269 research outputs found
Future ASKAP Studies of the Local Volume
The Australian SKA Pathfinder (ASKAP) will be a powerful instrument for
performing large-scale surveys of galaxies. Its frequency range and large field
of view makes it especially useful for an all-sky survey of Local Volume
galaxies, and will probably increase the number of known galaxies closer than
10 Mpc by a factor of two and increase, by at least an order of magnitude, the
number detected in HI. Implications for our knowledge of the HI mass function
for the very faintest galaxies and for the structure and dynamics of the Local
Volume are discussed.Comment: 4 pages, 2 figures. To appear in 'Galaxies in the Local Volume'
(Sydney, July 2007), eds. B. Koribalski & H. Jerjen, Astrophys. & Space
Scienc
HI Science with the Square Kilometre Array
The Square Kilometre Array (SKA) will be a formidable instrument for the
detailed study of neutral hydrogen (HI) in external galaxies and in our own
Galaxy and Local Group. The sensitivity of the SKA, its wide receiver bands,
and the relative freedom from radio frequency interference at the SKA sites
will allow the imaging of substantial number of high-redshift galaxies in HI
for the first time. It will also allow imaging of galaxies throughout the Local
Volume at resolutions of <100 pc and detailed investigations of galaxy disks
and the transition between disks, halos and the intergalactic medium (IGM) in
the Milky Way and external galaxies. Together with deep optical and
millimetre/sub-mm imaging, this will have a profound effect on our
understanding of the formation, growth and subsequent evolution of galaxies in
different environments. This paper provides an introductory text to a series of
nine science papers describing the impact of the SKA in the field of HI and
galaxy evolution. We propose a nested set of surveys with phase 1 of the SKA
which will help tackle much of the exciting science described. Longer commensal
surveys are discussed, including an ultra-deep survey which should permit the
detection of galaxies at z=2, when the Universe was a quarter of its current
age. The full SKA will allow more detailed imaging of even more distant
galaxies, and allow cosmological and evolutionary parameters to be measured
with exquisite precision.Comment: 9 pages, 1 figure, accepted by Proceedings of Science as the HI
Overview Chapter for "Advancing Astrophysics with the Square Kilometre Array
Cosmological Forecasts for Combined and Next Generation Peculiar Velocity Surveys
Peculiar velocity surveys present a very promising route to measuring the
growth rate of large-scale structure and its scale dependence. However,
individual peculiar velocity surveys suffer from large statistical errors due
to the intrinsic scatter in the relations used to infer a galaxy's true
distance. In this context we use a Fisher Matrix formalism to investigate the
statistical benefits of combining multiple peculiar velocity surveys. We find
that for all cases we consider there is a marked improvement on constraints on
the linear growth rate . For example, the constraining power of
only a few peculiar velocity measurements is such that the addition of the
2MASS Tully-Fisher survey (containing only galaxies) to the full
redshift and peculiar velocity samples of the 6-degree Field Galaxy Survey
(containing redshifts and velocities) can improve
growth rate constraints by . Furthermore, the combination of the
future TAIPAN and WALLABY+WNSHS surveys has the potential to reach a
error on , which will place tight limits on possible extensions to
General Relativity. We then turn to look at potential systematics in growth
rate measurements that can arise due to incorrect calibration of the peculiar
velocity zero-point and from scale-dependent spatial and velocity bias. For
next generation surveys, we find that neglecting velocity bias in particular
has the potential to bias constraints on the growth rate by over , but
that an offset in the zero-point has negligible impact on the velocity power
spectrum.Comment: 24 pages, 11 figures, 7 tables. Accepted for publication in MNRA
The Column Density Distribution Function at z=0 from HI Selected Galaxies
We have measured the column density distribution function, f(N), at z=0 using
21-cm HI emission from galaxies selected from a blind HI survey. f(N) is found
to be smaller and flatter at z=0 than indicated by high-redshift measurements
of Damped Lyman-alpha (DLA) systems, consistent with the predictions of
hierarchical galaxy formation. The derived DLA number density per unit
redshift, dn/dz =0.058, is in moderate agreement with values calculated from
low-redshift QSO absorption line studies. We use two different methods to
determine the types of galaxies which contribute most to the DLA cross-section:
comparing the power law slope of f(N) to theoretical predictions and analysing
contributions to dn/dz. We find that comparison of the power law slope cannot
rule out spiral discs as the dominant galaxy type responsible for DLA systems.
Analysis of dn/dz however, is much more discriminating. We find that galaxies
with log M_HI < 9.0 make up 34% of dn/dz; Irregular and Magellanic types
contribute 25%; galaxies with surface brightness > 24 mag arcsec^{-2} account
for 22% and sub-L* galaxies contribute 45% to dn/dz. We conclude that a large
range of galaxy types give rise to DLA systems, not just large spiral galaxies
as previously speculated.Comment: 13 pages, low resolution figures in the appendix, MNRAS accepte
Local Column Density Distribution Function from HI selected galaxies
The cross-section of sky occupied by a particular neutral hydrogen column
density provides insight into the nature of Lyman-alpha absorption systems. We
have measured this column density distribution at z=0 using 21-cm HI emission
from a blind survey. A subsample of HI Parkes All Sky Survey (HIPASS) galaxies
have been imaged with the Australia Telescope Compact Array (ATCA). The
contribution of low HI mass galaxies 10^7.5 to 10^8 M_solar is compared to that
of M_star (10^10 to 10^10.5 M_solar) galaxies. We find that the column density
distribution function is dominated by low HI mass galaxies with column
densities in the range 3x10^18 to 2x10^20 cm^-2. This result is not intuitively
obvious. M_star galaxies may contain the bulk of the HI gas, but the
cross-section presented by low HI mass galaxies 10^7.5 to 10^8 M_solar is
greater at moderate column densities. This result implies that moderate column
density Lyman-alpha absorption systems may be caused by a range of galaxy types
and not just large spiral galaxies as originally thought.Comment: 5 pages, including 1 figure. To appear in "Extragalactic Gas at Low
Redshift" (ASP Conf. Series, Weymann Conf.
Bulk flow and shear in the local Universe: 2MTF and COSMICFLOWS-3
The low-order kinematic moments of galaxies, namely bulk flow and shear,
enables us to test whether theoretical models can accurately describe the
evolution of the mass density field in the nearby Universe. We use the
so-called etaMLE maximum likelihood estimator in logdistance space to measure
thesemoments from a combined sample of the 2MASS Tully-Fisher (2MTF) survey and
the COSMICFLOWS-3 (CF3) compilation. Galaxies common between 2MTF and CF3
demonstrate a small zero-point difference of -0.016+-0.002 dex.We test the
etaMLE on 16 mock 2MTF survey catalogues in order to explore how well the
etaMLE recovers the true moments, and the effect of sample anisotropy. On the
scale size of 37 Mpc/h, we find that the bulk flow of the local Universe is 259
+- 15 km/h in the direction is (l,b)=(300+-4, 23+-3) (Galactic coordinates).
The average shear amplitude is 1.7+-0.4 h km/s/Mpc. We use a variable window
function to explore the bulk and shear moments as a function of depth. In all
cases, the measurements are consistent with the predictions of the L cold dark
matter (LCDM) model.Comment: 11 pages, 10+2 figures, published in MNRAS, Oct/201
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