Astron. Nachr. / AN 337, No. 1/2, 125 – 129 (2016) / DOI 10.1002/asna.201512277 The compact radio structure of radio-loud NLS1 galaxies and the relationship to CSS sources M. Gu 1,2, , Y. Chen 1,3 , S. Komossa 4 , W. Yuan 5 , and Z. Shen 1,3 1 Shanghai Astronomical Observatory, Chinese Academy of Science, Shanghai 200030, China 2 Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030, China 3 Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing, JiangSu 210008, China 4 Max-Planck-Institut f¨ ur Radioastronomie, Auf dem H¨ ugel 69, 53121 Bonn, Germany 5 Key Lab for Space Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China Received 2015 Oct 19, accepted 2015 Oct 21 Published online 2016 Feb 03 Key words galaxies: active – galaxies: jets – galaxies: Seyfert – radio continuum: galaxies Narrow-line Seyfert 1 galaxies are thought to be young AGNs with relatively small black hole masses and high accretion rates. Radio-loud narrow-line Seyfert 1 galaxies (RLNLS1s) are very special, because some of them show blazar-like characteristics, while others resemble compact steep-spectrum sources. Relativistic jets were shown to exist in a few RLNLS1s based on VLBI observations and confirmed by the gamma-ray flaring of some of them. These properties may possibly be contrary to typical radio-loud AGNs, in light of the low black-hole masses, and high accretion rates. We present the compact radio structure of fourteen RLNLS1 galaxies from Very Long Baseline Array observations at 5 GHz in 2013. Although all these sources are very radio-loud with R > 100, their jet properties are diverse, in terms of their milli- arcsecond (mas) scale (pc scale) morphology and their overall radio spectral shape. The core brightness temperatures of our sources are significantly lower than those of blazars, therefore, the beaming effect is generally not significant in our sources, compared to blazars. This implies that the bulk jet speed may likely be low in our sources. The relationship between RLNLS1s and compact steep-spectrum sources, and the implications on jet formation are discussed based on the pc-scale jet properties. c  2016 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim 1 Introduction Narrow-line Seyfert 1 galaxies (NLS1s) are a subclass of active galactic nuclei (AGNs), with strong permitted opti- cal/UV Fe ii emission lines, relatively weak forbidden-line emission (i.e. [O iii] 5007/Hβ< 3), and their broad lines are narrower than those of normal broad-line Seyfert 1 galax- ies with FWHM(Hβ) less than 2000 km s −1 (Osterbrock & Pogge 1985; Goodrich 1989). NLS1s are often thought to be young and still evolving AGNs with relatively small black hole masses and high accretion rates (see review by Komossa 2008). NLS1s as a class show a low probabil- ity to be radio loud at ∼ 7 %, compared to normal broad line AGNss (e.g., Komossa et al. 2006; Zhou et al. 2006). Interestingly, radio-loud NLS1s (RLNLS1s) are inhomoge- neous in their radio properties. As shown in Komossa et al. (2006), most RLNLS1s in their sample are compact, steep spectrum sources, and hence likely associated with com- pact steep-spectrum (CSS) radio sources, while only a few NLS1s showed blazar-like behaviour. On the other hand, observational evidence has recently shown that a signifi-  Corresponding author: gumf@shao.ac.cn cant fraction of RLNLS1s at the highest radio-loudnesses, does display the characteristics of blazars, including large- amplitude radio flux and spectral variability, compact ra- dio cores, very high variability brightness temperatures, en- hanced optical continuum emission, flat X-ray spectra, and blazar-like spectral energy distributions (e.g., Yuan et al. 2008). Gigahertz-Peaked-Spectrum (GPS, projected linear size D < 1 kpc) and the more extended CSS sources (D < 15 kpc) make up to ∼ 40 % of radio-loud AGNs (O’Dea 1998). The currently preferred theory to explain their compact- ness is the “youth scenario” (Fanti et al. 1995), in which the radio jet will not have expanded much, and will still be relatively small if we observe it young. Therefore, the GPS and CSS sources could represent the earliest stages in the radio-loud AGN life cycle, before they expand into the large-scale “classical” doubles. This scenario is supported by estimated dynamical ages for GPS sources of t dyn ∼ 10 2 – 10 3 yr (e.g. Owsianik, Conway & Polatidis 1998; Tschager et al. 2000), and by radio spectral ages for the larger CSS sources of t sp < 10 4 yr (Murgia et al. 1999). The RLNLS1 with properties similar to CSS sources, therefore, are pre- c  2016 WILEY-VCH Verlag GmbH& Co. KGaA, Weinheim