Ultraviolet Diagnostics for the Emission‐Line Gas in Active Galaxies

Allen, M. G.; Dopita, Michael A.; Tsvetanov, Z.

doi:10.1086/305145

Cited by 93 publications

(123 citation statements)

References 41 publications

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“…However, He ii is sensitive to even low levels of shock emission. We show that the C iii]/He ii vs. C iv λ1549/He ii diagnostic proposed for AGN (Villar-Martin et al 1997;Allen et al 1998;Feltre et al 2016) also separates star-forming regions from shock-ionized gas (Figure 13). …”

Section: Discussionsupporting

confidence: 54%

PHOTOIONIZATION MODELS FOR THE SEMI-FORBIDDEN C iii] 1909 EMISSION IN STAR-FORMING GALAXIES

Jaskot

Ravindranath

2016

ApJ

168

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The increasing neutrality of the intergalactic medium at z > 6 suppresses Lyα emission, and spectroscopic confirmation of galaxy redshifts requires detecting alternative UV lines. The strong [C iii] λ1907+C iii] λ1909 doublet frequently observed in low-metallicity, actively star-forming galaxies is a promising emission feature. We present CLOUDY photoionization model predictions for C iii] equivalent widths (EWs) and line ratios as a function of starburst age, metallicity, and ionization parameter. Our models include a range of C/O abundances, dust content, and gas density. We also examine the effects of varying the nebular geometry and optical depth. Only the stellar models that incorporate binary interaction effects reproduce the highest observed C iii] EWs. The spectral energy distributions from the binary stellar population models also generate observable C iii] over a longer timescale relative to single-star models. We show that diagnostics using C iii] and nebular He ii λ1640 can separate star-forming regions from shock-ionized gas. We also find that density-bounded systems should exhibit weaker C iii] EWs at a given ionization parameter, and C iii] EWs could therefore select candidate Lyman continuum-leaking systems. In almost all models, C iii] is the next strongest line at < 2700Å after Lyα, and C iii] reaches detectable levels for a wide range of conditions at low metallicity. C iii] may therefore serve as an important diagnostic for characterizing galaxies at z > 6.

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Section: Discussionsupporting

confidence: 54%

PHOTOIONIZATION MODELS FOR THE SEMI-FORBIDDEN C iii] 1909 EMISSION IN STAR-FORMING GALAXIES

Jaskot

Ravindranath

2016

ApJ

168

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“…Unfortunately, existing shock models (e.g., Allen et al 1998Allen et al , 2008 cannot be straightforwardly compared with the AGN and SF models considered here, because of the different ways in which the photoionized gas is parametrized. Allen et al (1998Allen et al ( , 2008 investigate ultraviolet emission-line diagnostics to discriminate between photoionization by shocks and nuclear activity. They propose the C ii]λ2326/C iii]λ1908 versus C iv λ1550/C iii]λ1908 diagram as one of the most suitable discriminant between AGN and shocks with velocities less than 400 km s −1 , while higher-velocity shocks overlap with AGN models in this diagram.…”

Section: Discussionmentioning

confidence: 99%

“…Diagnostics involving the relative intensities of these 3 lines, originally investigated by Villar- Martin et al (1997), are commonly invoked to distinguish between photoionization by AGN and shocks in galaxies. This is because, while AGN and shocks produce similar optical-line ratios, the C iv λ1550/He ii λ1640 and C iii]λ1908/He ii λ1640 ultraviolet-line ratios produced by shocks are predicted to be much larger than those produced by AGN (Allen et al 1998;Groves et al 2004b, and references therein). These ultraviolet-line ratios have also been used to investigate the metallicity of narrow-line regions of AGN (e.g., Nagao et al 2006).…”

Section: Diagnostics Based On the C Iv λ1550 He Ii λ1640 And C Iii]λmentioning

confidence: 99%

“…These studies have shown that important constraints on the density, ionization and metallicity of the photoionized gas can be derived from the relative intensities of ultraviolet and (mainly) optical emission lines. This has led to the identification of standard optical line-ratio diagnostic diagrams to separate nebular emission from active and inactive galaxies, such as the Baldwin, Phillips & Terlevich (1981, hereafter BPT) and Veilleux & Osterbrock (1987) Martin et al 1997;Allen et al 1998), while they have not been systematically investigated yet to discriminate between nuclear activity and star formation.…”

Section: Introductionmentioning

confidence: 99%

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Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths

Feltre¹,

Charlot²,

Gutkin³

2016

Mon. Not. R. Astron. Soc.

272

461

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In the context of observations of the rest-frame ultraviolet and optical emission from distant galaxies, we explore the emission-line properties of photoionization models of active and inactive galaxies. Our aim is to identify new line-ratio diagnostics to discriminate between gas photoionization by active galactic nuclei (AGN) and star formation. We use a standard photoionization code to compute the emission from AGN narrow-line regions and compare this with calculations of the nebular emission from star-forming galaxies achieved using the same code. We confirm the appropriateness of widely used optical spectral diagnostics of nuclear activity versus star formation and explore new diagnostics at ultraviolet wavelengths. We find that combinations of a collisionally excited metal line or line multiplet, such as C iv λλ1548, 1551,iii]λ1892 and [C iii]λ1907+C iii]λ1909, with the He ii λ1640 recombination line are individually good discriminants of the nature of the ionizing source. Diagrams involving at least 3 of these lines allow an even more stringent distinction between active and inactive galaxies, as well as valuable constraints on interstellar gas parameters and the shape of the ionizing radiation. Several line ratios involving Ne-based emission lines, such as [Ne iv]λ2424, [Ne iii]λ3343 and [Ne v]λ3426, are also good diagnostics of nuclear activity. Our results provide a comprehensive framework to identify the sources of photoionization and physical conditions of the ionized gas from the ultraviolet and optical nebular emission from galaxies. This will be particularly useful to interpret observations of high-redshift galaxies with future facilities, such as the James Webb Space Telescope and extremely large ground-based telescopes.

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“…Viegas-Aldrovandi & Contini 1989;Murayama & Taniguchi 1998;Allen et al 1998;Evans et al 1999;Contini & Viegas 2001;Collins et al 2009). Groves et al (2004) use a conventional range of powerlaw indices of α = −2.0 to α = −1.2, for a single powerlaw covering 0.005 to 1 keV.…”

Section: Introductionmentioning

confidence: 99%

A Physically Based Model of the Ionizing Radiation From Active Galaxies for Photoionization Modeling

Childress

Groves

Sutherland

et al. 2016

ApJ

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We present a simplified model of Active Galactic Nucleus (AGN) continuum emission designed for photoionization modeling. The new model oxaf reproduces the diversity of spectral shapes that arise in physically-based models. We identify and explain degeneracies in the effects of AGN parameters on model spectral shapes, with a focus on the complete degeneracy between the black hole mass and AGN luminosity. Our re-parametrized model oxaf removes these degeneracies and accepts three parameters which directly describe the output spectral shape: the energy of the peak of the accretion disk emission E peak , the photon power-law index of the non-thermal emission Γ, and the proportion of the total flux which is emitted in the non-thermal component p NT . The parameter E peak is presented as a function of the black hole mass, AGN luminosity, and 'coronal radius' of the optxagnf model upon which oxaf is based. We show that the soft X-ray excess does not significantly affect photoionization modeling predictions of strong emission lines in Seyfert narrow-line regions. Despite its simplicity, oxaf accounts for opacity effects where the accretion disk is ionized because it inherits the 'color correction' of optxagnf. We use a grid of mappings photoionization models with oxaf ionizing spectra to demonstrate how predicted emission-line ratios on standard optical diagnostic diagrams are sensitive to each of the three oxaf parameters. The oxaf code is publicly available in the Astrophysics Source Code Library.

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Ultraviolet Diagnostics for the Emission‐Line Gas in Active Galaxies

Cited by 93 publications

References 41 publications

PHOTOIONIZATION MODELS FOR THE SEMI-FORBIDDEN C iii] 1909 EMISSION IN STAR-FORMING GALAXIES

PHOTOIONIZATION MODELS FOR THE SEMI-FORBIDDEN C iii] 1909 EMISSION IN STAR-FORMING GALAXIES

Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths

A Physically Based Model of the Ionizing Radiation From Active Galaxies for Photoionization Modeling

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