📝 SummarXiv

Abstract

The remnants of galaxy mergers may host multiple off-nuclear massive black holes (MBHs), some of which may wander indefinitely within the host galaxy halos. Tracing the population of offset MBHs is essential for understanding how the distribution of MBHs in the Universe evolves through galaxy mergers, the efficiency of binary MBH formation, and the rates at which MBHs are seeded in low-mass satellite galaxies. Offset MBHs can be observationally traced if they are accreting and detectable as spatially offset active galactic nuclei (AGN). In this work, we build the largest uniform sample of spatially offset AGN candidates (328) by matching sources from the Very Large Array Sky Survey (VLASS) to galaxies in the Sloan Digital Sky Survey (SDSS). Based on the radio source surface density, 29+/-3% are unrelated chance projections. The offset AGN occupation fraction is positively correlated with host galaxy stellar mass, consistent with predictions that most offset MBHs will reside in massive halos. However, this trend vanishes, and may reverse, at the lowest stellar masses, potentially reflecting the weaker host galaxy gravitational potentials. The offset AGN occupation fraction shows no significant evolution with orbital radius, and the agreement with predictions suggests a binary MBH formation rate of <0.5 per merger. Finally, for offset MBHs down to masses of 10^5 Solar masses, the occupation fraction is ~30-70 times lower than the expected value assuming all accreted satellites host a MBH. This result may suggest a relatively low MBH seeding efficiency.