Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows

Context. Arp 220 is the prototypical ultra luminous infrared galaxy (ULIRG). Despite extensive studies, the structure at MHz-frequencies has remained unknown because of limits in spatial resolution. Aims. This work aims to constrain the flux and shape of radio emission from Arp 220 at MHz frequencies. Methods. We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz. Results. We present an image of Arp 220 at 150 MHz with resolution 0′. 65 × 0′. 35, sensitivity 0.15 mJy beam, and integrated flux density 394 ± 59 mJy. More than 80% of the detected flux comes from extended (6″∼ 2.2 kpc) steep spectrum (α = -0.7) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending 0.3″ (110 pc) and 0.9″ (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with q = 2.36, and we estimate a star formation rate of 220 M yr. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust. Conclusions. International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz.A. A. and M.A.P.T. acknowledge support from the Spanish MINECO through grants AYA2012-38491-C02-02 and AYA2015-63939-C2-1-P, partially funded by FEDER funds.The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement No. 283393 (RadioNet3).Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts.Peer Reviewe

A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger

Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5 × 10 erg at infrared and radio wavelengths but was not luminous at optical or x-ray wavelengths. We interpret this as a TDE with much of its emission reradiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH. Copyright © 2018, American Association for the Advancement of ScienceS.M. acknowledges financial support from the Academy of Finland (pmject 8120503). The research leading to these mats has received funding from the European.. Commission Seventh Framework Programme (FP/2007-2013) under grant agreement number & 227290, 283393 (RadioNc-t) and 60725 (HELP). AA., M.P.-T., N.R.-O. and R.H.T. acknowledge support from the Spanish MINECO through grants AYA2012-38491-002-02 and AYA2015 63939 C2 1 P. P.G.J. acknowledges support from European Research Council Consolidator Grant 647208. C.R.-C. acknowledges support by the Ministry of Economy, Development and Tourism's Millennium Science Initiative through grant 10120009, awarded to The Millennium. Institute of Astrophysics, MAS Chile, and from CONICYT through FONDECYT grant 3150238 and China-CON1CYT fund CAS160313. P.M. and M.A.A. acknowledge support from the ERC research grant CAMAP-250276, and partial support from the Spanish MINECO grant AYA2015-66889C2-1P Lard the local Valencia government ghat PROMETE0-11-2014069. FIE. acknowledges support from a Science Foundation Ireland-Royal Society University Research Fellowship. D.L.C. acknowledges support from grants ST/0001901/4 ST/J001368/1, ST/ K001051/1, and st/N0001138/1. P.V. acknowledges support from the National Research Foundation of South Africa.. J.H. acknowledges financial support tom the Finis h ChAth ral Fouridation and the Virile), YIP and Kahle Vais8I8 Foundation. J.K. acknowledges financial support from the Academy of Finland (grant 311138)

High angular resolution radio observations of luminous infrared galaxies

[EN] The overall goal of this thesis has been to improve our understanding of the radio emission and absorption processes taking place in luminous and ultra-luminous infrared galaxies (U/LIRGs) in the local Universe, using state-of-the-art radio interferometers that offer high angular resolution (better than 1 arcsecond) and sensitivity at multiple frequencies. These radio interferometric observations allow us to characterise the spectral energy distribution in the GHz frequency range, where synchrotron emission, as well as free-free thermal emission, is known to contribute significantly (Condon, 1992). I have presented results from the e-MERLIN LIRGI sample, whose overall objective is to characterise the phenomenological evolution of the core of a starburst. One of the immediate goals is to reveal the source responsible for the heating of the dust and gas in the nuclear regions of these galaxies (AGN or starburst). Although we present here preliminary results from LIRGI in the 5 GHz band, the results show the enormous potential of using radio interferometry at resolutions better than arcsecond to study outbursts and nuclear regions in the local Universe. In the first part of this paper we made a comprehensive study of the LIRG Arp 299, where we present the first observations of this LIRG from the Jansky Very Large Array (JVLA) at frequencies between 1.4 and 8.4 GHz combined with the first observations of this LIRG obtained with the LOw Frequency ARray (LOFAR), including the international stations. This work has included a study of its magnetic field, of its emission measure and therefore its corresponding electron density, of its spectral index, and of its structural characteristics at these frequencies. One of the most important results obtained from LOFAR observations, in combination with JVLA observations at various frequencies, is the characterisation of the interstellar medium in the cores of LIRGs. For this purpose, we fit the spectral energy distribution of the cores, between 150 MHz and 8.4 GHz, using two different models of the absorbing/emitting thermal gas: in one model, emitting/absorbing particles are uniformly distributed (continuous model) (Condon, 1992), while in the second one we assume a clumpy medium (Conway, Elitzur, and Parra, 2018), where there is a nonuniform distribution. Both models fit the existing data well. The continuum model can account for the SED of nuclei with a standard population of relativistic electrons subject to synchrotron, Bremsstrahlung and ionisation losses, which are expected to be significant due to the large densities found in the central regions of the U/LIRGs (Lacki, Thompson, and Quataert, 2010). The clumpy model can explain the data by a relativistic electron population with negligible energy losses, and predicts thermal fractions that are more typical of star-forming galaxies, compared to the continuum model. We propose LOFAR observations at frequencies below 100 MHz, or uGMRT observations at 600 MHz to discern between the two models. In any case, these results highlight the relevance of low-frequency, high angular resolution observations for tracing the diffuse interstellar medium in galaxies. In the second part of the thesis I have addressed the study of the physical properties of the LIRGI sample, as well as the characterisation of the emission and absorption processes in local galaxies, where we can do so in extraordinary detail. This will allow us to better understand the properties of galaxies with outbursts of star formation at cosmological distances (Magnelli et al., 2009), where we know that they were much more abundant, but the angular resolution does not allow us to resolve the structures in adequate detail. Finally, I studied the supernova remnant luminosity function (SNR) in normal (Chomiuk and Wilcots, 2009) galaxies, with the aim of constructing a universal luminosity function. To do so, I wrote a code that uses a non-uniform bin size, which avoids introducing biases in the study due to the small size of some SNR samples. The main result is to obtain a new (true) completeness limit on the global sample due to the presence of starburst galaxies.[ES] El objetivo general de esta tesis ha sido mejorar nuestra comprensión de los procesos de emisión y absorción en radio que tienen lugar en las galaxias luminosas y ultraluminosa en el infrarrojo (U/LIRGs) del universo local, utilizando radiointerferómetros de última generación que ofrecen una alta resolución angular (mejor que 1 segundo de arco) y gran sensibilidad en múltiples frecuencias. Estas observaciones radiointerferométricas permiten caracterizar la distribución de energía espectral en el rango de frecuencias de GHz, donde se sabe que la emisión sincrotrón, así como la emisión térmica libre-libre, contribuyen de modo significativo (Condon, 1992). He presentado resultados de la muestra e-MERLIN LIRGI, cuyo objetivo global es caracterizar la evolución fenomenológica del núcleo de un brote de formación estelar (starburst). Uno de los objetivos inmediatos es desvelar la fuente responsable del calentamiento del polvo y el gas en las regiones nucleares de estas galaxias (AGN o brote de formación estelar). Aunque aquí presentamos resultados preliminares de LIRGI en la banda de 5 GHz, los resultados evidencian el enorme potencial del uso de la radiointerferometría con resoluciones mejores que el segundo de arco para el estudio de brotes y regiones nucleares en el universo local. En la primera parte de este trabajo realizamos un estudio exhaustivo de la LIRG Arp 299, donde presentamos las primeras combinando observaciones del Jansky Very Large Array (JVLA) a frecuencias entre 1.4 y 8.4 GHz con las primeras observaciones de esta LIRG obtenidas con el LOw Frequency ARray (LOFAR), incluyendo las estaciones internacionales. Este trabajo engloba un estudio detallado del campo magnético, la emission measure y por lo tanto su correspondiente densidad electrónica, de su índice espectral, y de las características estructurales de los regiones nucleares de Arp 299 a estas frecuencias. Uno de los resultados más importantes obtenidos a partir de las observaciones de LOFAR, en combinación las del JVLA a varias frecuencias, es la caracterización del medio interestelar en los núcleos de las LIRGs. Para ello, ajustamos la distribución espectral de energía de los núcleos, entre 150 MHz y 8.4 GHz, utilizando dos modelos diferentes del gas térmico absorbente/emisor: en uno de los modelos, las partículas emisoras/absorbentes están distribuidas de modo uniforme (modelo continuo) (Condon, 1992), mientras que en el segundo se supone un medio con grumos (Conway, Elitzur, and Parra, 2018), donde la distribución del gas no es uniforme. Ambos modelos ajustan bien los datos existentes. El modelo continuo puede explicar la SED de los núcleos con una población estándar de electrones relativistas sometidos a pérdidas por sincrotrón, Bremsstrahlung e ionización , que se esperan sean significativas debido a las grandes densidades encontradas en las regiones centrales de las U/LIRG (Lacki, Thompson, and Quataert, 2010). El modelo de grumos puede explicar los datos mediante una población de electrones relativistas con pérdidas de energía insignificantes, y predice fracciones térmicas que son más típicas de las galaxias con formación estelar, en comparación con el modelo continuo. Nosotros proponemos observaciones con LOFAR a frecuencias menores de 100 MHz, o bien observaciones con el uGMRT a 600 MHz para discernir entre ambos modelos. En cualquier caso, estos resultados ponen de manifiesto la relevancia de las observaciones de baja frecuencia y alta resolución angular para trazar el medio interestelar difuso en galaxias. En la segunda parte de la tesis he abordado el estudio de las propiedades físicas de la muestra LIRGI, así como de la caracterización de los procesos de emisión y absorción en galaxias locales, donde podemos hacerlo con extraordinario detalle. Esto nos permitirá comprender mejor las propiedades de las galaxias con brotes de formación estelar a distancias cosmológicas (Magnelli et al., 2009), donde sabemos que fueron mucho más abundantes, pero la resolución angular no permite resolver las estructuras con el adecuado detalle. Por último, he estudiado la función de luminosidad de los remanentes de supernova (SNR) en galaxias normales (Chomiuk and Wilcots, 2009), con el objetivo de construir una función de luminosidad universal. Para ello, escribí un código que utiliza un tamaño no uniforme del ”bin”, lo que evita introducir sesgos en el estudio debido al pequeño tamaño de algunas muestras de SNR. El resultado principal es la obtención de un nuevo (verdadero) límite de completitud en la muestra global debido a la presencia de galaxias con estallido de estrellas.With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.Peer reviewe

Sub-arcsecond imaging of Arp 299-A at 150 MHz with LOFAR: Evidence for a starburst-driven outflow

We report on the first sub-arcsecond (0.44 × 0.41 arcsec) angular resolution image at 150 MHz of the A-nucleus in the luminous infrared galaxy Arp 299, from International Low Frequency Array (LOFAR) Telescope observations. The most remarkable finding is that of an intriguing two-sided, filamentary structure emanating from the A-nucleus, which we interpret as an outflow that extends up to at least 14 arcsec from the A-nucleus in the N-S direction (≈5 kpc deprojected size) and accounts for almost 40% of the extended emission of the entire galaxy system. We also discuss HST/NICMOS [FeII] 1.64 μm and H 2.12 μm images of Arp 299-A, which show similar features to those unveiled by our 150 MHz LOFAR observations, providing strong morphological support for the outflow scenario. Finally, we discuss unpublished Na I D spectra that confirm the outflow nature of this structure. From energetic arguments, we rule out the low-luminosity active galactic nucleus in Arp 299-A as a driver for the outflow. On the contrary, the powerful, compact starburst in the central regions of Arp 299-A provides plenty of mechanical energy to sustain an outflow, and we conclude that the intense supernova (SN) activity in the nuclear region of Arp 299-A is driving the observed outflow. We estimate that the starburst wind can support a mass-outflow rate in the range (11-63 M yr) at speeds of up to 370-890 km s, and is relatively young, with an estimated kinematic age of 3-7 Myr. Those results open an avenue to the use of low-frequency (150 MHz), sub-arcsecond imaging with LOFAR to detect outflows in the central regions of local luminous infrared galaxies.© ESO 2018.Peer Reviewe