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Register a new userMagnetic fields and the outer rotation curve of M31
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Beatriz Ruiz-Granados
Publication date
2010
fields
Physics
and research's language is
English
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Recent observations of the rotation curve of M31 show a rise of the outer part that can not be understood in terms of standard dark matter models or perturbations of the galactic disc by M31s satellites. Here, we propose an explanation of this dynamical feature based on the influence of the magnetic field within the thin disc. We have considered standard mass models for the luminous mass distribution, a NFW model to describe the dark halo, and we have added up the contribution to the rotation curve of a magnetic field in the disc, which is described by an axisymmetric pattern. Our conclusion is that a significant improvement of the fit in the outer part is obtained when magnetic effects are considered. The best-fit solution requires an amplitude of ~4 microG with a weak radial dependence between 10 and 38 kpc.
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We present Keck/HIRES spectra of 3 globular clusters in the outer halo of M31, at projected distances beyond ~80 kpc from M31. The measured recession velocities for all 3 globular clusters confirm their association with the globular cluster system of M31. We find evidence for a declining velocity dispersion with radius for the globular cluster system. Their measured internal velocity dispersions, derived virial masses and mass-to-light ratios are consistent with those for the bulk of the M31 globular cluster system. We derive old ages and metallicities which indicate that all 3 belong to the metal-poor halo globular cluster subpopulation. We find indications that the radial gradient of the mean metallicity of the globular cluster system interior to 50 kpc flattens in the outer regions, however it is still more metal-poor than the corresponding field stars at the same (projected) radius.
We present a measurement of the trigonometric parallax of IRAS 05168+3634 with VERA. The parallax is 0.532 +/- 0.053 mas, corresponding to a distance of 1.88 +0.21/-0.17 kpc. This is significantly closer than the previous distance estimate of 6 kpc based on a kinematic distance measurement. This drastic change in the source distance implies the need for revised values of not only the physical parameters of IRAS 05168+3634, but it also implies a different location in the Galaxy, placing it in the Perseus arm rather than the Outer arm. We also measured the proper motion of the source. A combination of the distance and proper motion with the systemic velocity yields a rotation velocity {Theta} = 227 +9/-11 km s^-1 at the source position, assuming {Theta}_0 = 240 km s^-1. Our result, combined with previous VLBI results for six sources in the Perseus arm, indicates that the sources rotate systematically more slowly than the Galactic rotation velocity at the local standard of rest. In fact, we derive peculiar motions in the disk averaged over the seven sources in the Perseus arm of (U_mean, V_mean) = (11 +/- 3, -17 +/- 3) km s^-1, which indicates that these seven sources are moving systematically toward the Galactic Center and lag behind the overall Galactic rotation.
We investigate the possibility of measuring intergalactic magnetic fields using the dispersion measures and rotation measures of fast radio bursts. With Bayesian methods, we produce probability density functions for values of these measures. We distinguish between contributions from the intergalactic medium, the host galaxy and the local environment of the progenitor. To this end, we use constrained, magnetohydrodynamic simulations of the local Universe to compute lines-of-sight integrals from the position of the Milky Way. In particular, we differentiate between predominantly astrophysical and primordial origins of magnetic fields in the intergalactic medium. We test different possible types of host galaxies and probe different distribution functions of fast radio burst progenitor locations inside the host galaxy. Under the assumption that fast radio bursts are produced by magnetars, we use analytic predictions to account for the contribution of the local environment. We find that less than 100 fast radio bursts from magnetars in stellar-wind environments hosted by starburst dwarf galaxies at redshift $z gtrsim 0.5$ suffice to discriminate between predominantly primordial and astrophysical origins of intergalactic magnetic fields. However, this requires the contribution of the Milky Way to be removed with a precision of $approx 1 rm~rad~m^{-2}$. We show the potential existence of a subset of fast radio bursts whose rotation measure carry information on the strength of the intergalactic magnetic field and its origins.
We report measurement of trigonometric parallax of IRAS 05168+3634 with VERA. The parallax is 0.532 +/- 0.053 mas, corresponding to a distance of 1.88+0.21/-0.17 kpc. This result is significantly smaller than the previous distance estimate of 6 kpc based on kinematic distance. This drastic change in the source distance revises not only physical parameters of IRAS 05168+3634, but also its location of the source, placing it in the Perseus arm rather than the Outer arm. We also measure proper motions of the source. A combination of the distance and the proper motions with systemic velocity yields rotation velocity ({Theta}) of 227+9/-11 km s-1 at the source, assuming {Theta}0 = 240 km s-1. Our result combined with previous VLBI results for six sources in the Perseus arm indicates that the sources rotate systematically slower than the Galactic rotation velocity at the LSR. In fact, we show observed disk peculiar motions averaged over the seven sources in the Perseus arm as (Umean, Vmean) = (11 +/- 3, -17 +/- 3) km s-1, indicating that these seven sources are systematically moving toward the Galactic center, and lag behind the Galactic rotation.
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