703 research outputs found
Polar Dust, Nuclear Obscuration and IR SED Diversity in Type-1 AGNs
Despite the hypothesized similar face-on viewing angles, the infrared
emission of type-1 AGNs has diverse spectral energy distribution (SED) shapes
that deviate substantially from the well-characterized quasar templates.
Motivated by the commonly-seen UV-optical obscuration and the discovery of
parsec-scale mid-IR polar dust emission in some nearby AGNs, we develop
semi-empirical SED libraries for reddened type-1 AGNs built on the quasar
intrinsic templates, assuming low-level extinction caused by an extended
distribution of large dust grains. We demonstrate that this model can reproduce
the nuclear UV-to-IR SED and the strong mid-IR polar dust emission of NGC 3783,
the type-1 AGN with the most relevant and robust observational constraints. In
addition, we compile 64 low- Seyfert-1 nuclei with negligible mid-IR star
formation contamination and satisfactorily fit the individual IR SEDs as well
as the composite UV to mid-IR composite SEDs. Given the success of these fits,
we characterize the possible infrared SED of AGN polar dust emission and
utilize a simple but effective strategy to infer its prevalence among type-1
AGNs. The SEDs of high- peculiar AGNs, including the extremely red quasars,
mid-IR warm-excess AGNs, and hot dust-obscured galaxies, can be also reproduced
by our model. These results indicate that the IR SEDs of most AGNs, regardless
of redshift or luminosity, arise from similar circumnuclear torus properties
but differ mainly due to the optical depths of extended obscuring dust
components.Comment: 37 pages, 22 figures, 5 tables; accepted for publication in The
Astrophysical Journal; the AGN templates can be retrieved from
https://github.com/karlan/AGN_template
The Intrinsic Far-infrared Continua of Type-1 Quasars
The range of currently proposed active galactic nucleus (AGN) far-infrared
templates results in uncertainties in retrieving host galaxy information from
infrared observations and also undermines constraints on the outer part of the
AGN torus. We discuss how to test and reconcile these templates. Physically,
the fraction of the intrinsic AGN IR-processed luminosity compared with that
from the central engine should be consistent with the dust-covering factor. In
addition, besides reproducing the composite spectral energy distributions
(SEDs) of quasars, a correct AGN IR template combined with an accurate library
of star-forming galaxy templates should be able to reproduce the IR properties
of the host galaxies, such as the luminosity-dependent SED shapes and aromatic
feature strengths. We develop tests based on these expected behaviors and find
that the shape of the AGN intrinsic far-IR emission drops off rapidly starting
at m and can be matched by an Elvis et al. (1994)-like template
with minor modification. Despite the variations in the near- to mid-IR bands,
AGNs in quasars and Seyfert galaxies have remarkably similar intrinsic far-IR
SEDs at - m, suggesting similar emission character of
the outermost region of the circumnuclear torus. The variations of the
intrinsic AGN IR SEDs among the type-1 quasar population can be explained by
the changing relative strengths of four major dust components with similar
characteristic temperatures, and there is evidence for compact AGN-heated dusty
structures at sub-kpc scales in the far-IR.Comment: Minor corrections to match the published version, 14 pages, 9
figures, 5 tables. The quasar intrinsic IR templates can be found at
http://u.arizona.edu/~jianwei/data/AGN_temp.ascii or in the published pape
Dust-Deficient Palomar-Green Quasars and the Diversity of AGN Intrinsic IR Emission
To elucidate the intrinsic broadband infrared (IR) emission properties of
active galactic nuclei (AGNs), we analyze the spectral energy distributions
(SEDs) of 87 z<0.5 Palomar-Green (PG) quasars. While the Elvis AGN template
with a moderate far-IR correction can reasonably match the SEDs of the AGN
components in ~60% of the sample (and is superior to alternatives such as that
by Assef), it fails on two quasar populations: 1) hot-dust-deficient (HDD)
quasars that show very weak emission thoroughly from the near-IR to the far-IR,
and 2) warm-dust-deficient (WDD) quasars that have similar hot dust emission as
normal quasars but are relatively faint in the mid- and far-IR. After building
composite AGN templates for these dust-deficient quasars, we successfully fit
the 0.3-500 {\mu}m SEDs of the PG sample with the appropriate AGN template, an
infrared template of a star-forming galaxy, and a host galaxy stellar template.
20 HDD and 12 WDD quasars are identified from the SED decomposition, including
seven ambiguous cases. Compared with normal quasars, the HDD quasars have AGN
with relatively low Eddington ratios and the fraction of WDD quasars increases
with AGN luminosity. Moreover, both the HDD and WDD quasar populations show
relatively stronger mid-IR silicate emission. Virtually identical SED
properties are also found in some quasars from z = 0.5 to 6. We propose a
conceptual model to demonstrate that the observed dust deficiency of quasars
can result from a change of structures of the circumnuclear tori that can occur
at any cosmic epoch.Comment: minor corrections to match the published versio
Auction-Based Distributed Resource Allocation for Cooperation Transmission in Wireless Networks
Cooperative transmission can greatly improve communication system performance
by taking advantage of the broadcast nature of wireless channels. Most previous
work on resource allocation for cooperation transmission is based on
centralized control. In this paper, we propose two share auction mechanisms,
the SNR auction and the power auction, to distributively coordinate the
resource allocation among users. We prove the existence, uniqueness and
effectiveness of the auction results. In particular, the SNR auction leads to a
fair resource allocation among users, and the power auction achieves a solution
that is close to the efficient allocation.Comment: To appear in the Proceedings of the IEEE IEEE Global Communications
Conference (GLOBECOM), Washington, DC, November 26 - 30, 200
Auction-based Resource Allocation for Multi-relay Asynchronous Cooperative Networks
Resource allocation is considered for cooperative transmissions in
multiple-relay wireless networks. Two auction mechanisms, SNR auctions and
power auctions, are proposed to distributively coordinate the allocation of
power among multiple relays. In the SNR auction, a user chooses the relay with
the lowest weighted price. In the power auction, a user may choose to use
multiple relays simultaneously, depending on the network topology and the
relays' prices. Sufficient conditions for the existence (in both auctions) and
uniqueness (in the SNR auction) of the Nash equilibrium are given. The fairness
of the SNR auction and efficiency of the power auction are further discussed.
It is also proven that users can achieve the unique Nash equilibrium
distributively via best response updates in a completely asynchronous manner.Comment: To appear in the Proceedings of the 2008 IEEE International
Conference on Acoustics, Speech and Signal Processing, Las Vegas, NV, March
30 to April 4, 200
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A portable device for studying the effects of fluid flow on degradation properties of biomaterials inside cell incubators.
A portable device was designed and constructed for studying the properties of biomaterials in physiologically relevant fluids under controllable flow conditions that closely simulate fluid flow inside the body. The device can fit entirely inside a cell incubator; and, thus, it can be used directly under standard cell culture conditions. An impedance-driven pump was built in the sterile flow loop to control the flow rates of fluids, which made the device small and portable for easy deployment in the incubator. To demonstrate the device functions, magnesium (Mg) as a representative biodegradable material was tested in the flow device for immersion degradation under flow versus static conditions, while the flow module was placed inside a standard cell incubator. The flow rate was controlled at 0.17 ± 0.06 ml/s for this study; and, the flow rate is adjustable through the controller module outside of incubators for simulating the flow rates in the ranges of blood flow in human artery (0.05 ∼0.43 ml/s) and vein (0.02 ∼0.08 ml/s). Degradation of Mg under flow versus static conditions was characterized by measuring the changes of sample mass and thickness, and Mg2+ ion concentrations in the immersion media. Surface chemistry and morphology of Mg after immersion under flow versus static conditions were compared. The portable impedance-driven flow device is easy to fit inside an incubator and much smaller than a peristaltic pump, providing a valuable solution for studying biomaterials and implants (e.g. vascular or ureteral stents) in body fluids under flow versus static conditions with or without cells
Kerr-AdS/CFT Correspondence in Diverse Dimensions
It was proposed recently that the near-horizon states of an extremal
four-dimensional Kerr black hole could be identified with a certain chiral
conformal field theory whose Virasoro algebra arises as an asymptotic symmetry
algebra of the near-horizon Kerr geometry. Supportive evidence for the proposed
duality came from the equality of the microscopic entropy of the CFT,
calculated by means of the Cardy formula, and the Bekenstein-Hawking entropy of
the extremal Kerr black hole. In this paper we examine the proposed Kerr/CFT
correspondence in a broader context. In particular, we show that the
microscopic entropy and the Bekenstein-Hawking entropy agree also for the
extremal Kerr-AdS metric in four dimensions, and also for the extremal Kerr-AdS
metrics in dimensions 5, 6 and 7. General formulae for all higher dimensions
are also presented.Comment: Latex, 19 pages, typos corrected and references adde
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