We present partial results from our monitoring of the nuclear region of the starburst galaxy IC 694 (=Arp 299-A) at radio wavelengths, aimed at discovering recently exploded CCSNe, as well as to determine their rate of explosion, which carries crucia
l information on star formation rates and starburst scenarios at work. Two epochs of eEVN observations at 5.0 GHz, taken in 2008, revealed the presence of a rich cluster of compact radio emitting sources in the central 150 pc of the nuclear starburst in Arp 299A. The large brightness temperatures observed for the compact sources indicate a non-thermal origin for the observed radio emission, implying that most, if not all, of those sources were young radio supernovae (RSNe) and supernova remnants (SNRs). More recently, contemporaneous EVN observations at 1.7 and 5.0 GHz taken in 2009 have allowed us to shed light on the compact radio emission of the parsec-scale structure in the nucleus of Arp 299-A. Namely, our EVN observations have shown 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. The morphology, radio luminosity, spectral index and ratio of radio-to-X-ray emission of the A1-A5 region allowed us to identify A1-A5 with long-sought AGN in Arp 299-A. This finding may suggest that both starburst and AGN are frequently associated phenomena in mergers. Finally, we also note that component A0, identified as a young RSN, exploded at the mere distance of two parsecs from the putative AGN in Arp 299-A, which makes this supernova one of the closest to a central supermassive black hole ever detected.
We present the results of an eight-year long monitoring of the radio emission from the Luminous Infrared Galaxy (LIRG) NGC 7469, using 8.4 GHz Very Large Array (VLA) observations at 0.3 resolution. Our monitoring shows that the late time evolution of
the radio supernova SN 2000ft follows a decline very similar to that displayed at earlier times of its optically thin phase. The late time radio emission of SN 2000ft is therefore still being powered by its interaction with the presupernova stellar wind, and not with the interstellar medium (ISM). Indeed, the ram pressure of the presupernova wind is rho_w v_w^2 approx 7.6E-9 dyn/cm^2, at a supernova age of approximately 2127 days, which is significantly larger than the expected pressure of the ISM around SN 2000ft. At this age, the SN shock has reached a distance r_{sh approx 0.06 pc, and our observations are probing the interaction of the SN with dense material that was ejected by the presupernova star about 5820 years prior to its explosion. From our VLA monitoring, we estimate that the swept-up mass by the supernova shock after about six years of expansion is approx 0.29 M_sun, assuming an average expansion speed of the supernova of 10000 km/s. We also searched for recently exploded core-collapse supernovae in our VLA images. Apart from SN 2000ft (S_ u approx 1760 microJy at its peak, corresponding to 1.1E28 erg/s/Hz, we found no evidence for any other radio supernova (RSN) more luminous than approx 6.0E26 erg/s/Hz, which suggests that no other Type IIn SN has exploded since 2000 in the circumnuclear starburst of NGC 7469.
We present high resolution 240 and 607 MHz GMRT radio observations, complemented with 74 MHz archival VLA radio observations of the Ophiuchus cluster of galaxies, whose radio mini-halo has been recently detected at 1400 MHz. We also present archival
Chandra and XMM-Newton data of the Ophiuchus cluster. Our observations do not show significant radio emission from the mini-halo, hence we present upper limits to the integrated, diffuse non-thermal radio emission of the core of the Ophiuchus cluster. The XMM-Newton observations can be well explained by a two-temperature thermal model with temperatures of ~=1.8 keV and ~=9.0 keV, respectively, which confirms previous results that suggest that the innermost central region of the Ophiuchus cluster is a cooling core. We also used the XMM-Newton data to set up an upper limit to the (non-thermal) X-ray emission from the cluster. The combination of available radio and X-ray data has strong implications for the currently proposed models of the spectral energy distribution (SED) from the Ophiuchus cluster. In particular, a synchrotron+IC model is in agreement with the currently available data, if the average magnetic field is in the range (0.02-0.3) microG. A pure WIMP annihilation scenario can in principle reproduce both radio and X-ray emission, but at the expense of postulating very large boost factors from dark matter substructures, jointly with extremely low values of the average magnetic field. Finally, a scenario where synchrotron and inverse Compton emission arise from PeV electron-positron pairs (via interactions with the CMB), can be ruled out, as it predicts a non-thermal soft X-ray emission that largely exceeds the thermal Bremsstrahlung measured by INTEGRAL.
We show preliminary results from a sample of Luminous and Ultra-Luminous Infrared Galaxies (LIRGs and ULIRGs, respectively) in the local universe, obtained from observations using the Very Large Array (VLA), the Multi-Element Radio Link Interferomete
r Network (MERLIN), and the European VLBI Network (EVN). The main goal of our high-resolution, high-sensitivity radio observations is to unveil the dominant gas heating mechanism in the central regions of local (U)LIRGs. The main tracer of recent star-formation in (U)LIRGs is the explosion of core-collapse supernovae (CCSNe), which are the endproducts of the explosion of massive stars and yield bright radio events. Therefore, our observations will not only allow us to answer the question of the dominant heating mechanism in (U)LIRGs, but will yield also the CCSN rate and the star-formation rate (SFR) for the galaxies of the sample.
High-resolution radio observations of nearby starburst galaxies have shown that the distribution of their radio emission consists of a compact (<150 pc), high surface brightness, central radio source immersed in a low surface brightness circumnuclear
halo. This radio structure is similar to that detected in bright Seyferts galaxies like NGC 7469 or Mrk 331, which display clear circumnuclear rings. While the compact, centrally located radio emission in these starbursts might be generated by a point-like source (AGN), or by the combined effect of multiple radio supernovae and supernova remnants (e.g., the evolved nuclear starburst in Arp~220), it seems well established that the circumnuclear regions of those objects host an ongoing burst of star-formation (e.g., NGC 7469; Colina et al. 2001, Alberdi et al. 2006). Therefore, high-resolution radio observations of Luminous Infra-Red Galaxies (LIRGs) in our local universe are a powerful tool to probe the dominant dust heating mechanism in their nuclear and circumnuclear regions. In this contribution, we show results obtained from VLA-A, MERLIN, and EVN (VLBI) radio observations of the galaxies NGC 7469 (D~70 Mpc) and IRAS 18293-3413 (D ~ 79 Mpc), where two extremely bright radio supernovae have been found. High-resolution studies of these and other LIRGs would allow us to determine the core-collapse supernova rate in them, as well as their star-formation rate.
We present new continuum VLA observations of the nearby Sy 1.5 galaxy NGC 5033, made at 4.9 and 8.4 GHz on 8 April 2003. Combined with VLA archival observations at 1.4 and 4.9 GHz made on 7 August 1993, 29 August 1999, and 31 October 1999, we sample
the galaxy radio emission at scales ranging from the nuclear regions (<~ 100 pc) to the outer regions of the disk (~ 40 kpc). The high-resolution VLA images show a core-jet structure for the Sy 1.5 nucleus. While the core has a moderately steep non-thermal radio spectrum (S_ u ~ u^alpha; alpha_{1.5}^{4.9} approx -0.4), the inner kpc region shows a steeper spectrum (alpha_{1.5}^{8.4} approx -0.9). This latter spectrum is typical of galaxies where energy losses are high, indicating that the escape rate of cosmic ray electrons in NGC 5033 is low. The nucleus contributes little to the total 1.4 GHz radio power of NGC 5033 and, based on the radio to far-infrared (FIR) relation, it appears that the radio and far-infrared emission from NGC 5033 are dominated by a starburst that during the last 10 Myr produced stars at a rate of ~2.8 M_sun/yr, yielding a supernova (type Ib/c and II) rate of 0.045 #/yr. This supernova rate corresponds to about 1 SN event every 22 yr. Finally, from our deep 8.4 GHz VLA-D image, we suggest the existence of a radio spur in NGC 5033, which could have been due to a hot superbubble formed as a consequence of sequential supernova explosions occurring during the lifetime of a giant molecular cloud.
