150 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
Self-Generated Magnetic Fields in Galactic Cooling Flows
Interstellar magnetic fields in elliptical galaxies are assumed to have their
origin in stellar fields that accompany normal mass loss from an evolving
population of old stars. The seed fields are amplified by interstellar
turbulence driven by stellar mass loss and supernova events. These disordered
fields are further amplified by time-dependent compression in the inward moving
galactic cooling flow and are expected to dominate near the galactic core.
Under favorable circumstances, fields similar in strength to those observed G can be generated solely from these natural
galactic processes. In general the interstellar field throughout elliptical
galaxies is determined by the outermost regions in the interstellar gas where
the turbulent dynamo process can occur. Because of the long hydrodynamic flow
times in galactic cooling flows, currently observed magnetic fields may result
from periods of intense turbulent field amplification that occurred in the
outer galaxy in the distant past. Particularly strong fields in ellipticals may
result from ancient galactic mergers or shear turbulence introduced at the
boundary between the interstellar gas and ambient cluster gas.Comment: 21 pages in AASTEX LaTeX with 2 figures; accepted by Astrophysical
Journa
Deep radio observations of 3C324 and 3C368: evidence for jet-cloud interactions
High resolution, deep radio images are presented for two distant radio
galaxies, 3C324 (z=1.206) and 3C368 (z=1.132), which are both prime examples of
the radio-optical alignment effect seen in powerful radio galaxies with
redshifts z > 0.6. Radio observations were made using the Very Large Array in
A-array configuration at 5 and 8 GHz, and using the MERLIN array at 1.4 and
1.65 GHz. Radio spectral index, radio polarisation, and rotation measure maps
are presented for both sources. Radio core candidates are detected in each
source, and by aligning these with the centroid of the infrared emission the
radio and the optical/infrared images can be related astrometrically with 0.1
arcsec accuracy. In each source the radio core is located at a minimum of the
optical emission, probably associated with a central dust lane. Both sources
also exhibit radio jets which lie along the directions of the bright strings of
optical knots seen in high resolution Hubble Space Telescope images. The
northern arm of 3C368 shows a close correlation between the radio and optical
emission, whilst along the jet direction of 3C324 the bright radio and optical
knots are co-linear but not co-spatial. These indicate that interactions
between the radio jet and its environment play a key role in producing the
excess ultraviolet emission of these sources, but that the detailed mechanisms
vary from source to source. 3C368 is strongly depolarised and has an average
rest-frame rotation measure of a few hundred rad/m^2, reaching about 1000
rad/m^2 close to the most depolarised regions. 3C324 has weaker depolarisation,
and an average rest-frame rotation measure of between 100 and 200 rad/m^2. Both
sources show large gradients in their rotation measure structures, with
variations of up to 1000 rad/m^2 over distances of about 10 kpc.Comment: 15 pages including 4 figures. LaTeX. Accepted for publication in
MNRA
MERLIN observations of relativistic ejections from GRS 1915+105
We present high resolution MERLIN radio images of multiple relativistic
ejections from GRS 1915+105 in 1997 October / November. The observations were
made at a time of complex radio behaviour, corresponding to multiple
optically-thin outbursts and several days of rapid radio flux oscillations. The
radio imaging resolved four major ejection events from the system. As
previously reported from earlier VLA observations of the source, we observe
apparent superluminal motions resulting from intrinsically relativistic motions
of the ejecta. However, our measured proper motions are significantly greater
than those observed on larger angular scales with the VLA. Under the assumption
of an intrinsically symmetric ejection, we can place an upper limit on the
distance to GRS 1915+105 of 11.2 +/- 0.8 kpc. Solutions for the velocities
unambiguously require a higher intrinsic speed by about 0.1c than that derived
from the earlier VLA observations, whilst the angle to the line-of-sight is not
found to be significantly different. At a distance of 11 kpc, we obtain
solutions of v = 0.98 (-0.05,+0.02)c and theta = 66 +/- 2 degrees. The jet also
appears to be curved on a scale which corresponds to a period of around 7 days.
We observe significant evolution of the linear polarisation of the approaching
component, with large rotations in position angle and a general decrease in
fractional polarisation. The power input into the formation of the jet is very
large, >10^38 erg/s at 11 kpc for a pair plasma. If the plasma contains a cold
proton for each electron, then the mass outflow rate, >10^18 g/sec is
comparable to inflow rates previously derived from X-ray spectral fits.Comment: 14 pages, 7 figures. Accepted for publication in MNRA
Why Are Rotating Elliptical Galaxies Less Elliptical at X-ray Frequencies?
If mass and angular momentum were conserved in cooling flows associated with
luminous, slowly rotating elliptical galaxies, the inflowing hot gas would spin
up, resulting in disks of cold gas and X-ray images that are highly flattened
along the equatorial plane out to several effective radii. Such X-ray
flattening is not observed at the spatial resolution currently available to
X-ray observations. Evidently mass and angular momentum are not in fact
conserved. If cooling flows are depleted by localized radiative cooling at
numerous sites distributed throughout the flows, then disks of cooled gas do
not form and the X-ray images appear nearly circular. However, the distribution
of young stars formed from the cooled gas is still somewhat flattened relative
to the stellar light. X-ray images of galactic cooling flows can also be
circularized by the turbulent diffusion of angular momentum away from the axis
of rotation, but the effective viscosity of known processes -- stellar mass
loss, supernovae, cooling site evolution, etc. -- is insufficient to
appreciably circularize the X-ray images. Radial gradients in the interstellar
iron abundance are unaffected by the expected level of interstellar turbulence
since these gradients are continuously re-established by Type Ia supernovae.Comment: 17 pages with 6 figures; accepted by Astrophysical Journa
MERLIN radio detection of an interaction zone within a binary Orion proplyd system
Presented here are high angular resolution MERLIN 5 GHz (6 cm) continuum
observations of the binary proplyd system, LV 1 in the Orion nebula, which
consists of proplyd 168--326SE and its binary proplyd companion 168--326NW
(separation 0.4 arcsec). Accurate astrometric alignment allows a detailed
comparison between these data and published HST PC Halpha and [Oiii] images.
Thermal radio sources coincide with the two proplyds and originate in the
ionized photoevaporating flows seen in the optical emission lines. Flow
velocities of approx 50 km/s from the ionized proplyd surfaces and \geq 100
km/s from a possible micro-jet have been detected using the Manchester Echelle
spectrometer.
A third radio source is found to coincide with a region of extended, high
excitation, optical line emission that lies between the binary proplyds
168--326SE/326NW . This is modelled as a bowshock due to the collision of the
photoevaporating flows from the two proplyds. Both a thermal and a non-thermal
origin for the radio emission in this collision zone are considered.Comment: 23 pages, 9 figures, accepted by Ap
HST and MERLIN observations of the jet in 3C273
We present red and blue images of the jet of the quasar 3C273 obtained with the WFPC2 on the Hubble Space Telescope as well as a new radio map made with the MERLIN array. The images are of significantly better quality than that of previous data. The two maps are aligned to an accuracy of 0.020''; this accuracy is achieved because both the quasar and the jet are contained in both the radio and optical images. The start of the optical jet is marked by an elongated knot which appears identical at radio and optical wavelengths. Other knots in the optical jet correspond to narrow oblique features within the radio outline. The total width of the smooth emission in the optical jet is~0.7''; the FWHM of the optical knots is~0.3''. The knots may trace the current location of a narrow, perhaps helical jet lying within the outlines of the older radio cocoon
Formation of Low Mass Stars in Elliptical Galaxy Cooling Flows
X-ray emission from hot (T = 10^7 K) interstellar gas in massive elliptical
galaxies indicates that 10^{10} M_sun has cooled over a Hubble time, but
optical and radio evidence for this cold gas is lacking. We provide detailed
theoretical support for the hypothesis that this gas has formed into low
luminosity stars. Within several kpc of the galactic center, interstellar gas
first cools to T = 10^4 K where it is heated by stellar UV and emits the
observed diffuse optical line emission. This cooling occurs at a large number
(10^6) of isolated sites. After less than a solar mass of gas has accumulated
(10^{-6} M_sun/yr) at a typical cooling site, a neutral (HI or H_2) core
develops in the HII cloud where gas temperatures drop to T = 15 K and the
ionization level (from thermal X-rays) is very low (x = 10^{-6}). We show that
the maximum mass of cores that become gravitationally unstable is only about 2
M_sun. No star can exceed this mass. Fragmentation of collapsing cores produces
a population of low mass stars with a bottom-heavy IMF and radial orbits.
Gravitational collapse and ambipolar diffusion are rapid. The total mass of
star-forming (dust-free) HI or H_2 cores in a typical bright elliptical is only
10^6 M_sun, below current observational thresholds.Comment: 23 pages in AASTEX LaTeX with 8 figures; accepted by Astrophysical
Journa
AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field
We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations
of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds.
Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects
two radio sources in a field that encompasses the HDF and part of the Hubble
Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013,
low-luminosity FR-I radio source located within the HDF itself) and VLA
J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system).
A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI
detections of all three sources suggest that most of the radio emission of
these particular sources (including the dusty starburst) is generated by an
embedded AGN.Comment: 4 pages, 1 figure; Accepted by Astron. & Astrophys Letters ... See
http://www.nfra.nl/~mag/hdf_evn.htm
Multi-wavelength observations of the Soft Gamma Repeater SGR 1900+14 during its April 2001 activation
The soft-gamma repeater SGR 1900+14 became active on 18 April 2001 after
about two years of quiescence; it had remained at a very low state of activity
since the fall of 1998, when it exhibited extraordinary flaring. We have
observed the source in the gamma and X rays with Ulysses and Chandra, and in
the radio with MERLIN. We report here the confirmation of a two component X-ray
spectrum (power law + blackbody), indicating emission from the neutron star
surface. We have determined that there is a dust halo surrounding the source
that extends up to >~ 100'' from the center of SGR 1900+14, due to scattering
in the interstellar medium.Comment: 13 pages 3 figures, accepted for publication in the ApJ
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