2006AJ....132..531K

We present the first systematic study of (non-radio-selected) radio-loud narrow-line Seyfert 1 (NLS1) galaxies. Cross-correlation of the Catalogue of Quasars and Active Nuclei with several radio and optical catalogs led to the identification of ∼11 radio-loud NLS1 candidates, including four previously known ones. This study almost triples the number of known radio-loud NLS1 galaxies if all candidates are confirmed. Most of the radio-loud NLS1 galaxies are compact, steep-spectrum sources accreting close to or above the Eddington limit. The radio-loud NLS1 galaxies of our sample are remarkable in that they occupy a previously rarely populated regime in NLS1 multiwavelength parameter space. While their [O III]/Hβ and Fe II/Hβ intensity ratios almost cover the whole range observed in NLS1 galaxies, their radio properties extend the range of radio-loud objects to those with small widths of the broad Balmer lines. Their black hole masses are generally at the upper observed end among NLS1 galaxies but are still unusually small in view of the radio loudness of the sources. Among the radio-detected NLS1 galaxies, the radio index R is distributed quite smoothly up to the critical value of R≃10 and covers about 4 orders of magnitude in total. Statistics show that ∼7% of the NLS1 galaxies are formally radio-loud, while only 2.5% exceed a radio index R>100. Implications for NLS1 models are discussed. Several mechanisms are considered as explanations for the radio loudness of the NLS1 galaxies and for the lower frequency of radio-loud galaxies among NLS1 galaxies than among quasars. While properties of most sources (with two to three exceptions) generally do not favor relativistic beaming, the combination of accretion mode and spin may explain the observations.