645 research outputs found
Biodiversity and Endemism within the Mount Canobolas Volcanic Complex
Mt Canobolas State Conservation Area (SCA) hosts a small remnant of sub-alpine vegetation consisting of seven recognisable communities with the heathlands on the rock plates appearing to be unique to the SCA. The SCA has a known biota of 884 native species that includes 14 threatened species and at least 10 endemic taxa. Some 200 species are regionally significant, being either rare or at the limits of known geographic range. The vascular flora is particularly species-rich being considerably more diverse than nearby regional reserves and over 12 fold richer than comparable areas of the Kosciusko National Park. One of three endangered ecological communities, the Mt Canobolas Xanthoparmelia Lichen Community, is unique to the volcanic province.
While there is some indication the endemic lithophytic lichens, the threatened Eucalyptus canobolensis and the heath communities may be substrate specific, there is no strong evidence of a geological association among other flora and fauna. We postulate that the presence of multiple endemic species reflects the geographic isolation which has provided an environment for species evolution by vicariance. Alternatively, Mt Canobolas has acted as a refugium for formerly widespread species that have become extinct elsewhere
The Evolution of X-ray Clusters and the Entropy of the Intra Cluster Medium
The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies
is determined by gravitational processes associated with shock heating,
adiabatic compression, and non-gravitational processes such as heating by SNe,
stellar winds, activity in the central galactic nucleus, and radiative cooling.
The effect of gravitational processes on the thermodynamics of the Intra
Cluster Medium (ICM) can be expressed in terms of the ICM entropy S ~
ln(T/\rho^{2/3}). We use a generalized spherical model to compute the X-ray
properties of groups and clusters for a range of initial entropy levels in the
ICM and for a range of mass scales, cosmic epochs and background cosmologies.
We find that the statistical properties of the X-ray clusters strongly depend
on the value of the initial excess entropy. Assuming a constant, uniform value
for the excess entropy, the present-day X-ray data are well fitted for the
following range of values K_* = kT/\mu m_p \rho^{2/3} = (0.4\pm 0.1) \times
10^{34} erg cm^2 g^{-5/3} for clusters with average temperatures kT>2 keV; K_*
= (0.2\pm 0.1) \times 10^{34} erg cm^2 g^{-5/3} for groups and clusters with
average temperatures kT<2 keV. These values correspond to different excess
energy per particle of kT \geq 0.1 (K_*/0.4\times 10^{34}) keV. The dependence
of K_* on the mass scale can be well reproduced by an epoch dependent external
entropy: the relation K_* = 0.8(1+z)^{-1}\times 10^{34} erg cm^2 g^{-5/3} fits
the data over the whole temperature range. Observations of both local and
distant clusters can be used to trace the distribution and the evolution of the
entropy in the cosmic baryons, and ultimately to unveil the typical epoch and
the source of the heating processes.Comment: 53 pages, LateX, 19 figures, ApJ in press, relevant comments and
references adde
Detection of the Entropy of the Intergalactic Medium: Accretion Shocks in Clusters, Adiabatic Cores in Groups
The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies
is linked to the entropy level of the intra cluster medium. In particular,
models that successfully reproduce the properties of local X-ray clusters and
groups require the presence of a minimum value for the entropy in the center of
X-ray halos. Such a minimum entropy is most likely generated by
non-gravitational processes, in order to produce the observed break in
self-similarity of the scaling relations of X-ray halos. At present there is no
consensus on the level, the source or the time evolution of this excess
entropy. In this paper we describe a strategy to investigate the physics of the
heating processes acting in groups and clusters. We show that the best way to
extract information from the local data is the observation of the entropy
profile at large radii in nearby X-ray halos (z~0.1), both at the upper and
lower extremes of the cluster mass scale. The spatially and spectrally resolved
observation of such X-ray halos provides information on the mechanism of the
heating. We demonstrate how measurements of the size of constant entropy
(adiabatic) cores in clusters and groups can directly constrain heating models,
and the minimum entropy value. We also consider two specific experiments: the
detection of the shock fronts expected at the virial boundary of rich clusters,
and the detection of the isentropic, low surface-brightness emission extending
to radii larger than the virial ones in low mass clusters and groups. Such
observations will be a crucial probe of both the physics of clusters and the
relationship of non-gravitational processes to the thermodynamics of the
intergalactic medium.Comment: ApJ accepted, 31 pages including 8 figures. Important material added;
references update
Cosmological Constraints from the ROSAT Deep Cluster Survey
The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of
X-ray selected galaxy clusters. Observables such as the flux number counts
n(S), the redshift distribution n(z) and the X-ray luminosity function (XLF)
over a large redshift baseline (z\lesssim 0.8) are used here in order to
constrain cosmological models. Our analysis is based on the Press-Schechter
approach, whose reliability is tested against N-body simulations. Following a
phenomenological approach, no assumption is made a priori on the relation
between cluster masses and observed X-ray luminosities. As a first step, we use
the local XLF from RDCS, along with the high-luminosity extension provided by
the XLF from the BCS, in order to constrain the amplitude of the power
spectrum, \sigma_8, and the shape of the local luminosity-temperature relation.
We obtain \sigma_8=0.58 +/- 0.06 for Omega_0=1 for open models at 90%
confidence level, almost independent of the L-T shape. The density parameter
\Omega_0 and the evolution of the L-T relation are constrained by the RDCS XLF
at z>0 and the EMSS XLF at z=0.33, and by the RDCS n(S) and n(z) distributions.
By modelling the evolution for the amplitude of the L-T relation as (1+z)^A, an
\Omega_0=1 model can be accommodated for the evolution of the XLF with 1<A<3 at
90% confidence level, while \Omega_0=0.4^{+0.3}_{-0.2} and \Omega_0<0.6 are
implied by a non--evolving L-T for open and flat models, respectively.Comment: 12 pages, 9 colour figures, LateX, uses apj.sty, ApJ, in press, May
20 issu
Evolution of bias in different cosmological models
We study the evolution of the halo-halo correlation function and bias in four
cosmological models (LCDM, OCDM, tauCDM, and SCDM) using very high-resolution
N-body simulations. The high force and mass resolution allows dark matter (DM)
halos to survive in the tidal fields of high-density regions and thus prevents
the ambiguities related with the ``overmerging problem.'' This allows us to
estimate for the first time the evolution of the correlation function and bias
at small (down to ~100/h kpc) scales. We find that at all epochs the 2-point
correlation function of galaxy-size halos xi_hh is well approximated by a
power-law with slope ~1.6-1.8. The difference between the shape of xi_hh and
the shape of the correlation function of matter results in the scale-dependent
bias at scales <7/h Mpc, which we find to be a generic prediction of the
hierarchical models. The bias evolves rapidly from a high value of ~2-5 at
z~3-7 to the anti-bias of b~0.5-1 at small <5/h Mpc scales at z=0. We find that
our results agree well with existing clustering data at different redshifts.
Particularly, we find an excellent agreement in both slope and the amplitude
between xi_hh(z=0) in our LCDM simulation and the galaxy correlation function
measured using the APM galaxy survey. At high redshifts, the observed
clustering of the Lyman-break galaxies is also well reproduced by the models.
The agreement with the data at high and low z indicates the general success of
the hierarchical models of structure formation in which galaxies form inside
the host DM halos. (Abridged)Comment: submitted to the Astrophys.Journal; 21 pages, LaTeX (uses
emulateapj.sty); full resolution versions of figs.1 and 2 are available at
http://astro.nmsu.edu/~akravtso/GROUP/group_publications.html or at
ftp://charon.nmsu.edu/pub/kravtsov/PAPERS/Bias
Forgotten Plotlanders: Learning from the survival of lost informal housing in the UK.
Colin Wardâs discourses on the arcadian landscape of âplotlanderâ housing are unique documentations of the anarchistic birth, life, and death of the last informal housing communities in the UK. Today the forgotten history of âplotlanderâ housing documented by Ward can be re-read in the context of both the apparently never-ending âhousing crisisâ in the UK, and the increasing awareness of the potential value of learning from comparable informal housing from the Global South. This papers observations of a previously unknown and forgotten plotlander site offers a chance to begin a new conversation regarding the positive potential of informal and alternative housing models in the UK and wider Westernised world
Density profiles of dark matter haloes: diversity and dependence on environment
(Abridged) We study the outer density profiles of dark matter haloes
predicted by a generalized secondary infall model and observed in a N-body
cosmological simulation of a \Lambda CDM model. We find substantial systematic
variations in shapes and concentrations of the halo profiles as well as a
strong correlation of the profiles with the environment. In the N-body
simulation, the average outer slope of the density profiles, \beta (\rho\propto
r^{-\beta}), of isolated haloes is \approx 2.9; 68% of these haloes have values
of \beta between 2.5 and 3.8. Haloes in dense environments of clusters are more
concentrated and exhibit a broad distribution of \beta with values larger than
for isolated haloes . Contrary to what one may expect, the haloes contained
within groups and galaxy systems are less concentrated and have flatter outer
density profiles than the isolated haloes. The concentration decreases with
M_h, but its scatter for a given mass is substantial. The mass and circular
velocity of the haloes are strongly correlated: M_h \propto V_m^{\alpha} with
\alpha ~ 3.3 (isolated) and ~3.5 (haloes in clusters). For M_h=10^12M_sun the
rms deviations from these relations are \Delta logM_h=0.12 and 0.18,
respectively. Approximately 30% of the haloes are contained within larger
haloes or have massive companions (larger than ~0.3 the mass of the current
halo) within 3 virial radii. The remaining 70% of the haloes are isolated
objects. The distribution of \beta as well as the concentration-mass and
M_h-V_m relations for the isolated haloes agree very well with the predictions
of our seminumerical approach which is based on a generalization of the
secondary infall model and on the extended Press-Schechter formalism.Comment: 14 pages, 11 figures included, uses mn.sty, accepted by MNRAS. Minor
modifications, new and updated reference
Black Holes in the Early Universe
The existence of massive black holes was postulated in the sixties, when the
first quasars were discovered. In the late nineties their reality was proven
beyond doubt, in the Milky way and a handful nearby galaxies. Since then,
enormous theoretical and observational efforts have been made to understand the
astrophysics of massive black holes. We have discovered that some of the most
massive black holes known, weighing billions of solar masses, powered luminous
quasars within the first billion years of the Universe. The first massive black
holes must therefore have formed around the time the first stars and galaxies
formed. Dynamical evidence also indicates that black holes with masses of
millions to billions of solar masses ordinarily dwell in the centers of today's
galaxies. Massive black holes populate galaxy centers today, and shone as
quasars in the past; the quiescent black holes that we detect now in nearby
bulges are the dormant remnants of this fiery past. In this review we report on
basic, but critical, questions regarding the cosmological significance of
massive black holes. What physical mechanisms lead to the formation of the
first massive black holes? How massive were the initial massive black hole
seeds? When and where did they form? How is the growth of black holes linked to
that of their host galaxy? Answers to most of these questions are work in
progress, in the spirit of these Reports on Progress in Physics.Comment: Reports on Progress in Physics, in pres
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