478 research outputs found
Isotropic Heating of Galaxy Cluster Cores via Rapidly Reorienting AGN Jets
AGN jets carry more than sufficient energy to stave off catastrophic cooling
of the intracluster medium (ICM) in the cores of cool-core clusters. However,
in order to prevent catastrophic cooling, the ICM must be heated in a
near-isotropic fashion and narrow bipolar jets with
ergs/s, typical of radio AGNs at cluster centres, are inefficient at heating
the gas in the transverse direction to the jets. We argue that due to existent
conditions in cluster cores, the SMBHs will, in addition to accreting gas via
radiatively inefficient flows, experience short stochastic episodes of enhanced
accretion via thin discs. In general, the orientation of these accretion discs
will be misaligned with the spin axis of the black holes and the ensuing
torques will cause the black hole's spin axis (and therefore, the jet axis) to
slew and rapidly change direction. This model not only explains recent
observations showing successive generations of jet-lobes-bubbles in individual
cool-core clusters that are offset from each other in the angular direction
with respect to the cluster center, but also shows that AGN jets {\it can} heat
the cluster core nearly isotropically on the gas cooling timescale. Our model
{\it does} require that the SMBHs at the centers of cool-core clusters be
spinning relatively slowly. Torques from individual misaligned discs are
ineffective at tilting rapidly spinning black holes by more than a few degrees.
Additionally, since SMBHs that host thin accretion discs will manifest as
quasars, we predict that roughly 1--2 rich clusters within should have
quasars at their centers.Comment: 10 pages; accepted in ApJ; updated to conform with the accepted
Journal versio
Cold gas in group-dominant elliptical galaxies
We present IRAM 30m telescope observations of the CO(1-0) and (2-1) lines in
a sample of 11 group-dominant elliptical galaxies selected from the CLoGS
nearby groups sample. Our observations confirm the presence of molecular gas in
4 of the 11 galaxies at >4 sigma significance, and combining these with data
from the literature we find a detection rate of 43+-14%, comparable to the
detection rate for nearby radio galaxies, suggesting that group-dominant
ellipticals may be more likely to contain molecular gas than their non-central
counterparts. Those group-dominant galaxies which are detected typically
contain ~2x10^8 Msol of molecular gas, and although most have low star
formation rates (<1 Msol/yr) they have short depletion times, indicating that
the gas must be replenished on timescales ~100 Myr. Almost all of the galaxies
contain active nuclei, and we note while the data suggest that CO may be more
common in the most radio-loud galaxies, the mass of molecular gas required to
power the active nuclei through accretion is small compared to the masses
observed. We consider possible origin mechanisms for the gas, through cooling
of stellar ejecta within the galaxies, group-scale cooling flows, and gas-rich
mergers, and find probable examples of each type within our sample, confirming
that a variety of processes act to drive the build up of molecular gas in
group-dominant ellipticals.Comment: 9 pages, 5 postscript figures, 4 tables, accepted by A&A. Revised
throughout in response to referee's comments, including updates to Table 1
and Figure 4, and addition of Figure
A decentralized proportional-integral sliding mode tracking controller for a 2 D.O.F robot arm
Trajectory tracking with high accuracy is a very challenging topic in direct drive robot control. This is due to the nonlinearities and input couplings present in the dynamics of the arm. This paper deals with the tracking control of a class of direct-drive robot manipulators. A robust Proportional-Integral (PI) sliding mode control law is derived so that the robot trajectory tracks a desired trajectory as closely as possible despite the highly non-linear and coupled dynamics. The controller is designed using the decentralized approaches. Application to a two degree of freedom direct drive robot arm is considered
A Low Upper Limit to the Lyman Continuum Emission of two galaxies at z 3
Long exposure, long-slit spectra have been obtained in the UV/optical bands
for two galaxies at z=2.96 and z=3.32 to investigate the fraction of ionizing
UV photons escaping from high redshifts galaxies. The two targets are among the
brightest galaxies discovered by Steidel and collaborators and they have
different properties in terms of Lyman-alpha emission and dust reddening. No
significant Lyman continuum emission has been detected. The noise level in the
spectra implies an upper limit of f_{rel,esc}\equiv 3 f(900)/f(1500)< 16% for
the relative escape fraction of ionizing photons, after correction for
absorption by the intervening intergalactic medium. This upper limit is 4 times
lower than the previous detection derived from a composite spectrum of 29 Lyman
break galaxies at z 3.4. If this value is typical of the escape fraction of the
z 3 galaxies, and is added to the expected contribution of the QSO population,
the derived UV background is in good agreement with the one derived by the
proximity effect.Comment: 16 pages, 2 figures, ApJ Letters in pres
The time-evolution of bias
We study the evolution of the bias factor b and the mass-galaxy correlation
coefficient r in a simple analytic model for galaxy formation and the
gravitational growth of clustering. The model shows that b and r can be
strongly time-dependent, but tend to approach unity even if galaxy formation
never ends as the gravitational growth of clustering debiases the older
galaxies. The presence of random fluctuations in the sites of galaxy formation
relative to the mass distribution can cause large and rapidly falling bias
values at high redshift.Comment: 4 pages, with 2 figures included. Typos corrected to match published
ApJL version. Color figure and links at http://www.sns.ias.edu/~max/bias.html
or from [email protected]
Stochasticity of Bias and Nonlocality of Galaxy Formation: Linear Scales
If one wants to represent the galaxy number density at some point in terms of
only the mass density at the same point, there appears the stochasticity in
such a relation, which is referred to as ``stochastic bias''. The stochasticity
is there because the galaxy number density is not merely a local function of a
mass density field, but it is a nonlocal functional, instead. Thus, the
phenomenological stochasticity of the bias should be accounted for by nonlocal
features of galaxy formation processes. Based on mathematical arguments, we
show that there are simple relations between biasing and nonlocality on linear
scales of density fluctuations, and that the stochasticity in Fourier space
does not exist on linear scales under a certain condition, even if the galaxy
formation itself is a complex nonlinear and nonlocal precess. The stochasticity
in real space, however, arise from the scale-dependence of bias parameter, .
As examples, we derive the stochastic bias parameters of simple nonlocal models
of galaxy formation, i.e., the local Lagrangian bias models, the cooperative
model, and the peak model. We show that the stochasticity in real space is also
weak, except on the scales of nonlocality of the galaxy formation. Therefore,
we do not have to worry too much about the stochasticity on linear scales,
especially in Fourier space, even if we do not know the details of galaxy
formation process.Comment: 24 pages, latex, including 2 figures, ApJ, in pres
On the Energy Required to Eject Processed Matter from Galaxies
We evaluate the minimum energy input rate that starbursts require for
expelling their newly processed matter from their host galaxies. Special
attention is given to the pressure caused by the environment in which a galaxy
is situated, as well as to the intrinsic rotation of the gaseous component. We
account for these factors and for a massive dark matter distribution, and
develop a self-consistent solution for the interstellar matter gas
distribution. Our results are in excellent agreement with the results of Mac
Low & Ferrara (1999) for galaxies with a flattened disk-like ISM density
distribution and a low intergalactic gas pressure ( 1
cm K). However, our solution also requires a much larger energy input
rate threshold when one takes into consideration both a larger intergalactic
pressure and the possible existence of a low-density, non-rotating, extended
gaseous halo component.Comment: 7 pages, 4 figures, 1 table, Accepted for publication in Ap
Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-mass systems
will be the first sites of star formation. As these objects have shallow
gravitational potential wells, the subsequent growth of their stellar
populations may be halted by heating and gas loss due to reionization. This
effect has been suggested to have profoundly influenced properties of
present-day dwarf galaxies, including their stellar populations and even
survival as visible galaxies. In this Letter we draw on results from
quantitative studies of Local Group dwarf galaxy star formation histories,
especially for Milky Way satellites, to show that no clear signature exists for
a widespread evolutionary impact from reionization. All nearby dwarf galaxies
studied in sufficient detail contain ancient populations indistinguishable in
age from the oldest Galactic globular clusters. Ancient star formation activity
proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced
fairly continuous star formation until just a few Gyr ago. Despite their
uniformly low masses, their star formation histories differ considerably. The
evolutionary histories of nearby dwarf galaxies appear to reflect influences
from a variety of local processes rather than a dominant effect from
reionization.Comment: Accepted by The Astrophysical Journal Letters. 5 pages, one figur
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