arXiv subjects
Publications and source records attributed to M. Ward.
We performed imaging on 49 type 2 Seyfert galaxies in 6 near- and mid-infrared bands (1-10$μ$m). We are separating the contribution of the torus from the host galaxy by radial profile fitting techniques and we will compare the observed spectral energy distributions with theoretical models of torus emission to constrain geometrical and physical parameters.
We present results from an imaging and spectroscopic study of the dust properties of Seyfert galaxies in the 1-10um range. The data are compared to state of the art models of torus emission to constrain geometrical and physical properties of the obscuring medium.
We present Bepposax observations of the starburst/Seyfert composite galaxy Mrk609. This enigmatic object has an optical spectrum dominated by the features of starburst galaxies, yet its X-ray luminosity (6.3$\times10^{42}$\lunits) is typical of an AGN. The X-ray spectrum of Mrk609 can be parameterised by a single power-law model with a photon index $Γ\sim1.6\pm0.1$ and no evidence for significant absorption above the Galactic value. Long term variability in both the 0.1-2 keV and 2-10 keV energy bands is detected, again suggesting that the X-ray emission is dominated by an AGN. The observed broad Ha component is a factor of 40 below that predicted by the X-ray flux implying a deficit of ionizing UV photons.
We present a detailed analysis of high-resolution Chandra observations of the merger system NGC3256, the most IR luminous galaxy in the nearby universe. The X-ray data show that several discrete sources embedded in complex diffuse emission contribute ~>20 of the total emission (L_x^tot ~8E41 ergs/s in the 0.5-10 keV energy range). The compact sources are hard and extremely bright and their emission is probably dominated by accretion driven processes. Both galaxy nuclei are detected with L_x ~3-10E40 ergs/s. No evidence is found for the presence of an active nucleus in the southern nucleus, contrary to previous speculation. Once the discrete sources are removed, the diffuse component has a soft spectrum which can be modelled by the superposition of 3 thermal plasma components with temperatures kT = 0.6, 0.9 and 3.9 keV. Alternatively, the latter component can be described as a power-law with index Gamma ~3. Some evidence is found for a radial gradient of the amount of absorption and temperature of the diffuse component. We compare the X-ray emission with optical, Halpha and NICMOS images of NGC3256 and find a good correlation between the inferred optical/near-IR and X-ray extinctions. Although Inverse Compton scattering could be important in explaining the hard X-rays seen in the compact sources associated with the nuclei, the observed diffuse emission is probably of thermal origin. The observed X-ray characteristics support a scenario in which the powerful X-ray emission is driven solely by the current episode of star formation.
We present Chandra observations of two star-forming galaxies (M82 and the Antennae) and three starburst/AGN composite galaxies (NGC1808, NGC6240, NGC7331). In both star-forming galaxies we detect a large number of sources with diverse properties. Some of them can be identified as X-ray binaries, based on their variability and spectra. However, there is a significant number of very soft and/or extended sources which could be supernova remnants. These observations confirm previous indications that there is a population of sources with X-ray luminosities much higher than the Eddington limit for a neutron star, suggesting that these objects are abundant in star-forming galaxies. We find the the X-ray luminosity functions of the discrete sources in these two galaxies are very similar. In the case of the composite galaxies we find that the AGN does not dominate their X-ray emission. A significant fraction of the emission from these objects is extended but there are also X-ray sources associated with circumnuclear star-formation.
We present low resolution mid infrared spectra of 16 ultraluminous infrared galaxies (ULIRGs) obtained with the CVF spectroscopy mode of ISOCAM on board the Infrared Space Observatory ISO. Our sample completes previous ISO spectroscopy of ultra- and hyperluminous infrared galaxies towards higher luminosities. The combined samples cover an infrared luminosity range of \~10^{12 -13.1} Lo. For about half of the high luminosity ULIRGs studied here, strong aromatic emission bands suggest starburst dominance. Other spectra are dominated by a strong AGN-related continuum with weak superposed emission features of uncertain nature. An improved method to quantitatively characterize the relative contribution of star formation and AGN activity to the mid-infrared emission of ULIRGs is presented. As dominant source of the bolometric luminosity, starbursts prevail at the lower end and AGNs at the higher end of this range. The transition between mostly starburst and mostly AGN powered occurs at ~10^{12.4}$ to $10^{12.5} Lo, and individual luminous starbursts are found up to ~10^{12.65} Lo.
We present imaging and spectroscopic observations of 2 nearby (z<0.1) ULIRGs from a larger sample, and address the question of whether the JHK continuum colours and slope might be effective probes of the nuclear region in searches for AGN. Certainly there is evidence for significant quantities of hot dust emission at temperatures >1000K; but it may be that rather than pointing to an AGN, this instead tells us more about the environment and evolution of the star formation.
The high XMM-Newton throughput allows a detailed investigation of the spatial, spectral and variability properties of the extended and point source emission of the starburst galaxy NGC 253 simultaneously. We characterize the brightest sources by their hardness ratios, detect a bright X-ray transient, and show the spectrum and light curve of the brightest point source, most likely a black-hole X-ray binary. The unresolved emission of two disk regions can be modeled by two thin thermal plasma components of 0.13 and 0.4 keV plus residual harder emission, with the lower temperature component originating from above the disk, the nuclear spectrum by a three temperature plasma (~0.6, 0.9, and 6 keV) with the higher temperatures increasingly absorbed. The high temperature component most likely originates from the starburst nucleus. No non-thermal component, that would point at a significant contribution from an active nucleus (AGN), is needed. Assuming that type IIa supernova remnants (SNRs) are mostly responsible for the E>4 keV emission, the detection with EPIC of the 6.7 keV line allows us to estimate a supernova rate within the nuclear starburst of 0.2 yr^-1. RGS spectra and EPIC images reveal that the limb-brightening of the plume is mostly seen in higher ionization emission lines, while in the lower ionization lines, and below 0.5 keV, the plume is more homogeneously structured. (abridged)
We present results from observations of the most famous starburst galaxy M82 with the High-Resolution Camera onboard the Chandra X-Ray Observatory. We found nine sources in the central 1' x 1' region, but no source was detected at the galactic center. Comparing the observations on 1999 October 28 and on 2000 January 20, we found four of the nine sources showed significant time variability. In particular, CXOM82J095550.2+694047, which is 9" away from the galactic center, showed extremely large time variability. We conclude that this source is the origin of the hard X-ray time variability of M82 detected with ASCA. Assuming a spectral shape obtained by the ASCA observation, its luminosity in the 0.5 -- 10 keV band changed from 1.2E40 erg/s on 1999 October 28 to 8.7E40 erg/s on 2000 January 20.
X-ray and multi-wavelength observations of the most luminous known local (z<0.3) AGN, the recently discovered radio-quiet quasar PDS 456, are presented. The spectral energy distribution shows that PDS 456 has a bolometric luminosity of 1e47 erg/s, peaking in the UV. The X-ray spectrum obtained by ASCA and RXTE shows considerable complexity. The most striking feature observed is a deep, highly-ionised, iron K edge (8.7 keV, rest-frame), originating via reprocessing from highly ionised material, possibly the inner accretion disk. PDS 456 was found to be remarkably variable for its luminosity; in one flare the X-ray flux doubled in just about 15 ksec. If confirmed this would be an unprecedented event in a high-luminosity source, with a light-crossing time corresponding to about 2RS. The implications are that either flaring occurs within the very central regions, or else that PDS 456 is a super-Eddington or relativistically beamed system.
We present near infrared data on the luminous interacting system Mkn 266 (NGC 5256), comprising 2um continuum, and Br_gamma and 1-0S(1) emission line images, together with K-band spectra. We have fitted stellar templates to the continuum, allowing us to account for all the stellar features and hence detect even faint gas excitation emission lines, including 8 and 11 H_2 lines in the SW and NE nuclei respectively. Population diagrams for the excited H_2 molecules indicate that most of the 1-0S(1) in each of the nuclei has a thermal origin. We discuss this with reference to the observed morphologies, especially that of the 1-0S(1) line. In particular, the core of 1-0S(1) in the NE nucleus is more compact than the 2um continuum; while in the SW nucleus the 1-0S(1) is significantly offset by 500pc from the continuum (and other) emission. Lastly we address the issue of the region midway between the two nuclei, where previously a strong source of radio continuum has been observed. These results are set in the context of interacting galaxies where shock excited emission might be expected to occur as a direct consequence of the interaction.
XEUS is under study by ESA as part of the Horizon 2000+ program to utilize the International Space Station (ISS) for astronomical applications. XEUS will be a long-term X-ray observatory with an initial mirror area of 6m2 at 1 keV that will be grown to 30m2 following a visit to the ISS. The 1 keV spatial resolution is expected to be 2-5'' HEW. XEUS will consist of separate detector and mirror spacecraft aligned by active control to provide a focal length of 50m. A new detector spacecraft, complete with the next generation of instruments, will also be added after visiting the ISS. The limiting sensitivity will then be ~4 10-18 erg/cm2/s - around 250 times better than XMM, allowing XEUS to study the properties of the hot baryons and dark matter at high redshift.
We present quasi-simultaneous ASCA and RXTE observations of the most luminous known AGN in the local (z<0.3) Universe, the recently discovered quasar PDS 456. Multiwavelength observations have been conducted which show that PDS 456 has a bolometric luminosity of 10^47 erg/s peaking in the UV part of the spectrum. In the X-ray band the 2-10 keV (rest-frame) luminosity is 10^45 erg/s. The broad-band X-ray spectrum obtained with ASCA and RXTE contains considerable complexity. The most striking feature observed is a very deep, ionised iron K edge, observed at 8.7 keV in the quasar rest-frame. We find that these features are consistent with reprocessing from highly ionised matter, probably the inner accretion disk. PDS 456 appeared to show a strong (factor of 2.1) outburst in just 17ksec, although non-intrinsic sources cannot be completely ruled out. If confirmed, this would be an unusual event for such a high-luminosity source, with the light-crossing-time corresponding to 2 Schwarzschild radii. The implication would be that flaring occurs within the very central regions, or else that PDS 456 is a `super-Eddington' or relativistically beamed system. Overall we conclude on the basis of the extreme blue/UV luminosity, the rapid X-ray variability and from the imprint of highly ionised material on the X-ray spectrum, that PDS 456 is a quasar with an unusually high accretion rate.