Emission-line diagnostics of low-metallicity active galactic nuclei

By B. Groves, T. Heckman, Guinevere Kauffmann Mpa, Garching, Germany Jhu, Baltimore., Md., Usa. 2006

213 citations recorded by the source corpus.

Current emission-line-based estimates of the metallicity of active galactic nuclei (AGN) at both high and low redshifts indicate that AGN have predominantly solar-to-supersolar metallicities. This leads to the question: do low-metallicity AGN exist? In this paper, we use photoionization models to examine the effects of metallicity variations on the narrow emission-lines from an AGN. We explore a variety of emission-line diagnostics that are useful for identifying AGN with low-metallicity gas. We find that line ratios involving [N II] are the most robust metallicity indicators in galaxies where the primary source of ionization is from the active nucleus. Ratios involving [S II] and [O I] are strongly affected by uncertainties in modelling the density structure of the narrow-line clouds. To test our diagnostics, we turn to an analysis of AGN in the Sloan Digital Sky Survey (SDSS). We find a clear trend in the relative strength of [N II] with the mass of the AGN-host galaxy. The metallicity of the ISM is known to be correlated with stellar mass in star-forming galaxies; our results indicate that a similar trend exists for AGN. We also find that the best-fitting models for typical Seyfert narrow-line regions (NLRs) have supersolar abundances. Although there is a mass-dependent range of a factor of 2‐3 in the NLR metallicities of the AGN in our sample, AGN with subsolar metallicities are very rare in the SDSS. Out of a sample of ∼23 000 Seyfert 2 galaxies, we find only ∼40 clear candidates for AGN with NLR abundances that are below solar.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/62220ef327e59a249f0c2087b255c0deb184b785

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Citations in the corpus

• Metallicities and Physical Conditions in Star-forming Galaxies at z ~ 1.0-1.5 (2008, methodology): “As discussed in Paper I and Groves et al. (2006), several causes may account for this offset, in terms of the prevailing physical conditions in the H II regions of high-redshift galaxies.”
• DWARF GALAXIES WITH OPTICAL SIGNATURES OF ACTIVE MASSIVE BLACK HOLES (2013, background): “While models of low-metallicity AGNs overlap with low-metallicity starbursts (Groves et al. 2006), only one of the broad-line sources in the star-forming part of the [O III]/Hβ vs. [N II]/Hα diagram also has narrow-line ratios consistent with a low-metallicity AGN (source B).”
• The Mass Function of Active Black Holes in the Local Universe (2007, background): “This has been observed in NGC 4395 (e.g., Kraemer et al. 1999), with low significance in the original Greene & Ho (2004) sample, and quite strikingly in our new sample of Type 2 low-mass BHs (A. J. Barth et al., in preparation; see also Groves et al. 2006).”
• PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT (2010, background): “Groves et al. (2006) have studied emission line diagnostics in lower metallicity AGN, finding that the [N II]/Hα ratio is much more metallicity-dependent than [O III]/Hβ, and that therefore low metallicity AGN move to the left on the diagram, with relatively little vertical shift.”
• Galaxy pairs in the Sloan Digital Sky Survey - IV: Interactions trigger AGN (2011, background): “…Veilleux & Osterbrock1987), line ratio diagrams have evolved into a standard tool for categorizing the dominant source of ionizing radiation in a galaxy (Dopita et al. 2000; Kewley et al. 2001, 2006b; Kauffmann et al. 2003a; Stasinska et al. 2006; Groves et al. 2006; Cid-Fernandes et al. 2010).”
• Accretion and star formation rates in low‐redshift type II active galactic nuclei (2009, methodology): “Groves et al. (2006a) and KH09 suggested somewhat different procedures that serve the same purpose.”
• A NEW DIAGNOSTIC OF ACTIVE GALACTIC NUCLEI: REVEALING HIGHLY ABSORBED SYSTEMS AT REDSHIFT >0.3 (2011, background): “Some attributed this trend to varying HII region conditions (Liu et al. 2008; Brinchmann et al. 2008; Hainline et al. 2009) while others claim that additional AGN contribution may be the driving factor (Groves et al. 2006; Wright et al. 2010).”
• A New Sample of Low-Mass Black Holes in Active Galaxies (2007, background): “The decrease in [NII ]/Hα is most easily explained as a decrease in the gas-phase metallicity of these AGNs (e.g., Kraemer et al. 1999; Groves et al. 2006).”
• A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies from High-resolution Radio Observations (2019, background): “…of AGNs, as [O I]/Hα is sensitive to the hardness of the radiation field, which moves AGN host galaxies to the right, while [N II]/Hα is primarily sensitive to metallicity and low-mass galaxies are generally metal-poor, which moves them to the left (e.g., Groves et al. 2006; Kewley et al. 2006).”
• RELATIONS BETWEEN CENTRAL BLACK HOLE MASS AND TOTAL GALAXY STELLAR MASS IN THE LOCAL UNIVERSE (2015, background): “…AGN, which are likely to reside in lowmass galaxies, have line ratios significantly different from Seyferts in more metal-rich systems and overlap with low-metallicity starbursts in the [OIII]/Hβ vs. [NII]/Hα (i.e., BPT) diagram (Groves et al. 2006; Kewley et al. 2013; Reines et al. 2014).”
• Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths (2015, background): “This finding should be modulated by the fact that the SDSS AGN sample is much larger than that with available ultraviolet data in Figs 5 and 6.”