211 research outputs found
A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231
Supermassive binary black holes (BBHs) are unavoidable products of galaxy
mergers and are expected to exist in the cores of many quasars. Great effort
has been made during the past several decades to search for BBHs among quasars;
however, observational evidence for BBHs remains elusive and ambiguous, which
is difficult to reconcile with theoretical expectations. In this paper, we show
that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as
emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and
an orbital period of ~1.2 year. The flat optical and UV continua are mainly
emitted from a circumbinary disk and a mini-disk around the secondary black
hole (BH), respectively; and the observed sharp drop off and flux deficit at
wavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by the
secondary BH migrating within the circumbinary disk. If confirmed by future
observations, this BBH will provide a unique laboratory to study the interplay
between BBHs and accretion flows onto them. Our result also demonstrates a new
method to find sub-parsec scale BBHs by searching for deficits in the
optical-to-UV continuum among the spectra of quasars.Comment: a typo in equation (2) and also in equation (5) of the Appendix is
fixed; 9 pages, 7 figure
Rapid Changes of Photospheric Magnetic Field after Tether-Cutting Reconnection and Magnetic Implosion
The rapid, irreversible change of the photospheric magnetic field has been
recognized as an important element of the solar flare process. This Letter
reports such a rapid change of magnetic fields during the 2011 February 13 M6.6
flare in NOAA AR 11158 that we found from the vector magnetograms of the
Helioseismic and Magnetic Imager with 12-min cadence. High-resolution
magnetograms of Hinode that are available at ~-5.5, -1.5, 1.5, and 4 hrs
relative to the flare maximum are used to reconstruct three-dimensional coronal
magnetic field under the nonlinear force-free field (NLFFF) assumption. UV and
hard X-ray images are also used to illuminate the magnetic field evolution and
energy release. The rapid change is mainly detected by HMI in a compact region
lying in the center of the magnetic sigmoid, where the mean horizontal field
strength exhibited a significant increase by 28%. The region lies between the
initial strong UV and hard X-ray sources in the chromosphere, which are
cospatial with the central feet of the sigmoid according to the NLFFF model.
The NLFFF model further shows that strong coronal currents are concentrated
immediately above the region, and that more intriguingly, the coronal current
system underwent an apparent downward collapse after the sigmoid eruption.
These results are discussed in favor of both the tether-cutting reconnection
producing the flare and the ensuing implosion of the coronal field resulting
from the energy release.Comment: 7 pages, 5 figures, accepted to the Astrophysical Journal Letter
Suppressed star formation in circumnuclear regions in Seyfert galaxies
Feedback from black hole activity is widely believed to play a key role in
regulating star formation and black hole growth. A long-standing issue is the
relation between the star formation and fueling the supermassive black holes in
active galactic nuclei (AGNs). We compile a sample of 57 Seyfert galaxies to
tackle this issue. We estimate the surface densities of gas and star formation
rates in circumnuclear regions (CNRs). Comparing with the well-known
Kennicutt-Schmidt (K-S) law, we find that the star formation rates in CNRs of
most Seyfert galaxies are suppressed in this sample. Feedback is suggested to
explain the suppressed star formation rates.Comment: 1 color figure and 1 table. ApJ Letters in pres
Evolution of gaseous disk viscosity driven by supernova explosion in star-forming galaxies at high redshift
Motivated by Genzel et al.'s observations of high-redshift star-forming
galaxies, containing clumpy and turbulent rings or disks, we build a set of
equations describing the dynamical evolution of gaseous disks with inclusion of
star formation and its feedback. Transport of angular momentum is due to
"turbulent" viscosity induced by supernova explosions in the star formation
region. Analytical solutions of the equations are found for the initial cases
of a gaseous ring and the integrated form for a gaseous disk, respectively. For
a ring with enough low viscosity, it evolves in a slow processes of gaseous
diffusion and star formation near the initial radius. For a high viscosity, the
ring rapidly diffuses in the early phase. The diffusion drives the ring into a
region with a low viscosity and start the second phase undergoing pile-up of
gas at a radius following the decreased viscosity torque. The third is a
sharply deceasing phase because of star formation consumption of gas and
efficient transportation of gas inward forming a stellar disk. We apply the
model to two galaxies BX 482 and BzK 6004, and find that they are
undergoing a decline in their star formation activity.Comment: To appear in ApJ, 5 figure
Star Formation in Self-Gravitating Disks in Active Galactic Nuclei. I. Metallicity Gradients in Broad-Line Regions
It has been suggested that the high metallicity generally observed in active galactic nuclei (AGNs) and quasars originates from ongoing star formation in the self-gravitating part of accretion disks around supermassive black holes (SMBHs). We designate this region as the star-forming (SF) disk, in which metals are produced from supernova explosions (SNexp) while at the same time inflows are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In this paper, an equation of metallicity governed by SNexp and radial advection is established to describe the metal distribution and evolution in the SF disk. We find that the metal abundance is enriched at different rates at different positions in the disk, and that a metallicity gradient is set up that evolves for steady-state AGNs. Metallicity as an integrated physical parameter can be used as a probe of the SF disk age during one episode of SMBH activity. In the SF disk, evaporation of molecular clouds heated by SNexp blast waves unavoidably forms hot gas. This heating is eventually balanced by the cooling of the hot gas, but we show that the hot gas will escape from the SF disk before being cooled, and diffuse into the broad-line regions (BLRs) forming with a typical rate of ~1 M☉ yr–1. The diffusion of hot gas from an SF disk depends on ongoing star formation, leading to the metallicity gradients in BLR observed in AGNs. We discuss this and other observable consequences of this scenario
Episodic Random Accretion and the Cosmological Evolution of Supermassive Black Hole Spins
The growth of supermassive black holes (BHs) located at the centers of their
host galaxies comes mainly from accretion of gas, but how to fuel them remains
an outstanding unsolved problem in quasar evolution. This issue can be
elucidated by quantifying the radiative efficiency parameter () as a
function of redshift, which also provides constraints on the average spin of
the BHs and its possible evolution with time. We derive a formalism to link
with the luminosity density, BH mass density, and duty cycle of quasars,
quantities we can estimate from existing quasar and galaxy survey data. We find
that has a strong cosmological evolution: at z~2, ,
and by it has decreased by an order of magnitude, to . We interpret this trend as evolution in BH spin, and we appeal to
episodic, random accretion as the mechanism for reducing the spin. The
observation that the fraction of radio-loud quasars decreases with increasing
redshift is inconsistent with the popular notion that BH spin is a critical
factor for generating strong radio jets. In agreement with previous studies, we
show that the derived history of BH accretion closely follows the cosmic
history of star formation, consistent with other evidence that BHs and their
host galaxies coevolve.Comment: 4 page, 2 color figures. Accepted by ApJ
Star formation in self-gravitating disks in active galactic nuclei. I. Metallicity gradients in broad line regions
It has been suggested that the high metallicity generally observed in active
galactic nuclei (AGNs) and quasars originates from ongoing star formation in
the self-gravitating part of accretion disks around the supermassive black
holes. We designate this region as the star forming (SF) disk, in which metals
are produced from supernova explosions (SNexp) while at the same time inflows
are driven by SNexp-excited turbulent viscosity to accrete onto the SMBHs. In
this paper, an equation of metallicity governed by SNexp and radial advection
is established to describe the metal distribution and evolution in the SF disk.
We find that the metal abundance is enriched at different rates at different
positions in the disk, and that a metallicity gradient is set up that evolves
for steady-state AGNs. Metallicity as an integrated physical parameter can be
used as a probe of the SF disk age during one episode of SMBH activity. In the
SF disk, evaporation of molecular clouds heated by SNexp blast waves
unavoidably forms hot gas. This heating is eventually balanced by the cooling
of the hot gas, but we show that the hot gas will escape from the SF disk
before being cooled, and diffuse into the BLRs forming with a typical rate of
\sim 1\sunmyr. The diffusion of hot gas from a SF disk depends on ongoing
star formation, leading to the metallicity gradients in BLR observed in AGNs.
We discuss this and other observable consequences of this scenario.Comment: 11 pages, 5 Figures, ApJ, Vol. 737, in pres
Application of Flexible Micro Temperature Sensor in Oxidative Steam Reforming by a Methanol Micro Reformer
Advances in fuel cell applications reflect the ability of reformers to produce hydrogen. This work presents a flexible micro temperature sensor that is fabricated based on micro-electro-mechanical systems (MEMS) technology and integrated into a flat micro methanol reformer to observe the conditions inside that reformer. The micro temperature sensor has higher accuracy and sensitivity than a conventionally adopted thermocouple. Despite various micro temperature sensor applications, integrated micro reformers are still relatively new. This work proposes a novel method for integrating micro methanol reformers and micro temperature sensors, subsequently increasing the methanol conversion rate and the hydrogen production rate by varying the fuel supply rate and the water/methanol ratio. Importantly, the proposed micro temperature sensor adequately controls the interior temperature during oxidative steam reforming of methanol (OSRM), with the relevant parameters optimized as well
Protective Effects of Total Saponins of Panax Notoginseng on Steroid-Induced Avascular Necrosis of the Femoral Head In Vivo and In Vitro
This research was designed to investigate the protective effects of TSPN on steroid-induced avascular necrosis of the femoral head (ANFH) and the likely mechanisms of those effects. As an in vivo study, TSPN was shown to be protective against steroid-induced ANFH due to the upregulation of VEGF-A. Furthermore, TSPN attenuated the apoptosis of osteocytes and reduced the expression of Caspase-3 relative to the model group. As an in vitro study, TSPN exerted a concentration-dependent protective effect against apoptosis in MC3T3-E1 cells. Moreover, TSPN (at a dose of 100 μg/mL) significantly reversed the dexamethasone-induced augmentation of Caspase-3 expression and activity. Therefore, our study demonstrated that TSPN had a protective effect against steroid-induced ANFH that was related to the upregulation of VEGF-A and the inhibition of apoptosis and Caspase-3 activation
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