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R. Beck - One of the best experts on this subject based on the ideXlab platform.
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magnetic fields and Spiral Arms in the galaxy m51
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: A Fletcher, R. Beck, Anvar Shukurov, E M Berkhuijsen, C HorellouAbstract:We use new multiwavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from 200 pc to several kpc. Interferometric and single-dish data are combined to obtain new maps at lambda lambda 3, 6 cm in total and polarized emission, and earlier lambda 20 cm data are rereduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the lambda 20 cm emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at lambda 6 cm must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy; the disc is dominated by a combination of azimuthal modes, m = 0 + 2, but in the halo only an m = 1 mode is required to fit the observations. We discuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the Spiral shocks. In the inner Spiral Arms, the strong arm-interarm contrasts in total and polarized radio emission are roughly consistent with expectations from shock compression of the regular and turbulent components of the magnetic field. However, the average arm-interam contrast, representative of the radii r > 2 kpc where the Spiral Arms are broader, is not compatible with straightforward compression: lower arm-interarm contrasts than expected may be due to resolution effects and decompression of the magnetic field as it leaves the Arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of 50 pc for the size of the turbulent eddies.
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magnetic fields and Spiral Arms in the galaxy m51
arXiv: Cosmology and Nongalactic Astrophysics, 2010Co-Authors: A Fletcher, R. Beck, Anvar Shukurov, E M Berkhuijsen, C HorellouAbstract:(Abridged) We use new multi-wavelength radio observations, made with the VLA and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51 on scales from $200\pc$ to several $\kpc$. Interferometric and single dish data are combined to obtain new maps at \wwav{3}{6} in total and polarized emission, and earlier \wav{20} data are re-reduced. We compare the spatial distribution of the radio emission with observations of the neutral gas, derive radio spectral index and Faraday depolarization maps, and model the large-scale variation in Faraday rotation in order to deduce the structure of the regular magnetic field. We find that the \wav{20} emission from the disc is severely depolarized and that a dominating fraction of the observed polarized emission at \wav{6} must be due to anisotropic small-scale magnetic fields. Taking this into account, we derive two components for the regular magnetic field in this galaxy: the disc is dominated by a combination of azimuthal modes, $m=0+2$, but in the halo only an $m=1$ mode is required to fit the observations. We disuss how the observed arm-interarm contrast in radio intensities can be reconciled with evidence for strong gas compression in the Spiral shocks. The average arm--interam contrast, representative of the radii $r>2\kpc$ where the Spiral Arms are broader, is not compatible with straightforward compression: lower arm--interarm contrasts than expected may be due to resolution effects and \emph{decompression} of the magnetic field as it leaves the Arms. We suggest a simple method to estimate the turbulent scale in the magneto-ionic medium from the dependence of the standard deviation of the observed Faraday rotation measure on resolution. We thus obtain an estimate of $50\pc$ for the size of the turbulent eddies.
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magnetism in the Spiral galaxy ngc 6946 magnetic Arms depolarization rings dynamo modes and helical fields
Astronomy and Astrophysics, 2007Co-Authors: R. BeckAbstract:Context. The Spiral galaxy NGC 6946 hosts magnetic Spiral Arms, highly aligned magnetic fields between the gas/optical Arms. Aims. The origin of the magnetic phenomena and their relation to the interstellar gas are investigated. Methods. NGC 6946 was observed in total intensity and linear polarization in five radio bands between 3 cm and 21 cm. Maps of spectral index, Faraday rotation and depolarization were derived. Results. At the inner edge of the inner gas Spiral arm the ordered magnetic field is only mildly compressed and turns smoothly, to become aligned along the gas arm. Hence the field is not shocked and is probably connected to the warm, diffuse gas. At larger radii, two bright magnetic Arms between the optical Arms are visible in polarized intensity. The field in the northern magnetic arm is almost totally aligned. Faraday rotation measures (RM) in these Arms are consistent with the superposition of two low azimuthal dynamo modes. Three more magnetic Arms are discovered in the outer galaxy, located between H I Arms. The RM structure function confirms large-scale coherent fields. The observed anti-correlation between the field’s pitch angles and the RM values is a possible signature of helical fields. – Due to strong Faraday depolarization the galaxy is not transparent to polarized waves at λ18 cm and λ20 cm. The large-scale asymmetry in depolarization with respect to the major axis may be another indication of large-scale helical fields. Three depolarization rings of almost zero polarization seen at λ20 cm are probably generated by differential Faraday rotation in H II complexes in NGC 6946 of 300−500 pc size. – In the gas/optical Spiral Arms, the total (mostly turbulent) magnetic field is amplified to � 20 µG. Its energy density is � 10 times larger than that of the ionized gas and is similar to that of the turbulent gas motions in the inner galaxy. The magnetic energy exceeds that of the turbulent energy in the outer galaxy. All energy densities in NGC 6946 are about one order of magnitude larger than those in the Milky Way. Conclusions. Density waves in the inner gaseous Spiral Arms mildly compress the field. Dynamo action probably generates the magnetic Spiral Arms. The magnetic field is dynamically important, interacts with the gas flow and possibly determines the properties of the gas Spiral Arms.
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Magnetism in the Spiral galaxy NGC 6946: magnetic Arms, depolarization rings, dynamo modes, and helical fields
'EDP Sciences', 2007Co-Authors: R. BeckAbstract:Context.The Spiral galaxy NGC 6946 hosts magnetic Spiral Arms, highly aligned magnetic fields between the gas/optical Arms. Aims.The origin of the magnetic phenomena and their relation to the interstellar gas are investigated. Methods.NGC 6946 was observed in total intensity and linear polarization in five radio bands between 3 cm and 21 cm. Maps of spectral index, Faraday rotation and depolarization were derived. Results.At the inner edge of the inner gas Spiral arm the ordered magnetic field is only mildly compressed and turns smoothly, to become aligned along the gas arm. Hence the field is not shocked and is probably connected to the warm, diffuse gas. At larger radii, two bright magnetic Arms between the optical Arms are visible in polarized intensity. The field in the northern magnetic arm is almost totally aligned. Faraday rotation measures (RM) in these Arms are consistent with the superposition of two low azimuthal dynamo modes. Three more magnetic Arms are discovered in the outer galaxy, located between ${\rm H\,\scriptstyle I}$ Arms. The RM structure function confirms large-scale coherent fields. The observed anti-correlation between the field's pitch angles and the RM values is a possible signature of helical fields. – Due to strong Faraday depolarization the galaxy is not transparent to polarized waves at $\lambda18\,{\rm cm}$ and $\lambda20\,{\rm cm}$. The large-scale asymmetry in depolarization with respect to the major axis may be another indication of large-scale helical fields. Three depolarization rings of almost zero polarization seen at $\lambda20\,{\rm cm}$ are probably generated by differential Faraday rotation in ${\rm H\,\scriptstyle II}$ complexes in NGC 6946 of 300-500 pc size. – In the gas/optical Spiral Arms, the total (mostly turbulent) magnetic field is amplified to $\simeq$20 μG. Its energy density is $\simeq$10 times larger than that of the ionized gas and is similar to that of the turbulent gas motions in the inner galaxy. The magnetic energy exceeds that of the turbulent energy in the outer galaxy. All energy densities in NGC 6946 are about one order of magnitude larger than those in the Milky Way. Conclusions.Density waves in the inner gaseous Spiral Arms mildly compress the field. Dynamo action probably generates the magnetic Spiral Arms. The magnetic field is dynamically important, interacts with the gas flow and possibly determines the properties of the gas Spiral Arms
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magnetic fields and the interstellar medium in Spiral Arms
Symposium - International Astronomical Union, 1996Co-Authors: M Urbanik, M Soida, R. BeckAbstract:We performed the high frequency radio studies of Spiral galaxies using the 100 m MPIfR radio telescope at 10.55 GHz. Two objects: NGC 4254 and NGC 3627 possess perturbed Spiral structures while two others, NGC 3521 and NGC 5055 are flocculent objects, lacking organized Spiral patterns. NGC 3521 possesses also a peculiar dust lane. For NGC 4254, NGC 3627 and NGC 5055 deep polarization maps were made, for NGC 3521 the total power data only were analyzed (see Urbanik et al. 1989).
M Haverkorn - One of the best experts on this subject based on the ideXlab platform.
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the outer scale of turbulence in the magnetoionized galactic interstellar medium
The Astrophysical Journal, 2008Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M MccluregriffithsAbstract:We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of the partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's Spiral Arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in Spiral Arms are present, and show power-law behavior with a shallow spectrum. These results suggest that in the Spiral Arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and superbubble explosions are the main sources of energy on scales on the order of 100 pc.
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the outer scale of turbulence in the magneto ionized galactic interstellar medium
arXiv: Astrophysics, 2008Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M MccluregriffithsAbstract:We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's Spiral Arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in Spiral Arms are present, and show power law behavior with a shallow spectrum. These results suggest that in the Spiral Arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and super bubble explosions are the main sources of energy on scales on the order of 100 parsecs.
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rotation measures of extragalactic sources behind the southern galactic plane new insights into the large scale magnetic field of the inner milky way
The Astrophysical Journal, 2007Co-Authors: J C Brown, M Haverkorn, B M Gaensler, N M Mccluregriffiths, N S Bizunok, A R Taylor, J M DickeyAbstract:We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253° ≤ l ≤ 356°, |b| ≤ 1.5°), and use these data in combination with published data to probe the large-scale structure of the Milky Way's magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic Spiral Arms. The observed pattern in RMs requires the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius-Carina and Scutum-Crux Spiral Arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the Spiral Arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina Spiral arm (as viewed from the north Galactic pole), but is oriented counterclockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counterclockwise in the Sagittarius-Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.
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enhanced small scale faraday rotation in the galactic Spiral Arms
The Astrophysical Journal, 2006Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M Mccluregriffiths, N S Bizunok, J M Dickey, A J GreenAbstract:We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the Spiral Arms and the interarm regions. Structure functions of RM are flat in the Spiral Arms, while they increase in the interArms. This indicates that there are no correlated RM fluctuations in the magnetoionized interstellar medium in the Spiral Arms on scales larger than ~05, corresponding to ~17 pc in the nearest Spiral arm probed. The nonzero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magnetoionic medium in the Milky Way Spiral Arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.
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enhanced small scale faraday rotation in the galactic Spiral Arms
arXiv: Astrophysics, 2005Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M Mccluregriffiths, N S Bizunok, J M Dickey, A J GreenAbstract:We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the Spiral Arms and the interarm regions. Structure functions of RM are flat in the Spiral Arms, while they increase in the interArms. This indicates that there are no correlated RM fluctuations in the magneto-ionized interstellar medium in the Spiral Arms on scales larger than ~ 0.5 deg, corresponding to ~ 17 pc in the nearest Spiral arm probed. The non-zero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magneto-ionic medium in the Milky Way Spiral Arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.
Junichi Baba - One of the best experts on this subject based on the ideXlab platform.
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eventful evolution of giant molecular clouds in dynamically evolving Spiral Arms
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Junichi Baba, Kana Morokumamatsui, Takayuki R. SaitohAbstract:The formation and evolution of giant molecular clouds (GMCs) in Spiral galaxies have been investigated in the traditional framework of the combined quasi-stationary density wave and galactic shock model. However, our understanding of the dynamics of Spiral Arms is changing from the traditional Spiral model to a dynamically evolving Spiral model. In this study, we investigate the structure and evolution of GMCs in a dynamically evolving Spiral arm using a three-dimensional N-body/hydrodynamic simulation of a barred Spiral galaxy at parsec-scale resolution. This simulation incorporated self-gravity, molecular hydrogen formation, radiative cooling, heating due to interstellar far-ultraviolet radiation, and stellar feedback by both HII regions and Type-II supernovae. In contrast to a simple expectation based on the traditional Spiral model, the GMCs exhibited no systematic evolutionary sequence across the Spiral arm. Our simulation showed that the GMCs behaved as highly dynamic objects with eventful lives involving collisional build-up, collision-induced star formation, and destruction via stellar feedback. The GMC lifetimes were predicted to be short, only a few tens of millions years. We also found that, at least at the resolutions and with the feedback models used in this study, most of the GMCs without HII regions were collapsing, but half of the GMCs with HII regions were expanding owing to the HII-region feedback from stars within them. Our results support the dynamic and feedback-regulated GMC evolution scenario. Although the simulated GMCs were converging rather than virial equilibrium, they followed the observed scaling relationship well. We also analysed the effects of galactic tides and external pressure on GMC evolution and suggested that GMCs cannot be regarded as isolated systems since their evolution in disc galaxies is complicated because of these environmental effects.
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short term dynamical evolution of grand design Spirals in barred galaxies
Monthly Notices of the Royal Astronomical Society, 2015Co-Authors: Junichi BabaAbstract:We investigate the short-term dynamical evolution of stellar grand-design Spiral Arms in barred Spiral galaxies using a three-dimensional (3D) $N$-body/hydrodynamic simulation. Similar to previous numerical simulations of unbarred, multiple-arm Spirals, we find that grand-design Spiral Arms in barred galaxies are not stationary, but rather dynamic. This means that the amplitudes, pitch angles, and rotational frequencies of the Spiral Arms are not constant, but change within a few hundred million years (i.e. the typical rotational period of a galaxy). We also find that the clear grand-design Spirals in barred galaxies appear it only when the Spirals connect with the ends of the bar. Furthermore, we find that the short-term behaviour of Spiral Arms in the outer regions ($R>$ 1.5--2 bar radius) can be explained by the swing amplification theory and that the effects of the bar are not negligible in the inner regions ($R<$ 1.5--2 bar radius). These results suggest that, although grand-design Spiral Arms in barred galaxies are affected by the stellar bar, the grand-design Spiral Arms essentially originate not as bar-driven stationary density waves, but rather as self-excited dynamic patterns. We imply that a rigidly rotating grand-design Spiral could not be a reasonable dynamical model for investigating gas flows and cloud formation even in barred Spiral galaxies.
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dawes review 4 Spiral structures in disc galaxies
Publications of the Astronomical Society of Australia, 2014Co-Authors: C L Dobbs, Junichi BabaAbstract:The majority of astrophysics involves the study of Spiral galaxies, and stars and planets within them, but how Spiral Arms in galaxies form and evolve is still a fundamental problem. Major progress in this field was made primarily in the 1960s, and early 1970s, but since then there has been no comprehensive update on the state of the field. In this review, we discuss the progress in theory, and in particular numerical calculations, which unlike in the 1960s and 1970s, are now commonplace, as well as recent observational developments. We set out the current status for different scenarios for Spiral arm formation, the nature of the Spiral Arms they induce, and the consequences for gas dynamics and star formation in different types of Spiral galaxies. We argue that, with the possible exception of barred galaxies, Spiral Arms are transient, recurrent and initiated by swing amplified instabilities in the disc. We suppose that unbarred m = 2 Spiral patterns are induced by tidal interactions, and slowly wind up over time. However the mechanism for generating Spiral structure does not appear to have significant consequences for star formation in galaxies.
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dynamics of non steady Spiral Arms in disk galaxies
The Astrophysical Journal, 2013Co-Authors: Junichi Baba, Takayuki R. Saitoh, Keiichi WadaAbstract:In order to understand the physical mechanisms underlying non-steady stellar Spiral Arms in disk galaxies, we analyzed the growing and damping phases of their Spiral Arms using three-dimensional N-body simulations. We confirmed that the Spiral Arms are formed due to a swing amplification mechanism that reinforces density enhancement as a seeded wake. In the damping phase, the Coriolis force exerted on a portion of the arm surpasses the gravitational force that acts to shrink the portion. Consequently, the stars in the portion escape from the arm, and subsequently they form a new arm at a different location. The time-dependent nature of the Spiral Arms originates in the continual repetition of this nonlinear phenomenon. Since a Spiral arm does not rigidly rotate, but follows the galactic differential rotation, the stars in the arm rotate at almost the same rate as the arm. In other words, every single position in the arm can be regarded as the corotation point. Due to interaction with their host Arms, the energy and angular momentum of the stars change, thereby causing radial migration of the stars. During this process, the kinetic energy of random motion (random energy) of the stars does not significantly increase, and the disk remains dynamically cold. Owing to this low degree of disk heating, short-lived Spiral Arms can recurrently develop over many rotational periods. The resultant structure of the Spiral Arms in the N-body simulations is consistent with the observational nature of Spiral galaxies. We conclude that the formation and structure of Spiral Arms in isolated disk galaxies can be reasonably understood by nonlinear interactions between a Spiral arm and its constituent stars.
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The origin of large peculiar motions of star-forming regions and Spiral structures of our Galaxy
The Astrophysical Journal, 2009Co-Authors: Junichi Baba, Yoshiharu Asaki, Junichiro Makino, Makoto Miyoshi, Takayuki R. Saitoh, Keiichi WadaAbstract:Recent Very Long Baseline Interferometer (VLBI) observations determined the distances and proper motions of star-forming regions in Spiral Arms directly. They showed that star-forming regions and young stars have large peculiar motions as large as 30 km s–1 with complex structures. Such a large peculiar motion is incompatible with the prediction of the standard theory of quasi-stationary Spiral Arms. We use a high-resolution, self-consistent N-body+hydrodynamical simulation to explore how the Spiral Arms are formed and maintained, and how star-forming regions move. We found that Arms are not quasi-stationary but transient and recurrent, as suggested in alternative theories of Spiral structures. Because of this transient nature of the Spiral Arms, star-forming regions exhibit a trend of large and complex non-circular motions, which is qualitatively consistent with the VLBI observations. Owing to this large non-circular motion, a kinematically estimated gas map of our Galaxy has large systematic errors of ~2-3 kpc in the distance from the Sun.
B M Gaensler - One of the best experts on this subject based on the ideXlab platform.
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the outer scale of turbulence in the magnetoionized galactic interstellar medium
The Astrophysical Journal, 2008Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M MccluregriffithsAbstract:We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of the partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's Spiral Arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in Spiral Arms are present, and show power-law behavior with a shallow spectrum. These results suggest that in the Spiral Arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and superbubble explosions are the main sources of energy on scales on the order of 100 pc.
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the outer scale of turbulence in the magneto ionized galactic interstellar medium
arXiv: Astrophysics, 2008Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M MccluregriffithsAbstract:We analyze Faraday rotation and depolarization of extragalactic radio point sources in the direction of the inner Galactic plane to determine the outer scale and amplitude of the rotation measure power spectrum. Structure functions of rotation measure show lower amplitudes than expected when extrapolating electron density fluctuations to large scales assuming a Kolmogorov spectral index. This implies an outer scale of those fluctuations on the order of a parsec, much smaller than commonly assumed. Analysis of partial depolarization of point sources independently indicates a small outer scale of a Kolmogorov power spectrum. In the Galaxy's Spiral Arms, no rotation measure fluctuations on scales above a few parsecs are measured. In the interarm regions fluctuations on larger scales than in Spiral Arms are present, and show power law behavior with a shallow spectrum. These results suggest that in the Spiral Arms stellar sources such as stellar winds or protostellar outflows dominate the energy injection for the turbulent energy cascade on parsec scales, while in the interarm regions supernova and super bubble explosions are the main sources of energy on scales on the order of 100 parsecs.
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rotation measures of extragalactic sources behind the southern galactic plane new insights into the large scale magnetic field of the inner milky way
The Astrophysical Journal, 2007Co-Authors: J C Brown, M Haverkorn, B M Gaensler, N M Mccluregriffiths, N S Bizunok, A R Taylor, J M DickeyAbstract:We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253° ≤ l ≤ 356°, |b| ≤ 1.5°), and use these data in combination with published data to probe the large-scale structure of the Milky Way's magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic Spiral Arms. The observed pattern in RMs requires the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius-Carina and Scutum-Crux Spiral Arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the Spiral Arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina Spiral arm (as viewed from the north Galactic pole), but is oriented counterclockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counterclockwise in the Sagittarius-Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.
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enhanced small scale faraday rotation in the galactic Spiral Arms
The Astrophysical Journal, 2006Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M Mccluregriffiths, N S Bizunok, J M Dickey, A J GreenAbstract:We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the Spiral Arms and the interarm regions. Structure functions of RM are flat in the Spiral Arms, while they increase in the interArms. This indicates that there are no correlated RM fluctuations in the magnetoionized interstellar medium in the Spiral Arms on scales larger than ~05, corresponding to ~17 pc in the nearest Spiral arm probed. The nonzero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magnetoionic medium in the Milky Way Spiral Arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.
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enhanced small scale faraday rotation in the galactic Spiral Arms
arXiv: Astrophysics, 2005Co-Authors: M Haverkorn, J C Brown, B M Gaensler, N M Mccluregriffiths, N S Bizunok, J M Dickey, A J GreenAbstract:We present an analysis of the rotation measures (RMs) of polarized extragalactic point sources in the Southern Galactic Plane Survey. This work demonstrates that the statistics of fluctuations in RM differ for the Spiral Arms and the interarm regions. Structure functions of RM are flat in the Spiral Arms, while they increase in the interArms. This indicates that there are no correlated RM fluctuations in the magneto-ionized interstellar medium in the Spiral Arms on scales larger than ~ 0.5 deg, corresponding to ~ 17 pc in the nearest Spiral arm probed. The non-zero slopes in interarm regions imply a much larger scale of RM fluctuations. We conclude that fluctuations in the magneto-ionic medium in the Milky Way Spiral Arms are not dominated by the mainly supernova-driven turbulent cascade in the global ISM but are probably due to a different source, most likely H II regions.
Mark Cropper - One of the best experts on this subject based on the ideXlab platform.
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Spiral and bar driven peculiar velocities in milky way sized galaxy simulations
Monthly Notices of the Royal Astronomical Society, 2015Co-Authors: Robert J J Grand, Daisuke Kawata, Arnaud Siebert, Benoit Famaey, Jo Bovy, Jason A S Hunt, Giacomo Monari, Mark CropperAbstract:We investigate the kinematic signatures induced by Spiral and bar structure in a set of simulations of Milky Way-sized Spiral disc galaxies. The set includes test particle simulations that follow a quasi-stationary density wave-like scenario with rigidly rotating Spiral Arms, and N-body simulations that host a bar and transient, co-rotating Spiral Arms. From a location similar to that of the Sun, we calculate the radial, tangential and line-of-sight peculiar velocity fields of a patch of the disc and quantify the fluctuations by computing the power spectrum from a two-dimensional Fourier transform. We find that the peculiar velocity power spectrum of the simulation with a bar and transient, co-rotating Spiral Arms fits very well to that of APOGEE red clump star data, while the quasi-stationary density wave Spiral model without a bar does not. We determine that the power spectrum is sensitive to the number of Spiral Arms, Spiral arm pitch angle and position with respect to the Spiral arm. However, it is necessary to go beyond the line of sight velocity field in order to distinguish fully between the various Spiral models with this method. We compute the power spectrum for different regions of the Spiral discs, and discuss the application of this analysis technique to external galaxies.
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Spiral arm pitch angle and galactic shear rate in n body simulations of disc galaxies
Astronomy and Astrophysics, 2013Co-Authors: Robert J J Grand, Daisuke Kawata, Mark CropperAbstract:Spiral galaxies are observed to exhibit a range of morphologies, in particular in the shape of Spiral Arms. A key diagnostic parameter is the pitch angle, which describes how tightly wound the Spiral Arms are. Observationally and analytically, a correlation between pitch angle and galactic shear rate has been detected. For the first time, we examine whether this effect is detected in N-body simulations by calculating and comparing pitch angles of both individual density waves and overall Spiral structure in a suite of N-body simulations. We find that higher galactic shear rates produce more tightly wound Spiral Arms, both in individual mode patterns (density waves) and in the overall density enhancement. Although the mode pattern pitch angles by construction remain constant with time, the overall logarithmic Spiral arm winds over time, which could help to explain the scatter in the relation between pitch angle versus shear seen from observations. The correlation between Spiral arm pitch angle and galactic shear rate that we find in N-body simulations may also explain why late Hubble type of Spiral galaxies tend to have more open Arms.
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dynamics of stars around Spiral Arms in an n body sph simulated barred Spiral galaxy
Monthly Notices of the Royal Astronomical Society, 2012Co-Authors: Robert J J Grand, Daisuke Kawata, Mark CropperAbstract:We run N-body smoothed particle hydrodynamics (SPH) simulations of a Milky Way-sized galaxy. The code takes into account hydrodynamics, self-gravity, star formation, supernova and stellar wind feedback, radiative cooling and metal enrichment. The simulated galaxy is a barred Spiral galaxy consisting of a stellar and gas disc, enveloped in a static dark matter halo. Similar to what is found in our pure N-body simulation of a non-barred galaxy in Grand et al., we find that the Spiral Arms are transient features whose pattern speeds decrease with radius, in such a way that the pattern speed is similar to the rotation of star particles. Compared to the non-barred case, we find that the Spiral arm pattern speed is slightly faster than the rotation speed of star particles: the bar appears to boost the pattern speed ahead of the rotational velocity. We trace particle motion around the Spiral Arms at different radii, and demonstrate that there are star particles that are drawn towards and join the arm from behind (in front of) the arm and migrate towards the outer (inner) regions of the disc until the arm disappears as a result of their transient nature. We see this migration over the entire radial range analysed, which is a consequence of the Spiral arm rotating at similar speeds to star particles at all radii, which is inconsistent with the prediction of classical density wave theory. The bar does not prevent this systematic radial migration, which is shown to largely preserve circular orbits. We also demonstrate that there is no significant offset of different star-forming tracers across the Spiral arm, which is also inconsistent with the prediction of classical density wave theory.
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dynamics of stars around Spiral Arms in an n body sph simulated barred Spiral galaxy
Proceedings of the International Astronomical Union 5 (S261) pp. 167-180. (2009), 2009Co-Authors: Robert J J Grand, Daisuke Kawata, Mark CropperAbstract:We run N-body smoothed particle hydrodynamics (SPH) simulations of a Milky Way-sized galaxy. The code takes into account hydrodynamics, self-gravity, star formation, supernova and stellar wind feedback, radiative cooling and metal enrichment. The simulated galaxy is a barred Spiral galaxy consisting of a stellar and gas disc, enveloped in a static dark matter halo. Similar to what is found in our pure N-body simulation of a non-barred galaxy in Grand et al., we find that the Spiral Arms are transient features whose pattern speeds decrease with radius, in such a way that the pattern speed is similar to the rotation of star particles. Compared to the non-barred case, we find that the Spiral arm pattern speed is slightly faster than the rotation speed of star particles: the bar appears to boost the pattern speed ahead of the rotational velocity. We trace particle motion around the Spiral Arms at different radii, and demonstrate that there are star particles that are drawn towards and join the arm from behind (in front of) the arm and migrate towards the outer (inner) regions of the disc untilthe arm disappears as a result of their transient nature. We see this migration over the entire radial range analysed, which is a consequence of the Spiral arm rotating at similar speeds to star particles at all radii, which is inconsistent with the prediction of classical density wave theory. The bar does not prevent this systematic radial migration, which is shown to largely preserve circular orbits. We also demonstrate that there is no significant offset of different star-forming tracers across the Spiral arm, which is also inconsistent with the prediction of classical density wave theory. © 2012 The Authors.