New Results from a Survey of Active Galactic Nuclei

Across the universe, luminous galactic centers are fueled by supermassive black holes that accrete gas, dust, and stars from their surroundings. These powerful active galactic nuclei (AGN) radiate across the electromagnetic spectrum and emit jets of energetic particles, potentially shaping the evolution of the galaxies they inhabit. Collecting spectra of AGN is key to understanding the structure of the material that surrounds them and the role they may play in galaxy evolution — and a new public data release from a spectroscopic survey of AGN discovered by the Neil Gehrels Swift Observatory has expanded our ability to probe these objects.

An illustration of the Neil Gehrels Swift Observatory in front of a gamma-ray burst

An illustration of the Neil Gehrels Swift Observatory in front of a gamma-ray burst. [Spectrum and NASA E/PO, Sonoma State University, Aurore Simonnet]

Since 2005, the Neil Gehrels Swift Observatory has monitored the sky from gamma-ray to optical wavelengths, primarily in pursuit of the sources of gamma-ray bursts: extragalactic explosions potentially caused by massive stars going supernova or compact objects merging. However, Swift sees far more than just gamma-ray bursts — its Burst Alert Telescope (BAT), which scans 80% of the sky each day at X-ray and gamma-ray energies (14–195 kiloelectronvolts), has discovered hundreds of AGN in the local universe.

But detecting AGN is just the first step toward understanding the nature and importance of these objects — dedicated spectroscopic follow-up is a critical next step. Enter the BAT AGN Spectroscopic Survey (BASS): a project that aims to survey the most powerful AGN that have been detected in high-energy X-rays by Swift Observatory. In a new special issue of the Astrophysical Journal Supplement Series, the BASS team presents the latest step toward their goal of producing an immense catalog of AGN spectra.

projection of the sky with symbols indicating the locations of the active galactic nuclei being surveyed

Locations of the objects surveyed in the second BASS data release. The symbol shape and color indicates the instrument and telescope used to collect that object’s spectrum. Click to enlarge. [Koss et al. 2022]

This data release contains 1,449 optical spectra — 1,181 of which have never been released before — and 233 near-infrared spectra, all from 858 AGN in the local universe. The just published special issue presents catalogs of spectra, derived quantities, and first science results, including the following important findings:

  • The objects discovered by the Burst Alert Telescope span a wide range of properties. Namely, the black hole masses, luminosities, accretion rates, and degree to which the targets are obscured by gas and dust all vary by at least five orders of magnitude, making this survey a useful probe of a wide variety of AGN. Additionally, few of the sources observed in this survey are contained in other surveys, making the BASS project a source of unique information.
  • For the first time, the black hole mass function and the Eddington ratio distribution function — how the black hole mass and AGN luminosity vary with other factors — have been determined directly for heavily obscured objects. The resulting distribution functions show that obscured AGN are intrinsically less luminous compared to their theoretical maximum luminosities than their unobscured counterparts. This suggests that radiation plays a large role in determining the structure of the material close to an AGN.
  • plot of number of active galactic nuclei versus redshift

    Redshift distribution of AGN in the BASS second data release. [Oh et al. 2022]

    The masses of the supermassive black holes at the heart of obscured AGN tend to be underestimated when the mass is determined from measurements of hydrogen emission lines. Researchers can reduce this effect in the future studies by applying multiplicative factors when estimating masses this way.
  • New diagnostics based on mid-infrared luminosities can distinguish between obscured and unobscured AGN, yielding new candidate sources that are heavily obscured. However, separating out star-forming galaxies remains a challenge.

The publicly available sample of spectra developed by the BASS team provides a valuable tool for researchers wishing to study AGN in the local universe. Looking forward, the team plans to supplement their current catalog with observations of fainter sources made by the Burst Alert Telescope, expanding our understanding of these cosmic engines.


Special ApJS Issue on the BAT AGN Spectroscopic Survey Second Data Release

“BAT AGN Spectroscopic Survey XXI: The Data Release 2 Overview,” Michael J. Koss et al 2022 ApJS 261 1. doi:10.3847/1538-4365/ac6c8f