We report deep EVN and eMERLIN observations of the Type Ia SN 2014J in the nearby galaxy M 82. Our observations represent, together with JVLA observations of SNe 2011fe and 2014J, the most sensitive radio studies of Type Ia SNe ever. By combining dat
a and a proper modeling of the radio emission, we constrain the mass-loss rate from the progenitor system of SN 2014J to $dot{M} lesssim 7.0times 10^{-10}, {rm M_{odot}, yr^{-1}}$ (3-$sigma$; for a wind speed of $100, {rm km s^{-1}}$). If the medium around the supernova is uniform, then $n_{rm ISM} lesssim 1.3 {rm cm^3}$ (3-$sigma$), which is the most stringent limit for the (uniform) density around a Type Ia SN. Our deep upper limits favor a double-degenerate (DD) scenario--involving two WD stars--for the progenitor system of SN 2014J, as such systems have less circumstellar gas than our upper limits. By contrast, most single-degenerate (SD) scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to a exploding WD, are ruled out by our observations. Our estimates on the limits to the gas density surrounding SN 2011fe, using the flux density limits from Chomiuk et al. (2012), agree well with their results. Although we discuss possibilities for a SD scenario to pass observational tests, as well as uncertainties in the modeling of the radio emission, the evidence from SNe 2011fe and 2014J points in the direction of a DD scenario for both.
A consortium of universities has recently been formed with the goal of using the decommissioned telecommunications infrastructure at the Goonhilly Earth Station in Cornwall, UK, for astronomical purposes. One particular goal is the introduction of on
e or more of the ~30-metre parabolic antennas into the existing e-MERLIN radio interferometer. This article introduces this scheme and presents some simulations which quantify the improvements that would be brought to the e-MERLIN system. These include an approximate doubling of the spatial resolution of the array, an increase in its N-S extent with strong implications for imaging the most well-studied equatorial fields, accessible to ESO facilities including ALMA. It also increases the overlap between the e-MERLIN array and the European VLBI Network. We also discuss briefly some niche science areas in which an e-MERLIN array which included a receptor at Goonhilly would be potentially world-leading, in addition to enhancing the existing potential of e-MERLIN in its role as a Square Kilometer Array pathfinder instrument.
Studying molecular gas properties in merging galaxies gives important clues to the onset and evolution of interaction-triggered starbursts. The CO/13CO 1-0 line intensity ratio can be used as a tracer of how dynamics and star formation processes impa
ct the gas properties. The Medusa (NGC~4194) merger is particularly interesting to study since its LFIR/LCO ratio rivals that of ultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity, indicating an exceptionally high star formation efficiency (SFE) in the Medusa merger. Interferometric OVRO observations of CO and 13CO 1-0 in the Medusa show the CO/13CO intensity ratio increases from normal, quiescent values (7-10) in the outer parts (r>2 kpc) of the galaxy to high (16 to >40) values in the central (r<1 kpc) starburst region. In the centre there is an east-west gradient where the line ratio changes by more than a factor of three over 5 (945 pc). The integrated 13CO emission peaks in the north-western starburst region while the central CO emission is strongly associated with the prominent crossing dust-lane. We discuss the central east-west gradient in the context of gas properties in the starburst and the central dust lane. We suggest that the central gradient is mainly caused by diffuse gas in the dust lane. In this scenario, the actual molecular mass distribution is better traced by the 13CO 1-0 emission than the CO. The possibilities of temperature and abundance gradients are also discussed. We compare the central gas properties of the Medusa to those of other minor mergers and suggest that the extreme and transient phase of the Medusa star formation activity has similar traits to those of high-redshift galaxies.
We report MERLIN, VLA, OCRA-p, VLBA, Effelsberg and GMRT observations beginning 4.5 days after the discovery of RS Ophiuchi undergoing its 2006 recurrent nova outburst. Observations over the first 9 weeks are included, enabling us to follow spectral
development throughout the three phases of the remnant development. We see dramatic brightening on days 4 to 7 at 6 GHz and an accompanying increase in other bands, particularly 1.46 GHz, consistent with transition from the initial free expansion phase to the adiabatic expansion phase. This is complete by day 13 when the flux density at 5 GHz is apparently declining from an unexpectedly early maximum (compared with expectations from observations of the 1985 outburst). The flux density recovered to a second peak by approximately day 40, consistent with behaviour observed in 1985. At all times the spectral index is consistent with mixed non-thermal and thermal emission. The spectral indices are consistent with a non-thermal component at lower frequencies on all dates, and the spectral index changes show that the two components are clearly variable. The estimated extent of the emission at 22 GHz on day 59 is consistent with the extended east and west features seen at 1.7 GHz with the VLBA on day 63 being entirely non-thermal. We suggest a two-component model, consisting of a decelerating shell seen in mixed thermal and non-thermal emission plus faster bipolar ejecta generating the non-thermal emission, as seen in contemporaneous VLBA observations. Our estimated ejecta mass of 4+/-2x10^{-7} M_odot is consistent with a WD mass of 1.4 M_odot. It may be that this ejecta mass estimate is a lower limit, in which case a lower WD mass would be consistent with the data.
A 10-arcmin field around the HDF(N) contains 92 radio sources >40 uJy, resolved by MERLIN+VLA at 0.2-2.0 resolution. 55 have Chandra X-ray counterparts including 18 with a hard X-ray photon index and high luminosity characteristic of a type-II (obscu
red) AGN. >70% of the radio sources have been classified as starbursts or AGN using radio morphologies, spectral indices and comparisons with optical appearance and MIR emission. Starbursts outnumber radio AGN 3:1. This study extends the VO methods previously used to identify X-ray-selected obscured type-II AGN to investigate whether very luminous radio and X-ray emission originates from different phenomena in the same galaxy. The high-redshift starbursts have typical sizes of 5--10 kpc and star formation rates of ~1000 Msun/yr. There is no correlation between radio and X-ray luminosities nor spectral indices at z>~1.3. ~70% of both the radio-selected AGN and the starburst samples were detected by Chandra. The X-ray luminosity indicates the presence of an AGN in at least half of the 45 cross-matched radio starbursts, of which 11 are type-II AGN including 7 at z>1.5. This distribution overlaps closely with the X-ray detected radio sources which were also detected by SCUBA. Stacked 1.4-GHz emission at the positions of radio-faint X-ray sources is correlated with X-ray hardness. Most extended radio starbursts at z>1.3 host X-ray selected obscured AGN. Radio emission from most of these ultra-luminous objects is dominated by star formation but it contributes less than 1/3 of their X-ray luminosity. Our results support the inferences from SCUBA and IR data, that at z>1.5, star formation is an order of magnitude more extended and more copious, it is closely linked to AGN activity and it is triggered differently, compared with star formation at lower redshifts.
