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Register a new userNarrow-Line Seyfert 1 Galaxies
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S. Komossa
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I provide a short review of the properties of Narrow-line Seyfert 1 (NLS1) galaxies across the electromagnetic spectrum and of the models to explain them. Their continuum and emission-line properties manifest one extreme form of Seyfert activity. As such, NLS1 galaxies may hold important clues to the key parameters that drive nuclear activity. Their high accretion rates close to the Eddington rate provide new insight into accretion physics, their low black hole masses and perhaps young ages allow us to address issues of black hole growth, their strong optical FeII emission places strong constraints on FeII and perhaps metal formation models and physical conditions in these emission-line clouds, and their enhanced radio quiteness permits a fresh look at causes of radio loudness and the radio-loud radio-quiet bimodality in AGN.
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This work studies the optical emission line properties and physical conditions of the narrow line region (NLR) of seven narrow-line Seyfert 1 galaxies (NLS1). Our results show that the flux carried out by the narrow component of H-beta is, on average, 50% of the total line flux. As a result, the [OIII] 5007/H-beta ratio emitted in the NLR varies from 1 to 5, instead of the universally adopted value of 10. This has strong implications for the required spectral energy distribution that ionizes the NLR gas. Photoionization models that consider a NLR composed of a combination of matter-bounded and ionization-bounded clouds are successful at explaining the low [OIII] 5007/H-beta ratio and the weakness of low-ionization lines of NLS1s. Variation of the relative proportion of these two type of clouds nicely reproduce the dispersion of narrow line ratios found among the NLS1 sample. Assuming similar physical model parameters of both NLS1s and the normal Seyfert 1 galaxy NGC 5548, we show that the observed differences of emission line ratios between these two groups can be explained in terms of the shape of the input ionizing continuum. Narrow emission line ratios of NLS1s are better reproduced by a steep power-law continuum in the EUV -- soft X-ray region, with spectral index alpha ~ -2. Flatter spectral indices (alpha ~ -1.5) match the observed line ratios of NGC 5548 but are unable to provide a good match to the NLS1 ratios. This result is consistent with ROSAT observations of NLS1s, which show that these objects are characterized by steeper power-law indices than those of Sy1 galaxies with strong broad optical lines.
Narrow line Seyfert 1 (NLSy1) galaxies constitute a class of active galactic nuclei characterized by the full width at half maximum (FWHM) of the H$beta$ broad emission line < 2000 km/s and the flux ratio of [O III] to H$beta$ < 3. Their properties are not well understood since only a few NLSy1 galaxies were known earlier. We have studied various properties of NLSy1 galaxies using an enlarged sample and compared them with the conventional broad-line Seyfert 1 (BLSy1) galaxies. Both the sample of sources have z $le$ 0.8 and their optical spectra from SDSS-DR12 that are used to derive various physical parameters have a median signal to noise (S/N) ratio >10 per pixel. Strong correlations between the H$beta$ and H$alpha$ emission lines are found both in the FWHM and flux. The nuclear continuum luminosity is found to be strongly correlated with the luminosity of H$beta$, H$alpha$ and [O III] emission lines. The black hole mass in NLSy1 galaxies is lower compared to their broad line counterparts. Compared to BLSy1 galaxies, NLSy1 galaxies have a stronger FeII emission and a higher Eddington ratio that place them in the extreme upper right corner of the $R_{4570}$ - $xi_{Edd}$ diagram. The distribution of the radio-loudness parameter (R) in NLSy1 galaxies drops rapidly at R > 10 compared to the BLSy1 galaxies that have powerful radio jets. The soft X-ray photon index in NLSy1 galaxies is on average higher (2.9 $pm$ 0.9) than BLSy1 galaxies (2.4 $pm$ 0.8). It is anti-correlated with the H$beta$ width but correlated with the Fe II strength. NLSy1 galaxies on average have a lower amplitude of optical variability compared to their broad lines counterparts. These results suggest Eddington ratio as the main parameter that drives optical variability in these sources.
We have monitored the R-band optical linear polarisation of ten jetted NLSy1 galaxies with the aim to quantify their variability and search for candidate long rotation of the polarisation plane. In all cases for which adequate datasets are available we observe significant variability of both the polarisation fraction and angle. In the best-sampled cases we identify candidate long rotations of the polarisation plane. We present an approach that assesses the probability that the observed phenomenology is the result of pure noise. We conclude that although this possibility cannot be excluded it is much more likely that the EVPA undergoes an intrinsic evolution. We compute the most probable parameters of the intrinsic event which forecasts events consistent with the observations. In one case we find that the EVPA shows a preferred direction which, however, does not imply any dominance of a toroidal or poloidal component of the magnetic field at those scales.
It is arguably in the X-ray regime that Narrow-line Seyfert 1 galaxies (NLS1s) exhibit the most extreme behaviour. Spectral complexity, rapid and large amplitude flux variations, and exceptional spectral variability are well known characteristics. However, NLS1s are not eccentric, but form a continuous sequence with typical Seyfert 1 galaxies. Understanding the extreme behaviour displayed by NLS1s will provide insight to the general AGN phenomenon. In this review, I will examine some of the important NLS1 X-ray discoveries over the past twenty years. I will then explore recent work that looks at the nature of the primary X-ray source (i.e. the corona) in NLS1s, demonstrating how the corona can be compact, dynamic, and in some cases consistent with collimated outflow. X-ray observations of NLS1s will be key in determining the nature of the corona, resolving the disc-jet connection, and determining the origin of the radio loud/quiet dichotomy in AGN.
We have studied the location of narrow-line Seyfert 1 (NLS1) galaxies and broad-line Seyfert 1 (BLS1) galaxies on the M_BH - sigma relation of non-active galaxies. We find that NLS1 galaxies as a class - as well as the BLS1 galaxies of our comparison sample - do follow the M_BH-sigma relation of non-active galaxies if we use the width of the [SII]6716,6731 emission lines as surrogate for stellar velocity dispersion, sigma_*. We also find that the width of [OIII]5007 is a good surrogate for sigma_*, but only after (a) removal of asymmetric blue wings, and, more important, after (b) excluding core [OIII] lines with strong blueshifts (i.e., excluding galaxies which have their [OIII] velocity fields dominated by radial motions, presumably outflows). The same galaxies which are extreme outliers in [OIII] still follow the M_BH - sigma relation in [SII]. We confirm previous findings that NLS1 galaxies are systematically off-set from the M_BH - sigma relation if the full [OIII] profile is used to measure sigma. We systematically investigate the influence of several parameters on the NSL1 galaxies location on the M_BH - sigma plane: [OIII]_core blueshift, L/L_Edd, intensity ratio FeII/H_beta, NLR density, and absolute magnitude. Implications for NLS1 models and for their evolution along the M_BH - sigma relation are discussed.
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