Classification of active galaxies and the prospect of a unified phenomenology

Lawrence, Andy

doi:10.1086/131989

Cited by 92 publications

(54 citation statements)

References 91 publications

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“…Several investigations in the literature have yielded results consistent with the predictions of this scheme, e.g., the featureless continuum is stronger in Seyfert 1 than in Seyfert 2 galaxies (Lawrence 1987;Mas-Hesse et al 1994). Kinney et al (1991) showed similar ultraviolet slopes for Seyfert 2 and Seyfert 1 galaxies.…”

Section: Introductionsupporting

confidence: 71%

Seyfert Galaxies: Nuclear Radio Structure and Unification

Lal

Shastri

Gabuzda

2011

ApJ

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A radio study of a carefully selected sample of 20 Seyfert galaxies that are matched in orientation-independent parameters, which are measures of intrinsic active galactic nucleus power and host galaxy properties, is presented to test the predictions of the unified scheme hypothesis. Our sample sources have core flux densities greater than 8 mJy at 5 GHz on arcsec scales due to the feasibility requirements. These simultaneous parsec-scale and kiloparsec-scale radio observations reveal (1) that Seyfert 1 and Seyfert 2 galaxies have an equal tendency to show compact radio structures on milliarcsecond scales, (2) the distributions of parsec-scale and kiloparsec-scale radio luminosities are similar for both Seyfert 1 and Seyfert 2 galaxies, (3) there is no evidence for relativistic beaming in Seyfert galaxies, (4) similar distributions of source spectral indices in spite of the fact that Seyferts show nuclear radio flux density variations, and (5) the distributions of the projected linear size for Seyfert 1 and Seyfert 2 galaxies are not significantly different as would be expected in the unified scheme. The latter could be mainly due to a relatively large spread in the intrinsic sizes. We also find that a starburst alone cannot power these radio sources. Finally, an analysis of the kiloparsec-scale radio properties of the CfA Seyfert galaxy sample shows results consistent with the predictions of the unified scheme.

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Section: Introductionsupporting

confidence: 71%

Seyfert Galaxies: Nuclear Radio Structure and Unification

Lal

Shastri

Gabuzda

2011

ApJ

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“…Our understanding of AGNs is based largely on an empirical foundation that suggests a differentiation based on luminosity (Lawrence 1987). We attempt to test for this differentiation by plotting the rest-frame core RM versus 15 GHz radio luminosity in Figure 36.…”

Section: Rm and Radio Luminositymentioning

confidence: 99%

A View through Faraday’s Fog. II. Parsec‐Scale Rotation Measures in 40 Active Galactic Nuclei

Zavala

Taylor

2004

ApJ

109

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“…As in biology, however, taxonomy derived from empirical observations can impose some order on the chaos. Table 1 shows the principal classes of AGN (adapted from Lawrence 1987Lawrence , 1993, organized according to their radio loudness and their optical spectra, i.e., whether they have broad emission lines (Type 1), only narrow lines (Type 2), or weak or unusual line emission. Within each of the groupings, different types of AGN are listed by increasing luminosity.…”

Section: Observed Properties and Empirical Classification Of Agnmentioning

confidence: 99%

Unified Schemes for Radio-Loud Active Galactic Nuclei

Urry¹,

Padovani²

1995

PASP

4,695

149

4,274

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ABSTRACT. The appearance of active galactic nuclei (AGN) depends so strongly on orientation that our current classification schemes are dominated by random pointing directions instead of more interesting physical properties. Light from the centers of many AGN is obscured by optically thick circumnuclear matter, particularly at optical and ultraviolet wavelengths. In radio-loud AGN, bipolar jets emanating from the nucleus emit radio through gamma-ray light that is relativistically beamed along the jet axes. Understanding the origin and magnitude of radiation anisotropies in AGN allows us to unify different classes of AGN; that is, to identify each single, underlying AGN type that gives rise to different classes through different orientations. This review describes the unification of radio-loud AGN, which include radio galaxies, quasars, and blazars. We describe the classification and general properties of AGN. We summarize the evidence for anisotropic emission caused by circumnuclear obscuration and relativistic beaming. We outline the two most plausible unified schemes for radio-loud AGN, one linking the high-luminosity sources (quasars and luminous radio galaxies) and one the low-luminosity sources (BL Lac objects and less luminous radio galaxies). Using the formalism appropriate to samples biased by relativistic beaming, we show that the population statistics for two schemes are in accordance with available data. We analyze the possible connections between low-and high-luminosity radio-loud AGN and conclude they probably are powered by similar physical processes, at least within the relativistic jet. We review potential difficulties with unification and conclude that none currently constitutes a serious problem. We discuss likely complications to unified schemes that are suggested by realistic physical considerations; these will be important to consider when more comprehensive data for larger complete samples become available. We conclude with a list of the ten questions we believe are the most pressing in this field.

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Classification of active galaxies and the prospect of a unified phenomenology

Cited by 92 publications

References 91 publications

Seyfert Galaxies: Nuclear Radio Structure and Unification

Seyfert Galaxies: Nuclear Radio Structure and Unification

A View through Faraday’s Fog. II. Parsec‐Scale Rotation Measures in 40 Active Galactic Nuclei

Unified Schemes for Radio-Loud Active Galactic Nuclei

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