck measurements give us a uniquely high-resolution look at the innermost regions of the active nucleus, and the UKIRT images are very powerful in disentangling the contributions of the host galaxy and the accretion disk in the interferometry data”, says Makoto Kishimoto, the paper’s leading author.

Astronomers have been trying to see directly how the supermassive black hole is eating up the surrounding gas and how the strong jet is being launched around the black hole. However, to spatially resolve such a distant object at infrared wavelengths, a telescope having a diameter of 100 m would be required. Instead of building such a huge telescope, a more practical way is to combine the beams from two or more telescopes that are far apart in order to detect an interference pattern of the two beams and infer what the black hole vicinity looks like.

“The technique we are using is very new and very demanding in terms of observing conditions and data analysis”, says Robert Antonucci from the University of California at Santa Barbara, co-author of the paper.

In the future, there will be many telescopes, or a telescope array extended over several kilometers. Such arrays have already been used at radio, but not yet at infrared or optical wavelengths. Optical/infrared interferometry is still in an early stage — currently using two or three telescopes. A prototype array is formed by the two Keck telescopes of 10m diameter each, the so-called Keck interferometer (KI).

UKIRT’s wide field camera (WFCAM) was designed primarily as a survey instrument, capable of observing an area of sky the size of the full moon in the space of a few minutes. However it is very well suited to observations of galaxies, and has sufficient resolution to allow astronomers to separate the bright nucleus of the galaxy from the disk of billions of stars in which it is embedded. Importantly, it also has filters spanning a wide range of wavelengths, from just below twice the wavelength of visible light to more than four times that wavelength. Kishimoto and team have used this to their advantage.

Professor Gary Davis, Director of UKIRT, says “This is a slightly unusual use of UKIRT’s WFCAM, in that it capitalizes on some of the lesser-known strengths of the instrument. The wide field of view allows us to observe entire galaxies up close, but it also gives the sharp images required to look at more distant objects. It is wonderful to see two observatories on Mauna Kea joining forces in this way: by combining UKIRT imaging with Keck interferometry we can get a full and detailed view of these esoteric objects.”

Until recently, only one AGN had been successfully observed with the KI. This galaxy, NGC 4151, is one of the brightest of these sources in the optical and infrared wavelengths. The new, more sensitive observations of four galaxies have led to a much clearer picture of what is being resolved — a ring-like emission of dust grains, co-existing in the accreting gas, which are hot enough to be sublimating.

Using different, independent measurements of the radius of this dust sublimation region (which come from the analysis of the variabilities of the optical and infrared light), the team thinks that they have also possibly started to probe how the accreting material is distributed radially from the black hole — i.e., how compact or how extended the material distribution is.

“While we have got the highest spatial resolution in the IR, this is still a relatively outer region of the central black hole system”, says Makoto Kishimoto. “We hope to achieve an even higher resolution using telescopes that are much further apart in order to get even closer to the center, and we also hope to observe many other supermassive black hole systems. When we do that, we will once again be calling on infrared imaging to help us disentangle the results.”


Captions and Credits

1. UKIRT infrared images of the four target galaxies. They show “near-infrared color” where the images at different IR wavelengths are assigned to represent red/green/blue colors. WFCAM on UKIRT allows very quick imaging of the entire galaxy, while the Keck interferometer resolves a part of the very inner structure of the bright nucleus. The inferred ring-like structure obtained for NGC4151 at the top-left (scale for 10,000 light-years is shown as an arrow) is depicted in the top-right panel (the ring radius is 0.13 light-years, corresponding to an extremely small ~0.5 milli-arcsecond angular size on the sky). The distance to each galaxy is indicated in million light-years, together with the redshift (z) of each galaxy. Image: M. Kishimoto, galaxy images with United Kingdom Infrared Telescope (UKIRT).

2. The Wide Field Camera (long black tube) on the United Kingdom Infrared Telescope on Mauna Kea, Hawaii. Credit: UKIRT/JAC.

3. The United Kingdom Infrared Telescope on Mauna Kea, Hawaii. Credit: UKIRT/JAC.

4. The Keck interferometer on Mauna Kea, Hawaii. The interferometer consists of two 10m telescopes in separate domes, about 85 m apart. Credit: Keck Observatory.