Abstract
ABSTRACT Numerical relativity predicts that the coalescence of a black hole (BH) binary causes the newly formed BH to recoil, and evidence for such recoils has been found in the gravitational waves observed during the merger of stellar-mass BHs. Recoiling (super)massive BHs are expected to reside in hypercompact stellar clusters (HCSCs). Simulations of galaxy assembly predict that hundreds of HCSCs should be present in the halo of a Milky Way (MW)-type galaxy, and a fraction of those around the MW should have magnitudes within the sensitivity limit of existing surveys. However, recoiling BHs and their HCSCs are still waiting to be securely identified. With the goal of enabling searches through recent and forthcoming data bases, we improve over existing literature to produce realistic renditions of HCSCs bound to BHs with a mass of 105 M⊙. Including the effects of a population of blue stragglers, we simulate their appearance in Pan-STARRS and in forthcoming Euclid images. We also derive broad-band spectra and the corresponding multiwavelength colours, finding that the great majority of the simulated HCSCs fall on the colour–colour loci defined by stars and galaxies, with their spectra resembling those of giant K-type stars. We discuss the clusters properties, search strategies, and possible interlopers.
Highlights
General relativity predicts that a burst of gravitational waves (GW) is emitted during the coalescence of two compact objects (e.g. Fitchett 1983; Redmount & Rees 1989; Wiseman 1992)
We simulated a set of hypothetical hypercompact stellar clusters (HCSCs) spectra for clusters with about 10000 stars - the number of stars expected for a cluster bound to a 105 M black hole ejected at low velocity, and relatively young
To simulate the integrated spectra of HCSCs we generated atmospheric models for the individual stars which make up the cluster, we derived the corresponding spectra via a spectral synthesis software, and we co-added the individual spectra to produce an integrated spectrum for the HCSC as a whole
Summary
General relativity predicts that a burst of gravitational waves (GW) is emitted during the coalescence of two compact objects (e.g. Fitchett 1983; Redmount & Rees 1989; Wiseman 1992). Abbott et al (2016) inferred that the emission originated from the merger of two black holes with masses 29+−44 and 36+−54 M. Asymmetries in the merging objects (i.e. different masses and spins) are expected to produce asymmetries in the GW emission, leading to a net flux of linear momentum. The merging binary and the resulting object recoil . The amplitude of such kick is largest for black hole (BH) binaries, and it must be computed numerically.
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