Radio Emission from Broad Absorption Line Quasars

TL;DR

This study investigates whether broad absorption line quasar (BALQ) winds produce radio emission via shocks as the outflows interact with the ambient medium. Using VLA A-configuration observations at 5.5 and 9 GHz of 13 radio-quiet BALQs with measured outflow kinetic power $\dot{E}_{\rm k}$, nine sources are detected with compact emission on <1–4 kpc scales; the observed radio spectral slopes range from steep ($\alpha_{5.5-9.0}< -0.5$) to flat/inverted, and the radio luminosity shows no clear correlation with $\dot{E}_{\rm k}$. Comparisons to typical radio-quiet PG quasars indicate BALQs are not systematically more radio luminous, suggesting the UV outflows do not dominantly power the observed radio emission. The data favor a mixed picture, including intrinsic compact coronal-like emission and free-free absorption by outflowing gas that can flatten spectra; follow-up high-resolution, multi-frequency radio observations can test the free-free absorption scenario and use radio data as a probe of AGN winds. Key results include: (i) radio emission is generally confined to <1–4 kpc; (ii) the presence of both steep and flat/inverted spectra among detected BALQs; (iii) no robust link between radio power and outflow kinetic power $\dot{E}_{\rm k}$; and (iv) flat-spectrum sources can be explained by free-free absorption with absorber distances estimated at tens of parsecs. These findings imply that BALQ winds are not the primary driver of radio emission in this sample and motivate future spatially-resolved, multi-frequency studies to map wind-absorber geometry and test absorption models.

Abstract

Broad Absorption Line Quasars (BALQs) generally exhibit significant outflows that may interact with the surrounding medium, resulting in radio emission. We selected a sample of 13 powerful radio-quiet (RQ) BALQs, where the UV outflow kinetic power is measurable, and detected nine of them with the Very Large Array A configuration at 5.5 GHz and 9.0 GHz. The radio emission is mostly unresolved and is generally constrained within a scale of < 1--4 kpc. In the nine detected objects, the radio spectral slope alpha_5.5-9.0 is steep (< -0.5) in five objects and is flat or inverted (> -0.5) in four objects. We discuss how the steep-slope emission can be associated with the UV outflows, and how the flat-slope emission can be intrinsically steep but flattened by free-free absorption from the UV outflowing gas. However, we find no correlation between the radio luminosity and the estimated outflow kinetic power, which suggests that the outflows are likely not a major source of the observed radio emission. In addition, the radio loudness of these RQ BALQs is comparable to that of typical RQ quasars, implying that the UV outflows likely do not produce stronger radio emission compared to non-BALQs. Follow-up radio observations can test the free-free absorption interpretation and can be used as a new probe for outflows in AGN.

Figures (6)

• Figure 1: The nine absorption-line quasars detected with the VLA A configuration at 5.5 and 9.0 GHz. J0240$-$1851 (upper panel): The contours are at [$-$3, 3, 5] $\times$ 0.058 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3, 5] $\times$ 0.066 mJy beam$^{-1}$ at 9.0 GHz. J0242+0049 (middle panel): The contours are at [$-$3, 3, 5] $\times$ 0.008 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3] $\times$ 0.007 mJy beam$^{-1}$ at 9.0 GHz. J0831+0354 (lower panel): The contours are at [$-$3, 3, 5, 8] $\times$ 0.009 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3] $\times$ 0.009 mJy beam$^{-1}$ at 9.0 GHz.
• Figure 1: Continued. J1042+1646 (upper panel): The contours are at [$-$3, 3, 6] $\times$ 0.010 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3, 5] $\times$ 0.010 mJy beam$^{-1}$ at 9.0 GHz. J1123+0137 (middle panel): The contours are at [$-$3, 3] $\times$ 0.012 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3] $\times$ 0.011 mJy beam$^{-1}$ at 9.0 GHz. J1135+1615 (lower panel): The contours are at [$-$3, 3, 5, 7, 10] $\times$ 0.010 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3] $\times$ 0.012 mJy beam$^{-1}$ at 9.0 GHz.
• Figure 1: Continued. J1209+1036 (upper panel): The contours are at [$-$3, 3, 5, 10, 20, 30] $\times$ 0.011 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3, 5, 10, 15] $\times$ 0.012 mJy beam$^{-1}$ at 9.0 GHz. J1329+5405 (middle panel): The contours are at [$-$3, 3, 5, 8] $\times$ 0.011 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3] $\times$ 0.012 mJy beam$^{-1}$ at 9.0 GHz. J1512+1119 (lower panel): The contours are at [$-$3, 3, 6] $\times$ 0.014 mJy beam$^{-1}$ at 5.5 GHz and [$-$3, 3, 5, 8, 11] $\times$ 0.010 mJy beam$^{-1}$ at 9.0 GHz.
• Figure 2: The radio loudness versus the BH mass. The red circles and down-arrows represent the BALQs observed in our VLA observations. The blue triangles represent the BALQs excluded in our VLA observations. The grey crosses represent the whole PG quasar sample. The $M_{\rm BH}$ uncertainty is typically $\pm$0.5 dex. The $R$ values of the RQ BALQs are comparable to those of the RQ PG quasars, suggesting that the winds in BALQs do not produce stronger radio emission than those in non-BALQs.
• Figure 3: The observed 5.5--9.0 GHz slope versus the Eddington ratio. The red circles represent the RQ BALQs detected in our VLA observations. The black crosses represent the 25 RQ PG quasars which the $\alpha_{5.5-9.0}$ was studied systematically Laor2019. The $L/L_{\rm Edd}$ uncertainty is typically $\pm$0.5 dex. The RQ BALQs generally does not follow the $\alpha_{5.5-9.0}$ and $L/L_{\rm Edd}$ correlation found in the RQ PG quasars, where high and lower $L/L_{\rm Edd}$ objects exhibit steep and flat slopes, respectively.