313 research outputs found
Evolution of central dark matter of early-type galaxies up to z ~ 0.8
We investigate the evolution of dark and luminous matter in the central
regions of early-type galaxies (ETGs) up to z ~ 0.8. We use a spectroscopically
selected sample of 154 cluster and field galaxies from the EDisCS survey,
covering a wide range in redshifts (z ~ 0.4-0.8), stellar masses ( ~ 10.5-11.5 dex) and velocity dispersions
( ~ 100-300 \, km/s). We obtain central dark matter (DM)
fractions by determining the dynamical masses from Jeans modelling of galaxy
aperture velocity dispersions and the from galaxy colours, and
compare the results with local samples. We discuss how the correlations of
central DM with galaxy size (i.e. the effective radius, ),
and evolve as a function of redshift, finding
clear indications that local galaxies are, on average, more DM dominated than
their counterparts at larger redshift. This DM fraction evolution with can
be only partially interpreted as a consequence of the size-redshift evolution.
We discuss our results within galaxy formation scenarios, and conclude that the
growth in size and DM content which we measure within the last 7 Gyr is
incompatible with passive evolution, while it is well reproduced in the
multiple minor merger scenario. We also discuss the impact of the IMF on our DM
inferences and argue that this can be non-universal with the lookback time. In
particular, we find the Salpeter IMF can be better accommodated by low redshift
systems, while producing stellar masses at high- which are unphysically
larger than the estimated dynamical masses (particularly for
lower- systems).Comment: 14 pages, 6 figures, 3 tables, MNRAS in pres
Orientation-dependent Casimir force arising from highly anisotropic crystals: application to Bi2Sr2CaCu2O8+delta
We calculate the Casimir interaction between parallel planar crystals of Au
and the anisotropic cuprate superconductor Bi2Sr2CaCu2O8+delta (BSCCO), with
BSCCO's optical axis either parallel or perpendicular to the crystal surface,
using suitable generalizations of the Lifshitz theory. We find that the strong
anisotropy of the BSCCO permittivity gives rise to a difference in the Casimir
force between the two orientations of the optical axis, which depends on
distance and is of order 10-20% at the experimentally accessible separations 10
to 5000 nm.Comment: 5 pages, 3 figures. Accepted for publication in Physical Review
The dark matter halo of NGC 1399 - CDM or MOND?
Central galaxies in galaxy clusters may be key discriminants in the
competition between the cold dark matter (CDM) paradigm and modified Newtonian
dynamics (MOND). We investigate the dark halo of NGC 1399, the central galaxy
of the Fornax cluster, out to a galactocentric distance of 80 kpc. The data
base consists of 656 radial velocities of globular clusters obtained with
MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We
performed a Jeans analysis for a non-rotating isotropic model. An NFW halo with
the parameters r_s = 50 kpc and rho_s = 0.0065 M_sun/pc^3 provides a good
description of our data, fitting well to the X-ray mass. More massive halos are
also permitted that agree with the mass of the Fornax cluster as derived from
galaxy velocities. We compare this halo with the expected MOND models under
isotropy and find that additional dark matter on the order of the stellar mass
is needed to get agreement. A fully radial infinite globular cluster system
would be needed to change this conclusion. Regarding CDM, we cannot draw firm
conclusions. To really constrain a cluster wide halo, more data covering a
larger radius are necessary. The MOND result appears as a small-scale variant
of the finding that MOND in galaxy clusters still needs dark matter.Comment: 4 pages, 2 figures, accepted for publication as a Letter in A&
Testing the nature of S0 galaxies using planetary nebula kinematics in NGC 1023
We investigate the manner in which lenticular galaxies are formed by studying
their stellar kinematics: an S0 formed from a fading spiral galaxy should
display similar cold outer disc kinematics to its progenitor, while an S0
formed in a minor merger should be more dominated by random motions. In a pilot
study to attempt to distinguish between these scenarios, we have measured the
planetary nebula (PN) kinematics of the nearby S0 system NGC 1023. Using the
Planetary Nebula Spectrograph, we have detected and measured the line-of-sight
velocities of 204 candidate PNe in the field of this galaxy. Out to
intermediate radii, the system displays the kinematics of a normal
rotationally-supported disc system. After correction of its rotational
velocities for asymmetric drift, the galaxy lies just below the spiral galaxy
Tully-Fisher relation, as one would expect for a fading system. However, at
larger radii the kinematics undergo a gradual but major transition to random
motion with little rotation. This transition does not seem to reflect a change
in the viewing geometry or the presence of a distinct halo component, since the
number counts of PNe follow the same simple exponential decline as the stellar
continuum with the same projected disc ellipticity out to large radii. The
galaxy's small companion, NGC 1023A, does not seem to be large enough to have
caused the observed modification either. This combination of properties would
seem to indicate a complex evolutionary history in either the transition to
form an S0 or in the past life of the spiral galaxy from which the S0 formed.
More data sets of this type from both spirals and S0s are needed in order to
definitively determine the relationship between these types of system.Comment: Accepted for publication in MNRAS. Version with full resolution
figure 1 can be found at
http://www.nottingham.ac.uk/~ppzmrm/N1023_PNS.accepted.pd
Dark-Matter Content of Early-Type Galaxies with Planetary Nebulae
We examine the dark matter properties of nearby early-type galaxies using
planetary nebulae (PNe) as mass probes. We have designed a specialised
instrument, the Planetary Nebula Spectrograph (PN.S) operating at the William
Herschel telescope, with the purpose of measuring PN velocities with best
efficiency. The primary scientific objective of this custom-built instrument is
the study of the PN kinematics in 12 ordinary round galaxies. Preliminary
results showing a dearth of dark matter in ordinary galaxies (Romanowsky et al.
2003) are now confirmed by the first complete PN.S datasets. On the other hand
early-type galaxies with a "regular" dark matter content are starting to be
observed among the brighter PN.S target sample, thus confirming a correlation
between the global dark-to-luminous mass virial ratio (f_DM=M_DM/M_star) and
the galaxy luminosity and mass.Comment: 5 pages, 2 figures. To appear in the proceedings of the IAU Symposium
244 "Dark Galaxies and Lost Baryons", Cardiff 25-29 June 2007, eds. J.I.
Davies & M.J. Disne
The dark matter halo of NGC 1399 - CDM or MOND?
Context: Central galaxies in galaxy clusters may be key discriminants in the competition between the cold dark matter (CDM) paradigm and modified Newtonian dynamics (MOND).
Aims: We investigate the dark halo of NGC 1399, the central galaxy of the Fornax cluster, out to a galactocentric distance of 80 kpc.
Methods: The data base consists of 656 radial velocities of globular clusters obtained with MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We performed a Jeans analysis for a non-rotating isotropic model.
Results: An NFW halo with the parameters rs = 50 kpc and gs = 0.0065 M⊙/pc3 provides a good description of our data, fitting well to the X-ray mass. More massive halos are also permitted that agree with the mass of the Fornax cluster as derived from galaxy velocities. We compare this halo with the expected MOND models under isotropy and find that additional dark matter on the order of the stellar mass is needed to get agreement. A fully radial infinite globular cluster system would be needed to change this conclusion.
Conclusions: Regarding CDM, we cannot draw firm conclusions. To really constrain a cluster wide halo, more data covering a larger radius are necessary. The MOND result appears as a small-scale variant of the finding that MOND in galaxy clusters still needs dark matter.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La Plat
Stellar mass-to-light ratio gradients in galaxies: correlations with mass
We analyse the stellar mass-to-light ratio (M/L) gradients in a large sample of local galaxies taken from the Sloan Digital Sky Survey, spanning a wide range of stellar masses and morphological types. As suggested by the well-known relationship between M/L values and colours, we show that M/L gradients are strongly correlated with colour gradients, which we trace to the effects of age variations. Stellar M/L gradients generally follow patterns of variation with stellar mass and galaxy type that were previously found for colour and metallicity gradients. In late-type galaxies M/L gradients are negative, steepening with increasing mass. In early-type galaxies M/L gradients are shallower, while presenting a twofold trend: they decrease with mass up to a characteristic mass of and increase at larger masses. We compare our findings with other analyses and discuss some implications for galaxy formation and for dark matter estimate
The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological? Dynamics of NGC 3311 and its globular cluster system
NGC 3311 is the central cD galaxy of the Hydra I cluster. We use globular
clusters around NGC 3311, combined with kinematical data of the galaxy itself,
to investigate the dark matter distribution in the central region of Hydra I.
Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask
spectroscopy, are used to calculate velocity dispersions which are well defined
out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of
study has been performed. We also determine velocity dispersions of the stellar
component from long slit spectroscopy out to 20 kpc. Moreover, we present a new
photometric model for NGC 3311 in the V-band. We search for a dark halo which
in the context of a spherical Jeans model. We also compare the radial velocity
distributions of globular clusters and planetary nebulae. The projected stellar
velocity dispersion rises from 185 km/s to 350 km/s at a radius of 20 kpc. The
globular cluster dispersion rises as well from 500 km/s at 10 kpc to about 800
km/s at 100 kpc, comparable to the velocity dispersion of the cluster galaxies.
A dark matter halo with a core reproduces well the velocity dispersions of
stars and globular clusters simultaneously under isotropy. The central stellar
velocity dispersions predicted by cosmological NFW halos are less good
representations, while the globular clusters allow a wide range of halo
parameters. A suspected radial anisotropy of the stellar population aggravates
the deviations. However, we find discrepancies with previous kinematical data,
which we cannot resolve and may indicate a more complicated velocity pattern.
Although one cannot conclusively demonstrate that the dark matter halo of NGC
3311 has a core rather than a cusp, a core seems to be preferred by the present
data. A more complete velocity field and an analysis of the anisotropy is
required to reach firm conclusions.Comment: 8 pages, 5 figures, abstract abridged, accepted for publication in
A&
Nuclear constraints on non-Newtonian gravity at femtometer scale
Effects of the non-Newtonian gravity on properties of finite nuclei are
studied by consistently incorporating both the direct and exchange contribution
of the Yukawa potential in the Hartree-Fock approach using a well-tested Skyrme
force for the strong interaction. It is shown for the first time that the
strength of the Yukawa term in the non-Newtonian gravity is limited to
within the length scale
of fm in order for the calculated properties of finite nuclei
not to be in conflict with accurate experimental data available.Comment: Additional discussions and references added; related Lab Talk is
available via http://iopscience.iop.org/0954-3899/labtalk-article/5229
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