3,474 research outputs found
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
Emission lines and optical continuum in low-luminosity radio galaxies
We present spectroscopic observations of a complete sub-sample of 13
low-luminosity radio galaxies selected from the 2Jy sample. The underlying
continuum in these sources is carefully modelled in order to make a much-needed
comparison between the emission line and continuum properties of FRIs with
those of other classes of radio sources. We find that 5 galaxies in the sample
show a measurable UV excess: 2 of the these sources are BL Lacs and in the
remaining 3 galaxies we argue that the most likely contributor to the UV excess
is a young stellar component. Excluding the BL Lacs, we therefore find that
\~30% of the sample show evidence for young stars, which is similar to the
results obtained for higher luminosity samples. We compare our results with
far-infrared measurements in order to investigate the far-infrared-starburst
link. The nature of the optical-radio correlations is investigated in light of
this new available data and, in contrast to previous studies, we find that the
FRI sources follow the correlations with a similar slope to that found for the
FRIIs. Finally, we compare the luminosity of the emission lines in the FRI and
BL Lac sources and find a significant difference in the [OIII] line
luminosities of the two groups. Our results are discussed in the context of the
unified schemes.Comment: 18 pages, 31 figures, MNRAS in press, (all enquiries to Clive
Tadhunter ([email protected])
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
- …