The Host Galaxy of the Recoiling Black Hole Candidate in 3C 186: An Old Major Merger Remnant at the Center of a z=1 Cluster

TL;DR

This study analyzes the host galaxy of the quasar 3C 186 at z=1.0685, revealing a significant spatial offset and mature stellar populations, supporting the recoiling black hole hypothesis over a recent merger scenario.

Contribution

The paper provides detailed imaging analysis of 3C 186's host galaxy, supporting the recoiling black hole scenario with evidence from spatial offset and stellar population age.

Findings

Significant spatial offset of 11.1 kpc from the quasar core.

Host galaxy shows mature stellar populations (>200 Myr).

Features consistent with late-stage merger remnants.

Abstract

3C186, a radio-loud quasar at $z=1.0685$, was previously reported to have both velocity and spatial offsets from its host galaxy, and has been considered as a promising candidate for a gravitational wave recoiling black hole triggered by a black hole merger. Another possible scenario is that 3C186 is in an on-going galaxy merger, exhibiting a temporary displacement. In this study, we present analyses of new deep HST/WFC3-IR and ACS images, aiming to characterize the host galaxy and test this alternative scenario. We carefully measure the light-weighted center of the host and reveal a significant spatial offset from the quasar core ($11.1\pm0.1$kpc). The direction of the confirmed offset aligns almost perpendicularly to the radio jet. We do not find evidence of a recent merger, such as a young starburst in disturbed outskirts, but only marginal light concentration in F160W at $\sim30$kpc. The host consists of matured ($>200$Myr) stellar populations and one compact star-forming region. We compare with hydro-dynamical simulations and find that those observed features are consistently seen in late-stage merger remnants. Taken together, those pieces of evidence indicate that the system is not an on-going/young merger remnant, suggesting that the recoiling black hole scenario is still a plausible explanation for the puzzling nature of 3C186.