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A supermassive black hole kicked out of its galaxy

13 mai 2022

When two galaxies merge, the massive black holes lurking in their nuclei will merge rapidly. Their relative orbital energy will be radiated away through gravitational waves. If the latter emission is strongly anisotropic, this will produce a recoil of the merged black hole remnant that will be kicked out. The event is difficult to detect, and only candidates have been observed. In a recent work, an international team including researchers from Paris Observatory – PSL, has detected the molecular gas of the remnant galaxy of the 3C186 candidate, which confirms that the massive black hole has been ejected from its galaxy at 2000km/s, and at a projected distance of 24 000 light years.

Galaxy mergers are fundamental mechanisms regulating galaxy growth and the evolution of super massive black holes (SMBH) at their centers. When the SMBH merge, a fraction of the SMBH binding energy is converted into gravitational waves (GWs). In case of anisotropy of this GW emission, the merged SMBH may receive a recoil kick. Measurable displacements of SMBHs from the center of their host galaxy have been theoretically predicted, as results of GW recoils. However, so far there has been no unambiguous detection of such offsets. Such a detection would have a tremendous impact on our understanding of the evolution of galaxies and large scale structures.

Figure 1 : Image HST en ultra-violet de la galaxie 3C 186 (Morishita et al. 2022 arXiv220412499). Les contours verts et bleus affichent la carte NOEMA aux longueurs d’onde millimétriques (Castignani et al. 2022 arXiv220405882). Ils montrent un net décalage de 8 kpc entre le réservoir de gaz moléculaire de la galaxie hôte (vert) et le continuum d’émission de la source radio/quasar (bleu).

In order to tackle this challenge, an international team of researchers has studied 3C 186, a powerful extra-galactic radio source, at a distance of 8 billion light years from the earth, which has emerged as a strong GW recoil candidate. The team has studied the molecular gas of the 3C 186 galaxy, using the Northern Extended Millimeter Array (NOEMA) at the Plateau de Bure, in the French Alps. In a just published Astronomy and Astrophysics Letter, they report new observations of 3C 186 at millimeter wavelengths, at an unprecedented high resolution and high sensitivity. They were able to detect the CO emission of the galaxy, and discover a molecular disk of 8 10^10 Msun, rotating around the galaxy center. This disk is clearly offset by 8kpc in projection from the radio source/quasar, revealed by the broad line regions around the massive black hole (cf Figure 1). Also the CO line reveals that the velocity of the galaxy and the quasar are offset by 2000km/s, along the line of sight. The study thus reports the first confirmation of a GW recoil via high-resolution millimeter observations. These interferometric observations were indeed vital to test the GW recoil scenario, as they probe both the molecular gas reservoir that is the fuel of star formation within the host galaxy as well as the continuum emission associated with the radio source, which is ultimately powered by the accretion onto the SMBH.

The observed displacement is in remarkable agreement with the theoretical predictions for GW kicks. This is interpreted by the team as the result of a strong gravitational wave recoil as two SMBHs coalesced after the merger of their host galaxies. This would be the most energetic event ever observed : a major merger of SMBHs of comparable mass.

The work opens new perspectives for next generation radio telescope facilities, such as the Square Kilometer Array (SKA), and the pulsar-timing array (PTA), a giant network of radio telescopes, monitoring hundreds of pulsars. They will be directly sensitive to GWs emitted from coalescing SMBHs of masses similar to those we expect to find in radio sources such as 3C 186.

Reference

Castignani, G., Meyer, E., Chiaberge, M., Combes F. et al : 2022, NOEMA observations support a recoiling black hole in 3C186, Astronomy & Astrophysics Letters, 661, L2 www.aanda.org

Contact

  • Francoise Combes
    LERMA, Observatoire de Paris - PSL, CNRS, Collège de France, Sorbonne Univ.
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