BAT AGN Spectroscopic Survey I: Spectral Measurements, Derived Quantities, and AGN Demographics (1707.08123v1)

Abstract: We present the first catalog and data release of the Swift-BAT AGN Spectroscopic Survey (BASS). We analyze optical spectra of the majority of AGN (77%, 641/836) detected based on their 14-195 keV emission in the 70-month Swift BAT all-sky catalog. This includes redshift determination, absorption and emission line measurements, and black hole mass and accretion rate estimates for the majority of obscured and un-obscured AGN (74%, 473/641) with 340 measured for the first time. With ~90% of sources at z<0.2, the survey represents a significant census of hard-X-ray selected AGN in the local universe. In this first catalog paper, we describe the spectroscopic observations and datasets, and our initial spectral analysis. The FWHM of the emission lines show broad agreement with the X-ray obscuration (~94%), such that Sy 1-1.8 have NH<10^21.9 cm^-2, and Seyfert 2, have NH>10^21.9 cm^-2. Seyfert 1.9 show a range of column densities. Compared to narrow line AGN in the SDSS, the X-ray selected AGN have a larger fraction of dusty host galaxies suggesting these types of AGN are missed in optical surveys. Using the most sensitive [OIII]/Hbeta and [NII]/Halpha emission line diagnostic, about half of the sources are classified as Seyferts, ~15% reside in dusty galaxies that lack an Hbeta detection, but for which the line upper limits imply either a Seyfert or LINER, ~15% are in galaxies with weak or no emission lines despite high quality spectra, and a few percent each are LINERS, composite galaxies, HII regions, or in known beamed AGN.

Overview of the Swift-BAT AGN Spectroscopic Survey (BASS) Paper by Koss et al.

The Swift-BAT AGN Spectroscopic Survey (BASS) paper by Koss and collaborators presents a comprehensive catalog and initial data release of active galactic nuclei (AGN) detected through the Swift's Burst Alert Telescope (BAT) in the 14–195 keV energy range. This survey provides essential insights into AGN demographics, spectral measurements, and associated derived properties, highlighted by its robust spectroscopic analysis.

The paper aims to bridge the gap in AGN science by exploiting the BAT's ability to observe high-energy X-rays largely unobscured by intervening columns of gas and dust, thus offering an unfiltered view of AGN activities. This approach significantly enhances our understanding of AGN, particularly in addressing the underrepresentation of heavily obscured systems in optical surveys, and contributes to a more complete census of the local AGN population.

Key Findings

1. Redshift and Luminosity Distribution: The survey predominantly covers nearby AGN (90% with z < 0.2), much of which was previously undetected in optical bands due to obscuration. The hard X-ray selection ensures that BASS captures more elusive AGN properties, thereby improving the known AGN luminosity function at lower redshifts.
2. AGN Classification: A significant contribution of BASS lies in its detailed optical classification of AGN subtypes, revealing that nearly half are type 2 systems (Seyfert 2 or 1.9), and about a quarter are intermediate-type Seyferts. The diagnostic of emission lines confirms these classifications and highlights discrepancies with older catalogs, offering a more nuanced view of AGN structure and orientation-based phenomena.
3. Emission Line Diagnostics: Utilizing line ratio diagnostics like [NII]/Hα and [OIII]/Hβ, the paper emphasizes that many X-ray selected AGN reside in environments missed by optical surveys, often due to extinction issues. It further uncovers a distinct population characterized by extreme Balmer decrements indicative of dusty host galaxies.
4. X-ray vs. Optical Properties: There is a strong correlation between the X-ray properties and traditional optical AGN classifications, validating the use of X-ray data as a reliable way to infer physical evolutionary stages even when broad lines are not visible.
5. Black Hole Mass and Accretion Rates: BASS reports on black hole masses and accretion rates derived from both the X-ray luminosities and broad line widths. The analysis yields a broad range of black hole masses and suggests that higher luminosity AGN, particularly at higher redshifts, tend toward higher accretion rates as expected from AGN evolution models.
6. Unusual AGN Characteristics: The survey has identified peculiar AGN types, including those lacking broad optical lines yet showing low X-ray obscuration ("naked" AGN), double broad line systems, and "optically elusive" systems, enriching the dialogue on the diversity in AGN phenomenology and the complexities of structure.

Implications and Future Directions

• Completeness in AGN Census: BASS's inclusion of heavily obscured and dusty AGN offers a more complete picture of AGN demographics, informing cosmological models of black hole growth and the role of AGN feedback in galaxy evolution.
• AGN Physics and Environment: Findings related to dusty and seemingly unobscured AGN challenge established paradigms about AGN structure. Such discoveries invite further investigation into the nature and geometry of the obscuring torus as well as the co-evolution of AGN and their host galaxies.
• Benchmarking and Calibration: The BASS catalog serves as a local template for extrapolating the behaviors of higher redshift AGN detected in deep field surveys, thereby acting as a vital calibration source for refining AGN models.

Future work building upon BASS data is expected to explore specific aspects such as correlations between various bands, understanding the implications of obscuration on AGN scaling relations, and further analysis of the physical mechanisms driving AGN diversity. The community’s access to high-quality spectroscopic data through BASS is poised to deepen insights into the multi-faceted nature of AGN and unlock new questions for astrophysical inquiry.