6,766 research outputs found
Local HI: Constraints on the evolution of the HI content of the Universe
Analyses of QSO absorption lines are showing that HI content has evolved over
the redshift range z=5 to z= 0. The 21cm line measurements of the z=0 HI
content avoid several biases inherent in the absorption line technique, such as
the influence of evolving dust content in the absorbers, and will produce a
reliable measure to anchor theories of galaxy evolution. Examples of important
questions to be addressed by local HI surveys are: (1) is there a significant
population of gas-rich galaxies or intergalactic clouds that are missing from
the census of optically selected galaxies? (2) is there an adequate reservoir
of neutral gas to substantially prolong star formation at its present rate? and
(3) are there massive objects of such low HI column density that they can have
escaped detection in the ``unbiased'' HI surveys that have been conducted so
far?Comment: 8 pages; Latex; 2 encapsulated postscript figures. Presented at the
workshop `HI in the Local Universe', Sydney, May 13-15 1996. Accepted for
publication by Publications of the Astronomical Society of Australia. Also
available from
http://www.atnf.csiro.au/Publications/HI_workshop/proceedings.htm
Radio observations of Titan
Saturn and Titan have been observed at three frequencies (1420, 2695, and 8085 MHz) with the NRAO interferometer. A clear positive Titan detection has been obtained at 8085 MHz. Assuming Titan's radius is 2500 km, the radio brightness temperature (i.e. for unit emissivity) is 115 + or - 35 K
HI Observations of the starburst galaxy NGC 2146
NGC 2146 is a peculiar spiral galaxy which is currently undergoing a major
burst of star formation and is immersed in a extended HI structure that has
morphological and kinematical resemblence to a strong tidal interaction. This
paper reports aperture synthesis observations carried out in the 21cm line with
the Very Large Array (VLA - The National Radio Astronomy Observatory (NRAO) is
operated by Associated Universities, Inc. under cooperative agreement with the
National Science Foundation.) of two fields positioned to optimally cover the
HI streams to the north and south of the galaxy, along with a 300 ft total
power spectral mapping program to recover the low surface brightness extended
emission. The observations reveal elongated streams of neutral hydrogen towards
both the north and the south of the optical galaxy extending out up to 6
Holmberg radii. The streams are not in the principle plane of rotation of the
galaxy, but instead are suggestive of a tidal interaction between NGC 2146 and
a LSB companion that was destroyed by the encounter and remains undetected at
optical wavelengths. Part of the southern stream is turning back to fall into
the main galaxy, where it will create a long-lived warp in the HI disk of NGC
2146. Analysis of the trajectory of the outlying gas suggests that the closest
encounter took place about 0.8 billion years ago and that infall of debris will
continue for a similar time span.Comment: To be published in A&
HI 21cm observations of the PG1216+069 sub-DLy-alpha absorber field at z=0.00632
The Westerbork Synthesis Radio Telescope finds a weak 21cm line emission
feature at the coordinates (RA-Dec-velocity) of the sub-Damped Lyman-alpha
absorber observed at z_abs=0.00632 in the spectrum of PG1216+069. The emission
feature, WSRT-J121921+0639, lies within 30" of the quasar sightline, is
detected at 99.8% (3 sigma) confidence level, has M_HI between 5 and 15x10^6
M_solar, and has velocity spread between 20 and 60 km/s. Other HI emitters in
the field include VCC297 at a projected distance of 86/h_75 kpc and a
previously unreported HI cloud, WSRT-J121919+0624 at 112/h_75 kpc with M_HI ~
3x10^8 M_solar. The optically identified, foreground galaxy that is closest to
the quasar sightline appears to be VCC339 (~L*/25) at 29/h_75 kpc and velocity
offset 292 km/s . A low surface brightness galaxy with the HI mass of the
sub-DLA absorber WSRT-J121921+0639 would likely have m_B ~ 17, and its diffuse
optical emission would need to compete with the light of both the background
QSO and a brighter foreground star ~10" from the QSO sight line.Comment: 10 pages, 2 figures, accepted for publication in ApJLet
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
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