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Searching for high-brightness temperature AGN cores in surveys using high-resolution radio data

Ann Njeri  

Extragalactic Survey fields provide a rich hunting ground for Active nuclei, but multiple resolution interferometers are needed to tell them apart from star formation. 

Deep radio surveys of extragalactic legacy fields trace a large range of spatial and brightness temperature sensitivity scales, and therefore have differing biases to radio-emitting physical components within galaxies. This is particularly true of radio surveys performed at ≲1 arcsec angular resolutions, and so robust comparisons are necessary to better understand the biases present in each survey.  

Ann Njeri and her team performed a multi-resolution analysis of 24 sources detected in a new Very Long Baseline Array (VLBA) survey of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey Great Observatories Origins Deep Survey-North field. This VLBA data were augmented by ~0.1-1 arcsec angular resolution data provided by the European VLBI Network, the Very Large Array (VLA) and the enhanced-Multi Element Remotely Linked Interferometry Network (eMERLIN).  

 

Figure 1: Thumbnails of the five low-SNR (∼5.5σ ) new VLBI sources detected in the VLBA survey. Columns 1 and 2 show the contour maps for the sources in the VLA and e-MERLIN, respectively, at contour levels 3σ × [−1, 1, 2, 3, 4, 5, 6,...., 11] overlaying their 3D-HST CANDELS optical counterparts in the F160W band, in greyscale. The VLBA contours are drawn at a central rms ∼ 15 μJy per beam multiplied by factors of [2, 3, 5, 7, 9, 11, 13]. The red cross on the VLA and e-MERLIN images indicate the VLBI source position.  

 

The high brightness temperatures of these sources (TB ≳ 106 K) confirm AGN cores as measured by VLBI. These sources would often be missed or ambiguous in lower-resolution radio data of the same sources. By combining VLA and VLBI measurements, they find evidence that most of the extended radio emission is also AGN dominated, with only ~13% of the sources showinglikehood of being star formation-dominated. Combining eMERLIN and VLBI reveals potentially hybrid systems that are missed in either individual surveys. This survey demonstrates the importance of wide-field multiresolution (arcsecond-milliarcsecond) coverage of the faint radio source population, for a complete picture of the multiscale processes within these galaxies. 

 

Figure 2: The brightness temperature TB (K) distribution across the VLBA,EVN, e-MERLIN and the VLA as a function of both the uv-distance (km) and the angular sizes (mas)^2​​​. 

 

The squares highlight the low-SNR (∼5.5σ ) sources. The error bars on the VLBA and EVN data are indicative of lower limits on the brightness temperature and upper limits on the deconvolved angular size  measurements. The parallelograms highlight the minimum and maximum brightness temperatures limits that should be ideally recovered in each survey by applying equations (3) and (4) from Lobanov (2015). The dashed line indicate the TB > 105 K threshold typically used to separate AGN and star formation powered radio emission (Condon et al. 1991). Applying VLBI observations resolves bright compact (AGN) cores that would otherwise remain unresolved in both the e-MERLIN and VLA and as such mis-classified as star formation. 

Article: https://academic.oup.com/mnras/article/528/4/6141/7602412