SDSS J143244.91+301435.3: a link between radio-loud narrow-line Seyfert 1 galaxies and compact steep-spectrum radio sources?

We present SDSS J143244.91+301435.3, a new case of a radio-loud narrow-line Seyfert 1 (RL NLS1) with a relatively high radio power (P-1.4 GHz = 2.1 x 10(25) W Hz(-1)) and large radio-loudness parameter (R-1.4 = 600 +/- 100). The radio source is compact with a linear size below similar to 1.4 kpc but, in contrast to most of the RL NLS1 discovered so far with such a high R-1.4, its radio spectrum is very steep (alpha = 0.93, S-nu alpha nu(-alpha)) and does not support a 'blazar-like' nature. Both the small mass of the central supermassive black hole and the high accretion rate relative to the Eddington limit estimated for this object (3.2 x 10(7) M-circle dot and 0.27, respectively, with a formal error of similar to 0.4 dex for both quantities) are typical of the NLS1 class. Through modelling the spectral energy distribution of the source, we have found that the galaxy hosting SDSS J143244.91+301435.3 is undergoing quite intense star formation (SFR = 50M(circle dot) yr(-1)), which, however, is expected to contribute only marginally (similar to 1 per cent) to the observed radio emission. The radio properties of SDSS J143244.91+301435.3 are remarkably similar to those of compact steep-spectrum (CSS) radio sources, a class of active galactic nuclei (AGN) mostly composed of young radio galaxies. This may suggest a direct link between these two classes of AGN, with CSS sources possibly representing the misaligned version (the so-called 'parent population') of RL NLS1 showing blazar characteristics.