116 research outputs found
Galaxy Selection and Clustering and Lyman alpha Absorber Identification
The effects of galaxy selection on our ability to constrain the nature of
weak Ly\alpha absorbers at low redshift are explored. Current observations
indicate the existence of a population of gas-rich, low surface brightness
(LSB) galaxies, and these galaxies may have large cross sections for Ly\alpha
absorption. Absorption arising in LSB galaxies may be attributed to HSB
galaxies at larger impact parameters from quasar lines of sight, so that the
observed absorption cross sections of galaxies may seem unreasonably large.
Thus it is not possible to rule out scenarios where LSB galaxies make
substantial contributions to Ly\alpha absorption using direct observations.
Less direct tests, where observational selection effects are taken into account
using simulations, should make it possible to determine the nature of Ly\alpha
absorbers by observing a sample of ~100 galaxies around quasar lines of sight
with well-defined selection criteria. Such tests, which involve comparing
simulated and observed plots of the unidentified absorber fractions and
absorbing galaxy fractions versus impact parameter, can distinguish between
scenarios where absorbers arise in particular galaxies and those where
absorbers arise in gas tracing the large scale galaxy distribution. Care must
be taken to minimize selection effects even when using these tests. Results
from such tests are likely to depend upon the limiting neutral hydrogen column
density. While not enough data are currently available to make a strong
conclusion about the nature of moderately weak absorbers, evidence is seen that
such absorbers arise in gas that is around or between galaxies that are often
not detected in surveys.Comment: 15 pages, 10 figures, accepted to the Astrophysical Journa
Convicts, Communication and Authority: Britain and New South Wales, 1810-1830
Knowledge of the convict period in New South Wales has been substantially
expanded and enriched through a number of revisionist scholarly studies in the last
quarter of the twentieth century. The cumulative result has been the establishment of
a number of new orthodoxies. These studies have drawn on a number of analytic
frameworks including feminism and cliometrics, successfully challenging the
previous historiography. The rich archival sources in New South Wales have been
utilised to reformulate the convict period by a number of scholars, demonstrating the
complexity of life in the penal colony.
Academic divisions between what are regarded as âAustralianâ history and
âBritishâ history have imposed their own agendas on writing about transportation.
This study challenges this imposition through an examination of petitionersâ
approaches to the home and colonial administrations. A lacuna in the scholarly
studies has been a lack of attention to transportationâs consequences for married
couples and their children. This study seeks to narrow that gap through these
petitions. The findings of the study demonstrate the continuation of links between
those who were transported and those who remained in Britain. It is argued that
these findings have important implications for future research within Britain, and
that what is disclosed by these petitions and the individuals who were involved in
on-going communications cannot be restricted either to Australian or convict
histories. Our knowledge of what transportation meant to individuals in the
periphery as well as those in the metropole is diminished if the focus remains firmly
on the settler community. Supplementary material from contemporary sources as
well as the official records passing between the two administrations has been utilised
and these supplementary sources suggest that there was a broad division between
official publicly stated policy and practice in respect of transporteesâ family
circumstances.
Chapter One establishes the architecture of the thesis and explains the
methodology adopted. Chapter Two offers a reinterpretation of the colonyâs
formation in 1788 and inserts the âconvict audienceâ of that day into the
historiography . Chapter Three examines two petitioners writing from different gaols
in Britain prior to their expected transportation. A resolution of the division between
cliometrics and this more qualitative humanist approach is proposed. Chapter Four
is a study of petitioners in Britain and a study of the process required for a reunion
and reconstitution of family units in New South Wales. Chapter Five seeks to a resiting
of male convicts as family members through an examination of a number of
contemporary sources. Chapter Six examines the petitions raised by husbands and
fathers for their wives and families to be given free passages to the colony. Chapter
Seven provides case studies of three transportees and their experiences of the
petitioning process. In Chapter Eight the focus broadens out from married men to
examine and provide a revision of convictsâ correspondence with their relatives and
friends in Britain. Such correspondence has previously provided the basis for
nationalist interpretations; the revision here suggests that such interpretations are
anachronistic. Chapter Nine is an extended metaphor drawing the material together
to the conclusions of the study
Measuring galaxy segregation using the mark connection function
(abridged) The clustering properties of galaxies belonging to different
luminosity ranges or having different morphological types are different. These
characteristics or `marks' permit to understand the galaxy catalogs that carry
all this information as realizations of marked point processes. Many attempts
have been presented to quantify the dependence of the clustering of galaxies on
their inner properties. The present paper summarizes methods on spatial marked
statistics used in cosmology to disentangle luminosity, colour or morphological
segregation and introduces a new one in this context, the mark connection
function. The methods used here are the partial correlation functions,
including the cross-correlation function, the normalised mark correlation
function, the mark variogram and the mark connection function. All these
methods are applied to a volume-limited sample drawn from the 2dFGRS, using the
spectral type as the mark. We show the virtues of each method to provide
information about the clustering properties of each population, the dependence
of the clustering on the marks, the similarity of the marks as a function of
the pair distances, and the way to characterise the spatial correlation between
the marks. We demonstrate by means of these statistics that passive galaxies
exhibit stronger spatial correlation than active galaxies at small scales (r
<20 Mpc/h). The mark connection function, introduced here, is particularly
useful for understanding the spatial correlation between the marks.Comment: 6 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Numerical propagation of high energy cosmic rays in the Galaxy I: technical issues
We present the results of a numerical simulation of propagation of cosmic
rays with energy above eV in a complex magnetic field, made in
general of a large scale component and a turbulent component. Several
configurations are investigated that may represent specific aspects of a
realistic magnetic field of the Galaxy, though the main purpose of this
investigation is not to achieve a realistic description of the propagation in
the Galaxy, but rather to assess the role of several effects that define the
complex problem of propagation. Our simulations of Cosmic Rays in the Galaxy
will be presented in Paper II. We identified several effects that are difficult
to interpret in a purely diffusive approach and that play a crucial role in the
propagation of cosmic rays in the complex magnetic field of the Galaxy. We
discuss at length the problem of the extrapolation of our results to much lower
energies where data are available on the confinement time of cosmic rays in the
Galaxy. The confinement time and its dependence on particles' rigidity are
crucial ingredients for 1) relating the source spectrum to the observed cosmic
ray spectrum; 2) quantifying the production of light elements by spallation; 3)
predicting the anisotropy as a function of energy.Comment: 29 pages, 12 figures, submitted to JCA
Galaxy And Mass Assembly (GAMA) : The mechanisms for quiescent galaxy formation at z<1
© 2016 The Authors. One key problem in astrophysics is understanding how and why galaxies switch off their star formation, building the quiescent population that we observe in the local Universe. From the Galaxy And Mass Assembly and VIsible MultiObject Spectrograph Public Extragalactic Redshift surveys, we use spectroscopic indices to select quiescent and candidate transition galaxies.We identify potentially rapidly transitioning post-starburst (PSB) galaxies and slower transitioning green-valley galaxies. Over the last 8Gyr, the quiescent population has grown more slowly in number density at high masses (M * > 10 11 M â ) than at intermediate masses (M * > 10 10.6 M â ). There is evolution in both the PSB and green-valley stellar mass functions, consistent with higher mass galaxies quenching at earlier cosmic times.At intermediatemasses (M * > 10 10.6 M â ), we find a green-valley transition time-scale of 2.6 Gyr. Alternatively, at z ~ 0.7, the entire growth rate could be explained by fast-quenching PSB galaxies, with a visibility time-scale of 0.5 Gyr. At lower redshift, the number density of PSBs is so low that an unphysically short visibility window would be required for them to contribute significantly to the quiescent population growth. The importance of the fast-quenching route may rapidly diminish at z 10 11 M â ), there is tension between the large number of candidate transition galaxies compared to the slow growth of the quiescent population. This could be resolved if not all high-mass PSB and green-valley galaxies are transitioning from star forming to quiescent, for example if they rejuvenate out of the quiescent population following the accretion of gas and triggering of star formation, or if they fail to completely quench their star formation
Prospects for Determining the Equation of State of the Dark Energy: What can be Learned from Multiple Observables?
The dark energy that appears to produce the accelerating expansion of the
universe can be characterized by an equation of state p=w\rho with w<-1/3. A
number of observational tests have been proposed to study the value or redshift
dependence of w, including SN Ia distances, the Sunyaev-Zel'dovich effect,
cluster abundances, strong and weak gravitational lensing, galaxy and quasar
clustering, galaxy ages, the \lya forest, and CMB anisotropies. The proposed
observational tests based on these phenomena measure either the
distance-redshift relation d(z), the Hubble parameter H(z), the age of the
universe t(z), the linear growth factor D_1(z), or some combination of these
quantities. We compute the evolution of these four observables, and of the
combination H(z)d(z) that enters the Alcock-Paczyznski anisotropy test, in
models with constant w, in quintessence models with some simple forms of the
potential V(\phi), and in toy models that allow more radical time variations of
w. Measurement of any of these quantities to precision of a few percent is
generally sufficient to discriminate between w=-1 and w=-2/3. However, the
time-dependence predicted in quintessence models is extremely difficult to
discern because the quintessence component is dynamically unimportant at the
redshifts where w departs substantially from its low-z value. Even for the toy
models that allow substantial changes in w at low redshift, there is always a
constant-w model that produces very similar evolution of all of the observables
simultaneously. We conclude that measurement of the effective equation of state
of the dark energy may be achieved by several independent routes in the next
few years, but that detecting time-variation in this equation of state will
prove very difficult except in specialized cases.Comment: 29 pages, 7 figures, many minor corrections, additions, and
clarifications, to appear in Ap
A blind HI survey of the M81 group
Results are presented of the first blind HI survey of the M81 group of
galaxies. The data were taken as part of the HI Jodrell All Sky Survey
(HIJASS). The survey reveals several new aspects to the complex morphology of
the HI distribution in the group. All four of the known dwarf irregular (dIrr)
galaxies close to M81 can be unambiguously seen in the HIJASS data. Each forms
part of the complex tidal structure in the area. We suggest that at least three
of these galaxies may have formed recently from the tidal debris in which they
are embedded. The structure connecting M81 to NGC2976 is revealed as a single
tidal bridge of mass approx. 2.1 x 10^8 Msol and projected spatial extent
approx. 80 kpc. Two `spurs' of HI projecting from the M81 complex to lower
declinations are traced over a considerably larger spatial and velocity extent
than by previous surveys. The dwarf elliptical (dE) galaxies BK5N and Kar 64
lie at the spatial extremity of one of these features and appear to be
associated with it. We suggest that these may be the remnants of dIrrs which
has been stripped of gas and transmuted into dEs by close gravitational
encounters with NGC3077. The nucleated dE galaxy Kar 61 is unambiguously
detected in HI for the first time and has an HI mass of approx.10^8 Msol,
further confirming it as a dE/dIrr transitional object. HIJASS has revealed one
new possible group member, HIJASS J1021+6842. This object contains approx. 2 x
10^7 Msol of HI and lies approx.105arcmin from IC2574. It has no optical
counterpart on the Digital Sky Survey.Comment: To be published in Astrophysical Journal Letters 9 pages, including 3
figure
Propagation of cosmic-ray nucleons in the Galaxy
We describe a method for the numerical computation of the propagation of
primary and secondary nucleons, primary electrons, and secondary positrons and
electrons. Fragmentation and energy losses are computed using realistic
distributions for the interstellar gas and radiation fields, and diffusive
reacceleration is also incorporated. The models are adjusted to agree with the
observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and
convection do not account well for the observed energy dependence of B/C, while
models with reacceleration reproduce this easily. The height of the halo
propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc
for diffusion/convection models and 4-12 kpc for reacceleration models. For
convection models we set an upper limit on the velocity gradient of dV/dz < 7
km/s/kpc. The radial distribution of cosmic-ray sources required is broader
than current estimates of the SNR distribution for all halo sizes. Full details
of the numerical method used to solve the cosmic-ray propagation equation are
given.Comment: 15 pages including 23 ps-figures and 3 tables, latex2e, uses
emulateapj.sty (ver. of 11 May 1998, enclosed), apjfonts.sty, timesfonts.sty.
To be published in ApJ 1998, v.509 (December 10 issue). More details can be
found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html Some references
are correcte
Diffuse continuum gamma rays from the Galaxy
A new study of the diffuse Galactic gamma-ray continuum radiation is
presented, using a cosmic-ray propagation model which includes nucleons,
antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of
the inverse Compton (IC) scattering includes the effect of anisotropic
scattering in the Galactic interstellar radiation field (ISRF) and a new
evaluation of the ISRF itself. Models based on locally measured electron and
nucleon spectra and synchrotron constraints are consistent with gamma-ray
measurements in the 30-500 MeV range, but outside this range excesses are
apparent. A harder nucleon spectrum is considered but fitting to gamma rays
causes it to violate limits from positrons and antiprotons. A harder
interstellar electron spectrum allows the gamma-ray spectrum to be fitted above
1 GeV as well, and this can be further improved when combined with a modified
nucleon spectrum which still respects the limits imposed by antiprotons and
positrons. A large electron/IC halo is proposed which reproduces well the
high-latitude variation of gamma-ray emission. The halo contribution of
Galactic emission to the high-latitude gamma-ray intensity is large, with
implications for the study of the diffuse extragalactic component and
signatures of dark matter. The constraints provided by the radio synchrotron
spectral index do not allow all of the <30 MeV gamma-ray emission to be
explained in terms of a steep electron spectrum unless this takes the form of a
sharp upturn below 200 MeV. This leads us to prefer a source population as the
origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal
(vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49
ps-figures, uses emulateapj.sty. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
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