41 research outputs found
AGN Feedback and Bimodality in Cluster Core Entropy
We investigate a series of steady-state models of galaxy clusters, in which
the hot intracluster gas is efficiently heated by active galactic nucleus (AGN)
feedback and thermal conduction, and in which the mass accretion rates are
highly reduced compared to those predicted by the standard cooling flow models.
We perform a global Lagrangian stability analysis. We show for the first time
that the global radial instability in cool core clusters can be suppressed by
the AGN feedback mechanism, provided that the feedback efficiency exceeds a
critical lower limit. Furthermore, our analysis naturally shows that the
clusters can exist in two distinct forms. Globally stable clusters are expected
to have either: 1) cool cores stabilized by both AGN feedback and conduction,
or 2) non-cool cores stabilized primarily by conduction. Intermediate central
temperatures typically lead to globally unstable solutions. This bimodality is
consistent with the recently observed anticorrelation between the flatness of
the temperature profiles and the AGN activity (Dunn & Fabian 2008) and the
observation by Rafferty et al. (2008) that the shorter central cooling times
tend to correspond to significantly younger AGN X-ray cavities.Comment: 4 pages, to appear in the proceedings of "The Monster's Fiery Breath:
Feedback in Galaxies, Groups, and Clusters", Eds. Sebastian Heinz, Eric
Wilcots (AIP conference series
Feedback Heating by Cosmic Rays in Clusters of Galaxies
Recent observations show that the cooling flows in the central regions of
galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles
are a leading candidate for suppressing cooling, usually via some form of
mechanical heating. At the same time, observed X-ray cavities and synchrotron
emission point toward a significant non-thermal particle population. Previous
studies have focused on the dynamical effects of cosmic-ray pressure support,
but none have built successful models in which cosmic-ray heating is
significant. Here we investigate a new model of AGN heating, in which the
intracluster medium is efficiently heated by cosmic-rays, which are injected
into the ICM through diffusion or the shredding of the bubbles by
Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal
conduction as well. Using numerical simulations, we show that the cooling
catastrophe is efficiently suppressed. The cluster quickly relaxes to a
quasi-equilibrium state with a highly reduced accretion rate and temperature
and density profiles which match observations. Unlike the conduction-only case,
no fine-tuning of the Spitzer conduction suppression factor f is needed. The
cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within
observational bounds. Cosmic ray heating is a very attractive alternative to
mechanical heating, and may become particularly compelling if GLAST detects the
gamma-ray signature of cosmic-rays in clusters.Comment: Revised version accepted for publication in MNRAS. Significantly
expanded discussion and new simulations exploring parameter space/model
robustness; conclusions unchange
The Fermi Bubbles. II. The Potential Roles of Viscosity and Cosmic Ray Diffusion in Jet Models
The origin of the Fermi bubbles recently detected by the Fermi Gamma-ray
Space Telescope in the inner Galaxy is mysterious. In the companion paper Guo &
Mathews (Paper I), we use hydrodynamic simulations to show that they could be
produced by a recent powerful AGN jet event. Here we further explore this
scenario to study the potential roles of shear viscosity and cosmic ray (CR)
diffusion on the morphology and CR distribution of the bubbles. We show that
even a relatively low level of viscosity (\mu_{visc} >~ 3 g cm^{-1} s^{-1}, or
~0.1% - 1% of Braginskii viscosity in this context) could effectively suppress
the development of Kelvin-Helmholtz instabilities at the bubble surface,
resulting in smooth bubble edges as observed. Furthermore, viscosity reduces
circulating motions within the bubbles, which would otherwise mix the
CR-carrying jet backflow near bubble edges with the bubble interior. Thus
viscosity naturally produces an edge-favored CR distribution, an important
ingredient to produce the observed flat gamma-ray surface brightness
distribution. Generically, such a CR distribution often produces a
limb-brightened gamma-ray intensity distribution. However, we show that by
incorporating CR diffusion which is strongly suppressed across the bubble
surface (as inferred from sharp bubble edges) but is close to canonical values
in the bubble interior, we obtain a reasonably flat gamma-ray intensity
profile. The similarity of the resulting CR bubble with the observed Fermi
bubbles strengthens our previous result in Paper I that the Fermi bubbles were
produced by a recent AGN jet event. Studies of the nearby Fermi bubbles may
provide a unique opportunity to study the potential roles of plasma viscosity
and CR diffusion on the evolution of AGN jets and bubbles.Comment: Revised version, accepted for publication in ApJ. 14 pages, 9 figure
Could AGN Outbursts Transform Cool Core Clusters?
The origin of the bimodality in cluster core entropy is still unknown. At the
same time, recent work has shown that thermal conduction in clusters is likely
a time-variable phenomenon. We consider if time-variable conduction and AGN
outbursts could be responsible for the cool-core (CC), non cool-core (NCC)
dichotomy. We show that strong AGN heating can bring a CC cluster to a NCC
state, which can be stably maintained by conductive heating from the cluster
outskirts. On the other hand, if conduction is shut off by the heat-flux driven
buoyancy instability, then the cluster will cool to the CC state again, where
it is stabilized by low-level AGN heating. Thus, the cluster cycles between CC
and NCC states. In contrast with massive clusters, we predict the CC/NCC
bimodality should vanish in groups, due to the lesser role of conductive
heating there. We find tentative support from the distribution of central
entropy in groups, though firm conclusions require a larger sample carefully
controlled for selection effects.Comment: Slightly revised version, accepted for publication in MNRAS. 9 pages,
3 figure
Asthma prevalence based on the Baidu index and China's Health Statistical Yearbook from 2011 to 2020 in China
BackgroundDue to environmental pollution, changes in lifestyle, and advancements in diagnostic technology, the prevalence of asthma has been increasing over the years. Although China has made early efforts in asthma epidemiology and prevention, there is still a lack of unified and comprehensive epidemiological research within the country. The objective of the study is to determine the nationwide prevalence distribution of asthma using the Baidu Index and China's Health Statistical Yearbook.MethodsBased on China's Health Statistical Yearbook, we analyzed the gender and age distribution of asthma in China from 2011 to 2020, as well as the length of hospitalization and associated costs. By utilizing the Baidu Index and setting the covering all 31 provinces and autonomous regions in China, we obtained the Baidu Index for the keyword 'asthma'. Heatmaps and growth ratios described the prevalence and growth of asthma in mainland China.ResultsThe average expenditure for discharged asthma (standard deviation) patients was ¥5,870 (808). The average length of stay (standard deviation) was 7.9 (0.38) days. During the period of 2011 to 2020, hospitalization expenses for asthma increased while the length of hospital stay decreased. The proportion of discharged patients who were children under the age of 5 were 25.3% (2011), 19.4% (2012), 16% (2013), 17.9% (2014), 13.9% (2015), 11.3% (2016), 10.2% (2017), 9.4% (2018), 8.1% (2019), and 7.2% (2020), respectively. The prevalence of asthma among boys was higher than girls before the age of 14. In contrast, the proportion of women with asthma was larger than men after the age of 14. During the period from 2011 to 2020, the median [The first quartile (Q1)-the third quartile (Q3)] daily asthma Baidu index in Guangdong, Beijing, Jiangsu, Sichuan, and Zhejiang were 419 (279–476), 328 (258–376), 315 (227–365), 272 (166–313), and 312 (233–362) respectively. Coastal regions showed higher levels of attention toward asthma, indicating a higher incidence rate. Since 2014, there has been a rapid increase in the level of attention toward asthma, with the provinces of Qinghai, Sichuan, and Guangdong experiencing the fastest growth.ConclusionThere are regional variations in the prevalence of asthma among different provinces in China, and the overall prevalence of asthma is increasing