The optical polarization of M82 and the local spiral arm

This thesis comprises two separate but related topics in the study of optical polarization of galaxies. In part I we investigate interstellar polarization within 2 kpc of the sun and attempt to quantify the local structure of the galactic magnetic field. In part II we report the results of polarization measurements of the peculiar galaxy M82, obtained using a new polarimeter and digital reduction techniques, and discuss models of the origin of the polarization. Measurements of the linear polarization of starlight have been collated into a catalogue containing the Stokes' parameters in galactic coordinates for those stars for which reliable distances could be determined. The catalogue is presented in the form of vector maps on the sky in several distance intervals. Assuming a magnetic alignment hypothesis we have investigated the direction and form of the galactic magnetic field through e-vector plots and from the periodicity of the Stokes' parameters Q(ɭ) and U(ɭ) with galactic longitude. The results show the existence of a longitudinal field directed towards ɭ = 45º ± 10º within 500 pc, and beyond this there is much confusion with a possible change in direction, associated with the bifurcation of the spiral arm, to ɭ = 74º ± 10º, There is no evidence for a field directed towards ɭ = 90º, It is clear however that a simple longitudinal model of the field is rather naive. The U(ɭ) plots show strong evidence for an inclination of the field by 15º to the plane and this is not associated with a helical structure. The possible significance of this conclusion to the origin of the field is discussed. Incremental polarization maps have been produced but show little correlation with the spiral structure of the galaxy. There is strong evidence for irregularities in the field. The polarization appears to saturate in all directions at about 1 kpc from the sun. We interpret this as an observational selection effect. The major part of this work is directed towards studying the importance of irregularities in the field structure. Autocorrelation techniques have been used and unlike previous authors we can find no coherence in this component on scales greater than 50 pc. In the second part of the thesis we describe an imaging polarimeter constructed for use with a McMullan electronographic camera and designed to operate at an f/15 focus. This is the first polarimeter to use electronographic detection and the principles, construction and method of operation are described. The instrument is intended for observations of galaxies and other nebulae to diameters of up to 8 minutes of arc and has been successfully used to observe the irregular galaxy M82 in the B-band. The results of these observations are reported in this thesis. The polarimeter enables the simultaneous measurement of the linear polarization at more than 1500 locations in a 40 mm field of view to be made. This information is obtained in a series of eight exposures, which enables the effect of cathode sensitivity variations to be removed. The method is independent of variations in background sky brightness and polarization, and in atmospheric transparency. A review of the existing designs of polarimeter, their advantages and disadvantages and the possible sources of systematic errors are discussed. The optical system is also suitable for use with two-dimensional digital detectors but so far none have been used with the instrument. In order to take full advantage of the vast amount of information contained on each electronograph an entirely new digital analysis technique has been developed. Attempts have been made to locate features such as stars, grid overlaps, scratches and dirt blobs automatically using a random search technique. This proved unsatisfactory, and possible explanations and refinement in the approach outlined. A simple contour method is shown to be a satisfactory means of carrying out the feature extraction with manual assistance. A Highly accurate image registration method capable of producing a picture to picture registration better than 2µ is developed and the method takes into account small scale flaws, saturation effects, cathode sensitivity variations and differing exposure times. The technique is vastly superior to conventional methods of plate analysis and future refinements are discussed. The performance of the instrument in the laboratory and at the telescope is reported, the existence of severe instrumental effects established, and corrections derived and applied to the polarization data. Their eradication from the instrument is described and results of calibration measurements of standard stars with the improved optics presented, showing the instrument is capable of reaching a precision of ±0.5% in p and ± 3º in ɵ. The results of polarization measurements of M82 are presented and compared with previous observations. These results have a spatial resolution of between 5 and 8 times that of previous observations, are 20 times as numerous and have comparable accuracy (± 2.5% in p and ± 4º in ɵ). These results represent the first complete mapping of the linear polarization in an extragalactic system at optical wavelengths. A review of the existing observational material on M82 is presented and the relevance of the current observations established. The predictions of simple scattering models for producing the observed polarization are compared with the observations and show moderate agreement. The active region of the galaxy is located and the evolutionary status and energetics of M82 are discussed. The current problems in our understanding of the galaxy and suggestions for future work are detailed

Polarization Profiles of Scattered Emission Lines. II. Upstream Dust Scattering in the HH 1 Jet

Detailed comparisons are made between observations of scattered light upstream of the head of the HH~1 jet and predictions of simple scattering models. It is shown that, in order to unambiguously determine the velocity of the head of the jet (bow shock) with respect to the upstream dust, existing spectroscopic observations are insufficient and that spectropolarimetric observations of the scattered light are necessary. Such an independent measure of the bow shock velocity is important in order to test ``multiple outflow'' theories of Herbig-Haro jets. It is also shown 2that the scattering dust must have a very forward-throwing scattering phase function (\langle\cos\theta\rangle\msim 0.7) and slight evidence is found for a dust-gas ratio that is higher than average.Comment: 11 pages, uuencoded compressed postscript (including 9 figures), accepted for publication in Ap.J., IAUNAM_contrib.#34

The evidence for jet-cloud interactions in a sample of high/intermediate-redshift radio galaxies

We present the result obtained from a study, based on long-slit spectroscopy, of the kinematics and ionization mechanisms of the line-emitting gas for a sample of four high/intermediate-redshift radio galaxies. In two of the galaxies (3C352 and 3C435A) the radio sources are of the same scale as the emission-line regions, whereas in the other two (3C34 and 3C330) the radio sources are extended on a larger scale than the emission-line structures. We see evidence for shock-acceleration of the emission-line gas in the extended regions of all the galaxies, even in the largest radio sources of our sample, in which the radio hot spots have passed the extended gas of the galaxies. The extended regions present highly disturbed kinematics (line-splitting and/or underlying broad components), which are difficult to explain if we do not consider a strong interaction between the radio-emitting components and the ambient gas. However, the dominant ionization mechanism of the line-emitting gas remains uncertain. We have compared the optical diagnostic line ratios of the galaxies in our sample with both AGN-photoionization and shock-ionization models. We find a lack of consistency in explaining the main ionization mechanism of the emission-line gas. This suggest that, if the extended regions are shock-ionized, some of the assumptions implicit in the shock models may need to be reconsidered. In addition, we have investigated the nebular continuum cointribution to the UV excess in the galaxies of our sample. We find a substantial nebular emission contribution to the UV continuum in all the cases. However, after the subtraction iof the nebular component, a significant UV excess remains in the extended nebulae of most of the objects.Comment: 33 pages, 24 figures, accepted for publication in MNRAS. (Abstract shortened for astro-ph

Spectroastrometry of rotating gas disks for the detection of supermassive black holes in galactic nuclei. III. CRIRES observations of the Circinus galaxy

We present new CRIRES spectroscopic observations of BrGamma in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H2O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc. The new method is capable of probing gas rotation at scales which are a factor ~3.5 smaller than those probed by the rotation curve analysis. The dynamical mass spatially unresolved with the spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ~4.5 larger than the BH mass measurement obtained with the H2O maser emission indicating that it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ~2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.Comment: A&A in press. We wish to honor the memory of our great friend and colleague David Axon. He will be greatly missed by all of us. arXiv admin note: text overlap with arXiv:1110.093

The morphology of Sersic-Pastoriza galaxies

The authors present the preliminary results of their radio-continuum and neutral hydrogen observations of Sersic-Pastoriza (S-P) galaxies. They show that the central regions contain a population of compact features thought to be young supernova remnants (SNRs) and discuss the overall morphology of the nuclei

Models relating the radio emission and ionised gas in Seyfert nuclei

Possible models are discussed in which the radio emitting components in Seyfert II nuclei can compress and accelerate the ambient nuclear medium to produce the characteristics of the narrow line region. A first order model, which considers only the expansion of the radio components, is briefly described. However, in many Seyfert nuclei it appears that the linear motion of the radio components is also important. This can result in shock heating of the ambient medium, and if the cooling time is long enough, can lead to a displacement between the radio component and the associated emission lines. This effect may be present in NGC 1068 and NGC 5929 and by considering ram pressure balance and the cooling length it is possible to estimate lobe velocities and ambient densities

The large scale distribution of warm ionized gas around nearby radio galaxies with jet-cloud interactions

Deep, narrow-band Halpha observations taken with the TAURUS Tunable Filter (TTF) on the 4.2m WHT telescope are presented for two nearby radio galaxies with strong jet-cloud interactions. Although the brightest emission line components are closely aligned with the radio jets --- providing nearby examples of the ``alignment effect'' most commonly observed in high redshift (z > 0.5) radio galaxies --- lower surface brightness emission line structures are detected at large distances (10's of kpc) from the radio jet axis. These latter structures cannot be reconciled with anisotropic illumination of the ISM by obscured quasar-like sources, since parts of the structures lay outside any plausible quasar ionization cones. Rather, the distribution of the emission lines around the fringes of the extended radio lobes suggests that the gas is ionized either by direct interaction with the radio components, or by the diffuse photoionizing radiation fields produced in the shocks generated in such interactions. These observations serve to emphasise that the ionizing effects of the radio components can extend far from the radio jet axes, and that deep emission line imaging observations are required to reveal the true distribution of warm gas in the host galaxies. We expect future deep imaging observations to reveal similar structures perpendicular to the radio axes in the high-z radio galaxies.Comment: 18 pages, 4 figures, to be published in MNRA

High Accuracy Near-infrared Imaging Polarimetry with NICMOS

The findings of a nine orbit calibration plan carried out during HST Cycle 15, to fully determine the NICMOS camera 2 (2.0 micron) polarization calibration to high accuracy, are reported. Recently Ueta et al. and Batcheldor et al. have suggested that NICMOS possesses a residual instrumental polarization at a level of 1.2-1.5%. This would completely inhibit the data reduction in a number of GO programs, and hamper the ability of the instrument to perform high accuracy polarimetry. We obtained polarimetric calibration observations of three polarimetric standards at three spacecraft roll angles separated by ~60deg. Combined with archival data, these observations were used to characterize the residual instrumental polarization in order for NICMOS to reach its full potential of accurate imaging polarimetry at p~1%. Using these data, we place an 0.6% upper limit on the instrumental polarization and calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, are seen to dominate the accuracy of p and theta. However, the updated coefficients do allow imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15deg. This work enables a new caliber of science with HST.Comment: 13 pages, 9 figures, PASP accepte