Overmassive Black Holes in Dwarf Galaxies Out to z ∼ 0.9 in the VIPERS Survey

Abstract

Supermassive black holes (SMBHs) are thought to originate from early universe seed black holes of mass M BH ∼ 102–105 M ⊙ and grown through cosmic time. Such seeds could be powering the active galactic nuclei (AGN) found in today’s dwarf galaxies. However, probing a connection between the early seeds and local SMBHs has not yet been observationally possible. Massive black holes hosted in dwarf galaxies at intermediate redshifts, on the other hand, may represent the evolved counterparts of the seeds formed at very early times. We present a sample of seven broad-line AGN in dwarf galaxies with a spectroscopic redshift ranging from z = 0.35 to z = 0.93. The sources are drawn from the VIPERS survey as having an Large Magellanic Cloud (LMC) like stellar mass (M ∗) derived from spectral energy distribution fitting, and they are all star-forming galaxies. Six of these sources are also X-ray AGN. The AGN are powered by SMBHs of >107 M ⊙, more massive than expected from the M BH–M ∗ scaling relation of AGN. Based on semianalytical simulations, we find that these objects are likely overmassive with respect to their hosts since early times (z > 4), independently of whether they formed as heavy (∼105 M ⊙) or light (∼102 M ⊙) seed black holes. In our simulations, these objects tend to grow faster than their host galaxies, contradicting models of synchronized growth. The host galaxies are found to possibly evolve into massive systems by z ∼ 0, indicating that local SMBHs in massive galaxies could originate in dwarf galaxies hosting seed black holes at higher z.