710 research outputs found
3C459: A highly asymmetric radio galaxy with a starburst
Multifrequency radio observations of the radio galaxy 3C459 using MERLIN, VLA
and the EVN, and an optical HST image using the F702W filter are presented. The
galaxy has a very asymmetric radio structure, a high infrared luminosity and a
young stellar population. The eastern component of the double-lobed structure
is brighter, much closer to the nucleus and is significantly less polarized
than the western one. This is consistent with the jet on the eastern side
interacting with dense gas, which could be due to a merged companion or dense
cloud of gas. The HST image of the galaxy presented here exhibits filamentary
structures, and is compared with the MERLIN 5-GHz radio map. EVN observations
of the prominent central component, which has a steep radio spectrum, show a
strongly curved structure suggesing a bent or helical radio jet. The radio
structure of 3C459 is compared with other highly asymmetric, Fanaroff-Riley II
radio sources, which are also good candidates for studying jet-cloud
interactions. Such sources are usually of small linear size and it is possible
that the jets are interacting with clouds of infalling gas that fuel the radio
source.Comment: 10 pages, 6 figures, 2 table
Arp 65 interaction debris: massive HI displacement and star formation
Context: Pre-merger interactions between galaxies can induce significant
changes in the morphologies and kinematics of the stellar and ISM components.
Large amounts of gas and stars are often found to be disturbed or displaced as
tidal debris. This debris then evolves, sometimes forming stars and
occasionally tidal dwarf galaxies. Here we present results from our HI study of
Arp 65, an interacting pair hosting extended HI tidal debris. Aims: In an
effort to understand the evolution of tidal debris produced by interacting
pairs of galaxies, including in situ star and tidal dwarf galaxy formation, we
are mapping HI in a sample of interacting galaxy pairs. The Arp 65 pair is one
of them. Methods: Our resolved HI 21 cm line survey is being carried out using
the Giant Metrewave Radio Telescope (GMRT). We used our HI survey data as well
as available SDSS optical, Spitzer infra-red and GALEX UV data to study the
evolution of the tidal debris and the correlation of HI with the star-forming
regions within it. Results: In Arp 65 we see a high impact pre-merger
interaction involving a pair of massive galaxies (NGC 90 and NGC 93) that have
a stellar mass ratio of ~ 1:3. The interaction, which probably occurred ~ 1.0
-- 2.5 10 yr ago, appears to have displaced a large fraction of
the HI in NGC 90 (including the highest column density HI) beyond its optical
disk. We also find extended ongoing star formation in the outer disk of NGC 90.
In the major star-forming regions, we find the HI column densities to be ~ 4.7
10 cm or lower. But no signature of star formation was
found in the highest column density HI debris, SE of NGC 90. This indicates
conditions within the highest column density HI debris remain hostile to star
formation and it reaffirms that high HI column densities may be a necessary but
not sufficient criterion for star formation.Comment: Accepted in A&
3C254: MERLIN observations of a highly asymmetric quasar
Multifrequency, high-resolution radio observations of the quasar 3C254 using
MERLIN are presented. The quasar has a highly asymmetric radio structure, with
the eastern component of the double-lobed structure being much closer to the
nucleus and significantly less polarized than the western one. However, the two
lobes are more symmetric in their total flux densities. The observations show
the detailed structure of the hotspots which are very different on opposite
sides of the radio core, reveal no radio jet and suggest that the
oppositely-directed jets may be intrinsically asymmetric.Comment: 8 pages, 5 figures, 1 table, accepted for publication in MNRA
6-[(Dimethylamino)methyleneamino]-1,3-dimethylpyrimidine-2,4(1H,3H)-dione dihydrate
Uracil, the pyrimidine nucleobase, which combined with adenine forms one of the major motifs present in the biopolymer RNA, is also involved in the self-assembly of RNA. In the title compound, C9H14N4O2·2H2O, the asymmetric unit contains one dimethylaminouracil group and two water molecules. The plane of the N=C—NMe2 side chain is inclined at 27.6 (5)° to the plane of the uracil ring. Both water molecules form O—H⋯O hydrogen bonds with the carbonyl O atoms of the uracil group. Additional water–water hydrogen-bond interactions are also observed in the crystal structure. The O—H⋯O hydrogen bonds lead to the formation of a two-dimensional hydrogen-bonded network cage consisting of two dimethylaminouracil groups and six water molecules
J1420--0545: The radio galaxy larger than 3C236
We report the discovery of the largest giant radio galaxy, J1420-0545: a FR
type II radio source with an angular size of 17.4' identified with an optical
galaxy at z=0.3067. Thus, the projected linear size of the radio structure is
4.69 Mpc (if we assume that H_{0}=71 km\s\Mpc, Omega_{m}=0.27, and
Omega_{\Lambda}=0.73). This makes it larger than 3C236, which is the largest
double radio source known to date. New radio observations with the 100 m
Effelsberg telescope and the Giant Metrewave Radio Telescope, as well as
optical identification with a host galaxy and its optical spectroscopy with the
William Herschel Telescope are reported. The spectrum of J1420-0545 is typical
of elliptical galaxies in which continuum emission with the characteristic
4000A discontinuity and the H and K absorption lines are dominated by evolved
stars. The dynamical age of the source, its jets' power, the energy density,
and the equipartition magnetic field are calculated and compared with the
corresponding parameters of other giant and normal-sized radio galaxies from a
comparison sample. The source is characterized by the exceptionally low density
of the surrounding IGM and an unexpectedly high expansion speed of the source
along the jet axis. All of these may suggest a large inhomogeneity of the IGM.Comment: 20 pages, 5 figures, 3 table
On the Evolution of and High-Energy Emission from GHz-Peaked-Spectrum Sources
Here we discuss evolution and broad-band emission of compact (< kpc) lobes in
young radio sources. We propose a simple dynamical description for these
objects, consisting of a relativistic jet propagating into a uniform gaseous
medium in the central parts of an elliptical host. In the framework of the
proposed model, we follow the evolution of ultrarelativistic electrons injected
from a terminal hotspot of a jet to expanding lobes, taking into account their
adiabatic energy losses as well as radiative cooling. This allows us to discuss
the broad-band lobe emission of young radio sources. In particular, we argue
that the observed spectral turnover in the radio synchrotron spectra of these
objects cannot originate from the synchrotron self-absorption process but is
most likely due to free-free absorption effects connected with neutral clouds
of interstellar medium engulfed by the expanding lobes and photoionized by
active centers. We also find a relatively strong and complex high-energy
emission component produced by inverse-Compton up-scattering of various
surrounding photon fields by the lobes' electrons. We argue that such high
energy radiation is strong enough to account for several observed properties of
GHz-peaked-spectrum (GPS) radio galaxies at UV and X-ray frequencies. In
addition, this emission is expected to extend up to GeV (or possibly even TeV)
photon energies and can thus be probed by several modern gamma-ray instruments.
In particular, we suggest that GPS radio galaxies should constitute a
relatively numerous class of extragalactic sources detected by GLAST.Comment: 32 pages, 3 figures included. Revised version, accepted for
publication in Ap
The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution
Despite compelling evidence that stellar bars drive gas into the inner 1--2
kpc or circumnuclear (CN) region of galaxies, there are few large, high
resolution studies of the CN molecular gas and star formation (SF). We study a
sample of local barred non-starbursts and starbursts with high-resolution CO,
optical, Ha, RC, Br-gamma, and HST data, and find the following. (1) The inner
kpc of bars differs markedly the outer disk and hosts molecular gas surface
densities Sigma-gas-m of 500-3500 Msun pc-2, gas mass fractions of 10--30 %,
and epicyclic frequencies of several 100--1000 km s-1 kpc-1.Consequently,
gravitational instabilities can only set in at high gas densities and grow on a
short timescale (few Myr). This high density, short timescale, `burst' mode may
explain why powerful starbursts tend to be in the CN region of galaxies. (2) We
suggest that the variety in CO morphologies is due to different stages of
bar-driven inflow. At late stages, most of the CN gas is inside the outer inner
Lindblad resonance (OILR), and has predominantly circular motions. Across the
sample, we find bar pattern speeds with upper limits of 43 to 115 km s-1 kpc-1
and OILR radii of > 500 pc. (3) Barred starbursts and non-starbursts have CN
SFRs of 3--11 and 0.1--2 Msun yr-1, despite similar CN gas mass. Sigma-gas-m in
the starbursts is larger (1000--3500 Msun pc-2) and close to the Toomre
critical density over a large region. (4) Molecular gas makes up 10%--30% of
the CN dynamical mass (6--30 x 10^9 Msun).In the starbursts, it fuels CN SFRs
of 3--11 Msun yr-1, building young, massive, high V/sigma components. We
present evidence for such a pseudo-bulge in NGC 3351. Implications for secular
evolution along the Hubble sequence are discussed.Comment: Accepted by the Astrophysical Journal. Paper length reduced to fit
within APJ page limits. Version of paper with high resolution figures is at
http://www.as.utexas.edu/~sj/papers/ms-hires-sj05a.ps.g
Circular Polarization Induced by Scintillation in a Magnetized Medium
A new theory is presented for the development of circular polarization as
radio waves propagate through the turbulent, birefringent interstellar medium.
The fourth order moments of the wavefield are calculated and it is shown that
unpolarized incident radiation develops a nonzero variance in circular
polarization. A magnetized turbulent medium causes the Stokes parameters to
scintillate in a non-identical manner. A specific model for this effect is
developed for the case of density fluctuations in a uniform magnetic field.Comment: 16 pages, 1 figure, Phys. Rev. E, accepte
PKS B1545-321: Bow shocks of a relativistic jet?
Sensitive, high resolution images of the double-double radio galaxy PKS
B1545-321 reveal detailed structure, which we interpret in the light of
previous work on the interaction of restarted jets with pre-existing relict
cocoons. We have also examined the spectral and polarization properties of the
source, the color distribution in the optical host and the environment of this
galaxy in order to understand its physical evolution. We propose that the
restarted jets generate narrow bow shocks and that the inner lobes are a
mixture of cocoon plasma reaccelerated at the bow shock and new jet material
reaccelerated at the termination shock. The dynamics of the restarted jets
implies that their hot spots advance at mildly relativistic speeds with
external Mach numbers of at least 5. The existence of supersonic hot spot Mach
numbers and bright inner lobes is the result of entrainment causing a reduction
in the sound speed of the pre-existing cocoon. The interruption to jet activity
in PKS B1545-321 has been brief - lasting less than a few percent of the
lifetime of the giant radio source. The host
galaxy is located at the boundary of a large scale filamentary structure, and
shows blue patches in color distribution indicative of a recent merger, which
may have triggered the Mpc-scale radio galaxy.Comment: 26 pages including 1 table and 16 figures. To appear in MNRA
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