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 $P_{\rm jet}=10^{44-45}$ 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 $z<0.5$ 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, $b$. 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 ($P_{IGM}/k$ $\leq$ 1 cm$^{-3}$ 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