196 research outputs found
The Black Hole Mass Scale of Classical and Pseudo Bulges in Active Galaxies
The mass estimator used to calculate black hole (BH) masses in broad-line
active galactic nuclei (AGNs) relies on a virial coefficient (the " factor")
that is determined by comparing reverberation-mapped (RM) AGNs with measured
bulge stellar velocity dispersions against the relation
of inactive galaxies. It has recently been recognized that only classical
bulges and ellipticals obey a tight relation;
pseudobulges have a different zero point and much larger scatter. Motivated by
these developments, we reevaluate the factor for RM AGNs with available
measurements, updated H RM lags, and new bulge
classifications based on detailed decomposition of high-resolution ground-based
and space-based images. Separate calibrations are provided for the two bulge
types, whose virial coefficients differ by a factor of :
for classical bulges and ellipticals and for pseudobulges. The
structure and kinematics of the broad-line region, at least as crudely encoded
in the factor, seems to related to the large-scale properties or formation
history of the bulge. Lastly, we investigate the bulge stellar masses of the RM
AGNs, show evidence for recent star formation in the AGN hosts that correlates
with Eddington ratio, and discuss the potential utility of the relation as a more promising alternative to the
conventionally used relation for future refinement of the
virial mass estimator for AGNs.Comment: 2014, ApJ, 789, 1
Correlation between Galaxy Mergers and Luminous AGN
It is not yet clear what triggers the activity of active galactic nuclei
(AGNs), but galaxy merging has been suspected to be one of the main mechanisms
fuelling the activity. Using deep optical images taken at various ground-based
telescopes, we investigate the fraction of galaxy mergers in 39 luminous AGNs
(M -22.6 mag) at 0.3 (a median redshift of 0.155), of
which the host galaxies are generally considered as early-type galaxies.
Through visual inspection of the images, we find that 17 of 39 AGN host
galaxies (43.6%) show the evidence for current or past mergers like tidal
tails, shells, and disturbed morphology. In order to see if this fraction is
abnormally high, we also examined the merging fraction of normal early-type
galaxies in the Sloan Digital Sky Survey (SDSS) Strip 82 data (a median
redshift of 0.04), of which the surface-brightness limit is comparable to our
imaging data. To correct for the effects related to the redshift difference of
the two samples, we performed an image simulation by putting a bright point
source as an artificial AGN in the images of SDSS early-type galaxies and
placing them onto the redshifts of AGNs. The merging fraction in this realistic
sample of simulated AGNs is only ( to of that of
real AGNs). Our result strongly suggests that luminous AGN activity is
associated with galaxy merging.Comment: 57 pages, 19 figures, published in Astrophysical Journa
Nanomechanical characterization of quantum interference in a topological insulator nanowire
The discovery of two-dimensional gapless Dirac fermions in graphene and
topological insulators (TI) has sparked extensive ongoing research toward
applications of their unique electronic properties. The gapless surface states
in three-dimensional insulators indicate a distinct topological phase of matter
with a non-trivial Z2 invariant that can be verified by angle-resolved
photoemission spectroscopy or magnetoresistance quantum oscillation. In TI
nanowires, the gapless surface states exhibit Aharonov-Bohm (AB) oscillations
in conductance, with this quantum interference effect accompanying a change in
the number of transverse one-dimensional modes in transport. Thus, while the
density of states (DOS) of such nanowires is expected to show such AB
oscillation, this effect has yet to be observed. Here, we adopt nanomechanical
measurements that reveal AB oscillations in the DOS of a topological insulator.
The TI nanowire under study is an electromechanical resonator embedded in an
electrical circuit, and quantum capacitance effects from DOS oscillation
modulate the circuit capacitance thereby altering the spring constant to
generate mechanical resonant frequency shifts. Detection of the quantum
capacitance effects from surface-state DOS is facilitated by the small
effective capacitances and high quality factors of nanomechanical resonators,
and as such the present technique could be extended to study diverse quantum
materials at nanoscale.Comment: 15+16 pages, 4+11 figure
Deficit of Hot Dust in Low-redshift Active Galactic Nuclei
We assemble a broad-band spectral energy distribution (SED) ranging from
optical to mid-infrared of nearby active galactic nuclei at . SED
fitting analysis is performed using semi-empirical templates derived from
Palomar-Green quasars to classify the sample into normal, warm-dust-deficient
(WDD), and hot-dust-deficient (HDD) AGNs. Kolmogorov-Smirnov tests reveal that
HDD AGNs exhibit, on average higher AGN luminosity than normal and WDD AGNs.
HDD fraction, on the other hand, is only weakly correlated with black hole mass
and inversely correlated with Eddington ratio. By fixing the other parameters,
we conclude that the HDD fraction is primarily connected with the AGN
luminosity. It implies that there is a causal connection between the covering
factor of the hot dust component and AGN luminosity, possibly due to the
sublimation of the innermost dust or the thickening of the intervening gas in
the broad-line region. Analysis of the outflow properties traced by the wing of
[O III] suggests that outflows may be related to the formation and
maintenance of the hot dust component. Finally, we demonstrate through
comparison with previous studies that the classification of HDD AGNs requires
careful subtraction of the host galaxy light.Comment: Accepted for publication in Ap
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