6,282 research outputs found
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
Sediment and fluvial particulate carbon flux from an eroding peatland catchment
Erosion and the associated loss of carbon is a major environmental concern in many peatlands and remains difficult to accurately quantify beyond the plot scale. Erosion was measured in an upland blanket peatland catchment (0.017 km2) in northern England using Structure‐from‐Motion (SfM) photogrammetry, sediment traps and stream sediment sampling at different spatial scales. A net median topographic change of –27 mm yr–1 was recorded by SfM over the 12‐month monitoring period for the entire surveyed area (598 m2). Within the entire surveyed area there were six nested catchments where both SfM and sediment traps were used to measure erosion. Substantial amounts of peat were captured in sediment traps during summer storm events after two months of dry weather where desiccation of the peat surface occurred. The magnitude of topographic change for the six nested catchments determined by SfM (mean value: 5.3 mm, standard deviation: 5.2 mm) was very different to the areal average derived from sediment traps (mean value: –0.3 mm, standard deviation: 0.1 mm). Thus direct interpolation of peat erosion from local net topographic change into sediment yield at the catchment outlet appears problematic. Peat loss measured at the hillslope scale was not representative of that at the catchment scale. Stream sediment sampling at the outlet of the research catchment (0.017 km2) suggested that the yields of suspended sediment and particulate organic carbon were 926.3 t km–2 yr–1 and 340.9 t km–2 yr–1 respectively, with highest losses occurring during the autumn. Both freeze–thaw during winter and desiccation during long periods of dry weather in spring and summer were identified as important peat weathering processes during the study. Such weathering was a key enabler of subsequent fluvial peat loss from the catchment
Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter VLBI Observations of Sagittarius A*
Millimeter very-long baseline interferometry (mm-VLBI) provides the novel
capacity to probe the emission region of a handful of supermassive black holes
on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole
at the center of the Milky Way, this provides access to the region in the
immediate vicinity of the horizon. Broderick et al. (2009) have already shown
that by leveraging spectral and polarization information as well as accretion
theory, it is possible to extract accretion-model parameters (including black
hole spin) from mm-VLBI experiments containing only a handful of telescopes.
Here we repeat this analysis with the most recent mm-VLBI data, considering a
class of aligned, radiatively inefficient accretion flow (RIAF) models. We find
that the combined data set rules out symmetric models for Sgr A*'s flux
distribution at the 3.9-sigma level, strongly favoring length-to-width ratios
of roughly 2.4:1. More importantly, we find that physically motivated accretion
flow models provide a significantly better fit to the mm-VLBI observations than
phenomenological models, at the 2.9-sigma level. This implies that not only is
mm-VLBI presently capable of distinguishing between potential physical models
for Sgr A*'s emission, but further that it is sensitive to the strong
gravitational lensing associated with the propagation of photons near the black
hole. Based upon this analysis we find that the most probable magnitude,
viewing angle, and position angle for the black hole spin are
a=0.0(+0.64+0.86), theta=68(+5+9)(-20-28) degrees, and xi=-52(+17+33)(-15-24)
east of north, where the errors quoted are the 1-sigma and 2-sigma
uncertainties.Comment: 15 pages, 10 figures, submitted to Ap
Sagittarius A* Accretion Flow and Black Hole Parameters from General Relativistic Dynamical and Polarized Radiative Modeling
We obtain estimates of Sgr A* accretion flow and black hole parameters by
fitting polarized sub-mm observations with spectra computed using
three-dimensional (3D) general relativistic (GR) magnetohydrodynamical (MHD)
(GRMHD) simulations. Observations are compiled from averages over many epochs
from reports in 29 papers for estimating the mean fluxes Fnu, linear
polarization (LP) fractions, circular polarization (CP) fractions, and electric
vector position angles (EVPAs). GRMHD simulations are computed with
dimensionless spins a_*=0,0.5,0.7,0.9,0.98 over a 20,000M time interval. We
perform fully self-consistent GR polarized radiative transfer using our new
code to explore the effects of spin a_*, inclination angle \theta, position
angle (PA), accretion rate Mdot, and electron temperature Te (Te is reported
for radius 6M). By fitting the mean sub-mm fluxes and LP/CP fractions, we
obtain estimates for these model parameters and determine the physical effects
that could produce polarization signatures. Our best bet model has a_*=0.5,
\theta=75deg, PA=115deg, Mdot=4.6*10^{-8}M_Sun/year, and Te=3.1*10^10K at 6M.
The sub-mm CP is mainly produced by Faraday conversion as modified by Faraday
rotation, and the emission region size at 230GHz is consistent with the VLBI
size of 37microas. Across all spins, model parameters are in the ranges
\theta=42deg-75deg, Mdot=(1.4-7.0)*10^{-8}M_Sun/year, and Te=(3-4)*10^10K.
Polarization is found both to help differentiate models and to introduce new
observational constraints on the effects of the magnetic field that might not
be fit by accretion models so-far considered.Comment: 19 pages, 11 figures, accepted to Ap
Jet-lag in Sgr A*: What size and timing measurements tell us about the central black hole in the Milky Way
The black hole at the Galactic Center, Sgr A*, is the prototype of a galactic
nucleus at a very low level of activity. Its radio through submm-wave emission
is known to come from a region close to the event horizon, however, the source
of the emission is still under debate. A successful theory explaining the
emission is based on a relativistic jet model scaled down from powerful
quasars. We want to test the predictive power of this established jet model
against newly available measurements of wavelength-dependent time lags and the
size-wavelength structure in Sgr A*. Using all available closure amplitude VLBI
data from different groups, we again derived the intrinsic wavelength-dependent
size of Sgr A*. This allowed us to calculate the expected frequency-dependent
time lags of radio flares, assuming a range of in- and outflow velocities.
Moreover, we calculated the time lags expected in the previously published
pressure-driven jet model. The predicted lags are then compared to radio
monitoring observations at 22, 43, and 350 GHz. The combination of time lags
and size measurements imply a mildly relativistic outflow with bulk outflow
speeds of gamma*beta ~ 0.5-2. The newly measured time lags are reproduced well
by the jet model without any major fine tuning. The results further strengthen
the case for the cm-to-mm wave radio emission in Sgr A* as coming from a mildly
relativistic jet-like outflow. The combination of radio time lag and VLBI
closure amplitude measurements is a powerful new tool for assessing the flow
speed and direction in Sgr A*. Future VLBI and time lag measurements over a
range of wavelengths will reveal more information about Sgr A*, such as the
existence of a jet nozzle, and measure the detailed velocity structure of a
relativistic jet near its launching point for the first time.Comment: Latex, 7 pages, accepted for publication in Astronomy & Astrophysic
The evolution of cluster dwarfs
We summarize the results from analyzing six clusters of galaxies at 0.14 < z
< 0.40 observed with the Hubble Space Telescope Advanced Camera for Surveys. We
derive deep composite luminosity functions in B,g,V,r,i and z down to an
absolute magnitude of -14 +5 log h mag. The luminosity functions are fitted by
a single Schechter function with M^*=-19.8,-20.9,-21.9, -22.0,-21.7 and -22.3 i
B,g,V.r,i,z respectively and alpha=-1.3 for all bands. The data suggest that
the red sequence dominates the luminosity function down to more than 6 mag.
below L*, the dwarf spheroidal regime. Hence, at least at z=0.3 the red
sequence is well established and galaxies down to dwarf spheroidals are fully
assembled within these clusters. We do not detect the faint-end upturn (M >
-16) that is observed in lower redshift clusters. If this is real, the
faint-end population has originated since z = 0.3.Comment: To be published in Astronomische Nachrichten, proceeding of JENAM
2008 Symposium 6. Includes an.cls classfil
A solution to the problems of cusps and rotation curves in dark matter halos in the cosmological standard model
We discuss various aspects of the inner structure formation in virialized
dark matter (DM) halos that form as primordial density inhomogeneities evolve
in the cosmological standard model. The main focus is on the study of central
cusps/cores and of the profiles of DM halo rotation curves, problems that
reveal disagreements among the theory, numerical simulations, and observations.
A method that was developed by the authors to describe equilibrium DM systems
is presented, which allows investigating these complex nonlinear structures
analytically and relating density distribution profiles within a halo both to
the parameters of the initial small-scale inhomogeneity field and to the
nonlinear relaxation characteristics of gravitationally compressed matter. It
is shown that cosmological random motions of matter `heat up' the DM particles
in collapsing halos, suppressing cusp-like density profiles within developing
halos, facilitating the formation of DM cores in galaxies, and providing an
explanation for the difference between observed and simulated galactic rotation
curves. The analytic conclusions obtained within this approach can be confirmed
by the N-body model simulation once improved spatial resolution is achieved for
central halo regions.Comment: 44 pages, 16 figures, 1 tabl
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