771 research outputs found
Extended reagions of radio emission not associated with the AGN phenomenon as sources of acceleration of cosmic rays: The case of cD galaxies
Diffuse, non-thermal extended emission not associated with the AGN
phenomenon, found in many clusters of galaxies hosted by an AGN, are related to
the acceleration of cosmic rays. In the current work we present preliminary
evidence of absence of such formations in clusters of galaxies hosted by
optically identified cD galaxies. Our subsample consists of three powerful low
redshift radiogalaxies, centered in poor clusters of galaxies. We have searched
for radio relics and (mini)halos which could be forming as a result of the
confinement of cosmic rays by bubbles creayed by the AGN. We report on the work
in progress.Comment: 4 pages, Conference HEPRO II
A Companion Galaxy to the Post-Starburst Quasar UN J1025-0040
UN J1025-0040 is a quasar at z = 0.6344 that shows an extremely bright post
starburst population of age ~ 400 Myr (Brotherton et al. 1999). Images of UN
J1025-0040 show a nearly stellar object 4.2 arcseconds SSW of the quasar. We
present imaging and spectroscopy that confirm that this object is a companion
galaxy at redshift z = 0.6341. We estimate an age of ~ 800 Myr for the dominant
stellar population in the companion. The companion appears to be interacting
with the quasar host galaxy, and this interaction may have triggered both the
starburst and the quasar activity in UN J1025-0040.Comment: 8 pages plus 4 figures (2 postscript, 2 jpg). Postscript version
available at http://www.ifa.hawaii.edu/~canaguby/preprints.html To appear in
the January 2000 issue of A
HST images and properties of the most distant radio galaxies
We present Hubble Space Telescope images of 11 high redshift radio galaxies
(between and ). The galaxies were observed with the WFPC2 camera
in a broad band filter (F606W or F707W, roughly equivalent to V or R-band), for
2 orbits each. We find that on the scale of the HST observations there is a
wide variety of morphological structures of the hosting galaxies: most objects
have a clumpy, irregular appearance, consisting of a bright nucleus and a
number of smaller components, suggestive of merging systems. Some observed
structures could be due (at least partly) to the presence of dust distributed
through the galaxies. The UV continuum emission is generally elongated and
aligned with the axis of the radio sources, however the characteristics of the
``alignment effect'' differ from case to case, suggesting that the phenomenon
cannot be explained by a single physical mechanism. We compare the properties
of our radio galaxies with those of the
UV dropout galaxies and conclude that (i) the most massive radio galaxies may
well evolve from an aggregate of UV dropout galaxies and (ii) high redshift
radio galaxies probably evolve into present day brightest cluster galaxies.Comment: 22 pages, 30 figures, accepted by A&
Approximating a Behavioural Pseudometric without Discount for<br> Probabilistic Systems
Desharnais, Gupta, Jagadeesan and Panangaden introduced a family of
behavioural pseudometrics for probabilistic transition systems. These
pseudometrics are a quantitative analogue of probabilistic bisimilarity.
Distance zero captures probabilistic bisimilarity. Each pseudometric has a
discount factor, a real number in the interval (0, 1]. The smaller the discount
factor, the more the future is discounted. If the discount factor is one, then
the future is not discounted at all. Desharnais et al. showed that the
behavioural distances can be calculated up to any desired degree of accuracy if
the discount factor is smaller than one. In this paper, we show that the
distances can also be approximated if the future is not discounted. A key
ingredient of our algorithm is Tarski's decision procedure for the first order
theory over real closed fields. By exploiting the Kantorovich-Rubinstein
duality theorem we can restrict to the existential fragment for which more
efficient decision procedures exist
Radiative Shock-Induced Collapse of Intergalactic Clouds
Accumulating observational evidence for a number of radio galaxies suggests
an association between their jets and regions of active star formation. The
standard picture is that shocks generated by the jet propagate through an
inhomogeneous medium and trigger the collapse of overdense clouds, which then
become active star-forming regions. In this contribution, we report on recent
hydrodynamic simulations of radiative shock-cloud interactions using two
different cooling models: an equilibrium cooling-curve model assuming solar
metallicities and a non-equilibrium chemistry model appropriate for primordial
gas clouds. We consider a range of initial cloud densities and shock speeds in
order to quantify the role of cooling in the evolution. Our results indicate
that for moderate cloud densities (>1 cm^{-3}) and shock Mach numbers (<20),
cooling processes can be highly efficient and result in more than 50% of the
initial cloud mass cooling to below 100 K. We also use our results to estimate
the final H_2 mass fraction for the simulations that use the non-equilibrium
chemistry package. This is an important measurement, since H_2 is the dominant
coolant for a primordial gas cloud. We find peak H_2 mass fractions of >0.01
and total H_2 mass fractions of >10^{-5} for the cloud gas. Finally, we compare
our results with the observations of jet-induced star formation in
``Minkowski's Object.'' We conclude that its morphology, star formation rate (~
0.3M_solar/yr) and stellar mass (~ 1.2 x 10^7 M_solar) can be explained by the
interaction of a 90,000 km/s jet with an ensemble of moderately dense (~ 10
cm^{-3}), warm (10^4 K) intergalactic clouds in the vicinity of its associated
radio galaxy at the center of the galaxy cluster.Comment: 30 pages, 7 figures, submitted to Astrophysical Journa
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