Heavy interstellar scattering toward the near end of the Galactic bar

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 ${\lambda }^2$ 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 $r_{\text{in}}\approx 1500$ km toward the source J1833+0015.

16 pages, 12 figures, accepted for publication in MNRAS

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