Keywords

Recent hydrodynamical models of supernova remnants (SNRs) demonstrate that their evolution depends heavily on the inhomogeneities of the surrounding medium. As SNRs expand, their morphologies are influenced by the nonuniform and turbulent structure of their environments, as reflected in their radio continuum emission. In this paper, we measure the asymmetries of 96 SNRs in radio continuum images from three surveys of the Galactic plane and compare these results to the SNRs' radii, which we use as a proxy for their age. We find that larger (older) SNRs are more elliptical/elongated and more mirror asymmetric than smaller (younger) SNRs, though the latter vary in their degrees of asymmetry. This result suggests that SNR shells become more asymmetric as they sweep up the interstellar medium (ISM), as predicted in hydrodynamical models of SNRs expanding in a multiphase or turbulent ISM.

Although the ${}^{2}{{\rm{P}}}_{3/2}-{}^{2}{{\rm{P}}}_{1/2}$ transition of [C ii] at λ ≃  158 $\,\mu {\rm{m}}$ is known to be an excellent tracer of active star formation, we still do not have a complete understanding of where within star formation regions the emission originates. Here, we use SOFIA upGREAT observations of [C ii] emission toward the H ii region complex Sh2-235 (S235) to better understand in detail the origin of [C ii] emission. We complement these data with a fully sampled Green Bank Telescope radio recombination line map tracing the ionized hydrogen gas. About half of the total [C ii] emission associated with S235 is spatially coincident with ionized hydrogen gas, although spectroscopic analysis shows little evidence that this emission is coming from the ionized hydrogen volume. Velocity-integrated [C ii] intensity is strongly correlated with Wide-field Infrared Survey Explorer (WISE) 12 $\,\mu {\rm{m}}$ intensity across the entire complex, indicating that both trace ultraviolet radiation fields. The 22 $\,\mu {\rm{m}}$ and radio continuum intensities are only correlated with [C ii] intensity in the ionized hydrogen portion of the S235 region and the correlations between the [C ii] and molecular gas tracers are poor across the region. We find similar results for emission averaged over a sample of external galaxies, although the strength of the correlations is weaker. Therefore, although many tracers are correlated with the strength of [C ii] emission, only WISE 12 $\,\mu {\rm{m}}$ emission is correlated on small scales of the individual H ii region S235 and also has a decent correlation at the scale of entire range of galaxies. Future studies of a larger sample of Galactic H ii regions would help to determine whether these results are truly representative.

Much data on the Galactic polarized radio emission has been gathered in the last five decades. All-sky surveys have been made, but only in narrow, widely spaced frequency bands—and the data are inadequate for the characterization of Faraday rotation, the main determinant of the appearance of the polarized radio sky at decimeter wavelengths. We describe a survey of polarized radio emission from the southern sky, aiming to characterize the magneto-ionic medium, particularly the strength and configuration of the magnetic field. This work is part of the Global Magneto-Ionic Medium Survey (GMIMS). We have designed and built a feed and receiver covering the band 300–900 MHz for the CSIRO Parkes 64 m Telescope. We have surveyed the entire sky between decl. −90° and +20°. We present data covering 300–480 MHz with angular resolution 81'–45'. The survey intensity scale is absolutely calibrated, based on measurements of resistors at known temperatures and on an assumed flux density and spectral index for Taurus A. Data are presented as brightness temperatures. We have applied Rotation Measure Synthesis to the data to obtain a Faraday depth cube of resolution 5.9 rad m⁻², sensitivity of 60 mK of polarized intensity, and angular resolution 135. The data presented in this paper are available at the Canadian Astronomy Data Centre.

We report observations, made with the Atacama Large Millimeter/submillimeter Array, of 3 mm dust continuum emission and molecular line emission in HCO⁺, CS, and N₂H⁺, toward G305.137+0.069, a massive, dense, and cold clump, in order to characterize its small-scale (∼2000 au) structure. The dust continuum observations reveal the presence of twelve compact structures (cores) with masses ranging from 3.3 to 50.6 M_⊙, radii from 1800 to 5300 au, and densities from 3.1 × 10⁶ to 3.1 × 10⁷ cm⁻³. The line observations show that the molecular emission arises from a bright central region, with an angular size of ∼12'', and an extended, weaker envelope best seen in the HCO⁺ line. The N₂H⁺ emission is best correlated with the continuum emission and hence with the cores. For cores with single line profiles, the line widths range from 1.9 to 3.1 km s⁻¹ with an average value of 2.6 km s⁻¹, indicating that they are dominated by nonthermal motions, either due to random turbulence or core-scale motions. The virial parameter of the three most massive cores are smaller than one, suggesting that they are undergoing collapse. We find that in the regime of masses probed by our observations (M > 3 M_⊙) the shape of the core-mass function is notably different from the initial mass function, showing an overpopulation of high-mass cores. We suggest that the formation and mass distribution of the dense cores within G305.137+0.069 can be explained as the result of hierarchical or turbulent fragmentation in a gravitationally collapsing clump.

We report on the first bird's-eye view of the innermost accretion disk around the high-mass protostellar object G353.273+0.641, taken by Atacama Large Millimeter/submillimeter Array long baselines. The disk traced by dust continuum emission has a radius of 250 au, surrounded by the infalling rotating envelope traced by thermal CH₃OH lines. This disk radius is consistent with the centrifugal radius estimated from the specific angular momentum in the envelope. The lower-limit envelope mass is ∼5–7 M_☉ and accretion rate onto the stellar surface is 3 × 10⁻³ M_☉ yr⁻¹ or higher. The expected stellar age is well younger than 10⁴ yr, indicating that the host object is one of the youngest high-mass objects at present. The disk mass is 2–7 M_☉, depending on the dust opacity index. The estimated Toomre's Q parameter is typically 1–2 and can reach 0.4 at the minimum. These Q values clearly satisfy the classical criteria for gravitational instability, and are consistent with recent numerical studies. Observed asymmetric and clumpy structures could trace a spiral arm and/or disk fragmentation. We found that 70% of the angular momentum in the accretion flow could be removed via the gravitational torque in the disk. Our study has indicated that the dynamical nature of a self-gravitating disk could dominate the early phase of high-mass star formation. This is remarkably consistent with the early evolutionary scenario of a low-mass protostar.

We present the 850 μm polarization observations toward the IC 5146 filamentary cloud taken using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and its associated polarimeter (POL-2), mounted on the James Clerk Maxwell Telescope, as part of the B-fields In STar forming Regions Observations. This work is aimed at revealing the magnetic field morphology within a core-scale (≲1.0 pc) hub-filament structure (HFS) located at the end of a parsec-scale filament. To investigate whether the observed polarization traces the magnetic field in the HFS, we analyze the dependence between the observed polarization fraction and total intensity using a Bayesian approach with the polarization fraction described by the Rice likelihood function, which can correctly describe the probability density function of the observed polarization fraction for low signal-to-noise ratio data. We find a power-law dependence between the polarization fraction and total intensity with an index of 0.56 in A_V ∼ 20–300 mag regions, suggesting that the dust grains in these dense regions can still be aligned with magnetic fields in the IC 5146 regions. Our polarization maps reveal a curved magnetic field, possibly dragged by the contraction along the parsec-scale filament. We further obtain a magnetic field strength of 0.5 ± 0.2 mG toward the central hub using the Davis–Chandrasekhar–Fermi method, corresponding to a mass-to-flux criticality of ∼1.3 ± 0.4 and an Alfvénic Mach number of <0.6. These results suggest that gravity and magnetic field are currently of comparable importance in the HFS and that turbulence is less important.

We have performed new 1.4 and 5 GHz observations of the Local Group galaxy M33 with the Jansky Very Large Array. Our survey has a limiting sensitivity of 20 μJy (4σ) and a resolution of 59 (FWHM), corresponding to a spatial resolution of 24 pc at 817 kpc. Using a new multiresolution algorithm, we have created a catalog of 2875 sources, including 675 with well-determined spectral indices. We detect sources at the position of 319 of the X-ray sources in the Tüllmann et al. Chandra survey of M33, the majority of which are likely to be background galaxies. The radio source coincident with M33 X-8, the nuclear source, appears to be extended. Along with numerous H ii regions or portions of H ii region complexes, we detect 155 of the 217 optical supernova remnants (SNRs) included in the lists of Long et al. and Lee & Lee, making this by far the largest sample of remnants at known distances with multiwavelength coverage. The remnants show a large dispersion in the ratio of radio to X-ray luminosity at a given diameter, a result that challenges the current generation of models for synchrotron radiation evolution in SNRs.

We present a search for late-time rebrightening of radio emission from three supernovae (SNe) with associated gamma-ray bursts (GRBs). It has been previously proposed that the unusually energetic SNe associated with GRBs should enter the Sedov–Taylor phase decades after the stellar explosion, and this SN "remnant" emission will outshine the GRB radio afterglow and be detectable at significant distances. We place deep limits on the radio luminosity of GRB 980425/SN 1998bw, GRB 030329/SN 2003dh, and GRB 060218/SN 2006aj, 10–18 yr after explosion, with our deepest limit being L_ν < 4 × 10²⁶ erg s⁻¹ Hz⁻¹ for GRB 980425/SN 1998bw. We put constraints on the density of the surrounding medium for various assumed values of the microphysical parameters related to the magnetic field and synchrotron-emitting electrons. For GRB 060218/SN 2006aj and GRB 980425/SN 1998bw, these density limits have implications for the density profile of the surrounding medium, while the nondetection of GRB 030329/SN 2003dh implies that its afterglow will not be detectable anymore at gigahertz frequencies.

The Event Horizon Telescope, a global 230 GHz very-long-baseline interferometry array, achieves angular resolution of $\approx 20\,\mu \mathrm{as}$, sufficient to resolve the supermassive black hole Sagittarius A* (Sgr A*). This resolution may soon enable measurements of the black hole "shadow" size and asymmetry, predicted to be ≈50 and ≲3 μas, respectively. Measurements that depart from these values could indicate a violation of the "no-hair theorem." However, refractive scattering by the turbulent ionized interstellar medium distorts the image of Sgr A*, affecting its apparent size and asymmetry. In this paper, we present a general analytic approach to quantify the expected image wander, distortion, and asymmetry from refractive scattering. If the turbulence in the scattering material of Sgr A* is close to Kolmogorov, we estimate the mean refractive image wander, distortion, and asymmetry to be 0.53, 0.72, and 0.52 μas at 230 GHz. However, alternative scattering models with flatter power spectra can yield larger values, up to 2.1, 6.3, and 5.0 μas, respectively. We demonstrate that these effects can be reduced by averaging images over multiple observations. For a small number of observations, the effects of scattering can be comparable to or greater than those from black hole spin, and they determine a fundamental limit for testing general relativity via images of Sgr A*.

We present a multiwavelength analysis based on archival radio, optical, and X-ray data of the complex radio source 3C 196.1, whose host is the brightest cluster galaxy of a z = 0.198 cluster. HST data show Hα+[N ii] emission aligned with the jet 8.4 GHz radio emission. An Hα+[N ii] filament coincides with the brightest X-ray emission, the northern hotspot. Analysis of the X-ray and radio images reveals cavities located at galactic and cluster scales. The galactic-scale cavity is almost devoid of 8.4 GHz radio emission and the southwestern Hα+[N ii] emission is bounded (in projection) by this cavity. The outer cavity is cospatial with the peak of 147 MHz radio emission, and hence we interpret this depression in X-ray surface brightness as being caused by a buoyantly rising bubble originating from an active galactic nuclei outburst ∼280 Myr ago. A Chandra snapshot observation allowed us to constrain the physical parameters of the cluster, which has a cool core with a low central temperature ∼2.8 keV, low central entropy index ∼13 keV cm² and a short cooling time of ∼500 Myr, which is < 0.05 of the age of the universe at this redshift. By fitting jumps in the X-ray density, we found Mach numbers between 1.4 and 1.6, consistent with a shock origin. We also found compelling evidence of a past merger, indicated by a morphology reminiscent of gas sloshing in the X-ray residual image. Finally, we computed the pressures, enthalpies E_cav and jet powers P_jet associated with the cavities: E_cav ∼ 7 × 10⁵⁸ erg, P_jet ∼ 1.9 × 10⁴⁴ erg s⁻¹ for the inner cavity and E_cav ∼ 3 × 10⁶⁰ erg, P_jet ∼ 3.4 × 10⁴⁴ erg s⁻¹ for the outer cavity.