64 research outputs found
Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations
We have developed an analytical model to describe the evolution of
anisotropic galactic outflows. With it, we investigate the impact of varying
opening angle on galaxy formation and the evolution of the IGM. We have
implemented this model in a Monte Carlo algorithm to simulate galaxy formation
and outflows in a cosmological context. Using this algorithm, we have simulated
the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM
universe. Starting from a Gaussian density field at redshift z=24, we follow
the formation of ~20,000 galaxies, and simulate the galactic outflows produced
by these galaxies. When these outflows collide with density peaks, ram pressure
stripping of the gas inside the peak may result. This occurs in around half the
cases and prevents the formation of galaxies. Anisotropic outflows follow the
path of least resistance, and thus travel preferentially into low-density
regions, away from cosmological structures (filaments and pancakes) where
galaxies form. As a result, the number of collisions is reduced, leading to the
formation of a larger number of galaxies. Anisotropic outflows can
significantly enrich low-density systems with metals. Conversely, the
cross-pollution in metals of objects located in a common cosmological
structure, like a filament, is significantly reduced. Highly anisotropic
outflows can travel across cosmological voids and deposit metals in other,
unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap
Reconciling LCROSS and Orbital Neutron Water Abundance Estimates in Cabeus Crater
The Lunar Prospector Neutron Spectrometer (LPNS) first revealed Cabeus crater (84.9 deg S, 35.5degW) as having the highest inferred hydrogen on the Moon. Because of the broad LPNS footprint (approximately 40 km FWHM), the apparent peak water-equivalent hydrogen (WEH) concentration is only approximately 0.25 wt%, but could be much higher in smaller areas than the spectrometer footprint. Earlier image reconstruction work suggested that areas within permanent shadow have abundances approximately 1 wt% WEH. However, the LCROSS impact yielded total water estimates, ice plus vapor, of between 3 and 10 wt%. The large disagreement between LCROSS and apparent orbital values imply that either the ice is buried, by perhaps as much as 50 to 100 cm; or the ice distribution within Cabeus is spatially inhomogeneous, or both. Modeling reveals that the areal extent of a "shallow permafrost zone" is far greater than the area of permanent shadow. Ice can be virtually stable for billions of years within a few tens of centimeters of the surface in these areas. However, the LCROSS impact took place in an area of permanent shadow. If stably-trapped volatiles can be found in locales that receive occasional, oblique sunlight, landed missions may target these sites and eventual resource exploitation may be done more easily. Are orbital neutron data consistent with areally-extensive, volatile-rich cold traps? Orbital epithermal neutron data over the northern half of Cabeus (near the LCROSS impact site) are consistent with 0.2 wt% WEH or less in the "permafrost zone" near the crater. On the other hand, pixon reconstructions that confine the hydrogen enhancements to permanent shadow result in higher abundance estimates -- around 1 wt% if homogeneously mixed. But if the PSR abundance is increased to 10 wt%, consistent with the sum of all H-bearing compounds seen by LCROSS, a much larger-than-observed reduction in neutron count rate would be seen from orbit. It is likely that volatiles are inhomogeneously distributed, due to both impact processes and emplacement history. Two possibilities may bring consistency to the orbital and LCROSS measurements. Inhomogeneous lateral distribution: Consider the extreme case of a bimodal distribution within the crater -- dry and wet. In this case the epithermal leakage flux seen from orbit is a mixture of two different values, weighted according to fractional areas. Two possible outcomes, depending on whether the inferred leakage flux for the PSR or "permafrost" areas are considered. In the first case, approximately 40% of the PSR may be "wet", the remainder dry (and LCROSS was slightly lucky). However, if the whole area of permafrost is considered, then as little as 20% of the area will be as "wet" as the LCROSS results (and LCROSS was quite lucky). Inhomogeneous depth distribution: The leakage flux of thermal and epithermal neutrons depends on depth of burial of an icy layer beneath dry ferroan anorthosite soil (FAn). For the Cabeus PSR, the pixon reconstruction values for the epithermal flux allows a range of abundance and burial depth, while that of the thermal+epi detector constrains this range. (Uncertainties in iron abundance in the FAn can have significant impact on thermal neutron leakage flux estimates.) Between 20% and 40% of the Cabeus floor may be "wet", or alternatively a 5-10 wt% "wet" layer exists between 50 and 100 cm beneath a layer of dry regolith within the PSR. But volatile abundances of 5 wt% or more, distributed uniformly and homogeneously throughout the Cabeus PSR do not agree with orbital measurement
Near-Infrared and Optical Luminosity Functions from the 6dF Galaxy Survey
Luminosity functions and their integrated luminosity densities are presented
for the 6dF Galaxy Survey (6dFGS). This ongoing survey ultimately aims to
measure around 150,000 redshifts and 15,000 peculiar velocities over almost the
entire southern sky at |b|>10 deg. The main target samples are taken from the
2MASS Extended Source Catalog and the SuperCOSMOS Sky Survey catalogue, and
comprise 138,226 galaxies complete to (K, H, J, rF, bJ) = (12.75, 13.00, 13.75,
15.60, 16.75). These samples are comparable in size to the optically-selected
Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey samples, and improve on
recent near-infrared-selected redshift surveys by more than an order of
magnitude in both number and sky coverage. The partial samples used in this
paper contain a little over half of the total sample in each band and are ~90
percent complete.
Luminosity distributions are derived using the 1/Vmax, STY and SWML
estimators, and probe 1 to 2 absolute magnitudes fainter in the near-infrared
than previous surveys. The effects of magnitude errors, redshift incompleteness
and peculiar velocities have been taken into account and corrected throughout.
Generally, the 6dFGS luminosity functions are in excellent agreement with those
of similarly-sized surveys. Our data are of sufficient quality to demonstrate
that a Schechter function is not an ideal fit to the true luminosity
distribution, due to its inability to simultaneously match the faint end slope
and rapid bright end decline. Integrated luminosity densities from the 6dFGS
are consistent with an old stellar population and moderately declining star
formation rate.Comment: 20 pages, 15 figures. MNRAS published. Replaces earlier version
carrying a typo in Table 6. High resolution versions of the figures can be
obtained from http://www.aao.gov.au/local/www/6df/Publication
Gridmapping the northern plains of Mars: Geomorphological, Radar and Water-Equivalent Hydrogen results from Arcadia Plantia
A project of mapping ice-related landforms was undertaken to understand the role of sub-surface ice in the northern plains. This work is the first continuous regional mapping from CTX (âConTeXt Cameraâ, 6 m/pixel; Malin et al., 2007) imagery in Arcadia Planitia along a strip 300 km across stretching from 30°N to 80°N centred on the 170° West line of longitude. The distribution and morphotypes of these landforms were used to understand the permafrost cryolithology. The mantled and textured signatures occur almost ubiquitously between 35° N and 78° N and have a positive spatial correlation with inferred ice stability based on thermal modelling, neutron spectroscopy and radar data. The degradational features into the LDM (Latitude Dependent Mantle) include pits, scallops and 100 m polygons and provide supporting evidence for sub-surface ice and volatile loss between 35-70° N in Arcadia with the mantle between 70-78° N appearing much more intact. Pitted terrain appears to be much more pervasive in Arcadia than in Acidalia and Utopia suggesting that the Arcadia study area had more wide-spread near-surface sub-surface ice, and thus was more susceptible to pitting, or that the ice was less well-buried by sediments. Correlations with ice stability models suggest that lack of pits north of 65-70° N could indicate a relatively young age (~1Ma), however this could also be explained through regional variations in degradation rates. The deposition of the LDM is consistent with an airfall hypothesis however there appears to be substantial evidence for fluvial processes in southern Arcadia with older, underlying processes being equally dominant with the LDM and degradation thereof in shaping the landscape
Impact of Systematic Errors in Sunyaev-Zel'dovich Surveys of Galaxy Clusters
Future high-resolution microwave background measurements hold the promise of
detecting galaxy clusters throughout our Hubble volume through their
Sunyaev-Zel'dovich (SZ) signature, down to a given limiting flux. The number
density of galaxy clusters is highly sensitive to cluster mass through
fluctuations in the matter power spectrum, as well as redshift through the
comoving volume and the growth factor. This sensitivity in principle allows
tight constraints on such quantities as the equation of state of dark energy
and the neutrino mass. We evaluate the ability of future cluster surveys to
measure these quantities simultaneously when combined with PLANCK-like CMB
data. Using a simple effective model for uncertainties in the cluster mass-SZ
flux relation, we evaluate systematic shifts in cosmological constraints from
cluster SZ surveys. We find that a systematic bias of 10% in cluster mass
measurements can give rise to shifts in cosmological parameter estimates at
levels larger than the statistical errors. Systematic errors are
unlikely to be detected from the mass and redshift dependence of cluster number
counts alone; increasing survey size has only a marginal effect. Implications
for upcoming experiments are discussed.Comment: 12 pages, 6 figures; accepted to JCAP; revised to match submitted
versio
The Unusual Infrared Object HDF-N J123656.3+621322
We describe an object in the Hubble Deep Field North with very unusual
near-infrared properties. It is readily visible in Hubble Space Telescope
NICMOS images at 1.6um and from the ground at 2.2um, but is undetected (with
signal-to-noise <~ 2) in very deep WFPC2 and NICMOS data from 0.3 to 1.1um. The
f_nu flux density drops by a factor >~ 8.3 (97.7% confidence) from 1.6 to
1.1um. The object is compact but may be slightly resolved in the NICMOS 1.6um
image. In a low-resolution, near-infrared spectrogram, we find a possible
emission line at 1.643um, but a reobservation at higher spectral resolution
failed to confirm the line, leaving its reality in doubt. We consider various
hypotheses for the nature of this object. Its colors are unlike those of known
galactic stars, except perhaps the most extreme carbon stars or Mira variables
with thick circumstellar dust shells. It does not appear to be possible to
explain its spectral energy distribution as that of a normal galaxy at any
redshift without additional opacity from either dust or intergalactic neutral
hydrogen. The colors can be matched by those of a dusty galaxy at z >~ 2, by a
maximally old elliptical galaxy at z >~ 3 (perhaps with some additional
reddening), or by an object at z >~ 10 whose optical and 1.1um light have been
suppressed by the intergalactic medium. Under the latter hypothesis, if the
luminosity results from stars and not an AGN, the object would resemble a
classical, unobscured protogalaxy, with a star formation rate >~ 100 M_sun/yr.
Such UV-bright objects are evidently rare at 2 < z < 12.5, however, with a
space density several hundred times lower than that of present-day L* galaxies.Comment: Accepted for publication in the Astrophysical Journal. 27 pages,
LaTeX, with 7 figures (8 files); citations & references updated + minor
format change
The impact of dark matter cusps and cores on the satellite galaxy population around spiral galaxies
(Abridged) We use N-body simulations to study the effects that a divergent
(i.e. "cuspy") dark matter (DM) profile introduces on the tidal evolution of
dwarf spheroidal galaxies (dSphs). Our models assume cosmologically-motivated
initial conditions where dSphs are DM-dominated systems on eccentric orbits
about a host galaxy composed of a dark halo and a baryonic disc. We find that
the resilience of dSphs to tidal stripping is extremely sensitive to the halo
cuspiness; whereas dwarfs with a cored profile can be easily destroyed by the
host disc, those with cusps always retain a bound remnant. For a given halo
profile the evolution of the structural parameters as driven by tides is
controlled solely by the total amount of mass lost. This information is used to
construct a semi-analytic code that simulates the hierarchical build-up of
spiral galaxies assuming different halo profiles and disc masses. We find that
tidal encounters with discs tend to decrease the average mass of satellites at
all galactocentric radii. Interestingly, satellites accreted before
re-ionization (z>6), which may be singled out by anomalous metallicity
patterns, survive only if haloes are cuspy. We show that the size-mass relation
established from Milky Way (MW) dwarfs strongly supports the presence of cusps
in the majority of these systems, as cored models systematically underestimate
the masses of the known Ultra-Faint dSphs. Our models also indicate that a
massive M31 disc may explain why many of its dSphs fall below the size-mass
relationship derived from MW dSphs. We use our models to constrain the mass
threshold below which star formation is suppressed in DM haloes, finding that
luminous satellites must be accreted with masses above 10^8--10^9 M_sol in
order to explain the size-mass relation observed in MW dwarfs.Comment: 17 pages, 14 figures, MNRAS accepted after minor revisio
Parameterization Effects in the analysis of AMI Sunyaev-Zel'dovich Observations
Most Sunyaev--Zel'dovich (SZ) and X-ray analyses of galaxy clusters try to
constrain the cluster total mass and/or gas mass using parameterised models and
assumptions of spherical symmetry and hydrostatic equilibrium. By numerically
exploring the probability distributions of the cluster parameters given the
simulated interferometric SZ data in the context of Bayesian methods, and
assuming a beta-model for the electron number density we investigate the
capability of this model and analysis to return the simulated cluster input
quantities via three rameterisations. In parameterisation I we assume that the
T is an input parameter. We find that parameterisation I can hardly constrain
the cluster parameters. We then investigate parameterisations II and III in
which fg(r200) replaces temperature as a main variable. In parameterisation II
we relate M_T(r200) and T assuming hydrostatic equilibrium. We find that
parameterisation II can constrain the cluster physical parameters but the
temperature estimate is biased low. In parameterisation III, the virial theorem
replaces the hydrostatic equilibrium assumption. We find that parameterisation
III results in unbiased estimates of the cluster properties. We generate a
second simulated cluster using a generalised NFW (GNFW) pressure profile and
analyse it with an entropy based model to take into account the temperature
gradient in our analysis and improve the cluster gas density distribution. This
model also constrains the cluster physical parameters and the results show a
radial decline in the gas temperature as expected. The mean cluster total mass
estimates are also within 1 sigma from the simulated cluster true values.
However, we find that for at least interferometric SZ analysis in practice at
the present time, there is no differences in the AMI visibilities between the
two models. This may of course change as the instruments improve.Comment: 19 pages, 13 tables, 24 figure
Detailed SZ study of 19 LoCuSS galaxy clusters: masses and temperatures out to the virial radius
We present 16-GHz AMI SZ observations of 19 clusters with L_X >7x10^37 W
(h50=1) selected from the LoCuS survey (0.142<z<0.295) and of A1758b, in the
FoV of A1758a. We detect 17 clusters with 5-23sigma peak surface brightnesses.
Cluster parameters are obtained using a Bayesian cluster analysis. We fit
isothermal beta-models to our data and assume the clusters are virialized (with
all the kinetic energy in gas internal energy). Our gas temperature, T_AMI, is
derived from AMI SZ data, not from X-ray spectroscopy. Cluster parameters
internal to r500 are derived assuming HSE. We find: (i) Different gNFW
parameterizations yield significantly different parameter degeneracies. (ii)
For h70 = 1, we find the virial radius r200 to be typically 1.6+/-0.1 Mpc and
the total mass M_T(r200) typically to be 2.0-2.5xM_T(r500).(iii) Where we have
found M_T X-ray (X) and weak-lensing (WL) values in the literature, there is
good agreement between WL and AMI estimates (with M_{T,AMI}/M_{T,WL}
=1.2^{+0.2}_{-0.3} and =1.0+/-0.1 for r500 and r200, respectively). In
comparison, most Suzaku/Chandra estimates are higher than for AMI (with
M_{T,X}/M_{T,AMI}=1.7+/-0.2 within r500), particularly for the stronger
mergers.(iv) Comparison of T_AMI to T_X sheds light on high X-ray masses: even
at large r, T_X can substantially exceed T_AMI in mergers. The use of these
higher T_X values will give higher X-ray masses. We stress that large-r T_SZ
and T_X data are scarce and must be increased. (v) Despite the paucity of data,
there is an indication of a relation between merger activity and SZ
ellipticity. (vi) At small radius (but away from any cooling flow) the SZ
signal (and T_AMI) is less sensitive to ICM disturbance than the X-ray signal
(and T_X) and, even at high r, mergers affect n^2-weighted X-ray data more than
n-weighted SZ, implying significant shocking or clumping or both occur even in
the outer parts of mergers.Comment: 45 pages, 33 figures, 13 tables Accepted for publication in MNRA
Galaxy groups in the 2dFGRS: the group-finding algorithm and the 2PIGG catalogue
The construction of a catalogue of galaxy groups from the Two-degree Field Galaxy Redshift Survey (2dFGRS) is described. Groups are identified by means of a friends-of-friends percolation algorithm which has been thoroughly tested on mock versions of the 2dFGRS generated from cosmological N-body simulations. The tests suggest that the algorithm groups all galaxies that it should be grouping, with an additional 40 per cent of interlopers. About 55 per cent of the âŒ190 000 galaxies considered are placed into groups containing at least two members of which âŒ29 000 are found. Of these, âŒ7000 contain at least four galaxies, and these groups have a median redshift of 0.11 and a median velocity dispersion of 260 km sâ1. This 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue represents the largest available homogeneous sample of galaxy groups. It is publicly available on the World Wide We
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