445 research outputs found
Induction methods used in low temperature physics
A study has been made of induction bridges used in low temperature physics.\ud
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In Part 1 the design of a mutual inductance bridge of the Hartshorn type is discussed. This design is based on a critical analysis of impurity effects of the different parts of the Hartshorn bridge. With this equipment frequencies up to 0.5 MHz can be used. Two methods have been developed to examine the secondary signal. In one of these use has been made of AD conversion techniques. In the other one, the secondary signal, produced by a superconducting sample, which is generally distorted, is analysed by using a Fourier expansion.\ud
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In Part 2 equipment is described which enables us to measure the phase and amplitude of the harmonics of the output signal of the bridge. For synchronous detection a reference signal of the same frequency of the harmonic of interest is required. This reference signal is generated from the input signal of the bridge by means of a digital frequency multiplier with programmable multiplication factor N.\ud
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In Part 3 some experimental results, showing the possibilities of the equipment, on some superconductors are presented
BiOCuS: A new superconducting compound with oxypnictide - related structure
The discovery of about 50 K superconductivity in the tetragonal Fe-based
pnictides has stimulated the search for superconductivity in a wide class of
materials with similar structure. Copper forms compounds isostructural to
LaOFeAs. Single phase BiOCuS can be prepared by a solid state reaction at
temperature lower than 500 C from a mixture of Bi2O3, Bi2S3 and Cu2S. The
samples have been characterized by means of EDX analysis, X-ray diffraction,
magnetic and electrical measurements. The cell parameters are a = 3.8708 A, c =
8.565 A. Charge carrier doping can be realized either by F substitutions for O,
or by Cu off-stoichiometry. The latter doping route leads to the occurrence of
superconductivity below Tc = 5.8 K
The black hole in IC 1459 from HST observations of the ionized gas disk
The peculiar elliptical galaxy IC 1459 (M_V = -21.19, D = 16.5 Mpc) has a
fast counterrotating stellar core, stellar shells and ripples, a blue nuclear
point source and strong radio core emission. We present results of a detailed
HST study of IC 1459, and in particular its central gas disk, aimed a
constraining the central mass distribution. We obtained WFPC2 narrow-band
imaging centered on the Halpha+[NII] emission lines to determine the flux
distribution of the gas emission at small radii, and we obtained FOS spectra at
six aperture positions along the major axis to sample the gas kinematics. We
construct different dynamical models for the Halpha+[NII] and Hbeta kinematics
that include a supermassive black hole, and in which the stellar mass
distribution is constrained by the observed surface brightness distribution and
ground-based stellar kinematics. All models are consistent with a black hole
mass in the range Mbh=1-4 x 10^8 Msun, and models without a black hole are
always ruled out at high confidence.Comment: 40 pages including 14 figures, Latex; submitted to A
The counterrotating core and the black hole mass of IC1459
The E3 giant elliptical galaxy IC1459 is the prototypical galaxy with a fast
counterrotating stellar core. We obtained one HST/STIS long-slit spectrum along
the major axis of this galaxy and CTIO spectra along five position angles. We
present self-consistent three-integral axisymmetric models of the stellar
kinematics, obtained with Schwarzschild's numerical orbit superposition method.
We study the dynamics of the kinematically decoupled core (KDC) in IC1459 and
we find it consists of stars that are well-separated from the rest of the
galaxy in phase space. The stars in the KDC counterrotate in a disk on orbits
that are close to circular. We estimate that the KDC mass is ~0.5% of the total
galaxy mass or ~3*10^9 Msun. We estimate the central black hole mass M_BH of
IC1459 independently from both its stellar and its gaseous kinematics. Some
complications probably explain why we find rather discrepant BH masses with the
different methods. The stellar kinematics suggest that M_BH = (2.6 +/-
1.1)*10^9 Msun (3 sigma error). The gas kinematics suggests that M_BH ~
3.5*10^8 Msun if the gas is assumed to rotate at the circular velocity in a
thin disk. If the observed velocity dispersion of the gas is assumed to be
gravitational, then M_BH could be as high as ~1.0*10^9 Msun. These different
estimates bracket the value M_BH = (1.1 +/- 0.3)*10^9 Msun predicted by the
M_BH-sigma relation. It will be an important goal for future studies to assess
the reliability of black hole mass determinations with either technique. This
is essential if one wants to interpret the correlation between the BH mass and
other global galaxy parameters (e.g. velocity dispersion) and in particular the
scatter in these correlations (believed to be only ~0.3 dex). [Abridged]Comment: 51 pages, LaTeX with 19 PostScript figures. Revised version, with
three new figures and data tables. To appear in The Astrophysical Journal,
578, 2002 October 2
Hubble Space Telescope Observations of NGC 6240: a Case Study of an Ultra-Luminous Infrared Galaxy with Obscured Activity
We present results from an HST study of the morphology and kinematics of NGC
6240. This merging galaxy with a double nucleus is one of the nearest and
best-studied ultraluminous infrared galaxies. HST resolves both nuclei into
seperate components. The distance between the northern and southern
optical/near-infrared components is greater than that observed in radio and
X-ray studies, arguing that even in K-band we may not be seeing all the way
through the dust to the true nuclei. The ionized gas does not display rotation
around either of the nuclei, or equilibrium motion in general. There is a
strong velocity gradient between the nuclei, similar to what is seen in CO
data. There is no such gradient in our stellar kinematics. The velocity
dispersion of the gas is larger than expected for a cold disk. We also map and
model the emission-line velocity field at an off-nuclear position where a steep
velocity gradient was previously detected in ground-based data. Overall, the
data indicate that line-of-sight projection effects, dust absorption,
non-equilibrium merger dynamics, and the possible influence of a wind may be
playing an important role in the observed kinematics. Chandra observations of
hard X-rays have shown that both of the nuclei contain an Active Galactic
Nucleus (AGN). The HST data show no clear sign of the two AGNs: neither
continuum nor narrow-band imaging shows evidence for unresolved components in
the nuclei, and there are no increased emission line widths or rapid rotation
near the nuclei. This underscores the importance of X-ray data for identifying
AGNs in highly dust-enshrouded environments.Comment: LaTeX, 32 pages, 9 figures, 2 tables, accepted for publication in The
Astronomical Journal (Jan 2004). Paper with high-resolution (non-compressed)
color figures in gzipped postscript format available at
http://www.stsci.edu/~marel/psgzdir/ngc6240v11.ps.g
Velocity profiles of Osipkov-Merritt models
A simple algorithm is presented for the calculation of the projected
line-of-sight velocity profiles (VPs) of non-rotating anisotropic spherical
stellar dynamical models with a phase-space distribution function of the
Osipkov-Merritt type. The velocity distribution in these models is isotropic
inside the anisotropy radius r_a and becomes increasingly radially biased at
larger radii. VP shape parameters are presented for a family of models in which
the luminous mass density has an r^{-gamma} power-law cusp at small radii and
an r^{-4} power-law fall-off at large radii. Self-consistent models and models
in which the luminous matter is embedded in a dark halo are discussed. The
effects of changes in the cusp slope gamma and in the anisotropy radius r_a are
documented, and the area in the (gamma,r_a)-plane that contains physical models
is delineated. The shapes of the VPs of the models show a considerable (and
observable) variation with projected galactocentric radius. These models will
be useful for interpreting the data on the VP shapes of elliptical galaxies
that are now becoming available.Comment: 10 pages, uuencoded compressed PostScript, includes 7 figure
The Three-Dimensional Mass Distribution in NGC 1700
A variety of modeling techniques is used with surface photometry from the
literature and recently acquired high-accuracy stellar kinematic data to
constrain the three-dimensional mass distribution in the luminous cuspy
elliptical galaxy NGC 1700. First, we model the radial velocity field and
photometry, and, using a Bayesian technique, estimate the triaxiality T and
short-to-long axis ratio c in five concentric annuli between approximately 1
and 3 effective radii. The results are completely consistent with T being
constant inside about 2.5 r_e (36 arcsec; 6.7/h kpc). Adding an assumption of
constant T as prior information gives an upper limit of T < 0.16 (95%
confidence); this relaxes to T < 0.22 if it is also assumed that there is
perfect alignment between the angular momentum and the galaxy's intrinsic short
axis. Near axisymmetry permits us then to use axisymmetric models to constrain
the radial mass profile. Using the Jeans (moment) equations, we demonstrate
that 2-integral, constant-M/L models cannot fit the data; but a 2-integral
model in which the cumulative enclosed M/L increases by a factor of roughly 2
from the center out to 12/h kpc can. Three-integral models constructed by
quadratic programming show that, in fact, no constant-M/L model is consistent
with the kinematics. Anisotropic 3-integral models with variable M/L, while not
uniquely establishing a minimum acceptable halo mass, imply, as do the moment
models, a cumulative M/L_B approximately 10 h at 12/h kpc. We conclude that NGC
1700 represents the best stellar dynamical evidence to date for dark matter in
elliptical galaxies.Comment: 26 pages, Latex, AASTeX v4.0, with 11 eps figures. To appear in The
Astronomical Journal, January 1999. Figures 1 and 3 are color but are
readable in b/
Evidence for a 3 x 10^8 solar mass black hole in NGC 7052 from HST observations of the nuclear gas disk
We present an HST study of the nuclear region of the E4 radio galaxy NGC
7052, which has a nuclear disk of dust and gas. The WFPC2 was used to obtain B,
V and I broad-band images and an H_alpha+[NII] narrow-band image. The FOS was
used to obtain H_alpha+[NII] spectra along the major axis, using a 0.26 arcsec
diameter circular aperture. The observed rotation velocity of the ionized gas
is V = 155 +/- 17 km/s at r = 0.2 arcsec from the nucleus. The Gaussian
dispersion of the emission lines increases from sigma = 70 km/s at r=1 arcsec,
to sigma = 400 km/s on the nucleus.
To interpret the gas kinematics we construct axisymmetric models in which the
gas and dust reside in a disk in the equatorial plane of the stellar body. It
is assumed that the gas moves on circular orbits, with an intrinsic velocity
dispersion due to turbulence. The circular velocity is calculated from the
combined gravitational potential of the stars and a possible nuclear black hole
(BH). Models without a BH predict a rotation curve that is shallower than
observed (V_pred = 92 km/s at r = 0.2 arcsec), and are ruled out at > 99%
confidence. Models with a BH of 3.3^{+2.3}_{-1.3} x 10^8 solar masses provide
an acceptable fit.
NGC 7052 can be added to the list of active galaxies for which HST spectra of
a nuclear gas disk provide evidence for the presence of a central BH. The BH
masses inferred for M87, M84, NGC 6251, NGC 4261 and NGC 7052 span a range of a
factor 10, with NGC 7052 falling on the low end. By contrast, the luminosities
of these galaxies are identical to within 25%. Any relation between BH mass and
luminosity, as suggested by independent arguments, must therefore have a
scatter of at least a factor 10.Comment: 39 pages, LaTeX, with 16 PostScript figures. Submitted to the
Astronomical Journal. Postscript version with higher resolution figures
available from http://sol.stsci.edu/~marel/abstracts/abs_R22.htm
Estimating Black Hole Masses in Triaxial Galaxies
Most of the super massive black hole mass estimates based on stellar
kinematics use the assumption that galaxies are axisymmetric oblate spheroids
or spherical. Here we use fully general triaxial orbit-based models to explore
the effect of relaxing the axisymmetric assumption on the previously studied
galaxies M32 and NGC 3379. We find that M32 can only be modeled accurately
using an axisymmetric shape viewed nearly edge-on and our black hole mass
estimate is identical to previous studies. When the observed 5 degrees
kinematical twist is included in our model of NGC 3379, the best shape is
mildly triaxial and we find that our best-fitting black hole mass estimate
doubles with respect to the axisymmetric model. This particular black hole mass
estimate is still within the errors of that of the axisymmetric model and
consistent with the M-sigma relationship. However, this effect may have a
pronounced impact on black hole demography, since roughly a third of the most
massive galaxies are strongly triaxial.Comment: Accepted for publication in MNRAS. 11 pages, 9 figures. PDFlate
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