10,326 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
Moon - Optical properties of Apollo 11 samples
Optical properties of lunar powder samples from Tranquility Bas
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
- …