Narrow-Line Seyfert 1s: 15 Years Later

The spectroscopic properties of the objects that came to be called Narrow-Line Seyfert 1s were first systematically described and named as such 15 years ago by Osterbrock & Pogge (1985). At the time, they were a relatively rare and peculiar subclass of Seyfert galaxies. Their discovery in large numbers in X-ray surveys, however, has elevated them to a role as important members of the AGN family, ones which may hold many keys to understanding the physics of AGN across the electromagnetic spectrum. This contribution reviews the spectral classification of the narrow-line Seyfert 1s, and describes some of the properties that make this unusual class of objects interesting. 1 Narrow-Line Seyfert 1s “This unusual object merits further observations...” Davidson & Kinman (1978) On the possible importance of Markarian 359 The discovery of a new class or subclass of astronomical objects almost never begins with the “definitive” paper, since that paper is more often than not the outcome of a more systematic investigation into previous reports that some objects might be interesting. So too with narrow-line Seyfert 1s (NLS1s). The watershed year in the prehistory of NLS1s is 1978. Davidson & Kinman (5) described the unusual and potentially important spectral properties of Markarian 359. Mrk 359 has a Seyfert 1-like spectrum with unusually narrow permitted lines; the FWHM of Hβ was 520±100 km s, comparable to what is seen in classic Seyfert 2s. The second curiosity of 1978 was Mrk 42. Koski (14) and Phillips (21) remarked that Markarian 42 had many of the spectral properties of Seyfert 1s, but that the Fe ii and H i lines were very narrow, like in Seyfert 2s. Preprint submitted to Elsevier Preprint 5 February 2008 In 1983, Osterbrock & Dahari (16) undertook a systematic classification of a sample of Seyferts and candidate Seyferts, and noted in their tables that four showed unusual properties, namely “narrow H i and Fe ii ...Fe ii strong” (Mrk 493), and “Very narrow H i ... but noticeably wider than [O iii]” (Mrk 783). It was work that led Don Osterbrock to undertake a systematic search for and study of those objects which, along the lines first noted by Davidson & Kinman, had all the basic spectral characteristics of Seyfert 1s but unusually narrow permitted lines. It was this project that Don invited me to join as a new graduate student at UC Santa Cruz in 1984. The rest, as they say, is history... In Osterbrock & Pogge (18), we defined NLS1s to be those galaxies whose nuclear spectra are generally like those of Seyfert 1s (strong Fe ii, [O iii] relatively weak compared to the Balmer lines), but with line widths much narrower than typical Seyfert 1s. The formal spectral classification criteria for NLS1 galaxies that has emerged since are: • Narrow permitted lines only slightly broader than the forbidden lines. • [O iii]/Hβ < 3, but exceptions are allowed if there is also strong [Fevii] and [Fex] present, unlike what is seen in Seyfert 2s. • FWHM(Hβ)<2000 km s. The first two criteria are from our original classification (18), while the maximum line-width criterion was introduced by Goodrich (8) in his spectropolarimetric study. The essence of the classification is shown graphically in Figure 1, which contrasts an NLS1 against Seyfert 1s and 2s. While the Hβ line of the NLS1 Mrk 42 is not much wider than the Seyfert 2 Mrk 1066, the other spectral lines, especially [O iii] and Fe ii, appear in about the same proportions, if with narrower Fe ii widths, as they do in Seyfert 1s like NGC3516. 2 I Zw 1 and the Elusive Type 2 QSOs “So, What is 1E0449.4−2834 Anyway?” Halpern, Eracleous, & Forster (1998) NLS1s appear to have analogs among the more luminous QSOs, so-called narrow-line QSOs. The prototype of the narrow-line QSOs is arguably I Zw1 (15). The luminosity of I Zw1 places it at the ragged boundary between the most luminous Seyfert 1s and the least-luminous QSOs, an arbitrary dividing line set at MB = −23mag (27). Sargent (25) had noted as early as 1968 that I Zw1 had unusually strong Fe ii emission. Phillips (19; 20) made landmark studies of the Fe ii lines in this object, assisted both by their strength and