796 research outputs found
Splotch: Visualizing Cosmological Simulations
We present a light and fast, public available, ray-tracer {\tt Splotch}
software tool which supports the effective visualization of cosmological
simulations data. We describe the algorithm it relies on, which is designed in
order to deal with point-like data, optimizing the ray-tracing calculation by
ordering the particles as a function of their ``depth'' defined as a function
of one of the coordinates or other associated parameter. Realistic
three-dimensional impressions are reached through a composition of the final
color in each pixel properly calculating emission and absorption of individual
volume elements. We describe several scientific as well as public applications
realized with {\tt Splotch}. We emphasize how different datasets and
configurations lead to remarkable different results in terms of the images and
animations. A few of these results are available online.Comment: 19 Pages, 8 Figures, to appear in New Journal of Physics, Focus Issue
on "Visualisation in Physics", edited by B. Sanders, T. Senden and V.
Springe
3D Velocity and Density Reconstructions of the Local Universe with Cosmicflows-1
This paper presents an analysis of the local peculiar velocity field based on
the Wiener Filter reconstruction method. We used our currently available
catalog of distance measurements containing 1,797 galaxies within 3000 km/s:
Cosmicflows-1. The Wiener Filter method is used to recover the full 3D peculiar
velocity field from the observed map of radial velocities and to recover the
underlying linear density field. The velocity field within a data zone of 3000
km/s is decomposed into a local component that is generated within the data
zone and a tidal one that is generated by the mass distribution outside that
zone. The tidal component is characterized by a coherent flow toward the
Norma-Hydra-Centaurus (Great Attractor) region while the local component is
dominated by a flow toward the Virgo Cluster and away from the Local Void. A
detailed analysis shows that the local flow is predominantly governed by the
Local Void and the Virgo Cluster plays a lesser role. The analysis procedure
was tested against a mock catalog. It is demonstrated that the Wiener Filter
accurately recovers the input velocity field of the mock catalog on the scale
of the extraction of distances and reasonably recovers the velocity field on
significantly larger scales. The Bayesian Wiener Filter reconstruction is
carried out within the ?CDM WMAP5 framework. The Wiener Filter reconstruction
draws particular attention to the importance of voids in proximity to our
neighborhood. The prominent structure of the Local Supercluster is wrapped in a
horseshoe collar of under density with the Local Void as a major component.Comment: Accepted for ApJ, August 6, 201
IRAS versus POTENT Density Fields on Large Scales: Biasing and Omega
The galaxy density field as extracted from the IRAS 1.2 Jy redshift survey is
compared to the mass density field as reconstructed by the POTENT method from
the Mark III catalog of peculiar velocities. The reconstruction is done with
Gaussian smoothing of radius 12 h^{-1}Mpc, and the comparison is carried out
within volumes of effective radii 31-46 h^{-1}Mpc, containing approximately
10-26 independent samples. Random and systematic errors are estimated from
multiple realizations of mock catalogs drawn from a simulation that mimics the
observed density field in the local universe. The relationship between the two
density fields is found to be consistent with gravitational instability theory
in the mildly nonlinear regime and a linear biasing relation between galaxies
and mass. We measure beta = Omega^{0.6}/b_I = 0.89 \pm 0.12 within a volume of
effective radius 40 h^{-1}Mpc, where b_I is the IRAS galaxy biasing parameter
at 12 h^{-1}Mpc. This result is only weakly dependent on the comparison volume,
suggesting that cosmic scatter is no greater than \pm 0.1. These data are thus
consistent with Omega=1 and b_I\approx 1. If b_I>0.75, as theoretical models of
biasing indicate, then Omega>0.33 at 95% confidence. A comparison with other
estimates of beta suggests scale-dependence in the biasing relation for IRAS
galaxies.Comment: 35 pages including 10 figures, AAS Latex, Submitted to The
Astrophysical Journa
Magnetic Fields in the Milky Way and in Galaxies
Most of the visible matter in the Universe is ionized, so that cosmic
magnetic fields are quite easy to generate and due to the lack of magnetic
monopoles hard to destroy. Magnetic fields have been measured in or around
practically all celestial objects, either by in-situ measurements of
spacecrafts or by the electromagnetic radiation of embedded cosmic rays, gas,
or dust. The Earth, the Sun, solar planets, stars, pulsars, the Milky Way,
nearby galaxies, more distant (radio) galaxies, quasars, and even intergalactic
space in clusters of galaxies have significant magnetic fields, and even larger
volumes of the Universe may be permeated by 'dark' magnetic fields. Information
on cosmic magnetic fields has increased enormously as the result of the rapid
development of observational methods, especially in radio astronomy. In the
Milky Way, a wealth of magnetic phenomena was discovered that are only partly
related to objects visible in other spectral ranges. The large-scale structure
of the Milky Way's magnetic field is still under debate. The available data for
external galaxies can well be explained by field amplification and ordering via
the dynamo mechanism. The measured field strengths and the similarity of field
patterns and flow patterns of the diffuse ionized gas give strong indication
that galactic magnetic fields are dynamically important. They may affect the
formation of spiral arms, outflows, and the general evolution of galaxies. In
spite of our increasing knowledge on magnetic fields, many important questions
on the origin and evolution of magnetic fields, like their first occurrence in
young galaxies, or the existence of large-scale intergalactic fields remained
unanswered. The present upgrades of existing instruments and several radio
astronomy projects have defined cosmic magnetism as one of their key science
projects.Comment: Revised version of Chapter 13 in "Planets, Stars and Stellar
Systems", Vol. 5: "Galactic Structure and Stellar Populations", ed. G.
Gilmore, Springer, Berlin 2013, ISBN 978-90-481-8817-
Overview on spectral line source finding and visualisation
Here I will outline successes and challenges for finding spectral line
sources in large data cubes that are dominated by noise. This is a 3D challenge
as the sources we wish to catalog are spread over several spatial pixels and
spectral channels. While 2D searches can be applied, e.g., channel by channel,
optimal searches take into account the 3-dimensional nature of the sources. In
this overview I will focus on HI 21-cm spectral line source detection in
extragalactic surveys, in particular HIPASS, the "HI Parkes All-Sky Survey" and
WALLABY, the "ASKAP HI All-Sky Survey". I use the original HIPASS data to
highlight the diversity of spectral signatures of galaxies and gaseous clouds,
both in emission and absorption. Among others, I report the discovery of a 680
km/s wide HI absorption trough in the megamaser galaxy NGC 5793. Issues such as
source confusion and baseline ripples, typically encountered in single-dish HI
surveys, are much reduced in interferometric HI surveys. Several large HI
emission and absorption surveys are planned for the Australian Square Kilometre
Array Pathfinder (ASKAP): here we focus on WALLABY, the 21-cm survey of the sky
(Dec < +30 degr; z < 0.26) which will take about one year of observing time
with ASKAP. Novel phased array feeds ("radio cameras") will provide 30 square
degrees instantaneous field-of-view. WALLABY is expected to detect more than
500 000 galaxies, unveil their large-scale structures and cosmological
parameters, detect their extended, low-surface brightness disks as well as gas
streams and filaments between galaxies. It is a precursor for future HI surveys
with SKA Phase I and II, exploring galaxy formation and evolution. The
compilation of highly reliable and complete source catalogs will require
sophisticated source-finding algorithms as well as accurate source
parametrisation.Comment: 14 pages, 6 figures, PASA Special Issue on "Source Finding &
Visualisation", submitte
A dynamically cold disk galaxy in the early Universe
The extreme astrophysical processes and conditions that characterize the
early Universe are expected to result in young galaxies that are dynamically
different from those observed today. This is because the strong effects
associated with galaxy mergers and supernova explosions would lead to most
young star-forming galaxies being dynamically hot, chaotic and strongly
unstable. Here we report the presence of a dynamically cold, but highly
star-forming, rotating disk in a galaxy at redshift () 4.2, when the
Universe was just 1.4 billion years old. Galaxy SPT-S J041839-4751.9 is
strongly gravitationally lensed by a foreground galaxy at , and it
is a typical dusty starburst, with global star-forming and dust properties that
are in agreement with current numerical simulations and observations of its
galaxy population. Interferometric imaging at a spatial resolution of about 60
pc reveals a ratio of rotational-to-random motions of ,
which is at least four times larger than expected from any galaxy evolution
model at this epoch, but similar to the ratios of spiral galaxies in the local
Universe. We derive a rotation curve with the typical shape of nearby massive
spiral galaxies, which demonstrates that at least some young galaxies are
dynamically akin to those observed in the local Universe, and only weakly
affected by extreme physical processes.Comment: Published in Nature on 12 August 2020. The published version is
available at http://www.nature.com/articles/s41586-020-2572-
The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter
A detailed analysis of the November 15, 2006 data release (Clowe et al.,
2006) X-ray surface density Sigma-map and the strong and weak gravitational
lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed
and the results are compared with the predictions of a modified gravity (MOG)
and dark matter. Our surface density Sigma-model is computed using a King
beta-model density, and a mass profile of the main cluster and an isothermal
temperature profile are determined by the MOG. We find that the main cluster
thermal profile is nearly isothermal. The MOG prediction of the isothermal
temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with
the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the
two-dimensional convergence kappa-map data are obtained without non-baryonic
dark matter, accounting for the 8-sigma spatial offset between the Sigma-map
and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the
kappa-map results in two baryonic components distributed across the Bullet
Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM)
gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on
average 76% dark matter (neglecting the indeterminant contribution due to the
galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a
statistically significant result when compared to the predicted Lambda-CDM
cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al.,
2006).Comment: Accepted for publication in Mon. Not. Roy. Astron. Soc. -- July 26,
2007. In press. 28 pages, 15 figures, 5 table
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