arXiv subjects

Andrew S. Wilson

Publications and source records attributed to Andrew S. Wilson.

42 records · Page 3Linked to original sources

arXiv1998

HST and VLA Observations of Seyfert 2 Galaxies: The Relationship between Radio Ejecta and the Narrow Line Region

We present HST/WFPC2 images, in narrow-band filters containing the [OIII] lambda5007 and Halpha+[NII] emission-lines and their adjacent continua, of a sample of seven Seyfert 2 galaxies selected on the basis of possessing either extended emission-line regions in ground-based observations or a hidden broad-line region in polarized light. Six of the galaxies have also been observed with the VLA to obtain radio maps with better quality and angular resolution than those in the literature. We find detailed correspondences between features in the radio and emission-line images that clearly indicate strong interactions between the radio jets and the interstellar medium. Such interactions play a major role in determining the morphology of the NLR, as the radio jets sweep up and compress ambient gas, producing ordered structures with enhanced surface brightness in line emission. In at least three galaxies, namely Mrk 573, ESO 428-G14, and Mrk 34, and perhaps also NGC 7212, off-nuclear radio lobes coincide with regions of low gaseous excitation (as measured by the [OIII]/(Halpha+[NII]) ratio). In Mrk 573 and NGC 4388, there is a clear trend for low brightness ionized gas to be of higher excitation. Those results may be understood if radio lobes and regions of high emission-line surface brightness are associated with high gas densities, reducing the ionization parameter. [OIII]/(Halpha+[NII]) excitation maps reveal bi-polar structures which can be interpreted as either the `ionization cones' expected in the unified scheme or widening, self-excited gaseous outflows. Only NGC 4388 and Mrk 573 show a clearly defined, straight-edged ionization cone.

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arXiv1997

Molecular Hydrogen and Paschen-alpha Emission in Cooling Flow Galaxies

We present near-infrared spectra obtained to search for Pa-alpha and molecular hydrogen lines in edge-darkened (FR I-type) radio galaxies with bright Halpha emission in the redshift range 0.0535<z<0.15. We find that all three galaxies in our sample (PKS 0745-191, PKS 1346+26, & PKS2322-12) which are associated with strong cooling flows also have strong Pa-alpha and H_2 (1-0) S(1) through S(5) emission, while other radio galaxies do not. Together with earlier observations this confirms claims that cooling flow galaxies are copious emitters of molecular hydrogen with large H_2 (1-0) S(3)/Pa-alpha ratios in the range 0.5 to 2. The emission is centrally concentrated within the inner few kiloparsec and could come from warm (T ~ 1000-1500 K) molecular material which is being deposited by the cooling flow. We speculate that the H_2 emission could be related to the interaction between the jets and this molecular gas.

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arXiv1997

The Nuclear Ionized Gas in the Radio Galaxy M84 (NGC 4374)

We present optical images of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C272.1) obtained with the Wide Field/Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). Our three images cover the H$α$ + [N II] emission lines as well as the V and I continuum bands. Analysis of these images confirms that the H$α$ + [N II] emission in the central 5'' (410 pc) is elongated along position angle (P.A.) $\approx 72\arcdeg$, which is roughly parallel to two nuclear dust lanes.Our high-resolution images reveal that the H$α$ + [N II] emission has three components, namely a nuclear gas disk,an `ionization cone', and outer filaments. The nuclear disk of ionized gas has diameter $\approx 1'' = 82$ pc and major axis P.A. $\approx 58\arcdeg \pm 6\arcdeg$. On an angular scale of $0\farcs5$, the major axis of this nuclear gas disk is consistent with that of the dust. However, the minor axis of the gas disk (P.A. $\approx 148\arcdeg$) is tilted with respect to that of the filamentary H$α$ + [N II] emission at distances > 2'' from the nucleus; the minor axis of this larger scale gas is roughly aligned with the axis of the kpc-scale radio jets (P.A. $\approx 170\arcdeg$). The ionization cone (whose apex is offset by $\approx 0\farcs3$ south of the nucleus) extends 2'' from the nucleus along the axis of the southern radio jet. This feature is similar to the ionization cones seen in some Seyfert nuclei, which are also aligned with the radio axes.

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arXiv1996

Helical Strands in the Jet-like Narrow Line Region of ESO 428-G14

We present HST/WFPC2 images of the narrow line region (NLR) of the Seyfert 2 galaxy ESO 428-G14 (0714-2914, M4-1). The NLR consists of many individual, thin strands, which are very closely related to the radio jet and produce a highly complex yet ordered structure. We find that the jet is two-sided with a double-helix of emission-line gas apparently wrapped around the NW side. To the SE, the jet seems to be deflected at a ridge of highly excited gas. The strands to the SE may also wrap around the radio jet, but here complete helices are not seen. The overall structure is reminiscent of the jet seen in NGC 4258. Faint symmetric features aligned with the nucleus could indicate the presence of a highly collimated beam of photons or plasma from the center.

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arXiv1994

The Difference between Radio-Loud and Radio-Quiet Active Galaxies

The recent development of unified theories of active galactic nuclei (AGN) has indicated that there are two physically distinct classes of these objects - radio-loud and radio-quiet. The primary observational distinctions between the two types are: (1) The radio-loud objects produce large scale radio jets and lobes, with the kinetic power of the jets being a significant fraction of the total bolometric luminosity. On the other hand, the weak radio ejecta of the radio-quiet objects are energetically insignificant. (2) The radio-loud objects are associated with elliptical galaxies which have undergone recent mergers, while the radio-quiets prefer spiral hosts. (3) The space density of the radio-louds at a given optical luminosity is $\approx$ 10 times lower than that of the radio-quiets. Despite these differences, the (probably) thermal emissions from the AGN (continua and lines from X-ray to infrared wavelengths) are quite similar in the two classes of object. We argue that this last result suggests that the black hole masses and mass accretion rates in the two classes are not greatly different, and that the difference between the classes is associated with the spin of the black hole. We assume that the normal process of accretion through a disk does not lead to rapidly spinning holes, and propose instead that galaxies (e.g. spirals) which have not suffered a recent major merger event contain non-rotating or only slowly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually coalesce. In the small fraction of mergers in which the two ``parent'' galaxies contain very massive holes of roughly equal mass, a rapidly spinning, very massive hole results. It is proposed that such mergers are the progenitors of powerful radio

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arXiv1994

The Radio Emission from the Ultra-Luminous Far-Infrared Galaxy NGC 6240

We present new radio observations of the ``prototypical'' ultra-luminous far- infrared galaxy NGC~6240, obtained using the VLA at 20~cm in `B' configuration and at 3.6~cm in `A' configuration. These data, along with those from four previous VLA observations, are used to perform a comprehensive study of the radio emission from NGC~6240. Approximately 70\% (~3 x 10$^{23}$ W~Hz$^{-1}$) of the total radio power at 20 cm originates from the nuclear region ( <~ 1.5 kpc), of which half is emitted by two unresolved ($R <~ 36 pc) cores and half by a diffuse component. The radio spectrum of the nuclear emission is relatively flat ($α$ =~ 0.6; $S_ν \propto ν^{-α}$). The supernova rate required to power the diffuse component is consistent with that predicted by the stellar evolution models of Rieke \etal (1985). If the radio emission from the two compact cores is powered by supernova remnants, then either the remnants overlap and form hot bubbles in the cores, or they are very young ( <~ 100 yr). Nearly all of the other 30\% of the total radio power comes from an ``arm-like'' region extending westward from the nuclear region. The western arm emission has a steep spectrum ($α$ =~ 1.0), suggestive of aging effects from synchrotron or inverse-Compton losses, and is not correlated with starlight; we suggest that it is synchrotron emission from a shell of material driven by a galactic superwind. Inverse-Compton scattering of far-infrared photons in the radio sources is expected to produce an X-ray flux of ~2-6 x 10$^{-14}$ erg/s/cm2 in the 2-10 keV band. No significant radio emission is detected from or near the possible ultra-massive ``dark core'' hypothesized by Bland-Hawthorn, Wilson \& Tully (1991).

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