Chao-Wei Tsai

Context . Face-on spiral galaxies offer a favorable geometry for studying magnetic-field structures and their impact on cosmic-ray propagation, as projection effects and structural overlap are significantly reduced. Aims . We aim to investigate the transport of cosmic-ray electrons (CREs) in the nearby face-on spiral galaxy NGC 2442 and assess how the galactic environment and magnetic-field structure influence their propagation. Methods . We combined radio continuum (RC) observations from ASKAP at 943 MHz, MeerKAT at 1.28 and 1.7 GHz, and ATCA at 5 GHz with optical H α and infrared data to characterize the galactic environment. We then compared these observations with 2D CRE transport simulations to evaluate the role of the magnetic field and the surrounding environment in shaping CRE propagation. Results . NGC 2442 exhibits an integrated RC spectrum that is steeper than those of most nearby galaxies, with spectral indices of α = −0.96 ± 0.04 for the total emission and α _nt = −1.21 ± 0.04 for the synchrotron emission over 408 MHz–5 GHz. The spectrum shows a break near 1 GHz, indicating substantial radiative aging of the CRE population. Under the equipartition assumption, we derive a mean magnetic-field strength of 10.8 µG. The RC–star formation rate smoothing analysis indicates effective CRE propagation lengths of ∼0.65–0.89 kpc at 943–1700 MHz and ∼0.44 kpc at 5 GHz, corresponding to effective diffusion coefficients on the order of 10 ²⁸ cm ² s ⁻¹ . We identify a steep-spectrum synchrotron “island” in the southeastern region, with an average synchrotron spectral index of α ∼ −1.09 and no clear H α , infrared, far-ultraviolet or near-ultraviolet counterpart, indicating that the CREs are unlikely to be injected in situ. Our 2D simulations using CRPropa show that anisotropic diffusion along ordered magnetic fields allows CREs to reach the island region much more efficiently than isotropic diffusion. Conclusions . NGC 2442 provides evidence that the combined effects of environmental disturbances and ordered magnetic fields can strongly regulate CRE propagation in disturbed spiral galaxies.

Context . Infrared-dark clouds (IRDCs) are ideal sites for investigating the initial conditions of massive star and cluster formation. The A Lei Of the Habitat and Assembly of Infrared Dark Clouds (ALOHA IRDCs), a James Clerk Maxwell Telescope Large Program, has mapped nearby IRDCs utilising the SCUBA-2 continuum camera at the highest achievable sensitivity. Complementary molecular line observations are needed to characterise the physical, kinematic, and chemical properties of the dense gas. Aims . We aim to determine the thermal, kinematic, and chemical properties of clumps identified in the ALOHA IRDCs, and to assess their evolutionary status and level of star-forming activity. Methods . We performed single-pointing K -band and W -band observations towards 56 clumps pf ALOHA IRDCs using the Effelsberg 100 m and Yebes 40 m telescopes, respectively. We derived NH ₃ kinetic temperatures using the hyperfine group ratio method and identified infall and shock signatures from HCO ⁺ , H ¹³ CO ⁺ , SiO, and HNCO profiles. Water masers and NH ₂ D emission were used as complementary tracers of chemical evolution and star formation. Results . The clumps exhibit kinetic temperatures of 15–29 K. We detect NH ₂ D emission towards 18 sources; the NH ₂ D centroid velocities are consistent with those of NH ₃ , indicating the two species trace the same dense gas component. More than half of the clumps display blue-asymmetric HCO ⁺ profiles, identifying them as infall candidates. Water masers with prominent velocity ranges and variability are detected in 22 sources. Broad SiO emission (≳20 km s ⁻¹ ) indicates strong shocks, while narrower emission (≲6 km s ⁻¹ ) likely traces large-scale interactions or low-velocity shocks. Conclusions . The widespread infall signatures, shock tracers, masers, and NH ₂ D emission suggest that relatively quiescent, chemically young material can coexist with dynamically active gas affected by early protostellar feedback, providing insight into the coupled physical and chemical evolution of massive IRDC clumps.

Context. The nearly edge-on starburst galaxy NGC 253 has been observed to exhibit extended halo emission in multiple bands, making it an ideal laboratory for studying the transfer of matter from the disk to the halo. Aims. We aim to determine how the cosmic-ray electrons (CREs) flow from the disk to the halo and understand what drives their propagation. Methods. By combining data from multiple observations, we generated improved total intensity images at 943 MHz with a resolution of 13″ from the Australian SKA Pathfinder (ASKAP), and at 216 MHz with a resolution of 45″ from the Murchison Widefield Array (MWA). The 1D advection and diffusion equations were solved, and the solutions were fitted to the observed synchrotron emission intensity and spectral-index profiles to constrain the propagation model and parameters. Results. The ASKAP total intensity map has an rms noise of 16 μJy beam ⁻¹ , reaching the classical confusion limit, and the MWA map has an rms noise of 1 mJy beam ⁻¹ . The sensitivities are significantly improved in comparison to previous observations at similar frequencies. In the ASKAP image, we identify a clear loop-like structure in the northwestern radio spur, extending vertically up to ∼9 kpc above the disk, while the southeastern spur reaches heights of ∼8 kpc. The synchrotron emission intensity profiles perpendicular to the disk can be fitted with exponential components in the central regions and with Gaussian components in the outer regions. This result implies that CREs in these two regions propagate differently. By jointly fitting the vertical synchrotron emission intensity profiles at 943 MHz and 216 MHz, together with the corresponding synchrotron spectral-index profiles, our results provide the clearest evidence to date that CREs are transported from the disk by advection in the central region and by diffusion elsewhere in NGC 253. The advection speed in the central region increases exponentially with height and reaches the escape speed to form a superwind of CREs at about 5.5 kpc. This superwind is associated with regions in the disk with active star formation and X-ray emission, indicating a bulk motion of baryons caused by the advection. The combined thermal, magnetic, cosmic-ray, and ram pressures exceed the gravitational pressure below | z | ≲ 5.5 kpc, and this overpressure condition accelerates the superwind. Conclusions. High-sensitivity low-frequency radio observations provide an important probe of the transport of CREs. With these observations, we have revealed a newly detailed view into the kinematic origin of the superwind from the center of NGC 253.

The Andromeda galaxy (M31) exhibits a systematically low star formation efficiency (SFE) compared to other star-forming galaxies. Star formation is thought to be regulated by the mass of gravitationally bound gas. To investigate whether the low SFE in M31 originates from a scarcity of such bound gas, we analyze the dynamical state of its giant molecular cloud (GMC) population by estimating their virial parameters ( α _vir ). Using a James Clerk Maxwell Telescope SCUBA2 850 μ m dust-continuum GMC catalog as the parent sample, we combine it with IRAM 30 m CO(1–0) data to define a working sample of 173 GMCs and derive their kinematic properties. Most analyzed GMCs exhibit high α _vir , indicating that they are predominantly gravitationally unbound at the ∼86 pc resolution of our observations. While no statistically significant global correlation is found between SFE and α _vir for the full cloud sample, the upper quantiles of the SFE distribution show progressively stronger negative trends with increasing α _vir . This upper-tail trend may arise from the combined effects of the virial parameter limiting the upper range of SFE values accessible to the GMC population and feedback-driven disruption reducing the number of high-SFE, high- α _vir CO-bright clouds in the observed sample. The substantial scatter in the SFE- α _vir plane likely reflects the diverse evolutionary stages of GMCs, the temporal mismatch between gas and star formation tracers, and the limitations of a cloud-averaged virial parameter as a tracer of internal dynamical structure, making a simple one-to-one correspondence between α _vir and instantaneous SFE difficult to recover for individual GMCs.

Recent studies have shown that the star formation rate (SFR) correlates tightly and linearly with the mass of gravitationally bound gas, which can be delineated from the power-law tail of the column-density probability distribution function ( N -PDF) derived from dust emission observations. This relationship holds across 4 orders of magnitude within the Milky Way—spanning low-mass to high-mass star-forming regions and encompassing the extreme environment of the Central Molecular Zone. Building on this framework, we present a new approach for estimating the mass of gravitationally bound gas in molecular clouds using multiline CO isotopologue observations. Our sample includes 16 molecular clouds with robust detections in ¹² CO, ¹³ CO, and C ¹⁸ O J = 1–0, spanning both massive inner Galaxy clouds and nearby star-forming regions. We find that the N -PDFs derived from combined CO isotopologue data recover the characteristic log-normal plus power-law profiles seen in dust-based studies. The mass and spatial distribution of the self-gravitating structures estimated from both dust-based and CO-based methods agree well throughout the sample. This indicates that the CO isotopologue combination can robustly trace the self-gravitating component via the N -PDF method and provides a reliable, scalable, and velocity-resolved alternative to dust emission for identifying the star-forming gas in molecular clouds.

We present centimeter-wave spectroscopic observations of the OH 18 cm lines in two bright comets, 12P/Pons-Brooks and C/2023 A3 (Tsuchinshan-ATLAS), conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) during their 2024 apparitions. For the Halley-type comet 12P/Pons-Brooks, five epochs of OH observations were obtained. The main OH lines at 1665 and 1667 MHz were robustly detected in absorption during one preperihelion epoch, while upper limits were derived for two postperihelion epochs. For the dynamically new Oort Cloud comet C/2023 A3 (Tsuchinshan-ATLAS), six epochs of OH observations were obtained. The 1665 and 1667 MHz lines were robustly detected in absorption during two epochs immediately following the comet’s closest approach to Earth. We also reported a tentative detection of the 1721 MHz satellite line in one of these two epochs. The low OH detection rates for these two bright comets are primarily attributable to their unfavorable heliocentric radial velocity during the observations, which resulted in the anti-maser negative inversion mode of the OH excitation. We calculated the OH production rates for both comets and found that, after considering the small beam size of FAST and the collisional quenching effect, production rates are consistent with the data from literature. Specifically, this matches the power-law fit of the OH production rates for comet 12P/Pons-Brooks, as well as the production rates at comparable heliocentric distances among comet C/2023 A3 (Tsuchinshan-ATLAS) and four other dynamically new Oort Cloud comets.

Fast radio bursts (FRBs) are millisecond-duration, highly energetic radio transients of uncertain origin. Repeating FRBs provide an excellent population for investigating their nature, particularly through studies of parameter evolution. Out of the 63 repeaters monitored by CHIME, we select the 19 sources with more than 10 detected bursts and examine their long-term dispersion measure (DM) evolution. Seven sources show statistically significant DM evolution and are classified as the golden sample. Of these, five exhibit a decreasing DM trend and two show an increasing trend. We then perform a binomial test under the null hypothesis that decreasing and increasing DM variation trends have equal probabilities. The current combined sample, including our golden sample and additional repeaters with reported DM change rate from the literature, gives a p -value of 0.033, supporting that decreasing DM trends are more common in the repeating FRB population. This statistical result is consistent with scenarios in that the local electron density around repeaters generally decreases with time, for example, due to expansion of a young supernova remnant (SNR). Finally, within the SNR expansion model, we provide an illustrative estimate of the SNR contributions to the DM for different ejecta masses.