arXiv subjects

M. J. Ward

Publications and source records attributed to M. J. Ward.

87 records · Page 5Linked to original sources

arXiv2000

The non-stellar infrared continuum of Seyfert galaxies

JHKL'M (1-5 micron) imaging of a sample of Seyfert (Sy) 2 galaxies is presented. We confirm that the 1-2.2 micron nuclear continuum of some Sy 2s is dominated by stellar emission, whereas the continuum emission at longer wavelengths (3-5 micron) is almost entirely non-stellar in origin. The IR non-stellar spectral energy distributions (SED) (up to 15 micron) of Sy galaxies show a variety of shapes, and they are well reproduced with the tapered disk models of Efstathiou & Rowan-Robinson (1995). We have used two models, one including an optically thin cone component and a coneless model. Although our modelling of the SEDs does not allow us to favor either model to account for all the observed SEDs, we find that the viewing angle towards the central source is well constrained by both models. We have also investigated non-stellar color-color diagrams. The colors of the Sy galaxies with viewing angles theta < 30 degree are better reproduced with the cone model. These diagrams provide a good means to separate Sy 2s with moderate obscuration (A_V < 20 mag, from hard X-ray observations) from those with high obscuration. The ground-based 4.8 micron and ISO 9.6 micron luminosities are well correlated with the hard X-ray luminosities of Sy 1s and 2s. These continuum emissions appear as a good indicator of the AGN luminosity, at least in the cases of hard X-ray Compton-thin Sy galaxies. We finally stress the finding that some Compton thick galaxies show bright non-stellar emission at 5 micron. This suggests that the near-IR emission in Sys is produced in an extended component illuminated by the central source, that is more visible from all viewing angles. We discuss possible implications of mid-IR surveys for the search of counterparts of highly obscured hard X-ray sources. (Abridged)

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arXiv2000

Chandra High-Resolution Camera Observations of the Luminous X-Ray Source in the Starburst Galaxy M82

We have analyzed Chandra High Resolution Camera observations of the starburst galaxy M82, concentrating on the most luminous x-ray source. We find a position for the source of $\rm R.A. = 09^h 55^m 50^s.2, decl. = +69^{\circ} 40\arcmin 46\arcsec.7$ (J2000) with a $1σ$ radial error of $0.7\arcsec$. The accurate x-ray position shows that the luminous source is not at the dynamical centre of M82 nor coincident with any suggested radio AGN candidate. The source is highly variable between observations, which suggests that the source is a compact object and not a supernova or remnant. There is no significant short term variability within the observations. Dynamical friction and the off-center position place an upper bound of $10^{5} - 10^{6} M_{\odot}$ on the mass of the object, depending on its age. The x-ray luminosity suggests a compact object mass of at least $500 M_{\odot}$. Thus, the luminous source in M82 may represent a new class of compact object with a mass intermediate between those of stellar mass black hole candidates and supermassive black holes.

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arXiv2000

Hubble Space Telescope Imaging of the Circinus Galaxy

We present an HST imaging study of the nearby (4 Mpc distant) Circinus galaxy, which contains the nearest type 2 Seyfert nucleus and prominent circumnuclear star formation. Images have been obtained in the [OIII]$λ$5007, H$α$ and H$_2$ v=1--0 S(1) emission lines, and in the green (5470 Å), red (8140 Å) and near infrared (2.04 and 2.15 $μ$m) continua. An image in the [Fe II] $λ$1.644 $μ$m line has been taken with a ground-based telescope. The [OIII] and H$α$ images reveal the detailed structure of the complex of streamers and knots of high-excitation gas, which extends out of the galaxy disk. The morphology some 250 pc from the nucleus strongly suggests that the high-excitation gas is concentrated on the surface of a hollow cone with apex close to the nucleus. Such a structure may result through entrainment of dense gas from a circumnuclear torus in the galaxy disk by a low density, outflowing wind or jet. Within 40 pc of the nucleus, the high-excitation gas takes the form of a striking, filled V-shaped structure extending in the same direction as the larger scale high-excitation emission. This `V' can be described as an ionization cone, though a matter-bounded structure is also possible. The implied collimation of the ionizing photons or gaseous outflow must occur within 2 pc of the apex of the cone, presumed to be the location of the nucleus. The H$α$ image shows a complex structure of HII regions, including the well known starburst ring of radius 150 - 270 pc. In addition, there is a more compact (40 pc radius), elliptical ring of HII regions around the `ionization cone'. We argue that this latter ring, which we call the nuclear ring, is intrinsically circular and located in the plane of the galaxy disk. Abstract truncated.

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arXiv1999

Searching for X-ray Luminous Starburst Galaxies

The existence or otherwise of X-ray luminous star-forming galaxies has been an open question since the era of the Einstein satellite. Various authors have claimed the discovery of X-ray luminous star-forming galaxies but in many cases more careful spectroscopic studies of these objects have shown that many of them are in fact obscured AGN. In order to investigate the possibility that such a class of galaxies do exist, we have carried out a cross-correlation between optical and IRAS samples of galaxies which are known to contain large numbers of star-forming galaxies and catalogs of sources detected in X-ray surveys. The selection criteria for the optical follow-up observations was based on their X-ray and infrared (IRAS) colors and their X-ray luminosities. We note that this sample is by no means complete or uniformly selected and hence cannot be used for statistical studies, but nevertheless confirmation of the existence of such a class of objects would be a important step, and would require us to understand the physical process responsible for such powerful X-ray emission. We have initiated an optical spectroscopic survey in order to obtain accurate spectroscopic classifications for all the objects which are claimed to be starburst galaxies. Here we present preliminary results from this survey. We have discovered a number of starburst galaxies with X-ray luminosities above ~10^(41)erg s^(-1) (for Ho=50km s^(-1)Mpc^(-1)). We investigate possible origins for the X-ray emission in individual cases.

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arXiv1999

X-ray luminous star-forming galaxies

We present results from the cross-correlation of the spectroscopic atlas of Ho etal (1995) with the ROSAT All-Sky Survey Bright Source Catalogue, in an attempt to understand the X-ray emission mechanisms in nearby galaxies. The resulting sample of 45 galaxies consists predominantly of AGN. However, there are several starforming galaxies spanning a wide range of X-ray luminosities (~10^{38} - 10^{42} erg s^{-1}). We have analyzed ROSAT and ASCA data for the two most luminous star-forming galaxies, namely NGC3310 and NGC3690. We find that their 0.1-10 keV X-ray spectra can be fitted by a soft thermal plasma of kT$\sim0.8$ keV and a harder component with kT~10-15 keV or a power-law with Gamma~1.6. These are very similar to the spectra of the archetypal star-forming galaxies NGC253 and M82.

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arXiv1999

ROSAT observations of the dwarf starforming galaxy Holmerg II (UGC 4305)

We present ROSAT PSPC and HRI observations of the dwarf irregular galaxy Holmberg II (UGC4305). This is one of the most luminous dwarf galaxies (Lx~ 10^{40} erg s^{-1} cm^{-2}) detected in the ROSAT All-Sky Survey. The X-ray emission comes from a single unresolved point source, coincident with a large HII region which emits intense radio emission. The source is variable on both year and day timescales, clearly favouring accretion into a compact object rather than a supernova remnant or a superbubble interpretation for the origin of the X-ray emission. However, its X-ray spectrum is well-fit by a a Raymond-Smith spectrum with kT~0.8 keV, lower than the temperature of X-ray binaries in nearby spiral galaxies.

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arXiv1999

The effect of an attenuated continuum on the coronal line spectrum of NGC1068 and the Circinus galaxy

We present photoionization models of the optical and IR coronal line spectrum in NGC1068 and the Circinus galaxy. The line fluxes have been calculated using (a) a non-thermal (nuclear) continuum source and (b) the non-thermal continuum plus a UV bump due to a stellar cluster. We take into account the effect of attenuation of these continua by gas with column density $10^{22} cm^{-2}$ located between the nucleus and the coronal line region. The calculated coronal line ratios are in a good agreement with those observed in NGC1068 for a model in which about 40% of the line emission comes from gas illuminated by unattenuated, non-thermal continuum, and about 60% from gas illuminated by attenuated, non-thermal continuum. The electron density of the coronal line emitting gas in NGC1068 is found to be $10^4 cm^{-3}$. In the Circinus galaxy the coronal line emission comes from gas with electron density $10^3 cm^{-3}$ illuminated entirely by attenuated, non-thermal continuum. The derived ionization parameters for both coronal line regions are very similar, but the different densities imply a higher ionizing photon flux in NGC1068, consistent with the higher observed excitation state of the line emitting gas in that galaxy. A possible geometry of the coronal line region of both galaxies is discussed, in which the distribution of the attenuating gas may be strongly affected by the relative strength of the nuclear radio-jet. The stronger radio-jet in NGC1068 may have cleared a channel through the NLR allowing some unattenuated nuclear continuum to illuminate part of the coronal line region.

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arXiv1998

ISOCAM Photometry of Narrow-Line X-ray Galaxies

Mid-infrared photometry of the hosts of Narrow-Line X-ray Galaxies at 6 microns and 12 microns has been attempted with ISOCAM. No conclusive detections have been made. This implies that these are quiescent objects with little or no active star-formation. Neither X-ray binaries nor starburst-driven superwinds are consistent explanations for the X-ray emission in these objects. We conclude that these NLXGs are predominantly AGN-powered.

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arXiv1998

ROSAT and ASCA Observations of X-ray Luminous starburst Galaxies : NGC3310 and NGC3690

We present ROSAT (HRI and PSPC) and ASCA observations of the two luminous (L_x ~10^{41-42} erg s^{-1}) star-forming galaxies NGC3310 and NGC3690. The HRI shows clearly that the sources are extended with the X-ray emission in NGC3690 coming from at least three regions. The combined 0.1-10 keV spectrum of NGC3310 can be described by two components, a Raymond-Smith plasma with temperature kT=0.81^{+0.09}_{-0.12} keV and a hard power-law, Gamma=1.44^{+0.20}_{-0.11}, (or alternatively a harder Raymond-Smith plasma with kT ~15 keV), while there is no substantial excess absorption above the Galactic. The soft component emission is probably due to a super-wind while the nature of the hard emission is more uncertain with likely origins, X-ray binaries, inverse Compton scattering of IR photons, an AGN or a very hot gas component (~10^8 K). The spectrum of NGC3690 is similar, with kT=0.83^{+0.02}_{-0.04} keV and Gamma=1.56^{+0.11}_{-0.11}. We also employ more complicated models such as a multi-temperature thermal plasma, a non-equilibrium ionization code or the addition of a third softer component which improve the fit but not at a statistically significant level (<2sigma). These results are similar to recent results on the archetypal star-forming galaxies M82 and NGC253.

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arXiv1998

Near and Mid Infrared Imaging Polarimetry of NGC1068

We present the results of a series of observations of the near- and mid-infrared polarisation properties of the Seyfert 2 galaxy NGC1068. Our data agree well with previously published results in showing the need for a separate polarisation mechanism in the near infrared apart from scattering. We find that the predictions of a simple model in which this component arises through absorptive dichroism due to aligned grains within the extended warm (~400K) dust fits the data reasonably if the obscured background source is itself due to dust emission (at temperature >1000K). By considering the change of polarisation with wavelength we show that the extinction to this hot dust region is in the range AV=20-40. Consideration of the observed data then leads us to the conclusion that if viewed face-on, NGC1068 would have a strong near-infrared excess similar to Seyfert 1 galaxies. Comparison with other independent measures of the extinction to the active nucleus itself lead us to the conclusion that the hot dust must provide screening equivalent to at least AV=40, and possibly much higher. We speculate that this component alone may be the `classical' torus discussed in terms of the unified model, and the more extensive mid-infrared emission may arise from circumnuclear molecular cloud material, and dust in the ionisation cones.

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arXiv1998

Stellar population in Active Galactic Nuclei. I. The Observations

Recent observations supported by theoretical models have lead to the view that giant and supergiant stars are over abundant, and/or a high metallicity component may be present, in the stellar populations at the centres of active galaxies. Here we attempt to quantify these effects by observing the strengths of the stellar absorption lines of Mg~b, NaI, CaII triplet as well as molecular bands such as CN and TiO. Using long-slit spectroscopic data we are able to separate the stellar populations in and around the nucleus, for a sample including, normal, LINER, starburst and Seyfert galaxies. In this paper we present the data, namely spectra of the nucleus and of a number of circum-nuclear regions. Comparisons reveal gradients in both the reddening and the stellar population within the central regions of most galaxies. Detailed stellar population synthesis is presented in a companion paper.

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arXiv1997

Modelling the ENLR in NGC1068

We present photoionization models of the high excitation gas in the Extended Narrow Line Region (ENLR) of NGC1068. The ENLR line fluxes have been calculated allowing for attenuation of the central-source ionizing continuum as a function of distance from the centre. Diffuse continuum emission from low density ENLR gas is included as an important secondary source of ionization. The observed high excitation emission further than 25 arcsec from the centre of NGC1068 can be fitted by photoionization models using a central-source luminosity of 3.6x10^44 erg/s between 10^14.6 and 10^18.4 Hz, with the continuum shape attenuated by nuclear gas with an integrated column density of N_h=10^22 cm^-2. The reflected soft X-ray continuum from the attenuating gas could be responsible for about 10% of the observed, resolved circumnuclear soft X-ray continuum extending out to 15 arcsec from the centre (Wilson et al 1992).

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arXiv1997

ROSAT HRI observations of the Local Group galaxies IC 10, NGC 147 and NGC 185

We report on pointed X-ray observations of IC 10, NGC 147 and NGC 185 made with the ROSAT High Resolution Imager (HRI). These are three Local Group galaxies that have never been previously studied in detail in the X-ray regime. IC 10 is the closest starburst galaxy to our own Galaxy, and NGC 147 and NGC 185 are companions to M31. We have discovered a variable X-ray source coincident with IC 10. The source is located near the centre of a large, non-thermal bubble of radio emission, and it is positionally coincident with an emission line star in IC 10 which has been classified as a WN type Wolf-Rayet star. We demonstrate that a confusing foreground or background source is improbable. The X-ray source is probably an X-ray binary in IC 10, and it may be a Wolf-Rayet + black hole binary. The source has mean and maximum 0.1--2.5 keV isotropic luminosities of about 2 and 4 times 10^{38} erg/s. We do not detect any sources in the central regions of NGC 147 or NGC 185. We place upper limits on their central X-ray emission, and we list all X-ray sources coincident with their outer extents. We also present the first X-ray detections of the well-studied Algol-type binary TV Cas and the W UMa-type binary BH Cas, which were both serendipitously observed during our IC 10 pointing.

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arXiv1997

A multi-wavelength study of the Seyfert 1 galaxy MCG-6-30-15

We present a multiwaveband spectroscopic study of the nearby Seyfert 1 galaxy MCG-6-30-15. New optical spectra from the Anglo-Australian Telescope are presented which clearly show the effects of dust extinction/reddening on both the emission line spectrum and the non-stellar AGN continuum. The reddening is constrained to be in the range E(B-V)=0.61-1.09. Spectroscopy in the X-ray band, with both ROSAT and ASCA, reveal absorption by the warm absorber but little or no neutral absorption expected to accompany the dust responsible for the optical reddening. The dusty warm absorber solution to this discrepancy is discussed and photoionization models of such warm absorbers are constructed. The optical spectrum also displays the relatively strong `coronal' lines of [FeX]6375, [FeXI]7892 and [FeXIV]5303. We show that these lines may plausibly originate from the outer regions of the warm absorber, although better calculations of the collision strengths for these transitions are required in order to conclusively address this issue. We also present new ultraviolet data from the International Ultraviolet Explorer and suggest that much of the observed UV flux is scattered into our line of sight (with a scattering fraction of 1-5 per cent). We conclude with a discussion of the global energetics of this system.

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arXiv1995

The extended narrow line region in Mkn79. I.Observations

We present deep long slit spectra of Mkn79 in position angles PA=12$^{o}$ and PA=50$^{o}$ obtained with the WHT. These data prove the existence of an extended narrow line region in PA=12$^{o}$, which coincides with the triplet radio structure (Ulvestad \& Wilson 1984) and the observed outflow of material from the nucleus at PA=10$^{o}$ (Whittle et al. 1988). The ratios of the high to low ionization lines indicate a higher level of gas excitation in PA=12$^{o}$ compared to PA=50$^{o}$. The [NII]$λ$6583/H$α$ and [SII]$λ$6717,31/H$α$ versus [OIII]$λ$5007/H$β$ line ratios are consistent with excitation by an AGN continuum rather than a HII region.

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