@article {2015A\&A...574A.114V, title = {{Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz}}, journal = {Astronomy and Astrophysics}, volume = {574}, year = {2015}, month = {feb}, pages = {A114}, abstract = {Context. The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio-bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at low frequencies where the effects of free-free absorption are most prominent.
Aims: The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low-frequency spectra of compact and diffuse emission in the central starburst region of M82 via high-resolution radio imaging at low frequencies.
Methods: The international LOFAR telescope was used to observe M82 at 110-126 MHz and 146-162 MHz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines.
Results: The 154 MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3{\textquoteright}{\textquoteright}) and sensitivity ({$\sigma$} = 0.15 mJy/beam) at low frequencies ({\l}t}327 MHz). We detected 16 objects at 154 MHz, six of these also at 118 MHz. Seven objects detected at 154 MHz have not been catalogued previously. For the nine objects previously detected, we obtained spectral indices and emission measures by fitting models to spectra (combining LOFAR with literature data). Four weaker but resolved features are also found: a linear (50 pc) filament and three other resolved objects, of which two show a clear shell structure. We do not detect any emission from either supernova 2008iz or from the radio transient source 43.78+59.3. The images obtained using remote baselines show diffuse emission, associated with the outflow in M82, with reduced brightness in the region of the edge-on star-forming disk.}, keywords = {galaxies: individual: M82, galaxies: star formation, galaxies: starburst, supernovae: individual: 2008iz, techniques: high angular resolution, techniques: interferometric}, doi = {10.1051/0004-6361/201425089}, url = {http://adsabs.harvard.edu/abs/2015A\%26A...574A.114V}, author = {Varenius, E. and Conway, J.~E. and Mart{\'\i}-Vidal, I. and Beswick, R. and Deller, A.~T. and Wucknitz, O. and Jackson, N. and Adebahr, B. and M. A. P{\'e}rez-Torres and Chy{\.z}y, K.~T. and Carozzi, T.~D. and Mold{\'o}n, J. and Aalto, S. and Beck, R. and Best, P. and Dettmar, R.-J. and van Driel, W. and Brunetti, G. and Br{\"u}ggen, M. and Haverkorn, M. and Heald, G. and Horellou, C. and Jarvis, M.~J. and Morabito, L.~K. and Miley, G.~K. and R{\"o}ttgering, H.~J.~A. and Toribio, M.~C. and White, G.~J.} } @article {219, title = {{The radio core structure of the luminous infrared galaxy NGC 4418. A young clustered starburst revealed?}}, journal = {\aap}, volume = {566}, year = {2014}, pages = {A15}, keywords = {galaxies: individual: NGC 4418, galaxies: Seyfert, galaxies: star formation}, doi = {10.1051/0004-6361/201323303}, author = {Varenius, E. and Conway, J.~E. and Mart{\'\i}-Vidal, I. and Aalto, S. and Beswick, R. and Costagliola, F. and Kl{\"o}ckner, H.-R.} } @article {2014A\&A...566A..15V, title = {{The radio core structure of the luminous infrared galaxy NGC 4418. A young clustered starburst revealed?}}, journal = {Astronomy and Astrophysics}, volume = {566}, year = {2014}, month = {jun}, pages = {A15}, abstract = {Context. The galaxy NGC 4418 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 NGC 4418 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 NGC 4418 visible with EVN and MERLIN consists of eight compact ({\l}t}49 mas i.e. {\l}t}8 pc) features spread within a region of 250 mas, i.e. 41 pc. We derive an inverted spectral index {$\alpha$} {\ge} 0.7 (S$_{nu}$ {\prop} {$ν$}$^{alpha}$) for the compact radio emission.
Conclusions: Brightness temperatures {\gt}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 NGC 4418. 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.}, keywords = {galaxies: individual: NGC 4418, galaxies: Seyfert, galaxies: star formation}, doi = {10.1051/0004-6361/201323303}, url = {http://adsabs.harvard.edu/abs/2014A\%26A...566A..15V}, author = {Varenius, E. and Conway, J.~E. and Mart{\'\i}-Vidal, I. and Aalto, S. and Beswick, R. and F. Costagliola and Kl{\"o}ckner, H.-R.} } @article {221, title = {{High-resolution mm and cm study of the obscured LIRG NGC 4418 . A compact obscured nucleus fed by in-falling gas?}}, journal = {\aap}, volume = {556}, year = {2013}, pages = {A66}, keywords = {galaxies: active, galaxies: individual: NGC 4418, galaxies: ISM, galaxies: kinematics and dynamics, galaxies: starburst}, doi = {10.1051/0004-6361/201220634}, author = {Costagliola, F. and Aalto, S. and Sakamoto, K. and Mart{\'\i}n, S. and Beswick, R. and Muller, S. and Kl{\"o}ckner, H.-R.} } @article {2013A\&A...556A..66C, title = {{High-resolution mm and cm study of the obscured LIRG NGC 4418 . A compact obscured nucleus fed by in-falling gas?}}, journal = {Astronomy and Astrophysics}, volume = {556}, year = {2013}, month = {aug}, pages = {A66}, abstract = {Context. Understanding the nature of the power-source in luminous infrared galaxies (LIRG) is difficult due to their extreme obscuration. Observations at radio and mm wavelengths can penetrate large columns of dust and gas and provide unique insights into the properties of the compact obscured nuclei of LIRGs.
Aims: The aim of this study is to constrain the dynamics, structure, and feeding of the compact nucleus of NGC 4418, and to reveal the nature of the main hidden power-source: starburst or active galactic nucleus (AGN).
Methods: We obtained high spatial resolution observations of NGC 4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA in very extended configuration. We used the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate, and the dust temperature. Emission lines were used to study kinematics through position-velocity diagrams. Molecular emission was studied with population diagrams and by fitting a local thermal equilibrium (LTE) synthetic spectrum.
Results: We detect bright 1-mm-line emission from CO, HC$_{3}$N, HNC, and C$^{34}$S and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s$^{-1}$. We detect vibrationally excited HC$_{3}$N and HNC, with T$_{vib}$ \~{} 300 K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160, and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 10$^{4}$M$_{\&sun;}$ pc$^{-2}$ is extremely high and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220.
Conclusions: Our observations confirm the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC 4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in the ULIRG Arp 220, with an age of 3-10 Myr and a star formation rate {\gt}10 M$_{\&sun;}$ yr$^{-1}$. If an AGN is present, it must be extremely Compton-thick. Appendix A is only available in electronic form at http://www.aanda.org}, keywords = {galaxies: active, galaxies: individual: NGC 4418, galaxies: ISM, galaxies: kinematics and dynamics, galaxies: starburst}, doi = {10.1051/0004-6361/201220634}, url = {http://adsabs.harvard.edu/abs/2013A\%26A...556A..66C}, author = {F. Costagliola and Aalto, S. and Sakamoto, K. and Mart{\'\i}n, S. and Beswick, R. and Muller, S. and Kl{\"o}ckner, H.-R.} }