57 research outputs found
Locally Cold Flows from Large-Scale Structure
We show that the "cold" Hubble flow observed for galaxies around the Milky
Way does not represent a problem in cosmology but is due to the particular
geometry and dynamics of our local wall. The behavior of the perturbed Hubble
flow around the Milky Way is the result of two main factors: at small scales (R
< 1 Mpc) the inflow is dominated by the gravitational influence of the Milky
Way. At large scales (R > 1 Mpc) the out flow reflects the expansion of our
local wall which "cools down" the peculiar velocities. This is an intrinsic
property of walls and is independent of cosmology. We find the dispersion of
the local Hubble flow (1 < R < 3 Mpc) around simulated "Milky Way" haloes
located at the centre of low-density cosmological walls to be {\sigma}_H ~ 30
km/s, in excellent agreement with observations. The expansion of our local wall
is also reflected in the value of the measured local Hubble constant. For
"Milky Way" haloes inside walls, we find super-Hubble flows with h_local \simeq
0.77 - 1.13. The radius of equilibrium (R_0) depends not only on the mass of
the central halo and the Hubble expansion but also on the dynamics given by the
local LSS geometry. The super-Hubble flow inside our local wall has the effect
of reducing the radius at which the local expansion balances the gravitational
influence of the Milky Way. By ignoring the dynamical effect of the local wall,
the mass of the Milky Way estimated from R_0 can be underestimated by as much
as ~ 30%.Comment: 5 pages, 3 figures, Submitted to MNRA
Unfolding the Hierarchy of Voids
We present a framework for the hierarchical identification and
characterization of voids based on the Watershed Void Finder. The Hierarchical
Void Finder is based on a generalization of the scale space of a density field
invoked in order to trace the hierarchical nature and structure of cosmological
voids. At each level of the hierarchy, the watershed transform is used to
identify the voids at that particular scale. By identifying the overlapping
regions between watershed basins in adjacent levels, the hierarchical void tree
is constructed. Applications on a hierarchical Voronoi model and on a set of
cosmological simulations illustrate its potential.Comment: 5 pages, 2 figure
Polar disk galaxy found in wall between voids
We have found an isolated polar disk galaxy in what appears to be a
cosmological wall situated between two voids. This void galaxy is unique as its
polar disk was discovered serendipitously in an HI survey of SDSS void
galaxies, with no optical counterpart to the HI polar disk. Yet the HI mass in
the disk is comparable to the stellar mass in the galaxy. This suggests slow
accretion of the HI material at a relatively recent time. There is also a hint
of a warp in the outer parts of the HI disk. The central, stellar disk appears
relatively blue, with faint near UV emission, and is oriented (roughly)
parallel to the surrounding wall, implying gas accretion from out of the voids.
The considerable gas mass and apparent lack of stars in the polar disk, coupled
with the general underdensity of the environment, supports recent theories of
cold flow accretion as an alternate formation mechanism for polar disk
galaxies.Comment: 5 pages, 5 figures, accepted for publication in ApJ Letter
Spin alignment of dark matter haloes in filaments and walls
The MMF technique is used to segment the cosmic web as seen in a cosmological
N-body simulation into wall-like and filament-like structures. We find that the
spins and shapes of dark matter haloes are significantly correlated with each
other and with the orientation of their host structures. The shape orientation
is such that the halo minor axes tend to lie perpendicular to the host
structure, be it a wall or filament. The orientation of the halo spin vector is
mass dependent. Low mass haloes in walls and filaments have a tendency to have
their spins oriented within the parent structure, while higher mass haloes in
filaments have spins that tend to lie perpendicular to the parent structure.Comment: 4 pages, 2 figure
The cosmic web for density perturbations of various scales
We follow the evolution of galaxy systems in numerical simulation. Our goal
is to understand the role of density perturbations of various scales in the
formation and evolution of the cosmic web. We perform numerical simulations
with the full power spectrum of perturbations, and with spectrum cut at long
wavelengths. Additionally, we have one model, where we cut the intermediate
waves. We analyze the density field and study the void sizes and density field
clusters in different models. Our analysis shows that the fine structure
(groups and clusters of galaxies) is created by small-scale density
perturbations of scale \Mpc. Filaments of galaxies and clusters are
created by perturbations of intermediate scale from to \Mpc,
superclusters of galaxies by larger perturbations. We conclude that the scale
of the pattern of the cosmic web is determined by density perturbations of
scale up to \Mpc. Larger perturbations do not change the pattern of
the web, but modulate the richness of galaxy systems, and make voids emptier.
The stop of the increase of the scale of the pattern of the cosmic web with
increasing scale of density perturbations can probably be explained as the
freezing of the web at redshift .Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
The Void Galaxy Survey: Optical Properties and H I Morphology and Kinematics
We have carefully selected a sample of 60 galaxies that reside in the deepest
underdensities of geometrically identified voids within the SDSS. HI imaging of
55 galaxies with the WSRT reveals morphological and kinematic signatures of
ongoing interactions and gas accretion. We probe a total volume of 485 Mpc^3
within the voids, with an angular resolution of 8 kpc at an average distance of
85 Mpc. We reach column density sensitivities of 5 x 10^19 cm^-2, corresponding
to an HI mass limit of 3 x 10^8 M_sun. We detect HI in 41 galaxies, with total
masses ranging from 1.7 x 10^8 to 5.5 x 10^9 M_sun. The upper limits on the 14
non-detections are not inconsistent with their luminosities, given their
expected HI mass to light ratios. We find that the void galaxies are generally
gas rich, low luminosity, blue disk galaxies, with optical and HI properties
that are not unusual for their luminosity and morphology. The sample spans a
range of absolute magnitudes (-16.1 > M_r > -20.4) and colors (0.06 < g-r <
0.87), and includes disk and irregular galaxies. We also identify three as
early type galaxies, all of which are not detected in HI. All galaxies have
stellar masses less than 3 x 10^10 M_sun, and many have kinematic and
morphological signs of ongoing gas accretion, suggesting that the void galaxy
population is still in the process of assembling. The small scale clustering in
the void, within 600 kpc and 200 km/s, is similar to that in higher density
regions, and we identify 18 HI rich neighboring galaxies in the voids. Most are
within 100 kpc and 100 km/s of the targeted galaxy, and we find no significant
population of HI rich low luminosity galaxies filling the voids, contrary to
what is predicted by simulations.Comment: 34 pages, 33 figures (including Atlas in Appendix), accepted for
publication in A
The Multiscale Morphology Filter: Identifying and Extracting Spatial Patterns in the Galaxy Distribution
We present here a new method, MMF, for automatically segmenting cosmic
structure into its basic components: clusters, filaments, and walls.
Importantly, the segmentation is scale independent, so all structures are
identified without prejudice as to their size or shape. The method is ideally
suited for extracting catalogues of clusters, walls, and filaments from samples
of galaxies in redshift surveys or from particles in cosmological N-body
simulations: it makes no prior assumptions about the scale or shape of the
structures.}Comment: Replacement with higher resolution figures. 28 pages, 17 figures. For
Full Resolution Version see:
http://www.astro.rug.nl/~weygaert/tim1publication/miguelmmf.pd
Only the Lonely: H I Imaging of Void Galaxies
Void galaxies, residing within the deepest underdensities of the Cosmic Web,
present an ideal population for the study of galaxy formation and evolution in
an environment undisturbed by the complex processes modifying galaxies in
clusters and groups, as well as provide an observational test for theories of
cosmological structure formation. We have completed a pilot survey for the HI
imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in
HI in local (d < 100 Mpc) voids. HI masses range from 3.5 x 10^8 to 3.8 x 10^9
M_sun, with one nondetection with an upper limit of 2.1 x 10^8 M_sun. Our
galaxies were selected using a structural and geometric technique to produce a
sample that is purely environmentally selected and uniformly represents the
void galaxy population. In addition, we use a powerful new backend of the
Westerbork Synthesis Radio Telescope that allows us to probe a large volume
around each targeted galaxy, simultaneously providing an environmentally
constrained sample of fore- and background control sample of galaxies while
still resolving individual galaxy kinematics and detecting faint companions in
HI. This small sample makes up a surprisingly interesting collection of
perturbed and interacting galaxies, all with small stellar disks. Four galaxies
have significantly perturbed HI disks, five have previously unidentified
companions at distances ranging from 50 to 200 kpc, two are in interacting
systems, and one was found to have a polar HI disk. Our initial findings
suggest void galaxies are a gas-rich, dynamic population which present evidence
of ongoing gas accretion, major and minor interactions, and filamentary
alignment despite the surrounding underdense environment.Comment: 53 pages, 18 figures, accepted for publication in AJ. High resolution
available at http://www.astro.columbia.edu/~keejo/kreckel2010.pd
The Void Galaxy Survey
The Void Galaxy Survey (VGS) is a multi-wavelength program to study 60
void galaxies. Each has been selected from the deepest interior regions of
identified voids in the SDSS redshift survey on the basis of a unique geometric
technique, with no a prior selection of intrinsic properties of the void
galaxies. The project intends to study in detail the gas content, star
formation history and stellar content, as well as kinematics and dynamics of
void galaxies and their companions in a broad sample of void environments. It
involves the HI imaging of the gas distribution in each of the VGS galaxies.
Amongst its most tantalizing findings is the possible evidence for cold gas
accretion in some of the most interesting objects, amongst which are a polar
ring galaxy and a filamentary configuration of void galaxies. Here we shortly
describe the scope of the VGS and the results of the full analysis of the pilot
sample of 15 void galaxies.Comment: 9 pages, 6 figures. This is an extended version of a paper to appear
in "Environment and the Formation of Galaxies: 30 years later", Proceedings
of Symposium 2 of JENAM 2010, eds. I. Ferreras, A. Pasquali, ASSP, Springer.
Version with highres figures at
http://www.astro.rug.nl/~weygaert/vgs_jenam_weygaert.col.pd
The Hierarchical Structure and Dynamics of Voids
Contrary to the common view voids have very complex internal structure and
dynamics. Here we show how the hierarchy of structures in the density field
inside voids is reflected by a similar hierarchy of structures in the velocity
field. Voids defined by dense filaments and clusters can de described as simple
expanding domains with coherent flows everywhere except at their boundaries. At
scales smaller that the void radius the velocity field breaks into expanding
sub-domains corresponding to sub- voids. These sub-domains break into even
smaller sub-sub domains at smaller scales resulting in a nesting hierarchy of
locally expanding domains. The ratio between the magnitude of the velocity
field responsible for the expansion of the void and the velocity field defining
the sub voids is approximately one order of magnitude. The small-scale
components of the velocity field play a minor role in the shaping of the voids
but they define the local dynamics directly affecting the processes of galaxy
formation and evolution. The super-Hubble expansion inside voids makes them
cosmic magnifiers by stretching their internal primordial density fluctuations
allowing us to probe the small scales in the primordial density field. Voids
also act like time machines by "freezing" the development of the medium-scale
density fluctuations responsible for the formation of the tenuous web of
structures seen connecting proto galaxies in computer simulations. As a result
of this freezing haloes in voids can remain "connected" to this tenuous web
until the present time. This may have an important effect in the formation and
evolution of galaxies in voids by providing an efficient gas accretion
mechanism via coherent low-velocity streams that can keep a steady inflow of
matter for extended periods of time.Comment: High-res version are related media here:
http://skysrv.pha.jhu.edu/~miguel/Papers/Hierarchy_voids/index.htm
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