Danielle Fenech

Deep MERLIN 5 GHz radio imaging of supernova remnants in the M82 starburst

The results of an extremely deep, 8-d long observation of the central kpc of the nearby starburst galaxy M82 using Multi-Element Radio Linked Interferometer Network (MERLIN) at 5 GHz are presented. The 17 mu Jy beam(-1) rms noise level in the naturally weighted image makes it the most sensitive high-resolution radio image of M82 made to date. Over 50 discrete sources are detected, the majority of which are supernova remnants, but with 13 identified as H II regions. Sizes, flux densities and radio brightnesses are given for all of the detected sources, which are all well resolved with a majority showing shell or partial shell structures. Those sources within the sample which are supernova remnants have diameters ranging from 0.3 to 6.7 pc, with a mean size of 2.9 pc. From a comparison with previous MERLIN 5-GHz observations made in 1992 July, which gives a 9.75-yr timeline, it has been possible to measure the expansion velocities of 10 of the more compact sources, eight of which have not been measured before. These derived expansion velocities range between 2200 and 10 500 km s(-1).

Wide-field Global VLBI and MERLIN combined monitoring of supernova remnants in M82

From a combination of Multi-Element Radio-Linked Interferometer Network (MERLIN) and global Very Long Baseline Interferometry (VLBI) observations of the starburst galaxy M82, images of 36 discrete sources at resolutions ranging from ∼3 to ∼80 mas at 1.7 GHz are presented. Of these 36 sources, 32 are identified as supernova remnants, two are H II regions and three remain unclassified. Sizes, flux densities and radio brightnesses are given for all of the detected sources. Additionally, global VLBI only data from this project are used to image four of the most compact radio sources. These data provide a fifth epoch of VLBI observations of these sources, covering a 19-yr time-line. In particular, the continued expansion of one of the youngest supernova remnants, 43.31+59.3, is discussed. The deceleration parameter is a power-law index used to represent the time evolution of the size of a supernova remnant. For the source 43.31+59.3, a lower limit to the deceleration parameter is calculated to be 0.53 ± 0.06, based on a lower limit of the age of this source.

Discovery of an unusual new radio source in the star-forming galaxy M82: faint supernova, supermassive black hole or an extragalactic microquasar?

A faint new radio source has been detected in the nuclear region of the starburst galaxy M82 using MERLIN radio observations designed to monitor the flux density evolution of the recent bright supernova SN2008iz. This new source was initially identified in observations made between 1-5th May 2009 but had not been present in observations made one week earlier, or in any previous observations of M82. In this paper we report the discovery of this new source and monitoring of its evolution over its first 9 months of existence. The true nature of this new source remains unclear, and we discuss whether this source may be an unusual and faint supernova, a supermassive blackhole associated with the nucleus of M82, or intriguingly the first detection of radio emission from an extragalactic microquasar.

Flux density variations of radio sources in M82 over the last three decades

This paper presents the results of the 2009-2010 monitoring sessions of the starburst galaxy M82, obtained with the Multi-Element Radio-Linked Interferometer Network (MERLIN) at 5GHz and e-MERLIN at 6GHz. Combining several 5GHz MERLIN epochs to form a map with 33.0 uJy/bm noise level, 52 discrete sources, mostly supernova remnants and HII regions, are identified. These include three objects which were not detected in the 2002 5GHz MERLIN monitoring session: supernova SN2008iz, the transient source 43.78+59.3, and a new supernova remnant shell. Flux density variations, in the long (1981 to 2010), medium (2002 to 2010) and short (2009 to 2010) term, are investigated. We find that flux densities of SNRs in M82 stay constant in most of the sample (~95%). In addition, aside from SN2008iz and the well-known variable source 41.95+57.5, two sources display short and medium term variations over the period 2009-2010. These sources being among the most compact SNR in M82, these flux density variations could be due to changes in the circumstellar and interstellar medium in which the shocks travel.

SERPent: Automated reduction and RFI-mitigation software for e-MERLIN

The Scripted E-merlin Rfi-mitigation PipelinE for iNTerferometry (SERPent) is an automated reduction and RFI-mitigation procedure utilising the SumThreshold methodology (Offringa et al., 2010a), originally developed for the LOFAR pipeline. SERPent is written in the Parseltongue language enabling interaction with the Astronomical Image Processing Software (AIPS) program. Moreover, SERPent is a simple ‘out of the box’ Python script, which is easy to set up and is free of compilers. In addition to the flagging of RFI affected visibilities, the script also flags antenna zero-amplitude dropouts and Lovell telescope phase calibrator stationary scans inherent to the e-MERLIN system. Both the flagging and computational performances of SERPent are presented here, for e-MERLIN commissioning datasets for both L-band (1.3–1.8 GHz) and C-band (4–8 GHz) observations. RFI typically amounts to <20%–25% for the more problematic L-band observations and <5% for the generally RFI quieter C-band. The level of RFI detection and flagging is more accurate and delicate than visual manual flagging, with the output immediately ready for AIPS calibration. SERPent is fully parallelised and has been tested on a range of computing systems. The current flagging rate is at 110 GB day−1 on a ‘high-end’ computer (16 CPUs, 100 GB memory) which amounts to ∼6.9 GB CPU−1 day−1, with an expected increase in performance when e-MERLIN has completed its commissioning. The refining of automated reduction and calibration procedures is essential for the e-MERLIN legacy projects and future interferometers such as the SKA and the associated pathfinders (MeerKAT and ASKAP), where the vast data sizes (>TB) make traditional astronomer interactions unfeasible.

A multiple-beam CLEAN for imaging intra-day variable radio sources

The CLEAN algorithm, widely used in radio interferometry for the deconvolution of radio images, performs well only if the raw radio image (dirty image) is, to good approximation, a simple convolution between the instrumental point-spread function (dirty beam) and the true distribution of emission across the sky. An important case in which this approximation breaks down is during frequency synthesis if the observing bandwidth is wide enough for variations in the spectrum of the sky to become significant. The convolution assumption also breaks down, in any situation but snapshot observations, if sources in the field vary significantly in flux density over the duration of the observation. Such time-variation can even be instrumental in nature, for example due to jitter or rotation of the primary beam pattern on the sky during an observation. An algorithm already exists for dealing with the spectral variation encountered in wide-band frequency synthesis interferometry. This algorithm is an extension of CLEAN in which, at each iteration, a set of N “dirty beams” are fitted and subtracted in parallel, instead of just a single dirty beam as in standard CLEAN. In the wide-band algorithm the beams are obtained by expanding a nominal source spectrum in a Taylor series, each term of the series generating one of the beams. In the present paper this algorithm is extended to images which contain sources which vary over both frequency and time. Different expansion schemes (or bases) on the time and frequency axes are compared, and issues such as Gibbs ringing and non-orthogonality are discussed. It is shown that practical considerations make it often desirable to orthogonalize the set of beams before commencing the cleaning. This is easily accomplished via a Gram-Schmidt technique.

A High-Resolution Radio Continuum Study Of The Dwarf Irregular Galaxy IC 10

This is the accepted manuscript version of an article accepted for publication in Monthly Notices of the Royal Astronomical Society. J. Westcott et al, A High-Resolution Radio Continuum Study Of The Dwarf Irregular Galaxy IC 10, MNRAS 000, 1-15 (2016). Preprint 22 January 2017, doi: https://doi.org/10.1093/mnras/stx188

High-velocity-resolution observations of OH main line masers in the M82 starburst

Using the Very Large Array, a series of high-velocity-resolution observations have been made of the M82 starburst at 1.6 GHz. These observations follow up on previous studies of the main line OH maser emission in the central kiloparsec of this starburst region, but with far greater velocity resolution, showing significant velocity structure in some of the maser spots for the first time. A total of 13 masers were detected, including all but one of the previously known sources. While some of these masers are still unresolved in velocity, these new results clearly show velocity structure in spectra from several of the maser regions. Position-velocity plots show good agreement with the distribution of H I including interesting velocity structure on the blueward feature in the west of the starburst which traces the velocity distribution seen in the ionized gas.

ALMA observations of the supergiant B[e] star Wd1-9

Mass-loss in massive stars plays a critical role in their evolution, although the precise mechanism(s) responsible – radiatively driven winds, impulsive ejection and/or binary interaction – remain uncertain. In this Letter, we present Atacama Large Millimetre/Submillimeter Array line and continuum observations of the supergiant B[e] star Wd1-9, a massive post-main-sequence object located within the starburst cluster Westerlund 1 (Wd1). We find it to be one of the brightest stellar point sources in the sky at millimetre wavelengths, with (serendipitously identified) emission in the H41α radio recombination line. We attribute these properties to a low velocity (∼100 km s-1 ) ionized wind, with an extreme mass-loss rate ≳6.4 × 105(d/5 kpc)1.5 Mȯyr-1. External to this is an extended aspherical ejection nebula indicative of a prior phase of significant mass-loss. Taken together, the millimetre properties of Wd1-9 show a remarkable similarity to those of the highly luminous stellar source MWC349A. We conclude that these objects are interacting binaries evolving away from the main sequence and undergoing rapid case-A mass transfer. As such they – and by extension the wider class of supergiant B[e] stars – may provide a unique window into the physics of a process that shapes the life-cycle of ∼70 per cent of massive stars found in binary systems.

LeMMINGs I. The eMERLIN legacy survey of nearby galaxies. 1.5-GHz parsec-scale radio structures and cores

We present the first data release of high-resolution (≤0.2 arcsec) 1.5-GHz radio images of 103 nearby galaxies from the Palomar sample, observed with the eMERLIN array, as part of the LeMMINGs survey. This sample includes galaxies which are active (low-ionization nuclear emission-line regions [LINER] and Seyfert) and quiescent (H ii galaxies and absorption line galaxies, ALGs), which are reclassified based upon revised emission-line diagrams. We detect radio emission ≳0.2 mJy for 47/103 galaxies (22/34 for LINERS, 4/4 for Seyferts, 16/51 for H ii galaxies, and 5/14 for ALGs) with radio sizes typically of ≲100 pc. We identify the radio core position within the radio structures for 41 sources. Half of the sample shows jetted morphologies. The remaining half shows single radio cores or complex morphologies. LINERs show radio structures more core-brightened than Seyferts. Radio luminosities of the sample range from 1032 to 1040 erg s−1: LINERs and H ii galaxies show the highest and lowest radio powers, respectively, while ALGs and Seyferts have intermediate luminosities. We find that radio core luminosities correlate with black hole (BH) mass down to ∼107 M⊙, but a break emerges at lower masses. Using [O iii] line luminosity as a proxy for the accretion luminosity, active nuclei and jetted H ii galaxies follow an optical Fundamental Plane of BH activity, suggesting a common disc–jet relationship. In conclusion, LINER nuclei are the scaled-down version of FR I radio galaxies; Seyferts show less collimated jets; H ii galaxies may host weak active BHs and/or nuclear star-forming cores; and recurrent BH activity may account for ALG properties.