Optical Polarization Maps Research Articles

Interstellar Polarization Survey. IV. Characterizing the Magnetic Field Strength and Turbulent Dispersion Using Optical Starlight Polarization in the Diffuse Interstellar Medium

Angular dispersion functions are typically used to estimate the fluctuations in polarization angle around the mean magnetic field orientation in dense regions, such as molecular clouds. The technique provides accurate turbulent-to-regular magnetic field ratios, 〈Bt2〉1/2/Bpos , which are often underestimated by the classic Davis–Chandrasekhar–Fermi method. We assess the technique's suitability to characterize the turbulent and regular plane-of-sky magnetic field in low-density structures of the nearby interstellar medium (ISM), particularly when the turbulence outer scale, δ, is smaller than the smallest scale observed, ℓ min. We use optical polarization maps of three intermediate-latitude fields (∣b∣ ≳ 7.°5) with dimensions of 0.°3 × 0.°3, sourced from the Interstellar Polarization Survey–General ISM catalog. We decomposed the H i emission detected by the Galactic All-Sky Survey within our fields to estimate the multiphase ISM properties associated with the structure coupled to the magnetic field. We produced maps of the plane-of-sky magnetic field strength (B pos), mass density (ρ), and turbulent velocity dispersion (σ v,turb). In the regions with well-defined structures at d < 400 pc, the average B – ranges from ∼3 μG to ∼9 μ G, depending on the method and physical properties. In the region where structures extend up to 1000 pc, B pos varies from ∼1 to ∼3 μ G. The results agree with previous estimations in the local, diffuse ISM. Finally, optical starlight polarization and thermal dust polarization at 353 GHz consistently reveal a highly regular plane-of-sky magnetic field orientation unfazed by diffuse dust structures observed at 12 μm.

Polarimetric imaging of interacting pairs

AbstractWe present optical polarization maps of a sample of four interacting pairs at different phases of encounter, from nearly unperturbed galaxies to on‐going mergers. Only the pair RR 24 shows a linear polarization pattern which extends in both galaxies for several kiloparsecs. The more perturbed member, RR 24b, is linearly polarized up to the level of ≈3%. No polarization is measured in the strongly perturbed late‐type pair members of RR 23 and RR 99. Also, in the central part of the double nuclei shell galaxy ESO 2400100 there is no significant polarization. We use the ionized gas velocity field of RR 24 to interpret its linear polarization structure. In RR 24a the quite regular gas kinematics reflect the unperturbed spirallike polarization structure. In RR 24b a strong velocity gradient in ionized gas could be associated with the polarization structure. We suggest that the large‐scale magnetic field of the RR 24 pair members still plays a role in shaping the polarization pattern. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Further evidence for vertical magnetic fields in the galaxy NGC 891

An optical polarization map of NGC 891, an edge-on galaxy with similar properties to the Galaxy, confirms that there are vertical magnetic fields present in the bulge/ halo above the inner part of the galactic disc. We find no evidence for large-scale magnetic fields in the central dust lane, or polarization created by scattering from dust grains within the dust lane and elsewhere in the galaxy.

Does Near-Infrared Polarimetry Reveal the Magnetic Field in Cold Dark Clouds?

view Abstract Citations (161) References (67) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Does Near-Infrared Polarimetry Reveal the Magnetic Field in Cold Dark Clouds? Goodman, Alyssa A. ; Jones, Terry J. ; Lada, Elizabeth A. ; Myers, Philip C. Abstract We present near-infrared (JHK) observations of the polarization of background starlight seen through the filamentary dark cloud L1755. The mean position angle and dispersion of the polarization vectors measured in the near-infrared, for stars along lines of sight passing through the densest portions (1 < Av < 10 mag) of L1755, are virtually identical to those in an optical polarization map of stars around the periphery (Av ∼ 1 mag) of L1755. Furthermore, the percentage of polarization is not seen to increase, at all, with extinction in the near-infrared observations. We surmise that much of the dust in the dark cloud is extinguishing background starlight significantly, but not polarizing it efficiently, and thus that the polarization map of background starlight cannot reliably trace the magnetic field associated with the dense interior of the dark cloud. Our results in L1755 are remarkably similar to what we found in the dark cloud B216-217 (Goodman et al. 1992), which also shows no change in the polarization map associated with the cloud, and no rise in percentage polarization with extinction. Using our multiwavelength polarimetric observations of L1755, we have estimated the wavelength of maximum polarization, λmax, for most of the 53 stars in our sample. We find an unusually broad distribution of λmax, with a mean at 0.88±0.34 μm. The large range of λmax leads us to the hypothesis that there is a wide range of grain sizes and/or shapes along the lines of sight through L1755. We conclude that only a small subset of grains is responsible for producing the polarization of background starlight, and that these grains may be critically underrepresented in the dense interiors of cold dark clouds. Publication: The Astrophysical Journal Pub Date: August 1995 DOI: 10.1086/176003 Bibcode: 1995ApJ...448..748G Keywords: INFRARED: ISM: CONTINUUM; ISM: CLOUDS; ISM: DUST; EXTINCTION; ISM: INDIVIDUAL ALPHANUMERIC: L1755; ISM: MAGNETIC FIELDS; POLARIZATION full text sources ADS | data products SIMBAD (60)

The starburst galaxy NGC1808: another M82?

An optical polarization map of the starburst galaxy NGC1808 shows that on kpc scales there is a coherent galactic magnetic field which follows the spiral arms on the galaxy. The inner 750pc surrounding the central region shows characteristics of a huge reflection nebula illuminated by the various hotspots within the starburst. This is very similar to the archetypal starburst galaxy M82.

Imaging polarimetry of the starburst galaxy NGC 1808: another M82?

An optical polarization map for the starburst galaxy NGC 1808 is presented. On kiloparsec scales the polarization orientations follow the spiral arms, indicative of a magnetic field that is coherent on a galactic scale. The inner parts (<750 pc) of the galaxy show a polarization pattern that is typical of a huge reflection nebula illuminated from within the central activity. Although the galactic nucleus is the dominant illuminating source of this nebula, the other hotspots make varying contributions to the illumination. The inner reflection nebula is very reminiscent of the one in the archetypal starburst galaxy M82

The illumination of the GGD 30 nebulosity

An optical polarization map of the GGD 30 nebulosity shows that, although it shares many of the morphological characteristics of classical cometary nebulae such as NGC 2261/R.Mon, it differs in that the star-like knot at its head is not the illluminating and exciting source as previously assumed. The knot is reflection nebulosity and the true source, completely hidden from direct view at optical wave-lengths, is located some 6 arcsec east of the knot

Star formation in dark globules in the Gum Nebula

Optical polarization maps of small nebulosities (Re5 and IRAS 08211-4158) associated with IRAS sources located in dark globules are presented and used to determine the positions of the illuminators. The IRAS 08211-4158 system is found to have two illuminators, one corresponding to the hidden IRAS source while the other is a visible star shrouded in nebulosity. These results are combined with earlier ones for CG30/HH120 and HH46/47 to look at the overall picture of low-mass star formation in the complex of dark globules within the Gum Nebula

A high spatial resolution polarization map of GL2591

A high spatial resolution optical polarization map of the nebulosity close to the IR source GL2591 is presented which shows that the pattern of vectors is significantly affects by dichroic extinction within the molecular cloud in which the system is located. After correction for these effects it is found that the dominant illuminator of the nebulosity is the IR source GL2591, but there is also a significant localized contribution from a visible star which may be embedded within the cloud

The illumination of the cometary nebula 1548C27

Optical polarization maps of the crescent-shaped 1548C27 nebulosity show that there are two illuminating stars rather than the single apical one as previously assumed. The origin and nature of the jet is questioned.

Optical polarization maps of star-forming regions in Perseus, Taurus, and Ophiuchus

New optical linear polarization maps are presented of the star-forming regions near L1506 in Taurus, L1755 in Ophiuchus, and the complex of dark cloud which extends from L1448 in B5 in Perseus. The former two show a well-defined peak magnetic field direction in the plane of the sky with a finite dispersion about that peak which is smaller than would be expected for a random distribution of field distributions. The dispersion in the position angle of filamentary clouds within these complexes implies that clouds which appear elongated on the plane of the sky are not all associated with a pattern of polarization vectors particularly 'parallel' or 'perpendicular' to their geometry. Instead, clouds tend to be oriented at the angle formed by their axis and the 'mean' direction of the local large-scale field. For the dark cloud complex, a bimodal distribution of the polarization vector angle is taken to result from at least two distributions of gas along the line of sight which appear as a 'complex' in projection.

Oblique Shocks in Extragalactic Jets

In the framework of the diffusive shock acceleration of relativistic electrons in extragalactic jets, we show that it is possible to derive the speed of the jet. For this purpose, we transform continuity relations through an oblique shock front into the reference frame of the observer. We apply these calculations to knot A of the M87 jet. Measuring the deviation of the fluid through the shock front from high-resolution radio maps and the deviation of the magnetic field from optical polarization maps (Fraix-Burnet et al., 1989), we derive speeds of about 0.01 c (Fraix-Burnet and Biermann, in preparation). The compression ratio is most probably 4 and the magnetic field is nearly parallel to the shock front both upstream and downstream.

Optical polarization studies of Herbig-Haro objects - IV. The HH34 complex

Optical polarization maps are presented for the various nebulosities of the HH34 complex in the L1641 dark cloud. The Herbig–Haro object HH34 and its associated optical jet are unpolarized but their source of excitation (HH34-IRS) is the illuminating star of a reflection nebula which envelops the jet and extends as far as HH34. The optical polarization of HH34-IRS suggests that it is surrounded by a circumstellar disc which collimates the outflows in some manner. There is other reflection nebulosity in the region illuminated by a faint star which we identify as the optical counterpart of HH34-IRS5. We have also discovered a small bipolar reflection nebula approximately 20 arcsec west of HH34-IRS.

The structure of NGC 1999

An optical polarization map is presented for NGC 1999, the reflection nebulosity surrounding V380 Orionis. This star is slightly embedded on the near side of an extensive dust cloud which has an oblique boundary in front of the star. The dark globule silhouetted against NGC 1999 is physically associated with the nebula.

Optical polarization map of Eta Carinae and the nature of its outburst

Optical polarization map of Eta Carinae and the nature of its outburst