Heavy interstellar scattering toward the near end of the Galactic bar

We present results of a pilot observational wide-field VLBI campaign on probing scattering properties of the partly ionized interstellar medium towards the Galactic plane sky region between $28^\circ<l<36^\circ$ and $|b|<1^\circ$. This covers the region where the Galactic bar connects to the spiral arms and where a lot of star formation is currently ongoing. The Very Long Baseline Array (VLBA) observations of the whole region were performed in a special mode with multiple phase centers at L-band (1.4 – 1.8 GHz) during April-June 2022 and a year later complemented by sessions at S (2.2 – 2.4 GHz) and C-band (4.6 – 5.0 GHz) partially covering the pilot region. We found compelling evidence that target sources are subject to scattering. The total detection rate in L, S and C-bands is 1.5, 3.4 and 9.2 per cent, respectively, and approximately scales with the square of the observation frequency. The low rate values imply that scattering is strong. Its power is non-uniform across the Galactic plane and it can be approximated by a Gaussian with a width of about $2^\circ$ peaking at the Galactic mid-plane. One of the brightest sources of the field shows anisotropic scattering, with a $λ^2$ dependence of its observed angular size, along a position angle of $26^\circ$ aligned with the line of constant Galactic latitude. We estimate the turbulence dissipation scale $r_\text{in}\approx1500$ km toward the source J1833+0015.

💡 Research Summary

This paper reports on a pilot very‑long‑baseline interferometry (VLBI) survey aimed at characterizing interstellar scattering toward the region where the near end of the Galactic bar connects to the spiral arms (Galactic longitudes 28°–36°, latitude |b| < 1°). Using the VLBA, the authors observed the entire 16 deg² field at L‑band (1.4–1.8 GHz) with a dense hexagonal grid of 107 pointings, each observed five times for a total of ~13 min integration per pointing. Phase referencing was performed with J1833+0115 and a fringe‑finder (2007+777). The data were correlated with the DiFX correlator in multi‑phase‑center mode, allowing simultaneous processing of 1 210 compact sources from the GLOSTAR catalog.

Because the initial L‑band images revealed almost no detections, the team allocated ~45 % of the remaining time to higher frequencies: a single S‑band (2.3 GHz) session covering ~8 % of the field and four C‑band (4.8 GHz) sessions covering ~24 % of the field, targeting the brightest sources and those within primary beams. Standard AIPS calibration steps (gain, system temperature, bandpass, global gain, phase referencing) were applied, and visibility amplitudes were examined directly to identify detections that may be resolved out on long baselines.

The detection statistics are striking: only 1.5 % of the 1 210 sources were detected at L‑band, 3.4 % at S‑band, and 9.2 % at C‑band. Fitting the detection fraction as a power law of frequency (∝ ν^a) yields a = 1.61 ± 0.28, indicating a strong but sub‑quadratic dependence on frequency. This behavior reflects a scattering angular size that scales roughly as θ_sc ∝ ν⁻¹·⁶, consistent with strong scattering that weakens at higher frequencies.

Spatially, the detected sources cluster around the Galactic mid‑plane and can be described by a Gaussian distribution with a full width of about 2°, peaking at b = 0°. This suggests that the scattering strength is highly non‑uniform across the plane, with a pronounced enhancement in the bar‑arm transition region. Most detections are classified as extragalactic candidates (likely quasars); one known pulsar and two confirmed quasars were also identified. A particularly bright source, J1833+0015, exhibits anisotropic scattering: its apparent size follows a λ² law and is elongated along a position angle of 26°, essentially parallel to the line of constant Galactic latitude. Modeling of its visibility function yields an inner turbulence dissipation scale r_in ≈ 1 500 km, considerably smaller than the typical outer scales inferred for Kolmogorov turbulence in the ionized ISM.

The authors discuss the implications of these findings. The steep frequency dependence of the detection rate confirms that scattering dominates source detectability in this region, and the Gaussian latitude profile aligns with previous all‑sky scattering maps that show enhancements near spiral arms, supernova remnants, and the Galactic center. The anisotropy and small inner scale hint at a thin, possibly magnetically aligned scattering screen, where turbulence cascades down to sub‑kilometer scales before being damped, perhaps by ion–neutral collisions or kinetic effects. The detection of refractive sub‑structure in another bright source (1849+005) points to the presence of larger‑scale plasma lenses that could produce extreme scattering events.

In conclusion, the pilot VLBI campaign demonstrates that interstellar scattering toward the near end of the Galactic bar is exceptionally strong, varies on degree scales, and exhibits measurable anisotropy and a small inner turbulence scale. These results provide new constraints on the distribution of free electrons and the nature of turbulence in a key region of the Milky Way. Future work should extend the frequency coverage (e.g., X‑band), increase sky coverage, and employ long‑term monitoring to map the three‑dimensional geometry of the scattering screens, refine the turbulence spectral index β, and explore the role of magnetic fields in shaping the observed anisotropy.