GA-NIFS: JWST discovers an offset AGN 740 million years after the Big Bang (2312.03589v2)

Abstract: A surprising finding of recent studies is the large number of Active Galactic Nuclei (AGN) associated with moderately massive black holes ($\rm \log(M_\bullet/M_\odot)\sim 6-8$), in the first billion years after the Big Bang ($z>5$). In this context, a relevant finding has been the large fraction of candidate dual AGN, both at large separations (several kpc) and in close pairs (less than a kpc), likely in the process of merging. Frequent black hole merging may be a route for black hole growth in the early Universe; however, previous findings are still tentative and indirect. We present JWST/NIRSpec-IFU observations of a galaxy at $z=7.15$ in which we find evidence for a $\rm \log(M_\bullet/M_\odot)\sim7.7$ accreting black hole, as traced by a broad component of H$\beta$ emission, associated with the Broad Line Region (BLR) around the black hole. This BLR is offset by 620 pc in projection from the centroid of strong rest-frame optical emission, with a velocity offset of $\sim$40 km/s. The latter region is also characterized by (narrow) nebular emission features typical of AGN, hence also likely hosting another accreting black hole, although obscured (type 2, narrow-line AGN). We exclude that the offset BLR is associated with Supernovae or massive stars, and we interpret these results as two black holes in the process of merging. This finding may be relevant for estimates of the rate and properties of gravitational wave signals from the early Universe that will be detected by future observatories like LISA.

• The paper reports the identification of an offset type 1 AGN using JWST's NIRSpec-IFU, with a broad-line region displaced by 620 pc from the galaxy's center.
• It finds a 40 km/s velocity offset between the broad-line and narrow-line regions, strongly supporting a merger scenario.
• The study implies that early universe mergers played a critical role in the rapid growth of supermassive black holes.

JWST Discovers an Offset AGN 740 Million Years After the Big Bang

The paper "GA-NIFS: JWST Discovers an Offset AGN 740 Million Years After the Big Bang" presents intriguing findings from the James Webb Space Telescope (JWST) regarding an Active Galactic Nucleus (AGN) discovered at a high redshift, specifically at $z=7.15$. This AGN adds critical insights into the presence and characteristics of massive black holes during the early cosmological periods, only 740 million years after the Big Bang.

Key Findings

The research utilized JWST's NIRSpec-IFU to observe a galaxy system known as ZS7. The primary discovery was the identification of a type 1 AGN marked by a broad H$\beta$ emission line. Significantly, this broad line region (BLR) is spatially offset by approximately 620 parsecs (pc) from the narrow-line emission center of the galaxy, ZS7. The velocity offset between the BLR and the narrow-line emission region is around 40 km/s, indicating a physical separation rather than just a spectroscopic peculiarity. The authors rule out other potential scenarios, such as supernovae or massive stellar populations, to conclude that this broad emission indeed stems from an accreting black hole.

The primary evidence suggesting a merger includes:

• The spatial offset and distinct kinematic properties of the BLR AGN.
• The presence of a significant companion galaxy structure indicating ongoing interactions.
• High-angular resolution images revealing a displacement in the near-infrared continuum from the galaxy to the AGN's location.

Implications and Theoretical Context

The identification of this offset AGN provides empirical support to theories suggesting high rates of black hole mergers in the early universe, posited as a mechanism for rapid black hole growth following the initial seeding phase. These findings are particularly significant given that current models and simulations imply a widespread population of dual AGNs at various evolutionary stages in the early universe.

From a broader theoretical perspective, the paper reinforces the potential contribution of merging black holes to the assembly of supermassive black holes as well as their role in shaping the evolutionary history of their host galaxies. Moreover, the paper postulates that such systems might be critical sources of gravitational waves detectable by future ground-based and space-based observatories, such as the Laser Interferometer Space Antenna (LISA).

Numerical Results

The researchers estimate the black hole’s mass to be around $10^{7.7} M_\odot$, based on the virial method applied to the broad H$\beta$ line. The AGN’s bolometric luminosity approximates $8.5 \times 10^{44}$ erg/s, accounting for accretion rates at about 14% of the Eddington limit. These measurements confirm the AGN is moderately massive yet that it resides in a highly active galaxy environment, indicative of conditions conducive to substantial growth via accretion and hierarchical merging.

Future Work and Prospects

This research opens numerous pathways for continued observation and paper. Further spectral and imaging data from JWST and other future telescopes will be crucial to confirm the prevalence and properties of dual and offset AGNs, thereby refining our understanding of high-redshift black hole dynamics and evolution. Additionally, more extensive surveys could potentially reveal similar systems, thereby offering a more statistical basis for evaluating the early universe's black hole merger rates.

This paper is a testament to JWST's capabilities and its pivotal role in advancing our understanding of cosmic history, particularly concerning the growth mechanisms of supermassive black holes and the observable universe's structure at early epochs. The insights provided not only enrich the narrative of cosmic evolution but also set a foundational base for the paper of gravitational waves from ancient cosmic epochs.