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

A Parsec-Scale Study of the 5/15 GHz Spectral Indices of the Compact Radio Sources in M82

Observations of the starburst galaxy, M82, have been made with the VLA in its A-configuration at 15 GHz and MERLIN at 5 GHz enabling a spectral analysis of the compact radio structure on a scale of < 0.1'' (1.6 pc). Crucial to these observations was the inclusion of the Pie Town VLBA antenna, which increased the resolution of the VLA observations by a factor of ~2. A number of the weaker sources are shown to have thermal spectra and are identified as HII regions with emission measures ~10^7 cm^-6 pc. Some of the sources appear to be optically thick at 5 GHz implying even higher emission measures of ~10^8 cm^-6 pc. The number of compact radio sources in M82 whose origin has been determined is now 46, of which 30 are supernova related and the remaining 16 are HII regions. An additional 15 sources are noted, but have yet to be identified, meaning that the total number of compact sources in M82 is at least 61. Also, it is shown that the distribution of HII regions is correlated with the large-scale ionised gas distribution, but is different from the distribution of supernova remnants. In addition, the brightest HII region at (B1950) 09h 51m 42.21s +69 54' 59.2'' shows a spectral index gradient across its resolved structure which we attribute to the source becoming optically thick towards its centre.Comment: Accepted for publication in MNRAS. 15 pages, 9 figure

Second Epoch Global VLBI Observations of Compact Radio Sources in the M82 Starburst Galaxy

We have presented the results of a second epoch of global Very Long Baseline Interferometry observations, taken on 23 February 2001 at a wavelength of 18 cm, of the central kiloparsec of the nearby starburst galaxy Messier 82. These observations were aimed at studying the structural and flux evolution of some of the compact radio sources in the central region that have been identified as supernova remnants. The objects 41.95+575 and 43.31+592 have been studied, expansion velocities of 2500 +/- 1200 km/s and 7350 +/- 2100 km/s respectively have been derived. Flux densities of 31.1 +/- 0.3 mJy and 17.4 +/- 0.3 mJy have been measured for the two objects. These results are consistent with measurements and predictions from previous epochs.Comment: 5 pages, 3 figures. To be published on the accompanying CD of the Proceedings of IAU Colloquium 192: Supernova

Monitoring of the prompt radio emission from the unusual supernova 2004dj in NGC2403

Supernova 2004dj in the nearby spiral galaxy NGC2403 was detected optically in July 2004. Peaking at a magnitude of 11.2, this is the brightest supernova detected for several years. Here we present Multi-Element Radio Linked Interferometer Network (MERLIN) observations of this source, made over a four month period, which give a position of R.A. = 07h37m17.044s, Dec =+65deg35'57.84" (J2000.0). We also present a well-sampled 5 GHz light curve covering the period from 5 August to 2 December 2004. With the exception of the unusual and very close SN 1987A, these observations represent the first detailed radio light curve for the prompt emission from a Type II-P supernova.Comment: (1) Jodrell Bank Observatory (2) University of Valencia (3) University of Sheffield 6 pages, 1 figure. To appear in ApJ letter

A possible radio supernova in the outer part of NGC 3310

As part of an on-going radio supernova monitoring program, we have discovered a variable, compact steep spectrum radio source ~65 arcsec (~4 kpc) from the centre of the starburst galaxy NGC 3310. If the source is at the distance of NGC 3310, then its 5 GHz luminosity is ~3 x 10^{19} WHz^-1. The source luminosity, together with its variability characteristics, compact structure (<17 mas) and its association with a group of HII regions, leads us to propose that it is a previously uncatalogued type II radio supernova. A search of archival data also shows an associated X-ray source with a luminosity similar to known radio supernova.Comment: 5 pages, 3 figures. Accepted by MNRA

15 years of VLBI observations of two compact radio sources in Messier 82

We present the results of a second epoch of 18cm global Very Long-Baseline Interferometry (VLBI) observations, taken on 23 February 2001, of the central kiloparsec of the nearby starburst galaxy Messier 82. These observations further investigate the structural and flux evolution of the most compact radio sources in the central region of M82. The two most compact radio objects in M82 have been investigated (41.95+575 and 43.31+592). Using this recent epoch of data in comparison with our previous global VLBI observations and two earlier epochs of European VLBI Network observations we measure expansion velocities in the range of 1500-2000km/s for 41.95+575, and 9000-11000km/s for 43.31+592 using various independent methods. In each case the measured remnant expansion velocities are significantly larger than the canonical expansion velocity (500km/s) of supernova remnants within M82 predicted from theoretical models. In this paper we discuss the implications of these measured expansion velocities with respect to the high density environment that the SNR are expected to reside in within the centre of the M82 starburst.Comment: Accepted for publication in MNRAS, 9 pages, 8 figure

The origin of the Narrow Line Region of Mrk 3: an overpressured jet cocoon

We have obtained HST FOC long-slit optical spectroscopy of the Narrow Line Region of the Seyfert 2 galaxy Mrk 3. In the region cospatial with the radio-jet the velocity field is highly perturbed and shows two velocity systems separated by as much as 1700 km/s. We interpret this to be the consequence of the rapid expansion of a cocoon of hot gas, shocked and heated by the radio-emitting outflow, which compresses and accelerates the ambient gas. The NLR itself is essentially a cylindrical shell expanding supersonically. From the size and velocity of the expanding region, we derive an upper limit to the radio-source age, ~ 2 E42 erg/s required to inflate the cocoon and estimate that the jet minimum advance speed is 3 E-3 pc per year. The total kinetic energy of the high velocity NLR gas can be estimated as ~6 E54 erg, comparable to the total energy carried by the jet over its lifetime and this quantitatively supports the idea that the NLR gas is accelerated by the jet. If the advance speed of Mrk 3 is representative of the Seyfert population then these sources must also be short lived and probably recurrent. The jet kinetic luminosity of Mrk 3 is between 2 and 3 orders of magnitude smaller than that derived for radio-loud AGNs with similar emission-line luminosity. On the other hand, the fraction of jet power dissipated in radio-emission is similar. We speculate that the main distinction between radio-quiet and radio-loud AGN is ascribed to a difference in jet power rather than to a different efficiency in synchrotron emission production.Comment: 13 pages, 8 figures, Astrophysical Journal in pres

The broad Fe K line profile in NGC 4151

We present an analysis of the Fe K line profile of NGC 4151 by using long ASCA observation data obtained in May 1995. The unprecedented good data quality, which is much better in the energy band around 6.4 keV than that of the famous 4.2-day ASCA observation of MCG -6-30-15 in July 1994, offers a unique opportunity to study the details of Fe K line profile. Apart from those characteristics already noticed in earlier ASCA observations on this object (Yaqoob et al. 1995): a broad and skewed profile, with a strong peak at about 6.4 keV and a large red wing extending to $\sim$4 - 5 keV, which is remarkably similar to that of MCG -6-30-15, we also find a weak blue wing extending to about 8 keV, thanks to the good quality of the data. When fitted by a relativistic accretion disk line plus a narrow core at 6.4 keV, the data constrain the accretion disk to be nearly face-on, contrary to the edge-on geometry inferred from optical and UV observations. However, the extended blue wing can not be well fitted even after we include corresponding Fe K$\beta$ components. Ni K$\alpha$ line emission by an amount of 12% of Fe K$\alpha$ is statistically required. An alternative explanation is a model consisting of a narrow core and two disk lines with inclinations of 58$^{\rm o}$ and 0$^{\rm o}$, respectively. We suppose that the component with inclination of 58$^{\rm o}$ was observed directly, consistent with its edge-on geometry, and the component with inclination of 0$^{\rm o}$ was scattered into our line of sight by a Compton mirror, which might be the cool accretion disk corona proposed by Poutanen et al. (1996).Comment: 15 pages, including 2 figures, aasms4.sty. To appear in ApJL 52