@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 {2014MNRAS.440.1067R, title = {{The nature of supernovae 2010O and 2010P in Arp 299 - II. Radio emission}}, journal = {Monthly Notices of the Royal Astronomical Society}, volume = {440}, year = {2014}, pages = {1067-1079}, abstract = {We report radio observations of two stripped-envelope supernovae (SNe), 2010O and 2010P, which exploded within a few days of each other in the luminous infrared galaxy Arp 299. Whilst SN 2010O remains undetected at radio frequencies, SN 2010P was detected (with an astrometric accuracy better than 1 milli arcsec in position) in its optically thin phase in epochs ranging from {\tilde}1 to {\tilde}3 yr after its explosion date, indicating a very slow radio evolution and a strong interaction of the SN ejecta with the circumstellar medium. Our late-time radio observations towards SN 2010P probe the dense circumstellar envelope of this SN, and imply dot$\{$M$\}$ [ $\{$M\_$\{${\sun}$\}$ yr\^{}$\{$-1$\}$$\}$] / v\_$\{$wind$\}$ [10 km s\^{}$\{$-1$\}$] =(3.0-5.1) {\times} 10$^{-5}$, with a 5 GHz peak luminosity of {\tilde}1.2 {\times} 10$^{27}$ erg s$^{- 1}$ Hz$^{- 1}$ on day {\tilde}464 after explosion. This is consistent with a Type IIb classification for SN 2010P, making it the most distant and most slowly evolving Type IIb radio SN detected to date.}, keywords = {galaxies: individual: Arp 299, galaxies: starburst, radio continuum: galaxies, supernovae: general, supernovae: individual: SN 2010O, supernovae: individual: SN 2010P}, doi = {10.1093/mnras/stu430}, url = {http://adsabs.harvard.edu/abs/2014MNRAS.440.1067R}, author = {Romero-Ca{\~n}izales, C. and R. Herrero-Illana and M. A. P{\'e}rez-Torres and Alberdi, A. and Kankare, E. and Bauer, F.~E. and Ryder, S.~D. and Mattila, S. and Conway, J.~E. and Beswick, R.~J. and Muxlow, T.~W.~B.} } @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 {2012A\&A...543A..72R, title = {{e-MERLIN and VLBI observations of the luminous infrared galaxy IC 883: a nuclear starburst and an AGN candidate revealed}}, journal = {Astronomy and Astrophysics}, volume = {543}, year = {2012}, month = {jul}, pages = {A72}, abstract = {Context. The high star formation rates of luminous infrared galaxies (LIRGs) make them ideal places for core-collapse supernova (CCSN) searches. Massive star formation can often be found in coexistence with an active galactic nucleus (AGN), contributing jointly to the energy source of LIRGs. At radio frequencies, where light is unaffected by dust extinction, it is possible to detect compact components within the innermost LIRG nuclear regions, such as SNe and SN remnants, as well as AGN buried deep in the LIRG nuclei.
Aims: Our study of the LIRG IC 883 aims at: (i) investigating the parsec-scale radio structure of the (circum-)nuclear regions of IC 883; (ii) detecting at radio frequencies the two recently reported circumnuclear SNe 2010cu and 2011hi, which were discovered by near-IR (NIR) adaptive optics observations of IC 883; and (iii) further investigating the nature of SN 2011hi at NIR wavelengths.
Methods: We used the electronic European very long baseline interferometry (VLBI) Network (e-EVN) at 5 GHz, and the electronic Multi-Element Remotely Linked Interferometer Network (e-MERLIN) at 6.9 GHz, to observe contemporaneously the LIRG IC 883 at high angular-resolution (from tens to hundreds of milliarcsec) and with high sensitivity ({\l}t}70 {$μ$}Jy), complemented by archival VLBI data at 5 GHz and 8.4 GHz. We also used the Gemini North telescope to obtain late-time JHK photometry for SN 2011hi.
Results: The circumnuclear regions traced by e-MERLIN at 6.9 GHz have an extension of \~{} 1 kpc, at a position angle of 130{\deg}, and show a striking double-sided structure, which very likely corresponds to a warped rotating ring, in agreement with previous studies. Our e-EVN observations at 5 GHz and complementary archival VLBI data at 5 GHz and 8.4 GHz, reveal various milliarcsec compact components in the nucleus of IC 883. A single compact source, an AGN candidate, dominates the emission at both nuclear and circumnuclear scales, as imaged with the e-EVN and e-MERLIN, respectively. The other milliarcsec components are strongly indicative of ongoing nuclear CCSN activity. Our e-EVN observations also provided upper limits to the radio luminosity of the two SNe in IC 883 recently discovered at NIR wavelengths. We refine the classification of SN 2011hi as a Type IIP SN according to our latest epoch of Gemini North observations acquired in 2012, in agreement with a low-luminosity radio SN nature. We estimate a CCSN rate lower limit of 1.1$_{-0.6}$$^{+1.3}$ yr$^{-1}$ for the entire galaxy, based on three nuclear radio SNe and the circumnuclear SNe 2010cu and 2011hi.}, keywords = {galaxies: individual: IC 883, galaxies: starburst, radiation mechanisms: non-thermal, radio lines: stars}, doi = {10.1051/0004-6361/201218816}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...543A..72R}, author = {Romero-Ca{\~n}izales, C. and M. A. P{\'e}rez-Torres and Alberdi, A. and Argo, M.~K. and Beswick, R.~J. and Kankare, E. and Batejat, F. and Efstathiou, A. and Mattila, S. and Conway, J.~E. and Garrington, S.~T. and Muxlow, T.~W.~B. and Ryder, S.~D. and V{\"a}is{\"a}nen, P.} }