5-apr-03 UKIRT reveals dark secrets at the hearts of galaxies ---------------------------------------------------- Marc Seigar (Joint Astronomy Centre/United Kingdom Infrared Telescope) Our Milky Way galaxy, with its hundreds of billions of stars, is just one of hundreds of billions of galaxies in the Universe. And yet every time we peer into the center of one of these galaxies, we find a supermassive black hole (with a mass of millions or even billions that of the sun) lurking at its core. Are these black holes a result of the birth of galaxies, or are they a seed that is needed for a galaxy to form in the first place? These are questions which currently remain unanswered. At the UK Infrared Telescope (UKIRT), here on Mauna Kea, I, along with a team of researchers and students, have been working on two projects designed to investigate the very centers of galaxies and their supermassive black holes. We have been taking advantage of the excellent image quality available at UKIRT. UKIRT (funded by the UK) has a 3.8 metre (152 inch) primary mirror and has undergone a rigorous program of upgrades in the 1990s to help achieve extremely good image and data quality. The first of these projects is a black hole search. This makes use of the latest instrument at UKIRT, the UKIRT Imaging Spectrometer (UIST). This project is designed to look into the centers of spiral galaxies and investigate the dynamics of the gas in between the stars. If something extremely massive is present in the center, then this gas will be affected by its gravitational pull, and from the motion of the gas, one can calculate the mass of the object that is present. The motion of the gas is estimated using an 'image slicer' or Integral Field Unit (IFU). This 'slices' the light from the galaxies into thin sections. Each slice is then spread out to make a spectrum (similar to a rainbow produced by light passing through a prism). These spectra can then be used to investigate the motion of gas in complex systems, such as galaxies. The second project is designed to look into the centers of the most massive galaxies in the Universe, giant elliptical galaxies. It has become apparent recently that there are two types of giant elliptical galaxy, and that there may be different processes responsible for their existence. One kind sits at the centers of clusters of hundreds of galaxies, and it seems that they form when two galaxies merge. This results in a double (or binary) black hole in the center of the newly formed galaxy. When this pair of black holes forms, stars are ejected from the galaxy's center, due to the huge tidal forces involved in the merging process. We have used detailed images obtained at UKIRT to investigate the nuclei of giant elliptical galaxies and have found a decrease in the density of stars in the cores of these galaxies, suggesting that indeed stars have been ejected from the centers of this type of galaxy and a binary black hole has been formed. The second type of giant elliptical galaxy are also found in clusters, but not at their centers. These galaxies are thought to evolve by consuming smaller galaxies in the cluster, a process often refered to as cannibalism. If these galaxies have been consuming smaller ones, binary black holes would not form, as one black hole is ejected from the resulting galaxy. As a result, stars would not be thrown out. Our UKIRT observations have shown that the density of stars continues to increase as one approaches the centers of these galaxies. This is different from the sparseness of stars seen in the centers of the first type of galaxies, and it suggests that this second type have NOT been ejecting stars. We can therefore tell that there is no binary black hole in the center, and that these galaxies do indeed form by cannibalising other galaxies. These projects are helping us to find out more about the mysterious supermassive black holes at the centers of galaxies. We also know more about how these galaxies form. Some arise from a merger of equals, producing pairs of black holes and violently ejecting stars, whilst others are the result of intergalactic cannibalism. On April 5th, I will be presenting this work as part of the Universe Tonight program at the Onizuka Center for International Astronomy Visitor Information Station on Mauna Kea. All are welcome to attend. Dr. Marc Seigar is a support astronomer at the UK Infrared Telescope/Joint Astronomy Centre, with expertise in near-infrared imaging and telescope optics. His main area of research is nearby galaxies and galaxy evolution. -- ------------------------------------------------------------ Marc Seigar: UKIRT Support Astronomer, Telescope Scientist Joint Astronomy Centre, & UFTI instrument scientist University Park, Email: m.seigar@jach.hawaii.edu 660 N. A'ohoku Place, Tel: 1-808-969-6565 Hilo, HI 96720, USA Cell: 1-808-990-0454 http://www.jach.hawaii.edu/~mseigar/ Fax: 1-808-961-6516 ------------------------------------------------------------