13CO 1-0 imaging of the Medusa merger, NGC4194

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 impact 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.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and Astrophysic

Star-formation in the central kpc of the starburst/LINER galaxy NGC1614

A high angular resolution, multi-wavelength study of the LINER galaxy NGC1614 has been carried out. OVRO CO 1-0 observations are presented together with extensive multi-frequency radio continuum and HI absorption observations with the VLA and MERLIN. Toward the center of NGC1614, we have detected a ring of radio continuum emission with a radius of 300 pc. This ring is coincident with previous radio and Paschen-alpha observations. The dynamical mass of the ring based on HI absorption is 3.1 x 10E9 Msun. The peak of the integrated CO 1-0 emission is shifted by 1" to the north-west of the ring center and a significant fraction of the CO emission is associated with a crossing dust lane. An upper limit to the molecular gas mass in the ring region is 1.7 x 10E9 Msun. Inside the ring, there is a north to south elongated 1.4GHz radio continuum feature with a nuclear peak. This peak is also seen in the 5GHz radio continuum and in the CO. We suggest that the R=300 pc star forming ring represents the radius of a dynamical resonance - as an alternative to the scenario that the starburst is propagating outwards from the center into a molecular ring. The ring-like appearance probably part of a spiral structure. Substantial amounts of molecular gas have passed the radius of the ring and reached the nuclear region. The nuclear peak seen in 5GHz radio continuum and CO is likely related to previous star formation, where all molecular gas was not consumed. The LINER-like optical spectrum observed in NGC1614 may be due to nuclear starburst activity, and not to an Active Galactic Nucleus (AGN). Although the presence of an AGN cannot be excluded.Comment: Accepted by Astronomy and Astrophysics, 12 pages, 10 figure

Exchange interactions and relaxation in paramagnets

This thesis presents some investigations into a variety of paramagnetic systems. Electron paramagnetic resonance is often used to investigate the behaviour of paramagnetic ions present as substitutional impurities in diamagnetic host crystals. Measurements have been carried out at liquid helium temperatures on single crystals of aluminium acetylacetonate doped to 2% with ruthenium 3+ ions. The spectrum at nitrogen temperatures has been reported by Jarrett (1957) who found two ruthenium sites

Sub-arcsecond imaging of the radio continuum and neutral hydrogen in the Medusa merger

We present sub-arcsecond, Multi-Element Radio Linked Interferometer (MERLIN) observations of the decimetre radio continuum structure and neutral hydrogen (HI) absorption from the nuclear region of the starburst galaxy NGC 4194 (the Medusa Merger). The continuum structure of the central kiloparsec of the Medusa has been imaged, revealing a pair of compact radio components surrounded by more diffuse, weak radio emission. Using the constraints provided by these observations and those within the literature we conclude that the majority of this radio emission is related to the ongoing star-formation in this merger system. With these observations we also trace deep HI absorption across the detected radio continuum structure. The absorbing HI gas structure exhibits large variations in column densities. The largest column densities are found toward the south of the nuclear radio continuum, co-spatial with both a nuclear dust lane and peaks in $^{12}$CO (1->0) emission. The dynamics of the HI absorption, which are consistent with lower resolution $^{12}$CO emission observations, trace a shallow north-south velocity gradient of ~320km/s/kpc. This gradient is interpreted as part of a rotating gas structure within the nuclear region. The HI and CO velocity structure, in conjunction with the observed gas column densities and distribution, is further discussed in the context of the fuelling and gas physics of the ongoing starburst within the centre of this merger.Comment: 12 pages, 5 figures, to appear in A&

A high-resolution mm and cm study of the obscured LIRG NGC 4418 - A compact obscured nucleus fed by in-falling gas?

The aim of this study is to constrain the dynamics, structure and feeding of the compact nucleous of NGC4418, and to reveal the nature of the main hidden power source: starburst or AGN. We obtained high spatial resolution observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz with the SMA very extended configuration. We use the continuum morphology and flux density to estimate the size of the emitting region, the star formation rate and the dust temperature. Emission lines are used to study the kinematics through position-velocity diagrams. Molecular emission is studied with population diagrams and by fitting an LTE synthetic spectrum. We detect bright 1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI. The CO 2-1 emission and HI absorption can be fit by two velocity components at 2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib 300K. Molecular excitation is consistent with a layered temperature structure, with three main components at 80, 160 and 300 K. For the hot component we estimate a source size of less than 5 pc. The nuclear molecular gas surface density of 1e4 Msun pc-2 is extremely high, and similar to that found in the ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the presence of a molecular and atomic in-flow, previously suggested by Herschel observations, which is feeding the activity in the center of NGC4418. Molecular excitation confirms the presence of a very compact, hot dusty core. If a starburst is responsible for the observed IR flux, this has to be at least as extreme as the one in Arp220, with an age of 3-10 Myr and a star formation rate >10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.Comment: 18 pages, 11 figures, Accepted for publication by A&A on 10/6/201

Deep MERLIN 5GHz Radio Imaging of Supernova Remnants in the M82 Starburst

The results of an extremely deep, 8-day long observation of the central kpc of the nearby starburst galaxy M82 using MERLIN (Multi-Element Radio Linked Interferometer Network) at 5 GHz are presented. The 17E-06 Jy/beam, rms noise level in the naturally weighted image make 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 HII 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 July 1992, which gives a 9.75 year timeline, it has been possible to measure the expansion velocities of ten of the more compact sources, eight of which have not been measured before. These derived expansion velocities range between 2200 and 10500 km/s.Comment: 34 pages, 10 figures. Accepted by MNRA