1,619 research outputs found
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 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
components. Ni K line emission by an amount of 12% of Fe K is
statistically required. An alternative explanation is a model consisting of a
narrow core and two disk lines with inclinations of 58 and 0, respectively. We suppose that the component with inclination of 58 was observed directly, consistent with its edge-on geometry, and the
component with inclination of 0 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
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