Heavy interstellar scattering toward the near end of the Galactic bar

TL;DR

This study maps heavy interstellar scattering toward the end of the Galactic bar by conducting a multi-frequency, wide-field VLBI campaign with the VLBA across L-, S-, and C-bands, examining 1210 compact sources from the GLOSTAR survey. The authors quantify the scattering strength and its non-uniform, plane-peaked distribution, detect 33 VLBI sources, and identify both anisotropic scattering (in J1850$-$002) and isotropic scattering toward the phase calibrator (J1833+0115), with a size-frequency scaling $k\approx2.1$ and an inner-scale dissipation around $r_{\text{in}} \sim 10^3$ km inferred for a specific sightline. While NE2001 explains angular sizes well for $|b|>1^{\circ}$, many $|b|<1^{\circ}$ sightlines show larger scattering, implying multiple proximate screens near the plane. The results underscore the need for shorter baselines to fully recover scattered structure and suggest practical paths forward (e.g., adding a VLA antenna or using MERLIN) to improve detection of strongly scattered sources and to refine 3D models of Galactic ISM turbulence.

Abstract

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.

Figures (17)

• Figure 1: The full $8^\circ\times2^\circ$ image of the pilot region from the combination of VLA D-configuration and Effelsberg single-dish data Brunthaler21 together with the proposed VLBA pointing positions. Shown are pointing locations with the primary beam of the VLBA at 1.6 GHz (white circles), the location of already known VLBI sources (white dots), and all compact sources discovered in the VLA B-configuration images (white crosses).
• Figure 2: Correlated flux density, $S_\text{c}$, as a function of baseline projection $r_{uv}$ for one of the target sources, 1847+007, detected at three frequency bands, C (left), S (middle) and L (right). Every point marks a vector average over a scan length of $\sim$2.5 min and over IFs on an individual interferometer baseline. Red diamonds represent visibilities with $\text{S/N}>6$, while lavender points show the measurements with a lower significance. Data were phase-calibrated to a calibrator. Error bars represent statistical $1\sigma$ uncertainties. Each plot legend shows the observing epoch, frequency and a source name in the internal notation PXX-X (note, sources have different internal names at different frequency bands, see \ref{['t:detected_sources']}). Rapid decrease of the visibility function amplitudes with baseline length indicates that the source is heavily resolved. The plots for all detected sources are shown in \ref{['fa:radplots_c']}, \ref{['fa:radplots_l']}, and \ref{['fa:radplots_s']}.
• Figure 3: Positions of the observed sources in the pilot region. Grey points show undetected objects. Red points represent detected sources. Light-green circles depict positions and sizes of 20 known SNR within the sky area Green25, blueish ones represent 42 SNR candidates from the GLOSTAR survey Dokara21, and 17 peach circles denote H ii regions Medina19. Detection rate increases with frequency as scatter broadening weakens.
• Figure 4: Dependence of the correlated flux density on baseline projection for unscattered (black solid line), strongly scattered (blue line), and extremely scattered (red line) Gaussian source with a total flux density 1 Jy observed at L-band. Dashed lines show the case of a weaker source, 30 mJy. Shaded area represents the baseline range provided by the VLBA.
• Figure 5: Naturally weighted VLBA map of J1853$-$0010 in total intensity at 5.06 GHz (IF4, C-band), with a pixel size of 0.3 mas and Gaussian tapering (value 0.3 at $uv$-radius 50 M$\lambda$) applied. The contours are plotted at increasing powers of $\sqrt{2}$, starting from the corresponding 3.5 rms level. The full-width at half maximum (FWHM) of the restoring beam is shown as a shaded ellipse in the lower left corner. The map parameters are given on the top of the image. The dashed green line denotes the line of constant Galactic latitude ($b=0\fdg468$) at $\mathrm{PA}=27\fdg1$ showing perfect alignment with the direction of the stretched source brightness distribution induced by anisotropic scattering.