Radio selection of heavily obscured AGN in the J1030 field: unraveling a missing Compton-thick population

Abstract

We tested the effectiveness of radio selection to discover heavily obscured AGNs, particularly at high-z, and we measured their abundance for the first time from a radio perspective. We consider the radio sources detected in the J1030 field, which is one of the fields with the deepest combination of 1.4 GHz radio and X-ray observations. We defined a radio excess parameter as the ratio between the star formation rate (SFR) that would correspond to the observed radio luminosity and the one directly derived from the spectral energy distribution (SED) fitting, $\rm REX=SFR_{1.4GHz}/SFR^{corr}_{SED}$. We then select as radio excess AGN those sources with $\rm REX>8.5$, corresponding to a $3σ$ excess above the median value. In this way, we find 145 radio-excess sources falling into the \textit{Chandra} X-ray image footprint but without X-ray detection. From the deep X-ray upper limits, we estimated a lower limit to the obscuration of each radio-excess AGN, finding on average $\log (N_H/\rm{cm^{-2}})>23.7$. A CTK AGN scenario is also supported by the results of the X-ray stacking analysis performed on sources at $z>1.5$, which revealed X-ray luminosities and hardness ratios compatible with very highly obscured AGN. Finally, we computed the number density of these radio-selected CTK AGN. While at $z\sim 2$ the radio number density agrees well with the CTK AGN predictions of different population synthesis models, at $z\sim3$ the radio selection returns a CTK AGN number density $\sim 2-3$ times larger than what is predicted by the X-ray models and observations. This result supports the effectiveness of radio emission in selecting the most obscured sources, unraveling a population of AGN potentially missed by X-rays surveys at $z>3$, paving the way to a synergistic use of the future radio and X-ray facilities such as the \textit{SKAO} and \textit{NewAthena}.

Figures (11)

• Figure 1: LBT/LBC z-band image of the field. The coverage of different surveys/instruments is shown. The entire field is also covered by XMM, MUSYC wide, and Spitzer/IRAC.
• Figure 2: SED fitting decomposition of three different sources at $z\sim2$ with different values of $REX$. The black line represents the best fit model, the yellow line the intrinsic stellar emission, the blue component is the attenuated stellar emission, while the brown component is the reprocessed dust emission (computed assuming energy balance).
• Figure 3: Redshift versus $REX$ parameter of all the radio sources in the J1030 field with a counterpart in the Ks-band selected multiwavelength catalog. The red squares represent the radio-excess sources, while gold symbols are used for X-ray detected AGN. The dashed line remarks the 3$\sigma$ deviation from the peak of the Gaussian distribution of the $REX$ parameter (upper panel), which is likely to be dominated by SFGs being $REX\simeq 1$. The histogram in the upper panel shows the distribution of the sources according to the $REX$ parameter, while the gray and gold lines are the cumulative distribution of all the radio sources and of the X-ray AGN, respectively. In the lower right corner we report the average errors of the data points on the two axis.
• Figure 4: Distribution of the radio sources with a counterpart in the J1030 multiwavelength catalog according to their redshift and to the SFR derived from the SED-fitting normalized to the expected main sequence SFR. Radio-excess sources are also color-coded by the stellar mass returned by the SED fitting. $SFR^{corr}_{SED}$ for radio-excess sources have been computed in a second run by adding the AGN component to the fit. The red and gray lines mark the median $SFR^{corr}_{SED}$/$SFR_{MS}$ with redshift for the populations of radio-excess and non radio excess sources, respectively. On the left is plotted the density distribution according to the MS-normalized SFR (red refers to radio-excess sources).
• Figure 5: Distribution of the lower limit to the value of $\log (N_H/\rm cm^{-2})$ for the population of radio-excess and X-ray non detected AGN candidates. The red histogram shows the distribution of sources classified as RQ, the blue histogram as RL, while the gold histogram shows the distribution of $\log (N_H/\rm cm^{-2})$ of the X-ray detected radio AGN of J1030 field as derived from the X-ray spectral analysis. The black dashed line marks the median value of the distribution of radio-excess AGN, corresponding to $\log (N_H/\rm cm^{-2})=23.7$.