Impact of ringdown higher-order modes on black-hole mergers in dense environments: the scalar field case, detectability and parameter biases

Dense environments hosting compact binary mergers can leave an imprint on the gravitational-wave emission which, in turn, can be used to identify the characteristics of the environment. To demonstrate such scenario, we consider a simple setup of binary black holes with an environment consisting of a scalar-field bubble. We use this as a proxy for more realistic environments and as an example of the simplest physics beyond the standard model. We perform Bayesian inference on the numerical relativity waveforms using state-of-the-art waveform templates for black-hole mergers. In particular, we perform parameter estimation and model selection on signals from black-hole mergers with different mass ratios, total mass and loudness, and hosted by scalar-field bubbles of varying field amplitude. We find that sub-dominant gravitational wave modes emitted during the coalescence and ringdown are key to identifying environmental effects. In particular, we find that for face-on signals dominated by the quadrupole mode, the environment is only detectable if both the ringdown and the late inspiral/early merger fall in the detector band, so that inconsistencies can be found between the inferred binary parameters and those of the final black hole. For edge-on mergers we find that the environment can be detected even if only the ringdown is in band, thanks to the information encoded in the quasi-normal mode structure of the final black-hole.