A Comprehensive Study of 2 000 Narrow Line Seyfert 1 Galaxies from the Sloan Digital Sky Survey: I. The Sample

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 $\sim 2~000$ NLS1. This sample dramatically increases the number of known NLS1 by a factor of $\sim 10$ 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 (abbreviated).