@article {2012A\&A...540L...5H, title = {{Evidence of nuclear disks in starburst galaxies from their radial distribution of supernovae}}, journal = {Astronomy and Astrophysics}, volume = {540}, year = {2012}, pages = {L5}, abstract = {Galaxy-galaxy interactions are expected to be responsible for triggering massive star formation and possibly accretion onto a supermassive black hole, by providing large amounts of dense molecular gas down to the central kiloparsec region. Several scenarios to drive the gas further down to the central \~{}100 pc, have been proposed, including the formation of a nuclear disk around the black hole, where massive stars would produce supernovae. Here, we probe the radial distribution of supernovae and supernova remnants in the nuclear regions of the starburst galaxies M 82, Arp 299-A, and Arp 220, by using high-angular resolution ({\l}sim} 0.{\textquoteright}{\textquoteright}1) radio observations published in the literature (for M 82 and Arp 220), or obtained by ourselves from the European VLBI Network (Arp 299-A). Our main goal was to characterize the nuclear starbursts in those galaxies and thus test scenarios that propose that nuclear disks of sizes \~{}100 pc form in the central regions of starburst galaxies. We obtained the radial distribution of supernovae (SNe) in the nuclear starbursts of M 82, Arp 299-A, and Arp 220, and derived scale-length values for the putative nuclear disks powering the bursts in those central regions. The scale lengths for the (exponential) disks range from \~{}20-30 pc for Arp 299-A and Arp 220, up to \~{}140 pc for M 82. The radial distribution of SNe for the nuclear disks in Arp 299-A and Arp 220 is also consistent with a power-law surface density profile of exponent {$\gamma$} = 1, as expected from detailed hydrodynamical simulations of nuclear disks. Our results support scenarios where a nuclear disk of size \~{}100 pc is formed in (U)LIRGs, and sustained by gas pressure, in which case the accretion onto the black hole could be lowered by supernova feedback. Appendices are available in electronic form at http://www.aanda.org}, keywords = {galaxies: individual: Arp 299-A, galaxies: luminosity function, galaxies: starburst, mass function, radiation mechanisms: non-thermal, radio continuum: stars, supernovae: general}, doi = {10.1051/0004-6361/201118545}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...540L...5H}, author = {R. Herrero-Illana and M. A. P{\'e}rez-Torres and Alberdi, A.} } @article {2012A\&A...539A.134B, title = {{The nuclear starburst in Arp 299-A: from the 5.0 GHz VLBI radio light-curves to its core-collapse supernova rate}}, journal = {Astronomy and Astrophysics}, volume = {539}, year = {2012}, pages = {A134}, abstract = {Context. The nuclear region of the luminous infrared galaxy (LIRG) Arp 299-A hosts a recent ( {\sime} 10 Myr) intense burst of massive star formation that is expected to lead to numerous core-collapse supernovae (CCSNe). Previous VLBI observations, carried out with the European VLBI Network (EVN) at 5.0 GHz and with the VLBA at 2.3 and 8.4 GHz, resulted in the detection of many compact, bright, non-thermal sources in a region {\l}sim} 150 pc in size.
Aims: We aim to establish the nature of all non-thermal compact components in Arp 299-A, as well as to estimate its core-collapse supernova rate. While the majority of the compact components are expected to be young radio supernovae (RSNe) and supernova remnants (SNRs), a definitive classification is still lacking. Yet, this is very relevant for eventually establishing the CCSN rate, as well as the star formation rate, for this galaxy.
Methods: We used multi-epoch EVN observations taken at 5.0 GHz to image the compact radio sources in the nuclear region of Arp 299-A with milliarcsecond resolution. We also used one single-epoch 5.0 GHz Multi-Element Radio Linked Interferometer Network (MERLIN) observation to image the extended emission in which these compact radio sources are embedded.
Results: We present the first 5.0 GHz radio light-curve (spanning \~{}2.5 yr) of all compact components in the nuclear starburst of Arp 299-A. Twenty-six compact sources are detected, eight of which are new objects. The properties of all detected objects are consistent with them being a mixed population of CCSNe and SNRs. We find clear evidence for at least two new CCSNe, implying a lower limit to the CCSN rate of {$ν$}$_{SN}$ {\gsim} 0.80 SN/yr, indicating that the bulk of the current star formation in Arp 299-A is taking place in the innermost \~{}150 pc.A few more objects show variability consistent with them being recently exploded SNe, but only future observations will clarify this point. Our MERLIN observations trace a region of diffuse extended emission that is cospatial to the region where all compact sources are found. From this diffuse non-thermal radio emission traced by MERLIN we obtain an independent estimate for the CCSN rate, which is in the range {$ν$}$_{SN}$ = 0.40-0.65 SN/yr, in agreement with previous estimates and our direct estimate of the CCSN rate from the compact radio emission.
Conclusions: Our \~{}2.5 yr monitoring of Arp 299-A has allowed us to obtain for the first time a direct estimate of the CCSN rate of {$ν$}$_{SN}$ {\gsim} 0.80 SN/yr for the innermost \~{}150 pc of Arp 299-A.}, keywords = {galaxies: individual: Arp 299, galaxies: luminosity function, galaxies: starburst, mass function, radiation mechanisms: non-thermal, radio continuum: stars, supernovae: general}, doi = {10.1051/0004-6361/201118446}, url = {http://adsabs.harvard.edu/abs/2012A\%26A...539A.134B}, author = {Bondi, M. and M. A. P{\'e}rez-Torres and R. Herrero-Illana and Alberdi, A.} } @article {2011A\&A...529A..47M, title = {{Electron cooling and the connection between expansion and flux-density evolution in radio supernovae}}, journal = {Astronomy and Astrophysics}, volume = {529}, year = {2011}, month = {may}, pages = {A47}, abstract = {{Radio supernovae (RSNe) are weak and rare events. Their typical maximum radio luminosities are of the order of only 10$^{27}$ erg s$^{-1}$ Hz$^{-1}$. There are, however, very few cases of relatively bright (and/or close) RSNe, from which the expansion of the shock and the radio light curves at several frequencies have been monitored covering several years. Applying the standard model of radio emission from supernovae, it is possible to relate the defining parameters of the modelled expansion curve to those of the modelled light curves in a simple algebraic way by assuming an evolution law for the magnetic field and for the energy density of the population of synchrotron-emitting electrons. However, cooling mechanisms of the electrons may considerably affect this connection between light curves and expansion curve, and lead to wrong conclusions on the details of the electron acceleration and/or on the CSM radial density profile. We study how electron cooling modifies the flux-density decay rate of RSNe for a set of plausible/realistic values of the magnetic field and for different expansion regimes. We use these results to estimate the magnetic fields of different RSNe observed to date and compare them to those obtained by assuming energy equipartition between particles and magnetic fields. For some of the best monitored RSNe, for which deceleration measurements, optically thin spectral index, and power-law time decay have been observed (SN 1979C, SN 1986J, SN 1993J, and SN 2008iz), we find self-consistent solutions for the index of the power-law circumstellar density profile (s = 2 for all cases), the index of the power-law relativistic electron population (rather steep values}, keywords = {acceleration of particles, radiation mechanisms: nonthermal, radio continuum: stars, supernovae: general}, doi = {10.1051/0004-6361/201016105}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...529A..47M}, author = {Mart{\'\i}-Vidal, I. and M. A. P{\'e}rez-Torres and Brunthaler, A.} } @article {2011A\&A...531A..52M, title = {{A model for the thermal radio-continuum emission from radiative shocks in colliding stellar winds}}, journal = {Astronomy and Astrophysics}, volume = {531}, year = {2011}, month = {jul}, pages = {A52}, abstract = {Context. In massive-star binary systems, the interaction of the strong stellar winds results in a wind collision region (WCR) between the stars, which is limited by two shock fronts. Besides the nonthermal emission resulting from the shock acceleration, these shocks emit thermal (free-free) radiation detectable at radio frequencies that increase the expected emission from the stellar winds. Observations and theoretical studies of these sources show that the shocked gas is an important, but not dominant, contributor to the total emission in wide binary systems, while it plays a very substantial role in close binaries.
Aims: The interaction of two isotropic stellar winds is studied in order to calculate the free-free emission from the WCR. The effects of the binary separation and the wind momentum ratio on the emission from the wind-wind interaction region are investigated.
Methods: We developed a semi-analytical model for calculating the thermal emission from colliding stellar winds. Assuming radiative shocks for the compressed layer, which are expected in close binaries, we obtained the emission measure of the thin shell. Then, we computed the total optical depth along each line of sight to obtain the emission from the whole configuration.
Results: Here, we present predictions of the free-free emission at radio frequencies from analytic, radiative shock models in colliding wind binaries. It is shown that the emission from the WCR mainly arises from the optically thick region of the compressed layer and scales as \~{}D$^{4/5}$, where D is the binary separation. The predicted flux density S$_{nu}$ from the WCR becomes more important as the frequency {$ν$} increases, showing higher spectral indices than the expected 0.6 value (S$_{nu}$ {\prop} {$ν$}$^{alpha}$, where {$\alpha$} = 0.6) from the unshocked winds. We also investigate the emission from short-period WR+O systems calculated with our analytic formulation. In particular, we apply the model to the binary systems WR 98 and WR 113 and compare our results with the observations. Our theoretical results are in good agreement with the observed thermal spectra from these sources.}, keywords = {binaries: close, radiation mechanisms: thermal, radio continuum: stars, stars: Wolf-Rayet}, doi = {10.1051/0004-6361/201015988}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...531A..52M}, author = {Montes, G. and Gonz{\'a}lez, R.~F. and Cant{\'o}, J. and M. A. P{\'e}rez-Torres and Alberdi, A.} } @article {2011A\&A...526A.142M, title = {{Radio emission of SN1993J: the complete picture. I. Re-analysis of all the available VLBI data}}, journal = {Astronomy and Astrophysics}, volume = {526}, year = {2011}, month = {feb}, pages = {A142}, abstract = {We have performed a complete re-calibration and re-analysis of all the available Very-Long-Baseline-Interferometry (VLBI) observations of supernova SN1993J, following an homogeneous and well-defined methodology. VLBI observations of SN1993J at 69 epochs, spanning 13 years, were performed by two teams, which used different strategies and analysis tools. The results obtained by each group are similar, but their conclusions on the supernova expansion and the shape and evolution of the emitting region differ significantly. From our analysis of the combined set of observations, we have obtained an expansion curve with unprecedented time resolution and coverage. We find that the data from both teams are compatible when analyzed with the same methodology. One expansion index (m$_{3}$ = 0.87 {\plusmn} 0.02) is enough to model the expansion observed at 1.7 GHz, while two expansion indices (m$_{1}$ = 0.933 {\plusmn} 0.010 and m$_{2}$ = 0.796 {\plusmn} 0.005), separated by a break time, t$_{br}$ = 390 {\plusmn} 30 days, are needed to model the data, at frequencies higher than 1.7 GHz, up to day \~{}4000 after explosion. We thus confirm the wavelength dependence of the size of the emitting region reported by one of the groups. We also find that all sizes measured at epochs later than day \~{}4000 after explosion are systematically smaller than our model predictions (i.e., an additional expansion index might be needed to properly model these data). We also estimate the fractional shell width (0.31 {\plusmn} 0.02, average of all epochs and frequencies) and the level of opacity to the radio emission by the ejecta. We find evidence of a spectral-index radial gradient in the supernova shell, which is indicative of a frequency-dependent ejecta opacity. Finally, we study the distribution and evolution of the azimuthal anisotropies (hot spots) found around the radio shell during the expansion. These anisotropies have intensities of \~{}20\% of the mean flux density of the shell, and appear to systematically evolve during the expansion.}, keywords = {galaxies: individual: M 81, radio continuum: stars, supernovae: general, supernovae: individual: SN1993J}, doi = {10.1051/0004-6361/200913831}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...526A.142M}, author = {Mart{\'\i}-Vidal, I. and Marcaide, J.~M. and Alberdi, A. and Guirado, J.~C. and M. A. P{\'e}rez-Torres and Ros, E.} } @article {2011A\&A...526A.143M, title = {{Radio emission of SN1993J: the complete picture. II. Simultaneous fit of expansion and radio light curves}}, journal = {Astronomy and Astrophysics}, volume = {526}, year = {2011}, pages = {A143}, abstract = {We report on a simultaneous modelling of the expansion and radio light curves of the supernova SN1993J. We developed a simulation code capable of generating synthetic expansion and radio light curves of supernovae by taking into consideration the evolution of the expanding shock, magnetic fields, and relativistic electrons, as well as the finite sensitivity of the interferometric arrays used in the observations. Our software successfully fits all the available radio data of SN 1993J with a standard emission model for supernovae, which is extended with some physical considerations, such as an evolution in the opacity of the ejecta material, a radial decline in the magnetic fields within the radiating region, and a changing radial density profile for the circumstellar medium starting from day 3100 after the explosion.}, keywords = {acceleration of particles, galaxies: individual: M 81, radiation mechanisms: non-thermal, radio continuum: stars, supernovae: general, supernovae: individual: SN1993J}, doi = {10.1051/0004-6361/201014517}, url = {http://adsabs.harvard.edu/abs/2011A\%26A...526A.143M}, author = {Mart{\'\i}-Vidal, I. and Marcaide, J.~M. and Alberdi, A. and Guirado, J.~C. and M. A. P{\'e}rez-Torres and Ros, E.} } @article {2010A\&A...519L...5P, title = {{Serendipitous discovery of the long-sought active galactic nucleus in Arp 299-A}}, journal = {Astronomy and Astrophysics}, volume = {519}, year = {2010}, month = {sep}, pages = {L5}, abstract = {Context. The dusty nuclear regions of luminous infra-red galaxies (LIRGs) are heated by either an intense burst of massive star formation, an active galactic nucleus (AGN), or a combination of both. Disentangling the contribution of each of those putative dust-heating agents is a challenging task, and direct imaging of the innermost few pc can only be accomplished at radio wavelengths, using very high-angular resolution observations.
Aims: We observed the nucleus A of the interacting starburst galaxy Arp 299, using very long baseline interferometry (VLBI) radio observations at 1.7 and 5.0 GHz. Our aim was to characterize the compact sources in the innermost few pc region of Arp 299-A, as well as to detect recently exploded core-collapse supernovae.
Methods: We used the European VLBI Network (EVN) to image the 1.7 and 5.0 GHz compact radio emission of the parsec-scale structure in the nucleus of Arp 299-A with milliarcsecond resolution.
Results: Our EVN observations show that one of the compact VLBI sources, A1, previously detected at 5.0 GHz, has a flat spectrum between 1.7 and 5.0 GHz and is the brightest source at both frequencies. Our 1.7 GHz EVN image shows also diffuse, low-surface brightness emission extending westwards from A1 and displays a prominent core-jet structure.
Conclusions: The morphology, radio luminosity, spectral index and ratio of radio-to-X-ray emission of the A1-A5 region is consistent with a low-luminosity AGN (LLAGN), and rules out the possibility that it is a chain of young radio supernovae (RSNe) and supernova remnants (SNRs). We therefore conclude that A1-A5 is the long-sought AGN in Arp 299-A. This finding may suggest that both starburst and AGN are frequently associated phenomena in mergers.}, keywords = {galaxies: individual: Arp 299, galaxies: starburst, Galaxy: nucleus, radiation mechanisms: non-thermal, radio continuum: stars}, doi = {10.1051/0004-6361/201015462}, url = {http://adsabs.harvard.edu/abs/2010A\%26A...519L...5P}, author = {M. A. P{\'e}rez-Torres and Alberdi, A. and Romero-Ca{\~n}izales, C. and Bondi, M.} }