448 research outputs found
100 deg Mock Galaxy Cone for HI Surveys with the Early SKA
We distribute an easy-to-use mock catalog of galaxies with detailed neutral
atomic hydrogen (HI) and auxiliary molecular and optical properties. The
catalog covers a field of 10-by-10 degrees and a redshift range of z=0-1.2. It
contains galaxies with 21cm peak flux densities down to 1uJy and is, within
this flux limit, complete for HI masses above 10^8 solar masses. Five random
realisations of the catalog in ASCII format (~4GB/file) and subtables with HI
flux limits of 10u Jy (~500MB/file) and 100uJy$ (~30MB/file) can be downloaded
at http://ict.icrar.org/store/staff/do/s3sax.Comment: 3 pages, 1 table, 2 figure
The Cosmic Decline in the H2/HI-Ratio in Galaxies
We use a pressure-based model for splitting cold hydrogen into its atomic
(HI) and molecular (H2)components to tackle the co-evolution of HI, H2, and
star formation rates (SFR) in ~3e7 simulated galaxies in the Millennium
simulation. The main prediction is that galaxies contained similar amounts of
HI at redshift z=1-5 than today, but substantially more H2, in quantitative
agreement with the strong molecular line emission already detected in a few
high redshift galaxies and approximately consistent with inferences from
studies of the damped Lyman-alpha absorbers seen in the spectra of quasars. The
cosmic H2/HI-ratio is predicted to evolve monotonically as Omega(H2)/Omega(HI)
(1+z)^1.6. This decline of the H2/HI-ratio as a function of cosmic time is
driven by the growth of galactic disks and the progressive reduction of the
mean cold gas pressure. Finally, a comparison between the evolutions of HI, H2,
and SFRs reveals two distinct cosmic epochs of star formation: an early epoch
(z>3), driven by the evolution of Omega(HI+H2), and a late epoch (z<3), driven
by the evolution of Omega(H2)/Omega(HI).Comment: 4 pages, 3 figure
Rebounds of deformed cavitation bubbles
Presented here are experiments clarifying how the deformation of cavitation
bubbles affects their rebound. Rebound bubbles carry the remaining energy of a
bubble following its initial collapse, which dissipates energy mainly through
shock waves, jets, and heat. The rebound bubble undergoes its own collapse,
generating such violent events anew, which can be even more damaging or
effective than at first bubble collapse. However, modeling rebound bubbles is
an ongoing challenge because of the lack of knowledge on the exact factors
affecting their formation. Here we use single-laser-induced cavitation bubbles
and deform them by variable gravity or by a neighboring free surface to
quantify the effect of bubble deformation on the rebound bubbles. Within a wide
range of deformations, the energy of the rebound bubble follows a logarithmic
increase with the bubble's initial dipole deformation, regardless of the origin
of this deformation
Evolution of the Milky Way in Semi-Analytic Models: Detecting Cold Gas at z=3 with ALMA and SKA
We forecast the abilities of the Atacama Large Millimeter/submillimeter Array
(ALMA) and the Square Kilometer Array (SKA) to detect CO and HI emission lines
in galaxies at redshift z=3. A particular focus is set on Milky Way (MW)
progenitors at z=3 for their detection within 24 h constitutes a key science
goal of ALMA. The analysis relies on a semi-analytic model, which permits the
construction of a MW progenitor sample by backtracking the cosmic history of
all simulated present-day galaxies similar to the real MW. Results: (i) ALMA
can best observe a MW at z=3 by looking at CO(3-2) emission. The probability of
detecting a random model MW at 3-sigma in 24 h using 75 km/s channels is
roughly 50%, and these odds can be increased by co-adding the CO(3-2) and
CO(4-3) lines. These lines fall into ALMA band 3, which therefore represents
the optimal choice towards MW detections at z=3. (ii) Higher CO transitions
contained in the ALMA bands geq6 will be invisible, unless the considered MW
progenitor coincidentally hosts a major starburst or an active black hole.
(iii) The high-frequency array of SKA, fitted with 28.8 GHz receivers, would be
a powerful instrument for observing CO(1-0) at z=3, able to detect nearly all
simulated MWs in 24 h. (iv) HI detections in MWs at z=3 using the low-frequency
array of SKA will be impossible in any reasonable observing time. (v) SKA will
nonetheless be a supreme ha survey instrument through its enormous
instantaneous field-of-view (FoV). A one year pointed HI survey with an assumed
FoV of 410 sqdeg would reveal at least 10^5 galaxies at z=2.95-3.05. (vi) If
the positions and redshifts of those galaxies are known from an
optical/infrared spectroscopic survey, stacking allows the detection of HI at
z=3 in less than 24 h.Comment: 14 pages, 5 figures, 5 table
A Virtual Sky with Extragalactic HI and CO Lines for the SKA and ALMA
We present a sky simulation of the atomic HI emission line and the first ten
CO rotational emission lines of molecular gas in galaxies beyond the Milky Way.
The simulated sky field has a comoving diameter of 500/h Mpc, hence the actual
field-of-view depends on the (user-defined) maximal redshift zmax; e.g. for
zmax=10, the field of view yields ~4x4 sqdeg. For all galaxies, we estimate the
line fluxes, line profiles, and angular sizes of the HI and CO emission lines.
The galaxy sample is complete for galaxies with cold hydrogen masses above 10^8
Msun. This sky simulation builds on a semi-analytic model of the cosmic
evolution of galaxies in a Lambda-cold dark matter (LCDM) cosmology. The
evolving CDM-distribution was adopted from the Millennium Simulation, an N-body
CDM-simulation in a cubic box with a side length of 500/h Mpc. This side length
limits the coherence scale of our sky simulation: it is long enough to allow
the extraction of the baryon acoustic oscillations (BAOs) in the galaxy power
spectrum, yet the position and amplitude of the first acoustic peak will be
imperfectly defined. This sky simulation is a tangible aid to the design and
operation of future telescopes, such the SKA, the LMT, and ALMA. The results
presented in this paper have been restricted to a graphical representation of
the simulated sky and fundamental dN/dz-analyzes for peak flux density limited
and total flux limited surveys of HI and CO. A key prediction is that HI will
be harder to detect at redshifts z>2 than predicted by a no-evolution model.
The future verification or falsification of this prediction will allow us to
qualify the semi-analytic models.Comment: 16 pages, 9 figures, 1 tabl
Compactness of Cold Gas in High-Redshift Galaxies
Galaxies in the early Universe were more compact and contained more molecular
gas than today. In this paper, we revisit the relation between these empirical
findings, and we quantitatively predict the cosmic evolution of the surface
densities of atomic (HI) and molecular (H2) hydrogen in regular galaxies. Our
method uses a pressure-based model for the H2/HI-ratio of the Interstellar
Medium, applied to ~3*10^7 virtual galaxies in the Millennium Simulation. We
predict that, on average, the HI-surface density of these galaxies saturates at
Sigma_HI<10 Msun/pc^2 at all redshifts (z), while H2-surface densities evolve
dramatically as Sigma_H2(1+z)^2.4. This scaling is dominated by a (1+z)^2
surface brightness scaling originating from the (1+z)^-1 size scaling of
galaxies at high z. Current measurements of Sigma_H2 at high z, derived from
CO-observations, tend to have even higher values, which can be quantitatively
explained by a selection bias towards merging systems. However, despite the
consistency between our high-z predictions and the sparse empirical data, we
emphasize that the empirical data potentially suffer from serious selection
biases and that the semi-analytic models remain in many regards uncertain. As a
case study, we investigate the cosmic evolution of simulated galaxies, which
resemble the Milky Way at z=0. We explicitly predict their HI- and
H2-distribution at z=1.5, corresponding to the CO-detected galaxy BzK-21000,
and at z=3, corresponding to the primary science goal of the Atacama Large
Millimeter/submillimeter Array (ALMA).Comment: 5 pages, 3 figures, 2 table
Gravitationally Lensed HI with MeerKAT
The SKA era is set to revolutionize our understanding of neutral hydrogen
(HI) in individual galaxies out to redshifts of z~0.8; and in the z > 6
intergalactic medium through the detection and imaging of cosmic reionization.
Direct HI number density constraints will, nonetheless, remain relatively weak
out to cosmic noon (z~2) - the epoch of peak star formation and black hole
accretion - and beyond. However, as was demonstrated from the 1990s with
molecular line observations, this can be overcome by utilising the natural
amplification afforded by strong gravitational lensing, which results in an
effective increase in integration time by the square of the total magnification
(\mu^2) for an unresolved source. Here we outline how a dedicated lensed HI
survey will leverage MeerKAT's high sensitivity, frequency coverage, large
instantaneous bandwidth, and high dynamic range imaging to enable a lasting
legacy of high-redshift HI emission detections well into the SKA era. This
survey will not only provide high-impact, rapid-turnaround MeerKAT science
commissioning results, but also unveil Milky Way-like systems towards cosmic
noon which is not possible with any other SKA precursors/pathfinders. An
ambitious lensed HI survey will therefore make a significant impact from
MeerKAT commissioning all the way through to the full SKA era, and provide a
more complete picture of the HI history of the Universe.Comment: 15 pages, 3 figures, accepted for publication, Proceedings of
Science, workshop on "MeerKAT Science: On the Pathway to the SKA", held in
Stellenbosch 25-27 May 2016. Comments welcom
Three-point phase correlations: A new measure of non-linear large-scale structure
We derive an analytical expression for a novel large-scale structure
observable: the line correlation function. The line correlation function, which
is constructed from the three-point correlation function of the phase of the
density field, is a robust statistical measure allowing the extraction of
information in the non-linear and non-Gaussian regime. We show that, in
perturbation theory, the line correlation is sensitive to the coupling kernel
F_2, which governs the non-linear gravitational evolution of the density field.
We compare our analytical expression with results from numerical simulations
and find a 1-sigma agreement for separations r<30 Mpc/h. Fitting formulae for
the power spectrum and the non-linear coupling kernel at small scales allow us
to extend our prediction into the strongly non-linear regime where we find a
1-sigma agreement with the simulations for r<2 Mpc/h. We discuss the advantages
of the line correlation relative to standard statistical measures like the
bispectrum. Unlike the latter, the line correlation is independent of the bias,
in the regime where the bias is local and linear. Furthermore, the variance of
the line correlation is independent of the Gaussian variance on the modulus of
the density field. This suggests that the line correlation can probe more
precisely the non-linear regime of gravity, with less contamination from the
power spectrum variance.Comment: 11 pages, 5 figures. v2: replacement of the low resolution
simulations, more precise quantification of the agreement with simulations,
references added. Matches published version. Our code to calculate the line
correlation is available at http://blue-shift.ch/phas
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