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The radio core structure of the luminous infrared galaxy NGC4418 A young clustered starburst revealed?

Eskil Varenius (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; John Conway (Institutionen för rymd- och geovetenskap, Onsala rymdobservatorium) ; Ivan Marti-Vidal (Institutionen för rymd- och geovetenskap, Onsala rymdobservatorium) ; Susanne Aalto (Institutionen för rymd- och geovetenskap, Radioastronomi och astrofysik) ; R. Beswick ; Francesco Costagliola ; H. R. Klockner
Astronomy and Astrophysics (0004-6361). Vol. 566 (2014),
[Artikel, refereegranskad vetenskaplig]

Context. The galaxy NGC4418 contains one of the most compact obscured nuclei within a luminous infrared galaxy (LIRG) in the nearby Universe. This nucleus contains a rich molecular gas environment and an unusually high ratio of infrared-to-radio luminosity (q-factor). The compact nucleus is powered by either a compact starburst or an active galactic nucleus (AGN). Aims. The aim of this study is to constrain the nature of the nuclear region (starburst or AGN) within NGC4418 via very-high-resolution radio imaging. Methods. Archival data from radio observations using the European Very Long Baseline Interferometry Network (EVN) and Multi-Element Radio Linked Interferometer Network (MERLIN) interferometers are imaged. Sizes and flux densities are obtained by fitting Gaussian intensity distributions to the image. The average spectral index of the compact radio emission is estimated from measurements at 1.4 GHz and 5.0 GHz. Results. The nuclear structure of NGC4418 visible with EVN and MERLIN consists of eight compact (<49 mas i.e. <8 pc) features spread within a region of 250 mas, i.e. 41 pc. We derive an inverted spectral index a >= 0.7 (S-nu proportional to nu(alpha)) for the compact radio emission. Conclusions. Brightness temperatures >10(4.8) K indicate that these compact features cannot be HII-regions. The complex morphology and inverted spectrum of the eight detected compact features is evidence against the hypothesis that an AGN alone is powering the nucleus of NGC4418. The compact features could be super star clusters with intense star formation, and their associated free-free absorption could then naturally explain both their inverted radio spectrum and the low radio-to-IR ratio of the nucleus. The required star formation area density is extreme, however, and close to the limit of what can be observed in a well-mixed thermal/non-thermal plasma produced by star formation, and is also close to the limit of what can be physically sustained.



Denna post skapades 2014-08-14. Senast ändrad 2016-04-11.
CPL Pubid: 201409

 

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Projekt

Denna publikation är ett resultat av följande projekt:


Advanced Radio Astronomy in Europe (RADIONET3) (EC/FP7/283393)