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

Neutral hydrogen at the present epoch: A constraint on the evolution of high redshift systems

Damped Lyman-alpha and metal absorption lines in the spectra of quasars indicate the presence of intervening gas-rich systems at high redshift (z greater than 2). These systems have characteristic size scales, velocity dispersions, and neutral hydrogen column densities (N(H1)) similar to present day spirals and are thus thought to be their progenitors. Constraints on galaxy evolution can be derived by comparing the H1 properties of high redshift systems to the present galaxy population. Good observational statistics on high redshift absorbers specify the number of these systems along the line of sight as a function of N(H1), the column density of neutral hydrogen per absorber. Similar statistics for nearby (z = 0) galaxies of which spirals are the only gas-rich systems that provide a significant cross-section for the interception of light from quasars is derived