arXiv subjects

A. S. Wilson

Publications and source records attributed to A. S. Wilson.

54 records · Page 3Linked to original sources

arXiv2000

Chandra X-ray Observations of Pictor A: High Energy Cosmic Rays in a Radio Galaxy?

We report X-ray observations of the nearby, powerful radio galaxy Pictor A with the Chandra Observatory and optical and near uv observations of its western radio hot spot with the Hubble Space Telescope. X-ray emission is detected from the nucleus, a 1.9 arcmin (110 kpc) long jet to the west of the nucleus, the western radio hot spot some 4.2 arcmin (240 kpc) from the nucleus, and the eastern radio lobe. The morphology of the western hot spot is remarkably similar to that seen at radio and optical wavelengths, where the emission is known to be synchrotron radiation. The X-ray spectrum of the hot spot is well described by an absorbed power law with photon index Γ= 2.07 (+/- 0.11). The X-ray jet coincides with a weak radio jet and is laterally extended by \simeq 2.0 arcsec (1.9 kpc). The observed jet is up to \simeq 15 times brighter in X-rays than any counter jet, a difference ascribed to relativistic boosting as the western radio lobe is probably the closer. The jet's spectrum is well modelled by an absorbed power law with Γ= 1.94 (+0.43/-0.49) and poorly fitted by a Raymond-Smith thermal plasma model. (Abstract truncated).

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arXiv2000

Slow Jets in Seyfert Galaxies: NGC 1068

We have used the Very Long Baseline Array at 5 GHz to image the nucleus of NGC 1068 at two epochs separated by 2.92 yr. No relative motion was detected between the high brightness-temperature knots within components NE and C relative to the nuclear component S1, placing an upper limit of 0.075 c on the relative component speeds at distances of 21 pc and 43 pc from the AGN. The low speed is consistent with the low bulk flow speed previously inferred from indirect arguments based on ram pressure at the bow shock and on line emission from the jet-cloud collision at cloud C. The components are probably shocks in the jet, and the bulk flow speed could conceivably be higher than the limit reported here.

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arXiv2000

Chandra Observations of Cygnus A: Magnetic Field Strengths in the Hot Spots of a Radio Galaxy

We report X-ray observations of the powerful radio galaxy Cygnus A with the Chandra X-ray Observatory. This letter focuses on the radio hot spots, all four of which are detected in X-rays with a very similar morphology to their radio structure. X-ray spectra have been obtained for the two brighter hot spots (A and D). Both are well described by a power law with photon index Γ= 1.8 \pm 0.2 absorbed by the Galactic column in the direction of Cygnus A. Thermal X-ray models require too high gas densities and may be ruled out. The images and spectra strongly support synchrotron self-Compton models of the X-ray emission, as proposed by Harris, Carilli & Perley on the basis of ROSAT imaging observations. Such models indicate that the magnetic field in each of the brighter hot spots is 1.5 \times 10^-4 gauss, with an uncertainty of a few tens of percent. This value is close to the equipartition field strengths assuming no protons are present. The possibility that the X-rays are synchrotron radiation is briefly discussed, but not favored. We speculate that production of the γ\sim 10^7 electrons necessary for X-ray synchrotron radiation from hot spots is inhibited when the external gas density is high, as is the case when the radio galaxy is within a cooling flow.

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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

Free-Free Absorption on Parsec Scales in Seyfert Galaxies

Seyfert galaxies come in two main types (types 1 and 2) and the difference is probably due to obscuration of the nucleus by a torus of dense molecular material. The inner edge of the torus is expected to be ionized by optical and ultraviolet emission from the active nucleus, and will radiate direct thermal emission (e.g. NGC 1068) and will cause free-free absorption of nuclear radio components viewed through the torus (e.g. Mrk 231, Mrk 348, NGC 2639). However, the nuclear radio sources in Seyfert galaxies are weak compared to radio galaxies and quasars, demanding high sensitivity to study these effects. We have been making sensitive phase referenced VLBI observations at wavelengths between 21 and 2 cm where the free-free turnover is expected, looking for parsec-scale absorption and emission. We find that free-free absorption is common (e.g. in Mrk 348, Mrk 231, NGC 2639, NGC 1068) although compact jets are still visible, and the inferred density of the absorber agrees with the absorption columns inferred from X-ray spectra (Mrk 231, Mrk 348, NGC 2639). We find one-sided parsec-scale jets in Mrk 348 and Mrk 231, and we measure low jet speeds (typically <= 0.1 c). The one-sidedness probably is not due to Doppler boosting, but rather is probably free-free absorption. Plasma density required to produce the absorption is N_e >= 2 x 10^5 cm^-3 assuming a path length of 0.1 pc, typical of that expected at the inner edge of the obscuring torus.

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arXiv1999

Parsec-Scale Images of Flat-Spectrum Radio Sources in Seyfert Galaxies

We present high angular resolution (~2 mas) radio continuum observations of five Seyfert galaxies with flat-spectrum radio nuclei, using the VLBA at 8.4 GHz. The goal of the project is to test whether these flat-spectrum cores represent thermal emission from the accretion disk, as inferred previously by Gallimore et al. for NGC 1068, or non-thermal, synchrotron self-absorbed emission, which is believed to be responsible for more powerful, flat-spectrum nuclear sources in radio galaxies and quasars. In four sources (T0109-383, NGC 2110, NGC 5252, Mrk 926), the nuclear source is detected but unresolved by the VLBA, indicating brightness temperatures in excess of 10^8 K and sizes, on average, less than 1 pc. We argue that the radio emission is non-thermal and synchrotron self-absorbed in these galaxies, but Doppler boosting by relativistic outflows is not required. Synchrotron self-absorption brightness temperatures suggest intrinsic source sizes smaller than ~0.05-0.2 pc, for these four galaxies, the smallest of which corresponds to a light-crossing time of ~60 light days or 10^4 gravitational radii for a 10^8 M_sun black hole. We also present MERLIN and VLA observations of NGC 4388, which was undetected by the VLBA, and argue that the observed, flat-spectrum, nuclear radio emission in this galaxy represents optically thin, free-free radiation from dense thermal gas on scales ~0.4 to a few pc. It is notable that the two Seyfert galaxies with detected thermal nuclear radio emission (NGC 1068 and NGC 4388) both have large X-ray absorbing columns, suggesting that columns in excess of \~10^{24} cm^{-2} are needed for such disks to be detectable. (Abridged)

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arXiv1999

Sub-Relativistic Radio Jets and Parsec-Scale Absorption in Two Seyfert Galaxies

The Very Long Baseline Array has been used at 15 GHz to image the milliarcsecond structure of the Seyfert galaxies Mrk 231 and Mrk 348 at two epochs separated by about 1.7 yr. Both galaxies contain parsec-scale double radio sources whose components have brightness temperatures of 10^9-10^{11} K, implying that they are generated by synchrotron emission. The nuclear components are identified by their strong variability between epochs, indicating that the double sources represent apparently one-sided jets. Relative component speeds are measured to be ~0.1c at separations of 1.1 pc or less (for H_0 = 65 km/s/Mpc), implying that parsec-scale Seyfert jets are intrinsically different from those in most powerful radio galaxies and quasars. The lack of observed counterjets is most likely due to free-free absorption by torus gas, with an ionized density n_e > 2 X 10^5 cm^{-3} at T~8000 K, or n_e > 10^7 cm^{-3} at T~10^{6.6} K, in the inner parsec of each galaxy. The lower density is consistent with values found from X-ray absorption measurements, while the higher temperature and density are consistent with direct radio imaging of the NGC 1068 torus by Gallimore et al.

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arXiv1999

X-ray Emission from the Prototypical LINER Galaxy NGC 1052

We examine the 0.1 to 10.0 keV X-ray spectrum of the bright nuclear LINER galaxy NGC 1052, one of two elliptical galaxies known to contain a luminous H2O maser. The observed 2.0-10.0 keV spectrum is unusually flat (photon index Gamma~0.2) and best described as intrinsically power-law shaped nuclear flux that is either (1) attenuated by a complex absorber with ~70% of the nuclear flux absorbed by a column density of NH ~ 3x10^23 cm^{-2} and ~30% absorbed by a column density of NH ~ 3-5x10^{22} cm^{-2}, or (2) reprocessed, with the nuclear source blocked and the X-rays Compton reflected in our direction by high column density (>=10^{24} cm^{-2}) gas. The moderate equivalent width of the Fe K-alpha line favors the dual absorption model as the most likely scenario. The 0.1-2.0 keV spectrum does not resemble the few times 10^6 to 10^7 K thermal emission typically found in other elliptical galaxies, but instead is best described as nuclear X-rays leaking through a patchy absorber or scattered in our direction by low-density, ionized gas with the thermal contribution limited to about 15% for solar abundances. The absorption-corrected 2-10 keV luminosity of the nuclear source is L_X ~ 8x10^{41} ergs s^{-1} or L_X ~ 2x10^{43} ergs s^{-1} for the dual-absorption and Compton-reflection models, respectively. The absorbing and H2O masing gases appear to be spatially separate in this galaxy.

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arXiv1999

Do Jet-Driven Shocks ionize the Narrow Line Regions of Seyfert Galaxies?

We consider a model in which the narrow line regions (NLRs) of Seyfert galaxies are photoionized ``in situ'' by fast (300 -- 1,000 km/s), radiative shock waves driven into the interstellar medium of the galaxy by radio jets from the active nucleus. Such shocks are powerful sources of soft X-rays. We compute the expected ratio of the count rates in the ROSAT PSPC and Einstein IPC detectors to the [OIII] λ5007 flux as a function of shock velocity, and compare these ratios with observations of type 2 Seyferts. If most of the observed soft X-ray emission from these galaxies originates in the NLR and the absorbing hydrogen column is similar to that inferred from the reddening of the NLR, a photoionizing shock model with shock velocity $\simeq$ 400 -- 500 km/s is compatible with the observed ratios. High angular resolution observations with AXAF are needed to isolate the X-ray emission of the NLR and measure its absorbing column, thus providing a more conclusive test. We also calculate the expected coronal iron line emission from the shocks. For most Seyfert 2s, the [Fe X] λ6374/H β$ ratio is a factor of 2 -- 14 lower than the predictions of 300 -- 500 km/s shock models, suggesting that less hot gas is present than required by these models.

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arXiv1998

The Radio Emission of the Seyfert Galaxy NGC 7319

We present VLA maps of the Seyfert 2 galaxy NGC 7319 at 3.6, 6, and 20 cm. Sub-arcsecond resolution is achieved at 3.6 and 6 cm. The radio emission exhibits a triple structure on a scale of ~4'' (1.7 kpc). All three components have steep spectra, consistent with synchrotron radiation. We have also analyzed an HST archival, broad-band red image, which contains structure related to the radio components. In particular, a V-shaped feature in the HST image some 3.7'' (1.6 kpc) southwest of the nucleus is associated with highly blueshifted emission lines seen in ground-based spectra. We interpret the V-shaped feature as emission from gas compressed by the bow shock driven by the outwardly moving radio plasmoid.

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arXiv1998

Extended Gas in Seyfert Galaxies: Near Infrared Observations of NGC 2110 and Circinus

We present results of near--IR long-slit spectroscopy in the J and K bands of the Seyfert 2 galaxies NGC 2110 and Circinus, investigating the gaseous distribution, excitation, reddening and kinematics. In NGC 2110, the emission line ratio [FeII]/Pa beta increases towards the nucleus (to ~ 7). The nuclear [Fe II]1.257 (microns) and Pa beta lines are broader (FWHM ~ 500 km/s) than the H2 (2.121) line (FWHM ~ 300 km/s). Both these results suggest that shocks, driven by the radio jet, are an important source of excitation of [Fe II]. The H2 excitation appears to be dominated by X-rays from the nucleus. In Circinus, both [FeII]/Pa beta and H2/Br gamma decrease from ~ 2 at 4 arcsec from the nucleus to nuclear values of ~ 0.6 and ~ 1, respectively, suggesting that the starburst dominates the nuclear excitation, while the AGN dominates the excitation further out (r > 2 arcsec). For both galaxies, the gaseous kinematics are consistent with circular rotation in the plane of the disk. Our rotation curves suggest that the nucleus (identified with the peak of the IR continuum) is displaced from the kinematic centre of the galaxies. This effect has been observed previously in NGC 2110 based on the kinematics of optical emission lines, but the displacement is smaller in the infrared, suggesting the effect is related to obscuration. The continuum J-K colours of the nuclear region indicate a red stellar population in NGC 2110 and a reddened young stellar population in Circinus. Right at the nucleus of both galaxies, the colours are redder, apparently a result of hot dust emission from the inner edge of a circumnuclear torus.

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arXiv1998

The Ionization Fraction in the Obscuring `Torus' of an Active Galactic Nucleus

In this paper, we report VLBA radio maps at three frequencies and an ASCA X-ray spectrum of the water megamaser galaxy NGC 2639, which is believed to contain an edge-on viewed accretion disk. The radio observations reveal a compact ($<$ 0.2 pc) nuclear source with a spectrum that turns over sharply near 5 GHz. The X-ray spectrum shows emission in excess of a power-law model at energies greater than 4 keV; we interpret this excess as compact, nuclear, hard X-ray emission with the lower energies photoelectrically absorbed by an equivalent hydrogen column of $\simeq$ 5 $\times$ 10$^{23}$ cm$^{-2}$. If we assume that the turnover in the radio spectrum is caused by free-free absorption and that both the free-free and photoelectric absorptions are produced by the same gaseous component, the ratio $\int n_{e}^{2} dl/\int n_{H} dl$ may be determined. If the masing molecular gas is responsible for both absorptions, the required ionization fraction is $\gtrsim 1.3 \times 10^{-5}$, which is comparable to the theoretical upper limit derived by Neufeld, Maloney & Conger (1994) for X-ray heated molecular gas. The two values may be reconciled if the molecular gas is very dense -- $n_{H_{2}} \gtrsim 10^{9}$ cm$^{-3}$. The measured ionization fraction is also consistent with the idea that both absorptions occur in a hot ($\sim$ 6,000K), weakly ionized (ionization fraction a few times 10$^{-2}$) atomic region that may co-exist with the warm molecular gas. If this is the case, the absorbing gas is $\sim$ 1 pc from the nucleus. If our line of sight passes through more than one phase, the atomic gas probably dominates the free-free absorption while the molecular gas may dominate the photoelectric absorption.

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arXiv1998

H_2O megamaser emission from FR I radio galaxies

A systematic search for 22 GHz H2O megamaser emission is reported for 50 nearby (z < 0.15) FR I galaxies. No detection was obtained, implying that ultraluminous H2O masers (L_H2O>10^3 L_sun) must be rare in early-type galaxies with FR I radio morphology. Despite higher radio core luminosities the detection rate for our sample is much lower than in similar surveys of late-type Seyfert galaxies. This puzzling difference between Seyferts and low-power radio galaxies could be explained in several ways: a) the maser emission is saturated and therefore independent of the radio core luminosity, b) the masers are unsaturated and the background continuum source is associated with the counter jet which is beamed away and relativistically dimmed in radio-galaxies, or c) the amount, kinematics, or the distribution of the molecular gas in the nuclei of Seyferts and radio galaxies is different. Further studies of maser properties may therefore hold a clue for morphological differences between active nuclei of Seyfert and early-type radio galaxies.

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arXiv1997

Relativistic Jets in Radio-Weak Quasars and LINER Galaxies

We discuss radio observations of a sample of radio-weak and radio-intermediate quasars which demonstrate that - just like their radio-loud counterparts - radio-weak quasars too have relativistic jets in their nuclei. Moreover, a VLA survey of nearby LINER galaxies reveals a relatively large number of flat-spectrum radio cores. It is suggested that those cores are also best explained by moderately relativistic jets (gamma~2) produced by a central engine.

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arXiv1997

Sgr A* and Company - Multiwavelength observations of Sgr A* and VLA search of ``Sgr A*'s'' in LINERs

We report first results from a multiwavelength campaign to measure the simultaneous spectrum of Sgr A* from cm to mm wavelengths. The observations confirm that the previously detected submm-excess is not due to variability; the presence of an ultracompact component with a size of a few Schwarzschild radii is inferred. In a VLA survey of LINER galaxies, we found Sgr A*-like nuclei in one quarter of the galaxies searched, suggesting a link between those low-power AGN and the Galactic Center.

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arXiv1997

A Survey for H$_2$O Megamasers in Active Galactic Nuclei -- II. A Comparison of Detected and Undetected Galaxies

A survey for H2O megamaser emission from 354 active galaxies has resulted in the detection of 10 new sources, making 16 known altogether. The galaxies surveyed include a distance-limited sample (covering Seyferts and LINERs with recession velocities < 7000 km s^{-1}) and a magnitude-limited sample (covering Seyferts and LINERs with m(B) <= 14.5). In order to determine whether the H2O-detected galaxies are "typical" AGN or have special properties which facilitate the production of powerful masers, we have accumulated a database of physical, morphological, and spectroscopic properties of the observed galaxies. The most significant finding is that H2O megamasers are detected only in Seyfert 2 and LINER galaxies, not Seyfert 1s. Implications for this finding are discussed. LINERs are detected at a similar rate to Seyfert 2s, constituting a strong argument that at least some nuclear LINERs are AGN rather than starbursts, since starbursts have not been detected as H2O megamasers. We preferentially detect H2O emission from the nearer galaxies and from those which are apparently brighter at mid- and far-infrared and centimeter radio wavelengths. There is also a possible trend for the H2O-detected galaxies to be more intrinsically luminous in nuclear 6 cm radio emission than the undetected ones, though these data are incomplete. We find evidence that Seyfert 2s with very high (N(H) > 10^{24} cm^{-2}) X-ray absorbing columns of gas are more often detected as H2O maser emitters than Seyfert 2s with lower columns. It may be that the probability of detecting H2O maser emission in Seyfert galaxies increases with increasing column of cool gas to the nucleus, from Seyfert 1s through NLXGs to Seyfert 2s.

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arXiv1997

Evidence For A Precessing Accretion Disk in the Nucleus of NGC 1097

We present new spectroscopic observations of the LINER (and now Seyfert 1) nucleus of NGC 1097, and discuss the evolution of its broad, double-peaked Balmer lines. When originally discovered in 1991, the red peak of the double-peaked H-alpha line was stronger than the blue, while by 1994 the H-alpha profile had become almost symmetric and the integrated line flux had decreased to half its original value. Our new spectrum, taken in 1996, shows that the broad, double-peaked lines have returned to almost their original strengths, the profiles of H-beta and H-alpha are identical to within errors, and the broad-line emitting region is unreddened. However, the profile of the Balmer lines is now such that the blue peak is stronger than the red, opposite to the asymmetry observed in 1991. Various models are considered for the observed behavior, all assuming that the emission lines originate in an accretion disk. We present a refined version of the precessing, planar, elliptical accretion ring model proposed by Storchi-Bergmann et al. and Eracleous et al. This model provides an acceptable fit to the line profiles. We also consider the possibility that the line profile evolution results from a precessing warp in the disk, induced by irradiation from the center, and show that the range of radii and precession time scales expected in this model are consistent with the observations. The sudden appearance of the "disk-like" broad line profiles in NGC 1097 could have resulted from the formation of a new accretion disk due to, for example, the tidal disruption of a star, or the illumination of a pre-existing disk by a transient ionizing source at the center of the disk.

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arXiv1997

The Temperature of Extended Gas in Active Galaxies -- Evidence for Matter-Bounded Clouds

We report measurements of the electron temperature at about a dozen locations in the extended emission-line regions of five active (Seyfert and radio) galaxies. Temperatures (T_{[OIII]} and T_{[NII]}) have been determined from both the I([OIII]\lambda4363)/I([OIII]\lambda5007) and I([NII]\lambda5755)/I[NII]\lambda6583) ratios. T_{[OIII]} lies in the range (1.0 -- 1.7) \times 10^{4}K. We find a strong trend for T_{[OIII]} to be higher than T_{[NII]}, with the difference typically being \approx 5,000K. Because the critical density for collisional de-excitation of the ^{1}D{_2} level in NII is lower than that of the same level in OIII, the deviations of the measured intensity ratios from those expected for T_{[OIII]} = T_{[NII]} in the low density limit are unlikely to result from collisional de-excitation. The measured values of T_{[OIII]} and the differences between T_{[OIII]} and T_{[NII]} are very similar to those found in Galactic planetary nebulae. It is argued that the dominant form of energy input to the clouds is photoionization, but detailed modelling indicates that the temperature difference is too large to be accounted for in terms of photoionization of ionization-bounded clouds. We propose instead that both matter- and ionization-bounded clouds are present in the extended emission-line regions, with most of the [OIII] emission originating from a hot zone in the matter-bounded clouds and essentially all of the [NII] from the ionization-bounded clouds.

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