3,054 research outputs found
The Luminosity Function and Surface Brightness Distribution of HI Selected Galaxies
We measure the z=0 B-band optical luminosity function (LF) for galaxies
selected in a blind HI survey. The total LF of the HI selected sample is flat,
with Schechter parameters M*=-19.38_{-0.62}^{+1.02} + 5 log h mag and
alpha=-1.03_{-0.15}^{+0.25}, in good agreement with LFs of optically selected
late-type galaxies. Bivariate distribution functions of several galaxy
parameters show that the HI density in the local Universe is more widely spread
over galaxies of different size, central surface brightness, and luminosity
than is the optical luminosity density. The number density of very low surface
brightness (>24.0 mag/arcsec^2) gas-rich galaxies is considerably lower than
that found in optical surveys designed to detect dim galaxies. This suggests
that only a part of the population of LSB galaxies is gas rich and that the
rest must be gas poor. However, we show that this gas-poor population must be
cosmologically insignificant in baryon content. The contribution of gas-rich
LSB galaxies (>23.0 mag/arcsec^2) to the local cosmological gas and luminosity
density is modest (18_{-5}^{+6} and 5_{-2}^{+2} per cent respectively); their
contribution to Omega_matter is not well-determined, but probably < 11 per
cent. These values are in excellent agreement with the low redshift results
from the Hubble Deep Field.Comment: Accepted for publication in MNRAS, 12 pages 6 figure
Parsec-scale HI absorption structure in a low-redshift galaxy seen against a Compact Symmetric Object
We present global VLBI observations of the 21-cm transition of atomic
hydrogen seen in absorption against the radio source J0855+5751. The foreground
absorber (SDSS~J085519.05+575140.7) is a dwarf galaxy at = 0.026. As the
background source is heavily resolved by VLBI, the data allow us to map the
properties of the foreground HI gas with a spatial resolution of 2pc. The
absorbing gas corresponds to a single coherent structure with an extent
35pc, but we also detect significant and coherent variations, including a
change in the HI optical depth by a factor of five across a distance of
6pc. The large size of the structure provides support for the Heiles &
Troland model of the ISM, as well as its applicability to external galaxies.
The large variations in HI optical depth also suggest that caution should be
applied when interpreting measurements from radio-detected DLAs. In
addition, the distorted appearance of the background radio source is indicative
of a strong jet-cloud interaction in its host galaxy. We have measured its
redshift ( = 0.54186) using optical spectroscopy on the William Herschel
Telescope and this confirms that J0855+5751 is a FRII radio source with a
physical extent of 1kpc and supports the previous identification of this
source as a Compact Symmetric Object. These sources often show absorption
associated with the host galaxy and we suggest that both HI and OH should be
searched for in J0855+5751.Comment: 14 pages and 10 figures. Accepted for publication in MNRA
Constraints on the Space Density of Extragalactic HVCs
High Velocity Clouds (HVCs) have recently attracted renewed attention as
being long lived, massive dark matter dominated clouds of primordial
composition distributed throughout the Local Group. In this picture the HVCs
would contain a few times 10^7 M_sun of HI and would be at distances of a few
hundred kpc to 1.5 Mpc from the Local Group barycenter. If this extragalactic
interpretation of HVCs is true, similar clouds are expected in other galaxy
groups and around galaxies. We discuss the limits blind HI surveys and QSO
absorption line studies put on this proposed population of clouds.Comment: 8 pages, 2 figures, to appear in proceedings of "Mapping the Hidden
Universe", held in Guanajuato, Mexico, Feb 200
Action in cognition: the case of language
Empirical research has shown that the processing of words and sentences is accompanied by activation of the brain's motor system in language users. The degree of precision observed in this activation seems to be contingent upon (1) the meaning of a linguistic construction and (2) the depth with which readers process that construction. In addition, neurological evidence shows a correspondence between a disruption in the neural correlates of overt action and the disruption of semantic processing of language about action. These converging lines of evidence can be taken to support the hypotheses that motor processes (1) are recruited to understand language that focuses on actions and (2) contribute a unique element to conceptual representation. This article explores the role of this motor recruitment in language comprehension. It concludes that extant findings are consistent with the theorized existence of multimodal, embodied representations of the referents of words and the meaning carried by language. Further, an integrative conceptualization of âfault tolerant comprehensionâ is proposed
An HI survey of the Centaurus and Sculptor Groups - Constraints on the space density of low mass galaxies
We present results of two 21-cm HI surveys performed with the Australia
Telescope Compact Array in the nearby Centaurus A and Sculptor galaxy groups.
These surveys are sensitive to compact HI clouds and galaxies with HI masses as
low as 3E+06 Msun, and are therefore among the most sensitive extragalactic HI
surveys to date. The surveys consist of sparsely spaced pointings that sample
approximately 2% of the groups' area on the sky. We detected previously known
group members, but we found no new HI clouds or galaxies down to the
sensitivity limit of the surveys. If the HI mass function had a faint end slope
of alpha = 1.5 below M_{HI} = 10^{7.5} Msun in these groups, we would have
expected ~3 new objects. Cold dark matter theories of galaxy formation predict
the existence of a large number low mass DM sub-halos that might appear as tiny
satellites in galaxy groups. Our results support and extend similar conclusions
derived from previous HI surveys that a HI rich population of these satellites
does not exist.Comment: Accepted for publication in A&
Where is the Molecular Hydrogen in Damped Lyman-Alpha Absorbers?
We show in this paper why molecular millimeter absorption line searches in
DLAs have been unsuccessful. We use CO emission line maps of local galaxies to
derive the H2 column density distribution function f(N_H2) at z=0. We show that
it forms a natural extension to f(N_HI): the H2 distribution exceeds f(N_HI) at
N_H ~ 10^22 cm^-2 and exhibits a power law drop-off with slope ~ -2.5.
Approximately 97% of the H2 mass density rho_H2 is in systems above N_H2=10^21
cm^-2. We derive a value rho_H2 = 1.1 x 10^7 h_70 M_sun Mpc^-3, which is ~25%
the mass density of atomic hydrogen. Yet, the redshift number density of H2
above this N_H2 limit is only ~3 x 10^-4, a factor 150 lower than that for HI
in DLAs at z=0. Furthermore, we show that the median impact parameter between a
N_H2>10^21 cm^-2 absorber and the centre of the galaxy hosting the H2 gas is
only 2.5 kpc. Based on arguments related to the Schmidt law, we argue that H2
gas above this column density limit is associated with a large fraction of the
integral star formation rate density. Even allowing for an increased molecular
mass density at higher redshifts, the derived cross-sections indicate that it
is very unlikely to identify the bulk of the molecular gas in present quasar
absorption lines samples. We discuss the prospects for identifying this
molecular mass in future surveys.Comment: 6 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies
We present a statistical method for measuring the average HI spin temperature
in distant galaxies using the expected detection yields from future wide-field
21cm absorption surveys. As a demonstrative case study we consider a simulated
all-southern-sky survey of 2-h per pointing with the Australian Square
Kilometre Array Pathfinder for intervening HI absorbers at intermediate
cosmological redshifts between and . For example, if such a survey
yielded absorbers we would infer a harmonic-mean spin temperature of
K for the population of damped Lyman
(DLAs) absorbers at these redshifts, indicating that more than
per cent of the neutral gas in these systems is in a cold neutral medium (CNM).
Conversely, a lower yield of only 100 detections would imply
K and a CNM fraction less than per
cent. We propose that this method can be used to provide independent
verification of the spin temperature evolution reported in recent 21cm surveys
of known DLAs at high redshift and for measuring the spin temperature at
intermediate redshifts below , where the Lyman- line is
inaccessible using ground-based observatories. Increasingly more sensitive and
larger surveys with the Square Kilometre Array should provide stronger
statistical constraints on the average spin temperature. However, these will
ultimately be limited by the accuracy to which we can determine the HI column
density frequency distribution, the covering factor and the redshift
distribution of the background radio source population.Comment: 11 pages, 9 figures, 1 table. Proof corrected versio
The Weak Clustering of Gas-Rich Galaxies
We examine the clustering properties of HI-selected galaxies through an
analysis of the HI Parkes All-Sky Survey Catalogue (HICAT) two-point
correlation function. Various sub-samples are extracted from this catalogue to
study the overall clustering of HI-rich galaxies and its dependence on
luminosity, HI gas mass and rotational velocity. These samples cover the entire
southern sky Dec < 0 deg, containing up to 4,174 galaxies over the radial
velocity range 300-12,700 km/s. A scale length of r_0 = 3.45 +/- 0.25 Mpc/h and
slope of gamma = 1.47 +/- 0.08 is obtained for the HI-rich galaxy real-space
correlation function, making gas-rich galaxies among the most weakly clustered
objects known. HI-selected galaxies also exhibit weaker clustering than
optically selected galaxies of comparable luminosities. Good agreement is found
between our results and those of synthetic HI-rich galaxy catalogues generated
from the Millennium Run CDM simulation. Bisecting HICAT using different
parameter cuts, clustering is found to depend most strongly on rotational
velocity and luminosity, while the dependency on HI mass is marginal. Splitting
the sample around v_rot = 108 km/s, a scale length of r_0 = 2.86 +/- 0.46 Mpc/h
is found for galaxies with low rotational velocities compared to r_0 = 3.96 +/-
0.33 Mpc/h for the high rotational velocity sample.Comment: Accepted for publication in the Astrophysical Journa
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