This is the first paper in a series dedicated to the study of the emission-line and continuum properties of narrow line Seyfert 1 galaxies (NLS1s). We carried out a systematic search for NLS1s from objects assigned as "QSOs" or "galaxies" in the spectroscopic sample of the Sloan Digital Sky Survey Data Release 3 (SDSS DR3) by a careful modeling of their emission lines and continua. The result is a uniform sample comprising ∼ 2 000 NLS1. This sample dramatically increases the number of known NLS1 by a factor of ∼ 10 over previous compilations. This paper presents the parameters of the prominent emission lines and continua, which were measured accurately with typical uncertainties < 10%. Taking advantage of such an unprecedented large and uniform sample with accurately measured spectral parameters, we carried out various statistical analysis, some of which were only possible for the first time. The main results found are as follows.(1) Within the overall Seyfert 1 population, the incidence of NLS1s is strongly dependent on the optical, X-ray, and radio luminosities as well as the radioloudness. The fraction of NLS1 peaks around SDSS g-band absolute magnitude M g ∼ −22 m in the optical and ∼ 10 43.2 erg s −1 in the soft X-ray band, and decreases quickly as the radio-loudness increases. (2) On average the relative FeII emission, R 4570 = F eII(4434 − 4684)/Hβ, in NLS1s is about twice that in normal AGNs, and is anti-correlated with the broad component width of the Balmer emission lines. (3) The well-known anti-correlation between the width of broad low-ionization lines and the soft X-ray spectral slope for broad line AGNs extends down to F W HM ∼ 1 000 km s −1 in NLS1s, but the trend appears to

We investigate the relationship between the linewidths of broad Mg II λ2800 and Hβ in active galactic nuclei (AGNs) to refine them as tools to estimate black hole (BH) masses. We perform a detailed spectral analysis of a large sample of AGNs at intermediate redshifts selected from the Sloan Digital Sky Survey, along with a smaller sample of archival ultraviolet spectra for nearby sources monitored with reverberation mapping (RM). Careful attention is devoted to accurate spectral decomposition, especially in the treatment of narrow-line blending and Fe II contamination. We show that, contrary to popular belief, the velocity width of Mg II tends to be smaller than that of Hβ, suggesting that the two species are not cospatial in the broad-line region. Using these findings and recently updated BH mass measurements from RM, we present a new calibration of the empirical prescriptions for estimating virial BH masses for AGNs using the broad Mg II and Hβ lines. We show that the BH masses derived from our new formalisms show subtle but important differences compared to some of the mass estimators currently used in the literature.

We report the identification of 2MASX J032441.19ϩ341045.9 (hereafter 2MASX J0324ϩ3410) with an appealing object that shows the dual properties of both a narrow-line Seyfert 1 galaxy (NLS1) and a blazar. Its optical spectrum, which has a Hb line width of about 1600 km s Ϫ1 (FWHM), an [O iii]-to-Hb line ratio of Ӎ0.12, and strong Fe ii emission, clearly fulfills the conventional definition of NLS1s. On the other hand, 2MASX J0324ϩ3410 also exhibits some behavior that is characteristic of blazars, including a flat radio spectrum above 1 GHz, a compact core plus a one-sided jet structure on milliarcsecond scale at 8.4 GHz, highly variable fluxes in the radio, optical, and X-ray bands, and a possible detection of TeV g-ray emission. On its optical image, obtained with the HST WFPC2, the active nucleus is displaced from the center of the host galaxy, which exhibits an apparent one-armed spiral structure extended to 16 kpc. The remarkable hybrid behavior of this object presents a challenge to current models of NLS1s and g-ray blazars.