arXiv subjects

J. H. Hough

Publications and source records attributed to J. H. Hough.

31 records · Page 2Linked to original sources

arXiv2002

A Spectropolarimetric Atlas of Seyfert 1 Galaxies

We present optical spectropolarimetry of the nuclei of 36 Seyfert 1 galaxies, obtained with the William Herschel and the Anglo-Australian Telescopes from 1996 to 1999. In 20 of these, the optical emission from the active nucleus is intrinsically polarized. We have measured a significant level of polarization in a further 7 objects but these may be heavily contaminated by Galactic interstellar polarization. The intrinsically polarized Seyfert 1s exhibit a variety of characteristics, with the average polarization ranging from < 0.5 to 5 per cent and many showing variations in both the degree and position angle of polarization across the broad H alpha emission line. We identify a small group of Seyfert 1s that exhibit polarization properties similar to those of Seyfert 2 galaxies in which polarized broad-lines have been discovered. These objects represent direct observational evidence that a Seyfert 2-like far-field polar scattering region is also present in Seyfert 1s. Several other objects have features that can be explained in terms of equatorial scattering of line emission from a rotating disk. We propose that much of the diversity in the polarization properties of Seyfert galaxies can be understood in terms of a model involving both equatorial and polar scattering, the relative importance of the two geometries as sources of polarized light being determined principally by the inclination of the system axis to the line-of-sight.

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arXiv2002

The Magnetic Field Structure in W51A

We present 850 micron imaging polarimetry of the W51A massive star forming region performed with SCUBA on the JCMT. From the polarimetry we infer the column-averaged magnetic field direction, projected onto the plane of the sky. We find that the magnetic field geometry in the region is complicated. We compare the field geometry with 6 cm and CS J=7-6 emission and determine that the magnetic field must be relatively weak and plays a passive role, allowing itself to be shaped by pressure forces and dynamics in the ionised and neutral gases. Comparisons are drawn between our data and 1.3 mm BIMA interferometric polarimetry data, from which we conclude that the magnetic field must increase in importance as we move to smaller scales and closer to sites of active star formation.

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arXiv2001

Spectropolarimetry of a Complete Infrared Selected Sample of Seyfert 2 Galaxies

We report the results of a spectropolarimetric survey of a complete far infrared selected sample of Seyfert 2 galaxies. We have found polarized broad Halpha emission in one new source, NGC5995. In the sample as a whole, there is a clear tendency for galaxies in which we have detected broad Halpha in polarized light to have warm mid--far infrared colours (F(60um)/F(25um)<4), in agreement with our previous results. However, a comparison of the optical, radio and hard x-ray properties of these systems leads us to conclude that this is a secondary consequence of the true mechanism governing our ability to see scattered light from the broad line region. We find a strong trend for galaxies showing such emission to lie above a critical value of the relative luminosity of the active core to the host galaxy (as measured from the [OIII] 5007A equivalent width) which varies as a function of the obscuring column density as measured from hard x-ray observations. The warmth of the infrared colours is then largely due to a combination of the luminosity of the active core, the obscuring column and the relative importance of the host galaxy in powering the far infrared emission, and not solely orientation as we inferred in our previous paper. Our data may also provide an explanation as to why the most highly polarized galaxies, which appear to have tori that are largely edge-on, are also the most luminous and have the most easily detectable scattered broad Halpha.

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arXiv2001

Submillimeter Imaging Polarimetry of the NGC 7538 Region

Imaging polarimetry of the 0.85mm continuum emission in the NGC 7538 region, obtained with the SCUBA Polarimeter, is presented. The polarization map is interpreted in terms of thermal radiation by magnetically aligned dust grains. Two prominent cores associated with IRS 1 and IRS 11, IRS 1(SMM) and IRS 11(SMM), are found in the surface brightness map. Although these cores look similar in surface brightness, their polarization shows striking differences. In IRS 11(SMM), the polarization vectors are extremely well ordered, and the degrees of polarization are quite high with an average of ~3.9 %. In IRS 1(SMM), on the other hand, the directions of polarization vectors are locally disturbed, and the degrees of polarization are much lower than those of IRS 11(SMM). These differences suggest that small scale fluctuations of the magnetic field are more prominent in IRS 1(SMM). This can be interpreted in terms of the difference in evolutionary stage of the cores. Inside IRS 1(SMM), which seems to be at a later evolutionary stage than IRS 11(SMM), substructures such as subclumps or a cluster of infrared sources have already formed. Small scale fluctuations in the magnetic field could have developed during the formation of these substructures. The distribution of magnetic field directions derived from our polarization map agrees well with those of molecular outflows associated with IRS 1(SMM) and IRS 11(SMM). Comparisons of energy densities between the magnetic field and the outflows show that the magnetic field probably plays an important role in guiding the directions of the outflows.

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arXiv2001

Circumstellar structure of RU Lupi down to au scales

We have used the technique of spectro-astrometry to study the milli-arcsecond scale structure of the emission lines in the T Tauri star RU Lupi. The wings of the H-alpha emission are found to be displaced from the star towards the south-west (blue wing) and north-east (red wing) with angular scales of 20-30 milli-arcsecs. This structure is consistent with a bipolar outflow from the star. From a study of the variability of the intensity and position spectra we argue that a combination of magnetically-driven bipolar outflow and accreting gas contribute to the H-alpha emission. On the other hand, the [OI] and [SII] emission are displaced from the star to the south-west but at much larger distances than the H-alpha, hundreds of milli-arcsecs for the high-velocity component (HVC) and down to 30 milli-arcsecs for the low-velocity components (LVC). The presence of both red-shifted and blue-shifted outflows in H-alpha but only a blue-shifted outflow in the forbidden lines can be explained if the disc obscures the red-shifted forbidden line outflow, but a disc gap with outer radius 3-4 au allows the red-shifted H-alpha to be seen. This gap could be induced by an unseen companion.

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arXiv2000

A Polarization Flare in 3C 273: A Clue to Jet Physics

We present UBVRIJHK polarization and flux density observations of the quasar 3C 273 obtained during a time of outburst over two weeks in 1988 February. We have modelled these data with two power law components, each with wavelength-independent position angle. These components are roughly perpendicular. The steeper-spectrum component has higher infrared polarized flux density, with the electric vector approximately transverse to the projected direction of the VLBI jet. The K-band polarized flux density and position angle, and possibly the spectral index of the two components, are correlated with a time lag of less than a day. We explain our results in terms of a shocked jet model with two nearly co-spatial components: a shock component with magnetic field approximately perpendicular to the jet and the other with magnetic field approximately parallel to the jet.

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arXiv2000

Axisymmetry in proto-planetary nebulae: using imaging polarimetry to investigate envelope structure

We use ground-based imaging polarimetry to detect and image the dusty circumstellar envelopes of a sample of proto-planetary nebulae (PPNe) at near-infrared wavelengths. We detect extended (up to 9 arcsec diameter) circumstellar envelopes around 15 out of 16 sources with a range of morphologies including bipolars and shells. The distribution of scattered light in combination with its polarization (up to 40 per cent) provides unambiguous evidence for axisymmetry in 14 objects showing this to be a common trait of PPNe. We suggest that the range of observed envelope morphologies results from the development of an axisymmetric dust distribution during the superwind phase at the end of the AGB. We identify shells seen in polarized light with scattering from these superwind dust distributions which allows us to provide constraints on the duration of the superwind phase. In one object (IRAS 19475+3119) the circumstellar envelope has a two-armed spiral structure which we suggest results from the interaction of the mass losing star with a binary companion.

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arXiv2000

HST infrared imaging polarimetry of Centaurus A: implications for the unified scheme and the existence of a mis-directed BL Lac nucleus

We report results from HST/NICMOS 2 micron imaging polarimetry of the central region of Centaurus A. In the vicinity of the nucleus we observe a complex polarization structure which we explain by a combination of scattering of nuclear light and dichroic polarization associated with the dust lane. The scattered nuclear radiation is found in an angular region which extends over ~ 70 degrees and thus it does not originate from a highly collimated beam, but is associated with more omni-directional nuclear illumination. These observations also show the presence of an unresolved, highly polarized (P = 11.1 %) nuclear source whose polarization angle PA = 148.2 degrees is perpendicular to the jet axis. We set an upper limit of 0.04'' (~0.8 pc) to its extent. The observed nuclear polarization is naturally accounted for if we are observing scattered light from an otherwise obscured nucleus provided that both the scattering region and the occulting torus are extremely compact, with an outer radius of less than ~ 1 pc. Alternatively, we might be directly seeing the infrared counterpart of the radio core, similar to those found in other low luminosity radio-galaxies observed with HST. We discuss these results in the framework of the FRI / BL Lac unifying model.

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arXiv2000

Detection of polarized mm and submm emission from SgrA*

We report the detection of linear polarization from SgrA* at 750, 850, 1350 and 2000 microns which confirms the contribution of synchrotron radiation. From the lack of polarization at longer wavelengths it appears to arise in the millimetre/sub-millimetre excess. There are large position angle changes between the millimetre and sub-millimetre results and these are discussed in terms of a polarized dust contribution in the sub-millimetre and various synchrotron models. In the model which best explains the data the synchrotron radiation from the excess is self-absorbed in the millimetre region and becomes optically thin in the sub-millimetre. This implies that the excess arises in an extremely compact source to the order of 2 Schwarzschild radii.

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arXiv2000

Modelling the Infrared continuum of Centaurus A

We present ISOPHOT_S, ISOSWS and 8 to 13 micron ground based observations of Centaurus A that show prominent PAH and silicate features. These and other data are used to construct a model for the infrared continuum. We find that in a nuclear sized aperture (~4 arcsec, ~60 pc) the SED is characteristic of emission from a starburst and AGN torus; in larger apertures an additional component of cirrus emission is required. Based on our model, the torus diameter is estimated to be 3.6 pc and the best fitting inclination angle of the torus is 45 degrees. This result has implications for the detectability of tori in low power AGN and in particular for the use of the IRAS 60/25 micron flux ratio as an indicator of the torus inclination.

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arXiv2000

Highly polarized structures in the near-nuclear regions of Cygnus A: intrinsic anisotropy within the cones?

We present near-IR imaging polarimetry observations of the nucleus of Cygnus A, taken with the NICMOS camera of the HST at a wavelength of 2.0 microns.These maps reveal a highly collimated region of polarized emission straddling the nucleus and extending to a radius of 1.2 arcseconds. Remarkably, this feature coincides with one, but only one, limb of the edge-brightened bicone structure seen in the total intensity image. The high degree (P_k ~ 25%) and orientation of the extended polarization feature are consistent with a scattering origin. Most plausibly, the detection of polarization along only one limb of the bicone is a consequence of intrinsic anisotropy of the near-IR continuum within the radiation cones, with the direction of maximum intensity of the near-IR radiation field significantly displaced from the direction of the radio axis. The unresolved nuclear core source is also highly polarized (P_k > 28%), with a position angle close to the perpendicular to the radio axis. Given that this high degree of nuclear polarization can only be explained in terms of dichroic extinction if the dichroic mechanism is unusually efficient in Cygnus A, it is more likely that the nuclear polarization is due to the scattering of nuclear light in an unresolved scattering region close to the AGN. In this case, the flux of the core source in the K-band is dominated by scattered rather than transmitted quasar light, and previous extinction estimates based on K-band photometry of the core substantially underestimate the true nuclear extinction.

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arXiv1999

The X-ray, Optical and Infrared Counterpart to GRB 980703

We report on X-ray, optical and infrared follow-up observations of GRB 980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power law decline we find for its decay index alpha < -0.91 (3-sigma). We invoke host galaxy extinction to match the observed spectral slope with the slope expected from `fireball' models. We find no evidence for a spectral break in the infrared to X-ray spectral range on 1998 July 4.4, and determine a lower limit of the cooling break frequency: nu_c > 1.3 x 10^{17} Hz. For this epoch we obtain an extinction of A_V = 1.50 +/- 0.11. From the X-ray data we estimate the optical extinction to be A_V = 20.2 +12.3 -7.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966 found by Djorgovski et al. (1998). We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, epsilon_B < 6x10^{-5}, is much lower in the case of GRB 980703, which is a consequence of the high frequency of the cooling break.

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arXiv1997

A Millimetre/Submillimetre Polarisation Survey of Compact Flat-Spectrum Radio Sources

We present the results of the first survey and monitoring study of the linear polarisation properties of compact, flat-spectrum radio sources at millimetre/submillimetre wavelengths and discuss the implications of the inferred magnetic field structure for the emission models involving shock waves in relativistic jets on sub-parsec scales. We find significant polarisation in most sources but, in general, the magnetic field on sub-parsec scales is less well ordered than on parsec scales. We observe no difference in polarisation properties between the BL Lac objects and compact flat-spectrum quasars at these wavelengths. Although we find the behaviour of some sources, particularly the most highly polarised, to be very consistent with the predictions of transverse shock-in-jet models, the detailed behaviour of most objects is not. Conical shock structures can more readily explain the observed diverse behaviour of the sample, although some degree of bending of the jet may still be necessary in some cases.

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