564 research outputs found
MERLIN observations of Stephan's Quintet
We present MERLIN L-band images of the compact galaxy group, Stephan's
Quintet. The Seyfert 2 galaxy, NGC 7319, the brightest member of the compact
group, is seen to have a triple radio structure typical of many extra-galactic
radio sources which have a flat spectrum core and two steep spectrum lobes with
hot spots. The two lobes are asymmetrically distributed on opposite sides of
the core along the minor axis of the galaxy. Ultraviolet emission revealed in a
high resolution HRC/ACS HST image is strongly aligned with the radio plasma and
we interpret the intense star formation in the core and north lobe as an event
induced by the collision of the north radio jet with over-dense ambient
material. In addition, a re-mapping of archive VLA L-band observations reveals
more extended emission along the major axis of the galaxy which is aligned with
the optical axis. Images formed from the combined MERLIN and archive VLA data
reveal more detailed structure of the two lobes and hot spots.Comment: Completely revised version with new HST data included, to appear in
MNRA
Evidence for extended, obscured starbursts in submm galaxies
We compare high-resolution optical and radio imaging of 12 luminous submm
galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable
spatial resolution, 0.3" (2kpc). The radio emission traces the likely
far-infrared morphology of these dusty, luminous galaxies. In ~30% of the
sample the radio appears unresolved, suggesting that the emission is compact:
either an obscured AGN or nuclear starburst. However, in the majority, ~70%
(8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc).
For these galaxies the radio morphologies are broadly similar to their
restframe UV emission seen by HST. We discuss the probable mechanisms for the
extended emission and conclude that their luminous radio and submm emission
arises from a large, spatially-extended starburst. The median SFRs are
1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of
45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely
associated with the submm population. A more detailed comparison of the
distribution of UV and radio emission shows that the broad similarities on
large scales are not carried through to smaller scales, where there is rarely a
one-to-one correspondance. We interpret this as resulting from highly
structured internal obscuration, suggesting that the vigorous activity is
producing wind-blown channels through the obscuration in these galaxies. If
correct this underlines the difficulty of using UV morphologies to understand
structural properties of this population and also may explain the surprising
frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure
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
The ruff of equatorial emission around the SS433 jets: its spectral index and origin
We present unique radio observations of SS433, using MERLIN, the VLBA, and
the VLA, which allow us to, for the first time, properly image and derive a
meaningful spectral index for the `ruff' of equatorial emission which surrounds
SS433's jet. We interpret this smooth ruff as a wind-like outflow from the
binary.Comment: 4 pages, 4 figures, to appear in Proceedings of the 4th Microquasar
Workshop, eds. Ph Durouchoux, Y. Fuchs and J. Rodrigue
Radio Weak Gravitational Lensing with VLA and MERLIN
We carry out an exploratory weak gravitational lensing analysis on a combined
VLA and MERLIN radio data set: a deep (3.3 micro-Jy beam^-1 rms noise) 1.4 GHz
image of the Hubble Deep Field North. We measure the shear estimator
distribution at this radio sensitivity for the first time, finding a similar
distribution to that of optical shear estimators for HST ACS data in this
field. We examine the residual systematics in shear estimation for the radio
data, and give cosmological constraints from radio-optical shear
cross-correlation functions. We emphasize the utility of cross-correlating
shear estimators from radio and optical data in order to reduce the impact of
systematics. Unexpectedly we find no evidence of correlation between optical
and radio intrinsic ellipticities of matched objects; this result improves the
properties of optical-radio lensing cross-correlations. We explore the
ellipticity distribution of the radio counterparts to optical sources
statistically, confirming the lack of correlation; as a result we suggest a
connected statistical approach to radio shear measurements.Comment: 16 pages with 19 figures, accepted for publication in MNRAS; Minor
corrections to section 6.3; 2 references adde
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