Carl Heiles

We present a joint analysis of H I absorption Zeeman measurements and the morphology of filamentary H I emission to investigate the 3D structure of the magnetic field in the diffuse neutral interstellar medium. Our analysis is based on the Arecibo Millennium Survey and new data from the Five-hundred-meter Aperture Spherical radio Telescope toward radio sources 3C 75, 3C 207, and 3C 409. Toward 3C 409, we make a 4σ Zeeman detection and infer B _LOS = 9.1 ± 1.9 μG, in agreement with Arecibo results. We quantify the dispersion of H I filaments at the locations and velocities of Zeeman components using GALFA-H I narrow-channel emission maps. Focusing on a subsample of 42 spectrally distinct components, we find a weak but statistically significant positive correlation (Spearman ρ = 0.3, p = 0.01) between ∣B _LOS∣ and the circular variance of H I filament orientation angles. To examine its origin, we characterize the environments probed by H I absorption using dust emission, 3D dust maps, OH absorption, and CO emission. We find evidence that existing H I absorption Zeeman measurements trace magnetic fields that are coherent on parsec scales, probe primarily local gas (100–500 pc, often at distances consistent with the Local Bubble wall), and exhibit systematic differences in the magnitude of B _LOS. We attribute the correlation between Zeeman measurements and filamentary H I morphology to large-scale variations in magnetic field strength and/or inclination angle across different Galactic environments, which could arise due to the Local Bubble geometry or enhanced total field strength in denser regions.

The young supernova remnant Cassiopeia A is currently interacting with the circumstellar medium expelled by its progenitor star. We analyzed the proper motion of dense circumstellar clumps, known as quasi-stationary flocculi (QSFs), embedded within the remnant using archived images from the Hubble Space Telescope (HST). Our study focused on approximately 40 QSFs that are bright in optical bands. We determined the proper motions within each QSF by cross-correlating the HST images taken 15 yr apart, and presented the results as vector maps that show both the proper-motion magnitudes and directions. For most QSFs, the proper-motion vectors are well aligned, but some show significant variations in expansion direction within the QSF. We derived the mean proper motion of each QSF and investigated its properties. The mean proper-motion vectors of the QSFs predominantly point radially away from the explosion center of Cas A, though not perfectly aligned with a strict radial direction, with corresponding velocities typically in the range of a few hundred kilometers per second. We found no clear correlation between the proper motion and the projected distance from the explosion center, nor between the proper motion and the line-of-sight velocity of the QSFs. This result indicates that the apparent expansion of QSFs is primarily driven by interactions with the supernova blast wave, rather than by their initial ejection. We present an exploratory model of the three-dimensional distribution of QSFs.

We present the polarization calibration of the 19-beam receiver at 1420 MHz within the full illumination of the Five-hundred-meter Aperture Spherical radio Telescope from 2018 October to 2023 March. We perform spider observations to characterize the on-axis Mueller matrix of the central beam. The calibrated polarization percentage and polarization angle of a source with strong linear polarization emission are about 0.2% and 0 . ° 5, respectively. Several parameters of the central-beam Mueller matrix show time variability from months to years, suggesting that relatively frequent polarization calibrations are needed. We obtain the Mueller matrix parameters of the 18 off-center beams with a combination of on-the-fly observations and spider observations. The polarization calibration provides consistent fractional Stokes parameters of the 19 beams, although the Mueller matrix parameters of the off-center beams are not as accurate as those of the central beam. The Mueller matrix parameters of the central beam do not show a strong dependence on the reflector surface. However, we notice different off-center Mueller matrix parameters between the eastern and western sides of the reflector surface. We provide average parameters of the 19-beam Mueller matrices that should be applicable to observations from 2020 to 2022, with several caveats. After applying the average parameters, on-axis fractional linear polarization measurements ≳10% and on-axis fractional circular polarization measurements ≳1.5% can be considered high-confidence detections. For sources with weak polarization, timely polarization calibrations using spider observations are required.

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) has been fully operational since 2020 January 11. We present a comprehensive analysis of the beam structure for each of the 19 feed horns on FAST's L-band receiver across the Stokes I, Q, U, and V parameters. Using an on-the-fly mapping pattern, we conducted simultaneous sky mapping using all 19 beams directed toward polarization calibrators J1407+2827 and J0854+2006 from 2020 to 2022. Electromagnetic simulations were also performed to model the telescope's beam patterns in all Stokes parameters. Our findings reveal a symmetrical Gaussian pattern in the Stokes I parameter of the central beam without strong sidelobes, while the off-center beams exhibit significant asymmetrical shapes that can be fitted using a combination of log-normal and Gaussian distributions. The inner beams have higher relative beam efficiencies and smaller beam sizes compared to those of the outer beams. The sidelobes of the inner beams contribute approximately 2% of the total flux in the main lobe, increasing to 5% for outer beams, with a peak at 6.8%. In Stokes U, a distinct four-lobed cloverleaf beam squash structure is observed, with similar intensity levels in both inner and outer beams. In Stokes V, a two-lobed beam squint structure is observed in the central beam, along with a secondary eight-lobed structure. The highest squint peak in Stokes V is about 0.3% of the Stokes I in the outer beams. These results align closely with the simulations, providing valuable insights for the design of radio multibeam observations.

We investigate the conditions for the H i-to-H₂ transition in the solar neighborhood by analyzing H i emission and absorption measurements toward 58 Galactic lines of sight (LOSs) along with ¹²CO(1–0) (CO) and dust data. Based on the accurate column densities of the cold and warm neutral medium (CNM and WNM), we first perform a decomposition of gas into atomic and molecular phases, and show that the observed LOSs are mostly H i-dominated. In addition, we find that the CO-dark H₂, not the optically thick H i, is a major ingredient of the dark gas in the solar neighborhood. To examine the conditions for the formation of CO-bright molecular gas, we analyze the kinematic association between H i and CO, and find that the CNM is kinematically more closely associated with CO than the WNM. When CNM components within CO line widths are isolated, we find the following characteristics: spin temperature < 200 K, peak optical depth > 0.1, CNM fraction of ∼0.6, and V-band dust extinction > 0.5 mag. These results suggest that CO-bright molecular gas preferentially forms in environments with high column densities where the CNM becomes colder and more abundant. Finally, we confront the observed CNM properties with the steady-state H₂ formation model of Sternberg et al. and infer that the CNM must be clumpy with a small volume filling factor. Another possibility would be that missing processes in the model, such as cosmic-rays and gas dynamics, play an important role in the H i-to-H₂ transition.

Measurements of starlight polarized by aligned interstellar dust grains are used to probe the relation between the orientation of the ambient interstellar magnetic field (ISMF) and the ISMF traced by the ribbons of energetic neutral atoms discovered by the Interstellar Boundary Explorer spacecraft. We utilize polarization data, many acquired specifically for this study, to trace the configuration of the ISMF within 40 pc. A statistical analysis yields a best-fit ISMF orientation, B _magpol, aligned with Galactic coordinates ℓ = 42°, b = 49°. Further analysis shows the ISMF is more orderly for “downfield” stars located over 90° from B _magpol. The data subset of downfield stars yields an orientation for the nearby ISMF at ecliptic coordinates λ, β ≈ 219° ± 15°, 43° ± 9° (Galactic coordinates l, b ≈ 40°, 56°, ±17°). This best-fit ISMF orientation from polarization data is close to the field direction obtained from ribbon models. This agreement suggests that the ISMF shaping the heliosphere belongs to an extended ordered magnetic field. Extended filamentary structures are found throughout the sky. A previously discovered filament traversing the heliosphere nose region, “Filament A,” extends over 300° of the sky, and crosses the upwind direction of interstellar dust flowing into the heliosphere. Filament A overlaps the locations of the Voyager kilohertz emissions, three quasar intraday variables, cosmic microwave background (CMB) components, and the inflow direction of interstellar grains sampled by Ulysses and Galileo. These features are likely located in the upstream outer heliosheath where ISMF drapes over the heliosphere, suggesting Filament A coincides with a dusty magnetized plasma. A filament 55° long is aligned with a possible shock interface between local interstellar clouds. A dark spot in the CMB is seen within 5° of the filament and within 10° of the downfield ISMF direction. Two large magnetic arcs are centered on the directions of the heliotail. The overlap between CMB components and the aligned dust grains forming Filament A indicates the configuration of dust entrained in the ISMF interacting with the heliosphere provides a measurable foreground to the CMB.

We present high-sensitivity CH 9 cm ON/OFF observations toward 18 extragalactic continuum sources that have been detected with OH 18 cm absorption in the Millennium survey with the Arecibo telescope. CH emission was detected toward 6 of the 18 sources. The excitation temperature of CH has been derived directly through analyzing all detected ON and OFF velocity components. The excitation temperature of CH 3335 MHz transition ranges from −54.5 to −0.4 K and roughly follows a log-normal distribution peaking within [−5, 0] K, which implies overestimation by 20% to more than 10 times during calculating CH column density by assuming the conventional value of −60 or −10 K. Furthermore, the column density of CH would be underestimated by a factor of 1.32 ± 0.03 when adopting local thermal equilibrium assumption instead of using the CH three hyperfine transitions. We found a correlation between the column density of CH and OH following log N(CH) = (1.80 ± 0.49) log N(OH) −11.59 ± 6.87. The linear correlation between the column density of CH and H₂ is consistent with that derived from visible wavelengths studies, confirming that CH is one of the best tracers of H₂ components in diffuse molecular gas.

Tracing the transition between the diffuse atomic interstellar medium and cold, dense gas is crucial for deciphering the star formation cycle in galaxies. Here we present Measuring Absorption by Cold Hydrogen (MACH), a new survey of cold neutral hydrogen (HI) absorption at 21 cm by the Karl G. Jansky Very Large Array. We target 42 bright background sources with 60 < l < 110°, 30 < b < 62°, significantly expanding the sample of publicly available, sensitive 21 cm absorption outside the Galactic plane. With matching 21 cm emission data from the Effelsberg-Bonn Hi (EBHiS) Survey, we measure the total column density and cold Hi fraction, and quantify the properties of individual Hi structures along each sightline via autonomous Gaussian decomposition. Combining the MACH sample with results from recent Hi absorption surveys, we produce a robust characterization of the cool atomic medium at high and intermediate Galactic latitudes. We find that MACH Hi has significantly smaller column density relative to samples at similar latitudes, and the detected cold Hi structures have smaller line widths, temperatures, and turbulent Mach numbers, suggesting that MACH probes a particularly quiescent region. Using all available observations, we compute the cumulative covering fraction (c) of cold Hi at local velocities outside the disk: structures with τ > 0.001 are ubiquitous (c ∼ 100%), whereas high optical depths (τ > 1) are extremely rare (c ∼ 0%).

Faraday rotation (FR) of MESSENGER spacecraft X-band transcoronal radio transmissions were studied to evaluate the mid-coronal magnetic field strength, with the line-of-sight closest solar approach at heliocentric altitudes ranging 1.647–1.817 R _⊙. A Community Coordinated Modeling Center (CCMC) CORHEL-MAS 3D coronal field map revealed this region to contain closed magnetic fields. By FR analysis we found a field strength of 118,000 nT at r = 1.647 R _⊙, with fall-off to 40,000 nT by r = 1.817 R _⊙. These values straddle estimates provided by the CCMC model. The mean value of 79,000 nT at r = 1.732 R _⊙ is comparable to the value provided by an established empirical model for average coronal magnetic field strength. FR can be used to evaluate middle coronal magnetic fields, but improved methods to constrain concurrent electron column density will be needed to produce the most accurate results.

Optical depth variations in the Galactic neutral interstellar medium (ISM) with spatial scales from hundreds to thousands of astronomical units have been observed through H i absorption against pulsars and continuum sources, while extremely small structures with spatial scales of tens of astronomical units remain largely unexplored. The nature and formation of such tiny-scale atomic structures (TSAS) need to be better understood. We report a tentative detection of TSAS with a signal-to-noise ratio of 3.2 toward PSR B1557−50 in the second epoch of two Parkes sessions just 0.36 yr apart, which are the closest-spaced spectral observations toward this pulsar. One absorption component showing marginal variations has been identified. Based on the pulsar’s proper motion of 14 mas yr⁻¹ and the component’s kinematic distance of 3.3 kpc, the corresponding characteristic spatial scale is 17 au, which is among the smallest sizes of known TSAS. Assuming a similar line-of-sight (LOS) depth, the tentative TSAS cloud detected here is overdense and overpressured relative to the cold neutral medium (CNM), and can radiatively cool fast enough to be in thermal equilibrium with the ambient environment. We find that turbulence is not sufficient to confine the overpressured TSAS. We explore the LOS elongation that would be required for the tentative TSAS to be at the canonical CNM pressure, and find that it is ∼5000—much larger than filaments observed in the ISM. We see some evidence of line width and temperature variations in the CNM components observed at the two epochs, as predicted by models of TSAS-like cloud formation colliding warm neutral medium flows.