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Black Holes & Matter in Extreme Environments
Kirpal Nandra (Professor), Elise Laird (STFC Postdoctoral Research Associate), James Aird (Postdoctoral Research Associate), Murray Brightman (PhD student), Shyam Bhayani (PhD student), and Jonathan Digby-North (PhD student).
We study the astrophysics of extreme environments, close to the black holes in active galactic nuclei (AGN). We are also exploring the interplay and relative importance of black hole accretion and star formation in galaxies, both in individual objects and in the cosmological context.
We work closely with several other members of the Imperial Astrophysics group, notably Dave Clements, Markos Trichas and Harsit Patel. Past members of Imperial Astrophysics who worked with the X-ray group include: Antonis Georgakakis, Paul O'Neill, Lisa Voigt, Anna Lia Longinotti, Daniel Rosa Gonzalez, Stuart Sim, and Ivan Valtchanov.
Various useful links are available, and a publications list.
In the news:
Data from the X-ray group have been included in Google Sky. Read the press release "X-ray images key to understanding evolution of galaxies".Current Projects
- AEGIS
- Data products for Chandra surveys
- Chandra ACIS-I PSF table
- International X-ray Observatory (IXO)
- Extreme ironing
- High-z star forming galaxies
- ELAIS-N1
- Tartarus
- SHEEP
- AGN X-ray timing
- Infrared galaxies
AEGIS
The All-Wavelength Extended Groth Strip International Survey (AEGIS) is a collaborative effort to obtain both deep imaging covering all major wavebands from X-ray to radio and optical spectroscopy over a large area of sky (0.5 - 1 deg2), with the aim of studying the panchromatic properties of galaxies over the last half of the Hubble time. As part of this survey we have obtained a total of 3.4 Ms of Chandra ACIS-I time to survey the region in the X-ray band. More information on the programme is available on our AEGIS web pages.
Probing accreting supermassive black holes with X-ray iron K lines
One of the most significant results from the ASCA satellite was the discovery of broad Fe K lines in some Seyfert galaxies, interpreted as arising from reflection off a relativistic accretion disc. ASCA showed that these features were quite common in Seyfert 1 galaxies, but new data from XMM-Newton are shedding new light on this phenomenon. We have recently completed the first comprehensive survey of the broad line phenomenon in AGN (Nandra et al. 2007). We also have a strong interest in the interpretation of narrow features from iron in the spectra of AGN. These have a number of origins and can provide powerful diagnostics of obscuration, rotation, inflow, and outflow. The figure shows the broad band spectrum and iron line profile of the Seyfert 1 galaxy Mrk 335 (from O'Neill et al. 2007).
Chandra observations of high redshift star forming galaxies
We have undertaken a programme to study the X-ray emission of high redshift star forming galaxies (such as Lyman break galaxies (LBGs) at z~3 and Balmer break galaxies (BBGs) at z~1) using deep Chandra data in several fields. The X-ray data are used to directly probe high energy processes within the galaxies, offering a view that is largely unaffected by large columns of dust and gas. As a result the X-ray data provide a highly effective way of uncovering low luminosity and obscured active galactic nuclei (AGN) within the galaxy populations (Laird et al 2006, Nandra, Laird & Steidel 2005). The X-ray data also provide an unobscured view of the star formation in commonly dusty galaxies that can be used to determine star formation rates and estimate UV extinction corrections (Laird et al. 2005, Laird et al 2006, Nandra et al. 2002).
Chandra survey of the SWIRE ELAIS-N1 field
ELAIS-N1 is one of the northern fields originally selected to be surveyed with the Infrared Space Observatory (ISO) as part of the European Large Area ISO Survey (ELAIS) (Rowan-Robinson et al. 2004) in 6.7, 15, 90 and 175 microns. Subsequent surveys of ELAIS-N1 with VLA (20 cm), INT (u',g',r',i',z'), Spitzer (3.6, 4.5, 5.8, 8.0, 70 and 160 microns), Gemini, Kitt peak, UKIRT (J,H,K) and GALEX (NUV, FUV) have provided us with a plethora of supporting data especially in the infrared. Previous ELAIS-N1 Chandra data consisted of only a single moderate depth (75 ks) ACIS-I pointing. We have extended this area by a factor of 12, by overlapping 30x5 ks ACIS-I exposures giving even coverage over 1 deg2. We have detected more than 400 X-ray sources with 100% overlap with the SWIRE (Spitzer Wide-area Infrared Extragalactic) survey. This will enable us to determine the broad band spectral energy distributions of X-ray selected AGN, necessary for investigating obscuration and unification schemes, exploring the connection between AGN and star-formation activity and assessing the radio loudness distribution.
Tartarus - a database of ASCA observations of AGN
The ASCA satellite was launched into a 96 minute orbit on 1993 February 20 and ceased observing on 2000 July 14. The satellite comprised four X-ray telescopes with four corresponding focal-plane instruments. There were two solid-state imaging spectrometers (0.5--10 keV) and two gas imaging spectrometers (0.7--10 keV).
The Tartarus database contains products for all ASCA observations with targets designated as AGN. The products comprise event files, images, spectra, and light curves. In collaboration with Jane Turner (Goddard Space Flight Centre/University of Maryland Baltimore County), we have released Tartarus (Version 3.2) at Imperial College London. This updated and improved version of Tartarus comprises 611 observing sequences. Tartarus is available also at the GSFC mirror site , and the products are included in the HEASARC archive.
SHEEP: The Search for the high energy extragalactic population
SHEEP (Nandra et al. 2003) is a large area (50 sq. deg) hard X-ray (5-10 keV) survey designed to discover relatively bright examples of the kind of object which make up the majority of the X-ray background. We have discovered 69 serendipitous hard X-ray objects in ASCA GIS images, the vast majority of which are AGN. A followup program of Chandra and optical observations is underway, and initial results (Nandra et al. 2004) show remarkable and unusual properties compared to AGN selected by more conventional means. They are hard to reconcile with standard unification schemes and models of the X-ray background. The real strength of SHEEP is in determining the detailed astrophysics of the objects, which are probably closer to the "true" AGN population than those discovered by surveys more prone to bias due to obscuration.
X-ray variability in AGN
The goal of X-ray variability studies is to provide additional constaints on models for the X-ray emission and to break the degeneracies inherent in time-averaged spectral modelling. Using data from the Tartarus database, we found (having taken into account the dependence on black hole mass) no clear correlation between the X-ray variability amplitude and either the X-ray photon index or mass accretion rate ( O'Neill et al. 2005 ). We are now conducting a systematic variability analysis of XMM-Newton observations of Seyfert galaxies with the goal of characterising the energy dependence of the variability. Simultaneous X-ray/UV observations are also useful. If the X-ray emission in AGN is the result of inverse Compton scattering of UV seed photons, then the X-ray and UV emission should be correlated. Using Chandra and Hubble Space Telescope observations we found such a correlation in the low mass Seyfert 1 galaxy NGC 4395 (left; O'Neill et al. 2006 ).
X-ray properties of luminous infrared galaxies
[Image credit: J. McDowell]
The precise nature of luminous infrared galaxies, including the balance between star formation and accretion power, has long been debated. New X-ray observations with Chandra and XMM-Newton are now making a great contribution to this debate. We are pursuing this issue, with detailed Chandra observations of the closest ULIRG, Arp 220 (left; Clements et al. 2002 ; McDowell et al. 2003 ), comparison of X-ray and SCUBA sources in deep surveys (e.g. HDF-N , Groth Strip - see above), and an XMM-Newton program studying the most extreme hyperluminous galaxies.