Narrow-Line Seyfert 1 Galaxies

It is generally supposed that a characteristic feature of active galactic nuclei (AGN), relating the X-ray and optical/UV regime, is the big blue bump (BBB) (e.g., Mathews & Ferland 1987), which may be associated with thermal accretion disk emission (Rees 1984). The presence of the BBB, which is located in the EUV range, is indicated by upturns in the far-UV and soft X-rays. The strength and location of the BBB likely depend on the black hole mass, the luminosity, and the accretion rate. However, because a direct observation of the EUV range is extremely difficult due to Galactic absorption, it is still an open question as to whether the BBB is as prominent as it is generally assumed or whether it is a shallower feature (e.g., Laor et al. 1997). The strength of a variety of emission lines in the optical domain and their emission line flux ratios that are sensitive to different parts of the ionizing continuum in the EUV can be used as tracers of the continuum shape and strength. Narrow-Line Seyfert 1 galaxies (NLS1s) may hold key information about the structure and evolution of AGN. Although the permitted emission line profiles in NLS1 spectra are unusually narrow, FWHM less than 2000 km/s, like in Seyfert 2 galaxies, strong optical and ultraviolet FeII emission, distinct X-ray continuum properties, and rapid, large amplitude X-ray variations unmask them as type 1 Seyfert galaxies (Osterbrock & Pogge 1985; Goodrich 1989; Boller et al. 1997; Mathur 2000; Pogge 2000; Grupe 2000,2004). Most of the observed properties of NLS1s can be explained in the framework of a low-mass black hole (few times 10^6 Msol) and a high accretion rate that is close to the Eddington limit. NLS1 galaxies are prime candidates to investigate the strength and shape of the soft X-ray - EUV continuum, since they show generally a strong soft X-ray excess (Boller et al. 1996; Leighly 2000) and also steeper hard X-ray continua than normal Seyfert 1 galaxies (Brandt et al. 1997; Leighly 1999). Recently, Wang & Netzer (2003) suggested that the soft X-ray properties of NLS1s are natural consequences of their super-Eddington accretion rates.

We have observed a sample of about 40 bright nearby NLS1 galaxies using the 1.5m telescope at Cerro Tololo Inter-American Observatory and the 2.1m telescope at Kitt Peak National Observatory. The observations recorded at CTIO were obtained as part of the Small and Moderate Aperture Research Telescopes (SMARTS) collaboration.

Fig.1 - Rest-frame spectra of the NLS1 galaxy NGC4748 - the flux is given in units of 10^-15 erg/s cm^2 A. The result of the decomposition of these NLS1 galaxy spectra is shown, i.e. a power law continuum, a host galaxy template spectrum, a Balmer continuum spectrum, and an appropriately broadened and scaled FeII emission spectrum. The resulting fit is shown as thick line. The residuum spectra is plotted in the bottom panel the figure.

Most of the F([NeV]3426)/F(Hb) and F(HeII4686)/F(Hb) emission line ratios which be measured for the NLS1 galaxies are consistent with the theoretical range that is expected for NLR gas photoionized by an AGN continuum as is indicated by the dotted line in Figure 2. The same trend is given by the small sample of classical NLS1 galaxies studied by Osterbrock & Pogge (1985). However, 4 NLS1 galaxies of our sample exhibit several times stronger [NeV]3426 emission relative to Hb than expected based on the relative strength of the HeII4686 emission line. To compare their distribution with those of the normal BLS1 galaxies we include measurements of these line ratios that are observed for BLS1s (Koski 1978; Cohen 1983; Kraemer et al.1999). In comparison with the NLS1 galaxies the normal BLS1 galaxies show a very similar distribution of the F(HeII4686)/F(Hb) vs. F([NeV]3426)/F(Hb) ratios. While most of the BLS1 galaxies also follow the expected relation for these two line ratios like the NLS1s, a few BLS1 galaxies deviate significantly from this trend. Therefore, NLS1 and BLS1 galaxies show a very similar distribution of the F(HeII4686)/F(Hb) and F([NeV]3426)/F(Hb) ratios. This indicates that the range of shapes and relative strengths of the ionizing EUV continuum in NLS1 and BLS1 galaxies are similar at least up to ~100 eV.

Fig.2 - Comparison of the emission line ratios F(HeII4686)/F(Hb) vs. F([NeV]3426)/F(Hb) which we measure for the NLS1 galaxies of our sample (Dietrich, Crenshaw, & Kraemer 2004, filled diamonds) with measurements provided by Osterbrock & Pogge (1985) (filled triangles) and for normal Seyfert 1 Galaxies (Koski 1978 (open diamonds); Cohen 1983 (open triangles); Kraemer & Crenshaw 1999 (asterix). The dotted line indicate the expected value of 5x F(HeII4686)/F(Hb) = F([NeV]3426)/F(Hb).

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This page is under construction. Last update: April 30, 2005.

This page created by Matthias Dietrich
Questions or comments should be sent to: dietrich@astronomy.ohio-state.edu

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