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Register a new userMagnetic Fields in Large Diameter HII Regions Revealed by the Faraday Rotation of Compact Extragalactic Radio Sources
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We present a study of the line-of-sight magnetic fields in five large-diameter Galactic HII regions. Using the Faraday rotation of background polarized radio sources, as well as dust-corrected H-alpha surface brightness as a probe of electron density, we estimated the strength and orientation of the magnetic field along 93 individual sight-lines through the HII regions. Each of the HII regions displayed a coherent magnetic field. The magnetic field strength (line-of-sight component) in the regions ranges from 2 to 6 microgauss, which is similar to the typical magnetic field strength in the diffuse interstellar medium. We investigated the relationship between magnetic field strength and electron density in the 5 HII regions. The slope of magnetic field vs. density in the low-density regime (0.8 < n_e < 30 per cubic cm) is very slightly above zero. We also calculated the ratio of thermal to magnetic pressure, beta_th, for each data point, which fell in the range 1.01 < beta_th < 25. Finally, we studied the orientation of the magnetic field in the solar neighborhood (d < 1.1 kpc) using our data from 5 HII regions along with existing measurements of the line-of-sight magnetic field strength from polarized pulsars whose distances have been determined from their annual parallax. We identify a net direction for the magnetic field in the solar neighborhood, but find no evidence for a preferred vertical direction of the magnetic field above or below the Galactic plane.
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Faraday rotation measures (RMs) of extragalactic radio sources provide information on line-of-sight magnetic fields, including contributions from our Galaxy, source environments, and the intergalactic medium (IGM). Looking at differences in RMs, $Delta$RM, between adjacent sources on the sky can help isolate these different components. In this work, we classify adjacent polarized sources in the NVSS as random or physical pairs. We recompute and correct the uncertainties in the NVSS RM catalog, since these were significantly overestimated. Our sample contains 317 physical and 5111 random pairs, all with Galactic latitudes $|b|ge20^{circ}$, polarization fractions $ge2%$, and angular separations between $1.^{},$ and $20^{}$. We find an rms $Delta$RM of $14.9pm0.4,$rad m$^{-2}$ and $4.6pm1.1,$rad m$^{-2}$ for random and physical pairs, respectively. This means polarized extragalactic sources that are close on the sky, but at different redshifts, have larger differences in RM than two components of one source. This difference of $sim10,$rad m$^{-2}$ is significant at $5sigma$, and persists in different data subsamples. While there have been other statistical studies of $Delta$RM between adjacent polarized sources, this is the first unambiguous demonstration that some of this RM difference must be extragalactic, thereby providing a firm upper limit on the RM contribution of the IGM. If the $Delta$RMs originate local to the sources, then the local magnetic field difference between random sources is a factor of two larger than between components of one source. Alternatively, attributing the difference in $Delta$RMs to the intervening IGM yields an upper limit on the IGM magnetic field strength of $40,$nG.
We present a polarization catalog of 533 extragalactic radio sources with 2.3 GHz total intensity above 420 mJy from the S-band Polarization All Sky Survey, S-PASS, with corresponding 1.4 GHz polarization information from the NRAO VLA Sky Survey, NVSS. We studied selection effects and found that fractional polarization, $pi$, of radio objects at both wavelengths depends on the spectral index, source magnetic field disorder, source size and depolarization. The relationship between depolarization, spectrum and size shows that depolarization occurs primarily in the source vicinity. The median $pi_{2.3}$ of resolved objects in NVSS is approximately two times larger than that of unresolved sources. Sources with little depolarization are $sim2$ times more polarized than both highly depolarized and re-polarized sources. This indicates that intrinsic magnetic field disorder is the dominant mechanism responsible for the observed low fractional polarization of radio sources at high frequencies. We predict that number counts from polarization surveys will be similar at 1.4 GHz and at 2.3 GHz, for fixed sensitivity, although $sim$10% of all sources may be currently missing because of strong depolarization. Objects with $pi_{1.4}approx pi_{2.3} ge 4%$ typically have simple Faraday structures, so are most useful for background samples. Almost half of flat spectrum ($alpha ge -0.5$) and $sim$25% of steep spectrum objects are re-polarized. Steep spectrum, depolarized sources show a weak negative correlation of depolarization with redshift in the range 0 $<$ z $<$ 2.3. Previous non-detections of redshift evolution are likely due the inclusion of re-polarized sources as well.
We report Faraday rotation measurements of 11 extragalactic radio sources with lines of sight through the Rosette Nebula, a prominent HII region associated with the star cluster NGC 2244. It is also a prototypical example of a stellar bubble produced by the winds of the stars in NGC 2244. The goal of these measurements is to better determine the strength and structure of the magnetic field in the nebula. We calculate the rotation measure (RM) through two methods, a least-squares fit to $chi$( $lambda^2$) and Rotation Measure Synthesis. In conjunction with our results from Savage et al. (2013), we find an excess RM due to the shell of the nebula of +40 to +1200 rad m$^{-2}$ above a background RM of +147 rad m$^{-2}$. We discuss two forms of a simple shell model intended to reproduce the magnitude of the observed RM as a function of distance from the center of the Rosette Nebula. The models represent different physical situations for the magnetic field within the shell of the nebula. The first assumes that there is an increase in the magnetic field strength and plasma density at the outer radius of the HII region, such as would be produced by a strong magnetohydrodynamic shock wave. The second model assumes that any increase in the RM is due solely to an increase in the density, and the Galactic magnetic field is unaffected in the shell. We employ a Bayesian analysis to distinguish between the two forms of the model.
We report on the detection of optically thick free-free radio sources in the galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret these sources as being young, embedded star birth regions, which are likely to be clusters of ultracompact HII regions. All 35 of the sources presented in this article have positive radio spectral indices alpha>0 suggesting an optically thick thermal bremsstrahlung emission arising in the HII region surrounding hot stars. Energy requirements indicate a range of a several to >500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF, this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly half of the sources in our sample, there is no obvious optical counterpart, giving further support for their deeply embedded nature. Their luminosities and radio spectral energy distributions are consistent with HII regions having electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We suggest that the less luminous of these sources are extragalactic ultracompact HII region complexes, those of intermediate luminosity are similar to W49 in the Galaxy, while the brightest will be counterparts to 30 Doradus. These objects constitute the lower mass range of extragalactic ``ultradense HII regions which we argue are the youngest stages of massive star cluster formation yet observed. This sample is beginning to fill in the continuum of objects between small associations of ultracompact HII regions and the massive extragalactic clusters that may evolve into globular clusters.
We present a catalog of Faraday rotation measures (RMs) and redshifts for 4003 extragalactic radio sources detected at 1.4 GHz, derived by identifying optical counterparts and spectroscopic redshifts for linearly polarized radio sources from the NRAO VLA Sky Survey. This catalog is more than an order of magnitude larger than any previous sample of RM vs. redshift, and covers the redshift range 0 < z < 5.3 ; the median redshift of the catalog is z = 0.70, and there are more than 1500 sources at redshifts z > 1. For 3650 of these sources at Galactic latitudes |b| >= 20 degrees, we present a second catalog in which we have corrected for the foreground Faraday rotation of the Milky Way, resulting in an estimate of the residual rotation measure (RRM) that aims to isolate the contribution from extragalactic magnetic fields. We find no significant evolution of RRM with redshift, but observe a strong anti-correlation between RRM and fractional polarization, p, that we argue is the result of beam depolarization from small-scale fluctuations in the foreground magnetic field or electron density. We suggest that the observed variance in RRM and the anti-correlation of RRM with p both require a population of magnetized intervening objects that lie outside the Milky Way but in the foreground to the emitting sources.
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