New insights from deep VLA data on the potentially recoiling black hole CID-42 in the COSMOS field

TL;DR

This study uses deep 3 GHz VLA observations and multi-frequency data to analyze the radio properties of the galaxy CID-42, providing insights into its potential recoiling black hole and the nature of its radio emission.

Contribution

The paper presents the first detailed radio spectrum of CID-42 across multiple frequencies, supporting the recoiling black hole scenario with new high-resolution VLA data.

Findings

The entire 3 GHz radio emission is associated with the South-Eastern component.

The radio spectrum indicates a radio-quiet, flat-spectrum AGN in the South-Eastern component.

Data are consistent with a recoiling black hole but do not exclude an obscured SMBH in the North-Western component.

Abstract

We present deep 3 GHz VLA observations of the potentially recoiling black hole CID-42 in the COSMOS field. This galaxy shows two optical nuclei in the HST/ACS image and a large velocity offset of ~ 1300 km/s between the broad and narrow H beta emission line although the spectrum is not spacially resolved (Civano et al. 2010). The new 3 GHz VLA data has a bandwidth of 2 GHz and to correctly interpret the flux densities imaging was done with two different methods: multi-scale multi-frequency synthesis and spectral windows stacking. The final resolutions and sensitivities of these maps are 0.7" with rms = 4.6 muJy/beam and 0.9" with rms = 4.8 muJy/beam respectively. With a 7 sigma detection we find that the entire observed 3 GHz radio emission can be associated with the South-Eastern component of CID-42, coincident with the detected X-ray emission. We use our 3 GHz data combined with other radio data from the literature ranging from 320 MHz to 9 GHz, which include the VLA, VLBA and GMRT data, to construct a radio synchrotron spectrum of CID-42. The radio spectrum suggests a type I unobscured radio-quiet flat-spectrum AGN in the South-Eastern component which may be surrounded by a more extended region of old synchrotron electron population or shocks generated by the outflow from the supermassive black hole. Our data are consistent with the recoiling black hole picture but cannot rule out the presence of an obscured and radio-quiet SMBH in the North-Western component.