3,340 research outputs found
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
Galaxy evolution in groups and clusters: satellite star formation histories and quenching timescales in a hierarchical Universe
Satellite galaxies in groups and clusters are more likely to have low star
formation rates (SFR) and lie on the red-sequence than central (field)
galaxies. Using galaxy group/cluster catalogs from SDSS DR7, together with a
cosmological N-body simulation to track satellite orbits, we examine the star
formation histories and quenching timescales of satellites of M_star > 5 x 10^9
M_sun at z=0. We first explore satellite infall histories: group preprocessing
and ejected orbits are critical aspects of satellite evolution, and properly
accounting for these, satellite infall typically occurred at z~0.5, or ~5 Gyr
ago. To obtain accurate initial conditions for the SFRs of satellites at their
time of first infall, we construct an empirical parametrization for the
evolution of central galaxy SFRs and quiescent fractions. With this, we
constrain the importance and efficiency of satellite quenching as a function of
satellite and host halo mass, finding that satellite quenching is the dominant
process for building up all quiescent galaxies at M_star < 10^10 M_sun. We then
constrain satellite star formation histories, finding a 'delayed-then-rapid'
quenching scenario: satellite SFRs evolve unaffected for 2-4 Gyr after infall,
after which star formation quenches rapidly, with an e-folding time of < 0.8
Gyr. These quenching timescales are shorter for more massive satellites but do
not depend on host halo mass: the observed increase in satellite quiescent
fraction with halo mass arises simply because of satellites quenching in a
lower mass group prior to infall (group preprocessing), which is responsible
for up to half of quenched satellites in massive clusters. Because of the long
time delay before quenching starts, satellites experience significant stellar
mass growth after infall, nearly identical to central galaxies. This fact
provides key physical insight into the subhalo abundance matching method.Comment: 25 pages, 13 figures. Accepted for publication in MNRAS, matches
published versio
Variations in hospital standardised mortality ratios (HSMR) as a result of frequent readmissions
BACKGROUND: We investigated the impact that variations in the frequency of readmissions had upon a hospital's standardised mortality ratio (HSMR). An adapted HSMR model was used in the study. Our calculations were based on the admissions of 70 hospitals in The Netherlands during the years 2005 to 2009. METHODS: Through a retrospective analysis of routinely collected hospital data, we calculated standardised in-hospital mortality ratios both by hospital and by diagnostic group (H/SMRs) using two different models. The first was the Dutch 2010 model while the second was the same model but with an additional adjustment for the readmission frequency. We compared H/SMR outcomes and the corresponding quality metrics in order to test discrimination (c-statistics), calibration (Hosmer-Lemeshow) and explanatory power (pseudo-R2 statistic) for both models. RESULTS: The SMR outcomes for model 2 compared to model 1, varied between -39% and +110%. On the HSMR level these variations ranged from -12% to +11%. There was a substantial disagreement between the models with respect to significant death on the SMR level as well as the HSMR level (~ 20%). All quality metrics comparing both models were in favour of model 2. The susceptibility to adjustment for readmission increased for longer review periods. CONCLUSIONS: The 2010 HSMR model for the Netherlands was sensitive to adjustment for the frequency of readmissions. A model without this adjustment, as opposed to a model with the adjustment, produced substantially different HSMR outcomes. The uncertainty introduced by these differences exceeded the uncertainty indicated by the 95% confidence intervals. Therefore an adjustment for the frequency of readmissions should be considered in The Netherlands, since such a model showed more favourable quality metric characteristics compared to a model without such an adjustment. Other countries could well benefit from a similar adjustment to their models. A review period of the data collected over the last three years, at least, is advisable. (aut.ref.
The dynamical distance and intrinsic structure of the globular cluster omega Centauri
We determine the dynamical distance D, inclination i, mass-to-light ratio M/L
and the intrinsic orbital structure of the globular cluster omega Cen, by
fitting axisymmetric dynamical models to the ground-based proper motions of van
Leeuwen et al. and line-of-sight velocities from four independent data-sets. We
correct the observed velocities for perspective rotation caused by the space
motion of the cluster, and show that the residual solid-body rotation component
in the proper motions can be taken out without any modelling other than
assuming axisymmetry. This also provides a tight constraint on D tan i.
Application of our axisymmetric implementation of Schwarzschild's orbit
superposition method to omega Cen reveals no dynamical evidence for a
significant radial dependence of M/L. The best-fit dynamical model has a
stellar V-band mass-to-light ratio M/L_V = 2.5 +/- 0.1 M_sun/L_sun and an
inclination i = 50 +/- 4 degrees, which corresponds to an average intrinsic
axial ratio of 0.78 +/- 0.03. The best-fit dynamical distance D = 4.8 +/- 0.3
kpc (distance modulus 13.75 +/- 0.13 mag) is significantly larger than obtained
by means of simple spherical or constant-anisotropy axisymmetric dynamical
models, and is consistent with the canonical value 5.0 +/- 0.2 kpc obtained by
photometric methods. The total mass of the cluster is (2.5 +/- 0.3) x 10^6
M_sun. The best-fit model is close to isotropic inside a radius of about 10
arcmin and becomes increasingly tangentially anisotropic in the outer region,
which displays significant mean rotation. This phase-space structure may well
be caused by the effects of the tidal field of the Milky Way. The cluster
contains a separate disk-like component in the radial range between 1 and 3
arcmin, contributing about 4% to the total mass.Comment: 37 pages (23 figures), accepted for publication in A&A, abstract
abridged, for PS and PDF file with full resolution figures, see
http://www.strw.leidenuniv.nl/~vdven/oc
Publiek-private samenwerking : 'we moeten leren leven met zout'
In Leven met Zout werken Wageningse onderzoekers samen met andere onderzoeksinstellingen en het bedrijfsleven aan een kosteneffectieve mix van waterbeheer, watertechnologie en agrotechnologie, om deltaâs overal ter wereld leefbaar te houden ondanks stijgende zoutgehalten in zoet water
Constraints on Assembly Bias from Galaxy Clustering
We constrain the newly-introduced decorated Halo Occupation Distribution
(HOD) model using SDSS DR7 measurements of projected galaxy clustering or
r-band luminosity threshold samples. The decorated HOD is a model for the
galaxy-halo connection that augments the HOD by allowing for the possibility of
galaxy assembly bias: galaxy luminosity may be correlated with dark matter halo
properties besides mass, Mvir. We demonstrate that it is not possible to rule
out galaxy assembly bias using DR7 measurements of galaxy clustering alone.
Moreover, galaxy samples with Mr < -20 and Mr < -20.5 favor strong central
galaxy assembly bias. These samples prefer scenarios in which
high-concentration are more likely to host a central galaxy relative to
low-concentration halos of the same mass. We exclude zero assembly bias with
high significance for these samples. Satellite galaxy assembly bias is
significant for the faintest sample, Mr < -19. We find no evidence for assembly
bias in the Mr < -21 sample. Assembly bias should be accounted for in galaxy
clustering analyses or attempts to exploit galaxy clustering to constrain
cosmology. In addition to presenting the first constraints on HOD models that
accommodate assembly bias, our analysis includes several improvements over
previous analyses of these data. Therefore, our inferences supersede
previously-published results even in the case of a standard HOD analysis.Comment: 15 pages, 8 figures. To be submitted to MNRAS. Comments Welcome.
Python scripts to perform this analysis and MCMC chains will all be made
publicly availabl
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