48 research outputs found
Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae
Recent observations of high-redshift supernovae seem to suggest that the
global geometry of the Universe may be affected by a `cosmological constant',
which acts to accelerate the expansion rate with time. But these data by
themselves still permit an open universe of low mass density and no
cosmological constant. Here we derive an independent constraint on the lower
bound to the mass density, based on deviations of galaxy velocities from a
smooth universal expansion. This constraint rules out a low-density open
universe with a vanishing cosmological constant, and together the two favour a
nearly flat universe in which the contributions from mass density and the
cosmological constant are comparable. This type of universe, however, seems to
require a degree of fine tuning of the initial conditions that is in apparent
conflict with `common wisdom'.Comment: 8 pages, 1 figure. Slightly revised version. Letter to Natur
The inevitable youthfulness of known high-redshift radio galaxies
Radio galaxies can be seen out to very high redshifts, where in principle
they can serve as probes of the early evolution of the Universe. Here we show
that for any model of radio-galaxy evolution in which the luminosity decreases
with time after an initial rapid increase (that is, essentially all reasonable
models), all observable high-redshift radio-galaxies must be seen when the
lobes are less than 10^7 years old. This means that high-redshift radio
galaxies can be used as a high-time-resolution probe of evolution in the early
Universe. Moreover, this result helps to explain many observed trends of
radio-galaxy properties with redshift [(i) the `alignment effect' of optical
emission along radio-jet axes, (ii) the increased distortion in radio
structure, (iii) the decrease in physical sizes, (iv) the increase in radio
depolarisation, and (v) the increase in dust emission] without needing to
invoke explanations based on cosmology or strong evolution of the surrounding
intergalactic medium with cosmic time, thereby avoiding conflict with current
theories of structure formation.Comment: To appear in Nature. 4 pages, 2 colour figures available on request.
Also available at http://www-astro.physics.ox.ac.uk/~km
The Imprint of Galaxy Formation on X-ray Clusters
It is widely believed that structure in the Universe evolves hierarchically,
as primordial density fluctuations, amplified by gravity, collapse and merge to
form progressively larger systems. The structure and evolution of X-ray
clusters, however, seems at odds with this hierarchical scenario for structure
formation. Poor clusters and groups, as well as most distant clusters detected
to date, are substantially fainter than expected from the tight relations
between luminosity, temperature and redshift predicted by these models. Here we
show that these discrepancies arise because, near the centre, the entropy of
the hot, diffuse intracluster medium (ICM) is higher tha possible if the ICM
is heated at modest redshift (z \ltsim 2) but prior to cluster collapse,
indicating that the formation of galaxies precedes that of clusters and that
most clusters have been assembled very recently.Comment: 5 pages, plus 2 postscript figures (one in colour), accepted for
publication in Natur
Galaxy And Mass Assembly (GAMA): the halo mass of galaxy groups from maximum-likelihood weak lensing
We present a maximum-likelihood weak lensing analysis of the mass distribution in optically selected spectroscopic Galaxy Groups (G3Cv5) in the Galaxy And Mass Assembly (GAMA) survey, using background Sloan Digital Sky Survey (SDSS) pho-tometric galaxies. The scaling of halo mass, Mh, with various group observables is investigated. Our main results are: 1) the measured relations of halo mass with group luminosity, virial volume and central galaxy stellar mass,M⋆, agree very well with predictions from mock group catalogues constructed from a GALFORM semi-analytical galaxy formation model implemented in the Millennium _CDM N-body simulation; 2) the measured relations of halo mass with velocity dispersion and projected half-abundance radius show weak tension with mock predictions, hinting at problems in the mock galaxy dynamics and their small scale distribution; 3) the median Mh|M⋆ measured from weak lensing depends more sensitively on the lognormal dispersion in M⋆ at fixed Mh than it does on the median M⋆|Mh. Our measurements suggest an intrinsic dispersion of σlog(M⋆) _ 0.15; 4) Comparing our mass estimates with those in the catalogue, we find that the G3Cv5 mass can give biased results when used to select subsets of the group sample. Of the various new halo mass estimators that we calibrate using our weak lensing measurements, group luminosity is the best single-proxy estimator of group mass
Galaxy And Mass Assembly (GAMA): the halo mass of galaxy groups from maximum-likelihood weak lensing
We present a maximum-likelihood weak lensing analysis of the mass distribution in optically selected spectroscopic Galaxy Groups (G3Cv5) in the Galaxy And Mass Assembly (GAMA) survey, using background Sloan Digital Sky Survey (SDSS) pho-tometric galaxies. The scaling of halo mass, Mh, with various group observables is investigated. Our main results are: 1) the measured relations of halo mass with group luminosity, virial volume and central galaxy stellar mass,M⋆, agree very well with predictions from mock group catalogues constructed from a GALFORM semi-analytical galaxy formation model implemented in the Millennium _CDM N-body simulation; 2) the measured relations of halo mass with velocity dispersion and projected half-abundance radius show weak tension with mock predictions, hinting at problems in the mock galaxy dynamics and their small scale distribution; 3) the median Mh|M⋆ measured from weak lensing depends more sensitively on the lognormal dispersion in M⋆ at fixed Mh than it does on the median M⋆|Mh. Our measurements suggest an intrinsic dispersion of σlog(M⋆) _ 0.15; 4) Comparing our mass estimates with those in the catalogue, we find that the G3Cv5 mass can give biased results when used to select subsets of the group sample. Of the various new halo mass estimators that we calibrate using our weak lensing measurements, group luminosity is the best single-proxy estimator of group mass
Formation of Supermassive Black Holes
Evidence shows that massive black holes reside in most local galaxies.
Studies have also established a number of relations between the MBH mass and
properties of the host galaxy such as bulge mass and velocity dispersion. These
results suggest that central MBHs, while much less massive than the host (~
0.1%), are linked to the evolution of galactic structure. In hierarchical
cosmologies, a single big galaxy today can be traced back to the stage when it
was split up in hundreds of smaller components. Did MBH seeds form with the
same efficiency in small proto-galaxies, or did their formation had to await
the buildup of substantial galaxies with deeper potential wells? I briefly
review here some of the physical processes that are conducive to the evolution
of the massive black hole population. I will discuss black hole formation
processes for `seed' black holes that are likely to place at early cosmic
epochs, and possible observational tests of these scenarios.Comment: To appear in The Astronomy and Astrophysics Review. The final
publication is available at http://www.springerlink.co
The Main Belt Comets and ice in the Solar System
We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies
The evolution of X-ray clusters in a low-density universe
We present results of N-body/gasdynamical simulations designed to investigate
the evolution of X-ray clusters in a flat, low-density, cold dark matter (CDM)
cosmogony. The density profile of the dark matter component can be fitted
rather accurately by the simple formula originally proposed by Navarro, Frenk &
White to describe the structure of clusters in a CDM universe with .
In projection, the shape of the dark matter radial density profile and the
corresponding line-of-sight velocity dispersion profile are in very good
agreement with the observed profiles for galaxies in the CNOC sample of
clusters. The gas in our simulated clusters is less centrally concentrated than
the dark matter, and its radial density profile is well described by the
familiar -model. The total mass and velocity dispersion of our clusters
can be accurately inferred (with uncertainty) from their X-ray
emission-weighted temperature. We generalize Kaiser's scaling relations for
scale-free universes and show that the clusters in our simulations generally
follow these relations. The agreement between the simulations and the
analytical results provides a convincing demonstration of the soundness of our
gasdynamical numerical techniques. The slope of the luminosity-temperature
relation implied by the scaling relations, and obeyed by the simulations, is in
disagreement with observations. This suggests that non-gravitational effects
such as preheating or cooling must have played an important role in determining
the properties of the observed X-ray emission from galaxy clusters.Comment: Minor changes, final version in press in the ApJ. 43 pages with 17
figures. Full PS file can be obtained from
http://penedes.as.arizona.edu/~jfn/preprints/lamclus.ps.g