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We present the results of global VLBI observations of the BL Lac object PKS 0003-066 at 2.3 and 8.6 GHz, over a 10 year period using data from new and archived VLBA, geodetic VLBI, and global VLBI (including the use of new disk-based recording and software correlation systems) observations. Inter-continental baselines resolve this source into a compact core and three jet components that move away from the core with proper motions ranging between \~0.1 mas/yr for the outer jet component and ~1.4 mas/yr for the fastest inner jet component, i.e. apparent speeds of ~2.0 c to ~21.6 c respectively. This result is in contrast to previous studies of this source which did not reveal significant jet motions from limited data sets. We find that the fast inner jet components catch up to the more slowly moving outer jet components and interact strongly, causing local brightening of the jet in the interaction. We document the body of VLBI data for this source, as well as other supporting radio observations, as a basis for future work that will investigate the underlying physics of the pc-scale jet in PKS 0003-066., Comment: 25 pages, 13 figures, submitted to ApJ Supplement Series

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 ApJLett

We have detected the four 18cm OH lines from the $z \sim 0.765$ gravitational lens toward PMN J0134-0931. The 1612 and 1720 MHz lines are in conjugate absorption and emission, providing a laboratory to test the evolution of fundamental constants over a large lookback time. We compare the HI and OH main line absorption redshifts of the different components in the $z \sim 0.765$ absorber and the $z \sim 0.685$ lens toward B0218+357 to place stringent constraints on changes in $F \equiv g_p [\alpha^2/\mu]^{1.57}$. We obtain $[\Delta F/F] = (0.44 \pm 0.36^{\rm stat} \pm 1.0^{\rm syst}) \times 10^{-5}$, consistent with no evolution over the redshift range $0 < z < 0.7$. The measurements have a $2 \sigma$ sensitivity of $[\Delta \alpha/\alpha] < 6.7 \times 10^{-6}$ or $[\Delta \mu/\mu] < 1.4 \times 10^{-5}$ to fractional changes in $\alpha$ and $\mu$ over a period of $\sim 6.5$ Gyr, half the age of the Universe. These are among the most sensitive current constraints on changes in $\mu$., Comment: 4 pages, 3 figures. Final version, with minor changes to match the version in print in Phys. Rev. Lett

A comprehensive analysis of 355 high-quality WSRT HI 21-cm line maps of nearby galaxies shows that the properties and incident rate of Damped Lyman-alpha (DLA) absorption systems observed in the spectra of high redshift QSOs are in good agreement with DLAs originating in gas disks of galaxies like those in the z~0 population. Comparison of low-z DLA statistics with the HI incidence rate and column density distribution f(N) for the local galaxy sample shows no evidence for evolution in the integral "cross section density" below z~1.5, implying that there is no need for a hidden population of galaxies or HI clouds to contribute significantly to the DLA cross section. Compared with z~4, our data indicates evolution of a factor of two in the comoving density along a line of sight. We find that dN/dz(z=0)=0.045 +/- 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, are in good agreement with what is expected from local galaxies. Approximately 87% of low z DLA galaxies are expected to be fainter than L* and 37 per cent have impact parameters less than 1'' at z=0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow up surveys. The well-known metallicity-luminosity relation in galaxies, in combination with metallicity gradients in galaxy disks, cause the expected median metallicity of low redshift DLAs to be low (~1/7 solar), which is also in good agreement with observations of low z DLAs. (Abridged), Comment: 22 pages, 22 figures. Accepted for publication in MNRAS. Fixed typos

We calculate in detail the expected properties of low redshift DLA systems under the assumption that they arise in the gaseous disks of galaxies like those in the z=0 population. A sample of 355 nearby galaxies is analysed, for which high quality HI 21-cm emission line maps are available as part of an extensive survey with the Westerbork telescope (WHISP). We find that expected luminosities, impact parameters between quasars and DLA host galaxies, and metal abundances are in good agreement with the observed properties of DLAs and DLA galaxies. The measured redshift number density of z=0 gas above the DLA limit is dN/dz=0.045 +/- 0.006, which compared to higher redshift measurements implies that there is no evolution in the comoving density of DLAs along a line of sight between z=1.5 and z=0, and a decrease of only a factor of two from z=4 to the present time. We conclude that the local galaxy population can explain all properties of low redshift DLAs., Comment: 6 pages, 2 figures. To appear in "Island Universes - Structure and Evolution of Disk Galaxies", Terschelling (Netherlands), July 2005

This overview provides an interface between lines of thought on rfi mitigation in the fields of radio astronomy and signal processing. The goal is to explore the commonality of different approaches to help researchers in both fields interpret each others' concepts and jargon. The paper elaborates on the astronomers' concept of gain closure relations and how they may be used in a "self-calibrating" system of rfi cancellation. Further discussion of the eigen decomposition method in terms of rfi power and antenna gains introduces adaptive nulling and rfi cancellation through matrix partitioning. While multipath scattering appears at first glance to be fatal to methods of rfi cancellation, its effect is easily incorporated in frequency dependent gain coefficients under many circumstances., Comment: Radio Science, in press. 11 pages, 9 figures

Neutral intergalactic clouds are so greatly out numbered by galaxies that their integral HI content is negligible in comparison to that contained in optically luminous galaxies. In fact, no HI cloud that is not associated with a galaxy or grouping of galaxies has yet been identified. This points to a causal relationship that relies on gravitational potentials that bind galaxies also being responsible for confining HI clouds to sufficient density that they can become self-shielding to the ionizing background radiation. Unconfined clouds of low density become ionized, but confined clouds find themselves vulnerable to instability and collapse, leading to star formation., Comment: IAU Symp 217, July 2003, invited review

We report on our experiences using the Metsahovi Radio Observatory's (MRO) VLBI Standard Interface (VSI, Whitney 2002) recorder in a number of astronomical applications. The PC-EVN device is a direct memory access (DMA) interface which allows 512 megabit per second (Mbps) or better recording to "off the shelf" PC components. We have used this setup to record at 640 Mbps for a pulsar coherent dispersion system and at 256 Mpbs for a global VLBI session. We have also demonstrated recording at 512 Mbps and will shortly form cross correlations between the CPSR-II and the PC-EVN systems., Comment: 4 pages. 3 figures. Proceedings of the 7th European VLBI Network Symposium held in Toledo, Spain on October 12-15, 2004. Editors: R. Bachiller, F. Colomer, J.-F. Desmurs, P. de Vicente (Observatorio Astronomico Nacional), p. 253-256. Needs evn2004.cls

The epoch of reionization (EoR) sets a fundamental benchmark in cosmic structure formation, corresponding to the formation of the first luminous objects that act to ionize the neutral intergalactic medium (IGM). Recent observations at near-IR and radio wavelengths imply that we are finally probing into this key epoch of galaxy formation at $z \ge 6$. The Square Kilometer Array (SKA) will provide critical insight into the EoR in a number of ways. First, the ability of the SKA to image the neutral IGM in 21cm emission is a truly unique probe of the process of reionization, and is recognized as the next necessary and fundamental step in our study of the evolution of large scale structure and cosmic reionization. Second, study of HI 21cm absorption toward the first radio loud objects probes small to intermediate scale structure in the neutral 'cosmic web', as well as HI in the first collapsed structures (proto-disks and mini-halos). And third, the incomparable sensitivity of the SKA allows for the study of the molecular gas, dust, and star formation activity in the first galaxies, as well as the radio continuum emission from the first accreting massive black holes. Such objects will be obscured at optical wavelengths due to absorption by the neutral IGM., Comment: espcrc2.sty, 12 pages. to appear in: "Science with the Square Kilometer Array," New Astronomy Reviews, eds. C. Carilli, S. Rawlings (Elsevier: Amsterdam)

HI 21~cm absorption spectroscopy provides an excellent probe of the neutral gas content of absorbing galaxies, yielding information on their kinematics, mass, physical size and ISM conditions. The high sensitivity, unrivaled frequency coverage and RFI suppression techniques of the SKA will enable it to use HI absorption to study the ISM of high column density intervening systems along thousands of lines of sight out to high redshifts. Blind SKA 21~cm surveys will yield large, unbiased absorber samples, tracing the evolution of normal galaxies and active galactic nuclei from $z \gtrsim 6$ to the present epoch. It will thus be possible to directly measure the physical size and mass of typical galaxies as a function of redshift and, hence, to test hierarchical models of structure formation., Comment: 12 pages, 7 figures. Uses espcrc2.sty. To appear in "Science with the Square Kilometer Array," eds. C. Carilli and S. Rawlings, New Astronomy Reviews (Elsevier: Amsterdam)

We have used the Westerbork Synthesis Radio Telescope to detect HI 21cm absorption at $z \sim 0.7645$ in the gravitational lens system towards PMN J0134-0931. The 21cm profile has two broad components, with peak optical depths of $0.047 \pm 0.007$ and $0.039 \pm 0.007$, at heliocentric redshifts $0.76470 \pm 0.00006$ and $0.76348 \pm 0.00006$, respectively. The redshift of the stronger component matches that of CaII H and K absorption detected earlier. The absorption has a total velocity width of $\sim 500$ km/s (between nulls) and an equivalent width of $7.1 \pm 0.08$ km/s. This would imply a total HI column density of $2.6 \pm 0.3 \times 10^{21}$ per cm$^2$, for a spin temperature of 200 K and a covering factor of unity. The high estimated HI column density is consistent with the presence of large amounts of dust at the lens redshift; the intervening dust could be responsible for the extremely red colour of the background quasar., Comment: 4 pages, 4 figures. Accepted for publication in A&A Letters

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&A

Westerbork Radio Synthesis Telescope observation of the galaxy VV29=Arp188=UGC10214 shows that there are at least three distinct dynamical components whose kinematics can be traced in 21cm line emission. The system appears to be the result of a galaxy-galaxy interaction. We identify a sufficient number of dynamical elements containing baryons (stars and neutral gas) that there is no compelling reason to postulate the presence of an additional dark matter halo that is devoid of detectable baryons. The central galaxy VV29a is massive (V_rot = 330 km/s) and gas rich (M_HI} = 6x10^9 Msolar). The distinctive optical plume (VV29b), which extends eastward from the main galaxy, is also gas rich (M_HI = 3x10^9 Msolar) and has a very low gradient in line of sight velocity (<30 km/s) over 70kpc. On the western side, there is an HI feature of M_HI = 4x10^8 Msolar that participates strongly in orbital motion about the host in the same sense of rotation as the VV29a itself. A blue, less massive, gas-rich galaxy "VV29c" (M_HI = 9x10^8 Msolar) appears clearly in the HI maps as an 170 km/s wide spectral feature, seen in projection against or, more likely, behind the west side of the host disk. Its high recessional velocity is counter to the host rotation direction. The optical images of Trentham et al (2001) show signs of this blue dwarf against the redder VV29a disk. The companion galaxy CGCG27-021=MGC09-26-54 (at projected distance 115 kpc) is not detected in 21cm line emission (M_HI<10^9Msolar)., Comment: 7 pages, 13 figures, accepted for publication in Astronomy and Astrophysics

As part of a larger program to identify low redshift radio analogues of the damped Lyman-alpha (DLA) absorbers seen in the spectra of high redshift quasars, Westerbork Synthesis Radio Telescope (WSRT) observations have discovered two new HI 21cm absorption lines at z=0.394 and z=0.437 in the spectra of the radio sources B 0248+430 and B 1243-072 respectively. These sightlines and redshifts were selected for study on the basis of the previously known low ionization absorption lines of MgII, and neither has been observed in the Lyman-alpha line. The 21cm line observations provide information on column densities, temperatures and kinematics of the thickest cold neutral clouds in the absorbers., Comment: 6 pages incl. 2 figures, accepted for publication in the Astrophysical Journal Letters, missing reference added

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 figures

Westerbork Radio Synthesis Telescope observations of the redshifted 21cm line absorber against the z_em = 0.871 double lobed quasar 3C196 show that the intervening absorber is an L approximately equal L* spiral galaxy (3C196-G1) and that the absorbing layer of cold gas extends to radii of at least 30 h_50^-1 kpc. The new data solve several long standing puzzles about this system by (1) discovering a second 21cm absorption feature, corresponding to absorption against the NE lobe of the background radio source and (2) spatially ``resolving'' the two absorption features to isolate the absorption along the two lines of sight to the opposing radio lobes. These findings resolve the disagreement in redshift between the UV metal and 21cm lines, and as well as demonstrating that the neutral layer does absorb both lobes of the background radio source. Simple kinematic models with an inclined, rotating gas disk match the observed 21cm profile and are also compatible with both the redshift and velocity spread of the absorption measured in UV resonance lines along a third, independent line of sight to the quasar nucleus and with the lack of 21cm absorption in as earlier VLBI experiment that was sensitive to opacity against the hot spot in the northern lobe. The inferred rotation speed and luminosity for the galaxy are compatible with the z approximately 0 Tully-Fisher Relation. This system illustrates well how 21cm absorption against extended background radio sources is a powerful tool in determining the nature of the damped Lyman-alpha class of QSO absorption line system., Comment: 9 pages, accepted at A&A

We present measurements of absorption by the 21cm hyperfine transition of neutral hydrogen toward radio sources at substantial look-back times. These data are used in combination with observations of rotational transitions of common interstellar molecules, to set limits on the evolution of the fine structure constant: (dot alpha)/alpha < 3.5e-15/yr to a look-back time of 4.8 Gyr. The neutral hydrogen observations employed Very Long Baseline Interferometry in order to mitigate the substantial uncertainty arising from the fact that observations at very different wavelengths may probe different lines-of-site due to frequency dependent structure of the background source. We discuss the implication of these results on theories unifying natural forces based on compact quantum dimensions. In the context of string theory, the limit on the secular evolution of the scale factor of these compact dimensions, R, is (dot R)/R < 1e-15/yr. Including terrestrial and other astronomical measurements places limits (2sigma) on slow oscillations of R from the present to the epoch of cosmic nucleosynthesis, just seconds after the big bang, of (Delta R)/R < 1e-5., Comment: Physical Review Letters 85, 2511, fixed typo. fixed typo 2

The growing level of radio frequency interference (RFI) is a recognized problem for research in radio astronomy. This paper describes an intuitive but powerful RFI cancellation technique that is suitable for radio spectroscopy where time-averages are recorded. An RFI ``reference signal,'' is constructed from the cross power spectrum of the signals from the two polarizations of a reference horn pointed at the source of the RFI signal. The RFI signal paths obey simple phase and amplitude closure relations, which allows computation of the RFI contamination in the astronomical data and the corrections to be applied to the astronomical spectra. Since the method is immune to the effects of multipath scattering in both the astronomy and reference signal channels, ``clean copies'' of the RFI signal are not required. The method could be generalized (1) to interferometer arrays, (2) to correct for scattered solar radiation that causes spectral ``standing waves'' in single-dish spectroscopy, and (3) to pulsar survey and timing applications where a digital correlator plays an important role in broadband pulse dedispersion. Future large radio telescopes, such as the proposed LOFAR and SKA arrays, will require a high degree of RFI suppression and could implement the technique proposed here with the benefit of faster electronics, greater digital precision and higher data rates., Comment: 15 pages, 13 figures, accepted at AJ

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&A

Neutral hydrogen traces gravitational potentials. In the nearby universe, 21cm emission-line surveys show that the bulk of the HI resides in well-formed, optically-luminous galaxies. At high redshift, 21cm line absorption against background radio quasars occurs in gas-rich systems identified with the highest HI column densities -- the ``damped Lyman alpha'' quasar absorption-line systems. High spatial-resolution observations of the redshifted 21cm line absorbers measure sizes and kinematics of the neutral absorbers., Comment: 8 pages, 2 figures. Invited review to appear in Proceedings of the XIXth Recontres de Moriond, on "Building the Galaxies: from the Primordial Universe to the Present," Editors F. Hammer, T.X. Thuan, V. Cayatte, B. Guiderdoni and J. Tran Thanh Van, Editions Frontieres

The Arecibo HI Strip Survey probed the halos of ~300 cataloged galaxies and the environments of ~14 groups with sensitivity to neutral hydrogen masses > 10^7 M_sun. The survey detected no objects with properties resembling the High Velocity Clouds (HVCs) associated with the Milky Way or Local Group. If the HVCs were typically M_HI=10^{7.5} M_sun objects distributed throughout groups and galaxy halos at distances of 1 Mpc, the survey should have made ~70 HVC detections in groups and ~250 detections around galaxies. The null detection implies that HVCs are deployed at typical distances of <200 kpc from the galaxies or group barycenters. If the clouds are in virial equilibrium, their average dark matter fraction must be 98% or higher., Comment: Accepted for publication in ApJL. 7 pages, 2 figures

We present a detailed study of the HI 21cm absorption system at z=0.0912 towards the radio quasar B0738+313. The uncommonly narrow main absorption line and weak secondary line are resolved for the first time. In addition we find it necessary to add a third, broader shallow component to obtain a good fit to the spectrum. Although the harmonic mean spin temperature calculated by comparison of the 21cm lines to the damped Ly-alpha line is T_s = 775 K, the thermal kinetic temperatures of the two narrow components, calculated from their widths, are much lower: T_k \leq 297 and \leq 103 K respectively. This is the first case of a redshifted absorption system for which T_k is measured to be less than T_s. We discuss this result in the context of a two phase gas model, in which the damped Ly-alpha gas is sensitive to a significant neutral column density of warm phase gas as well as the cold phase gas of the narrow 21cm lines. The third component is interpreted as representing the warm phase gas with with T_k \leq 5050 K. The combined column density of the three 21cm components is approximately equal to that derived from fits to the damped Ly-alpha line., Comment: 15 pages, incl. 4 figures, accepted for publication in ApJ

A Square Kilometer Array radio telescope will detect tens of thousands of galaxies per square degree in the 21cm emission line of neutral hydrogen. The telescope will be sensitive to ordinary galaxy populations at redshifts z>3 when the mass density in neutral gas greatly exceeded the mass in luminous stars. Spectroscopy in the HI line will trace the kinematics of evolving galactic potentials, as well as monitoring consumption of the fuel for star formation. The OH megamaser population may be a significant indicator of merger and interaction rates for galaxies over cosmic time. Spectroscopy of the redshifted HI and OH lines provides a major incentive for designing a large radio telescope with continuous frequency coverage below 2 GHz., Comment: 13 pages, 6 figures, to appear in "Radio Astronomy: Scientific Imperatives at cm and m Wavelengths" (Dwingeloo: NFRA), Edited by: M.P. van Haarlem & J.M. van der Hulst

A radio spectroscopic driftscan survey in the 21cm line with the Nancay Radio Telescope of 0.08 steradians of sky in the direction of the constellation Canes Venatici covering a heliocentric velocity range of -350 < V_hel < 2350 km/s produced 53 spectral features, which was further reduced to a sample of 33 reliably detected galaxies by extensive follow-up observations. With a typical noise level of rms = 10 mJy after Hanning smoothing, the survey is -- depending on where the detections are located with regard to the center of the beam -- sensitive to M(HI)=1-2x10^8 Msun at 23 Mpc and to M(HI)=4-8x10^7 Msun throughout the CVn Groups (Ho=100). The survey region had been previously examined on deep optical plates by Binggeli et al. 1990 and contains loose groups with many gas-rich galaxies as well as voids. No galaxies that had not been previously identified in these deep optical surveys were uncovered in our HI survey, neither in the groups nor the voids. The implication is that no substantial quantity of neutral hydrogen contained in gas-rich galaxies has been missed in these well-studied groups. All late-type members of our sample are listed in the Fisher and Tully (1981b) optically selected sample of nearby late-type galaxies; the only system not contained in Fisher and Tullys' Catalog is the S0 galaxy NGC 4203. Within the well--sampled CVn group volume with distances corrected for flow motions, the HI mass function is best fitted with the Zwaan et al. (1997) HI mass function (alpha=-1.2) scaled by a factor of f=4.5 in account of the locally overdense region., Comment: Accepted for publication in A&A, 18 pages, 6 tables and 7 encapsulated ps-figures (Fig.1 in reduced format only)

Surveys in the 21cm line of neutral hydrogen are testing the completeness of the catalogs of nearby galaxies. The remarkable observational fact is that the potential wells that confine gas to sufficient density that it can remain neutral in the face of ionizing radiation also provide sites for star formation, so that there are no known cases of neutral intergalactic clouds without associated star light., Comment: 8 pages, 2 figures, LaTeX. To Appear in the proceeding of the IAU colloquium 171, "The Low Surface Brightness Universe", held in July 1998 in Cardiff, U.K; Typo in Eq. 2 corrected

The present generation of radio telescopes, combined with powerful new spectrometers, is opening a new age of redshifted radio absorption-line studies. Outfitting of arrays of antennas, such as the European VLBI Network and the upgraded VLA, with flexibly tuned receivers, will measure sizes and kinematics of intervening galaxies as a function of cosmic time., Comment: 12 pages, 6 figures, to appear in "Highly redshifted radio lines," eds. Carilli, Radford, Menten, and Langston (San Francisco: ASP Conference Series)

We report the discovery of two low redshift HI 21cm absorbers, one at z = 0.2212 towards the z_{em} = 0.630 quasar OI 363 (B0738+313), and the other at z = 0.3127 towards PKS B1127-145 (z_{em} = 1.187). Both were found during a survey of MgII selected systems at redshifts 0.2 < z < 1 using the new UHF-high system at the Westerbork Synthesis Radio Telescope (WSRT). New HST/FOS observations also identify both systems as damped Ly-alpha (DLa) absorbers. By comparing the column density from the DLa line with that from the HI 21cm line, we calculate the spin temperature, and find that T_s is approximately 1000 K for both of these low redshift absorbers. We briefly discuss some implications of this result., Comment: 10 pages incl. 4 eps figures, accepted by AJ

Bright OH megamaser galaxies, radiating 1667/1665 MHz lines, could be detected at redshifts from z from 1 to 3 in moderate integration times with existing radio telescopes. The superluminous FIR galaxies that host the megamasers are relatively rare at z equal 0, but they may have been more common at high redshift, if the galaxy merger rate increases steeply with redshift. Therefore, blind radio spectroscopic surveys at frequencies of 400 to 1000 Mhz can form an independent test of the galaxy merger rate as a function of time over the redshift interval z = 4 to 0.7. The redshift range z = 0.17 to 0.4 will be difficult to survey for OH masers, since spectroscopic survey signals will be confused with HI emission from normal galaxies at redshifts less than 0.3. In fact, the signals from OH masers are likely to dominate over 21cm line emission from normal galaxies at frequencies below 1200 MHz (i.e. large redshifts z_HI > 0.18 and z_OH > 0.4). Surveyors of nearby galaxies in the 21cm line may find that OH masers form a contaminant to deep, blind HI surveys for redshift velocities less than a few hundred kilometers per second. At frequencies just above 1420 MHz, sensitive sky surveys might detect OH masers, which could be mistaken for a population of ``infalling, compact High Velocity Clouds'' but would ultimately be traced to luminous FIR background galaxies at z equal 0.17 once optical and IR follow-up has been performed., Comment: 9 pages, 8 figures, latex, accepted for publication in Astronomy and Astrophysics with minor enhancements

The Fisher-Tully `10 Mpc Catalog of Late-Type Galaxies' (FT, 1981) is remarkably complete. Despite the considerable effort that has been spent searching for and cataloging low surface brightness (LSB) galaxies, almost no new HI-rich galaxies have been added to the volume to which the FT observations were both sensitive (d < 10 (M_{HI}/10^{8.2} Msolar)^{5/12} Mpc) and intended to be complete (d < 10 Mpc). It has not yet been demonstrated by the new surveys that HI-rich LSB galaxies are not properly represented in this catalog of nearby galaxies. Although new optical surveys are discovering heretofore uncatalogued galaxies at greater distances than the depth of the FT Catalog, they do not alter the completeness of the FT Catalog for HI-rich LSB objects. Thus, unless the galaxy population of the local volume is atypical, there is at present no evidence for a significant, new population of gas-rich galaxies that has escaped recognition until now., Comment: 10 pages, 1 figure, 1 table, latex. To appear in ApJL

The space density of low surface brightness and tiny gas-rich dwarf galaxies are estimated for two recent catalogs: The Arecibo Survey of Northern Dwarf and Low Surface Brightness Galaxies (Schneider, Thuan, Magri & Wadiak 1990) and The Catalog of Low Surface Brightness Galaxy, List II (Schombert, Bothun, Schneider & McGaugh 1992). The goals are (1) to evaluate the additions to the completeness of the Fisher and Tully (1981) 10 Mpc Sample and (2) to estimate whether the density of galaxies contained in the new catalogs adds a significant amount of neutral gas mass to the the inventory of HI already identified in the nearby, present-epoch universe. Although tiny dwarf galaxies (M_HI < ~10^7 solar masses) may be the most abundant type of extragalactic stellar system in the nearby Universe, if the new catalogs are representative, the LSB and dwarf populations they contain make only a small addition (<10%) to the total HI content of the local Universe and probably constitute even smaller fractions of its luminous and dynamical mass., Comment: 27 pages; LaTex; 8 encapsulated postscript figures; 1 table. Accepted for publication in The Astrophysical Journal

Radio spectroscopy at 323 MHz using the Arecibo Telescope and the VLA has produced a tentative detection of the 21cm line of neutral hydrogen at Z_abs=3.38716+-0.00007 in absorption against the radio continuum of the QSO MG0201+113 (Z_em = 3.61). This redshift roughly agrees with one determined by the Westerbork Synthesis Radio Telescope and reported earlier; however the observations at different telescopes produce different results for width and optical depth of the line. If the detection holds, it provides evidence for a high column density of neutral gas that is confined to a dynamically cold layer with velocity dispersion ~10 km/s. Although the interpretation is uncertain due to a lack of detailed knowledge of the extended radio structure of the background quasar and the relative quantities of neutral gas in the cold and turbulent components, the observations specify high spin temperatures, T_s > 1000 K for both this 21cm line absorbing cloud and the turbulent component, which together have N_HI ~ 10^21.4 cm^-2. Results of optical spectroscopy require the additional presence of metal-enriched clouds of still broader velocity dispersion than the 21cm line, creating a picture which is consistent with this system being a young disk galaxy that is sufficiently evolved by z=3.4 to have collapsed to a flattened system and produced a population of stars that have aged to pollute a turbulent halo. The observations constrain the neutral gas mass of a possible 21cm line emitter associated with the intervening absorber to be less than ~(Delta V/200 km/s) 10^13 Msolar for velocity widths Delta V (FWHM) in the range 200 to 1200 km/s (H_o=100 km/s/Mpc, Omega_o=1)., Comment: 15 pages; Latex; 5 encapsulated postscript figures; 1 table; accepted for publication in Astronomical Journal

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.html

Driftscan methods are highly efficient, stable techniques for conducting extragalactic surveys in the 21cm line of neutral hydrogen. Holding the telescope still while the beam scans the sky at the sidereal rate produces exceptionally stable spectral baselines, increased stability for RFI signals, and excellent diagnostic information about system performance. Data can be processed naturally and efficiently by grouping long sequences of spectra into an image format, thereby allowing thousands of individual spectra to be calibrated, inspected and manipulated as a single data structure with standard tools that already exist in astronomical software. The behavior of spectral standing waves (multi-path effects) can be appraised and excised in this environment, making observations possible while the Sun is up. The method is illustrated with survey data from Arecibo and Nancay., Comment: 7 pages; Latex; 1 encapsulated postscript figure; 4 jpeg images; 1 gif image. 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. Complete manuscript with encapsulated figures is available at http://www.atnf.csiro.au/Publications/HI_workshop/proceedings.html

Results from the Arecibo HI Strip Survey, an unbiased extragalactic HI survey, combined with optical and 21cm follow-up observations, determine the HI Mass Function and the cosmological mass density of HI at the present epoch. Both are consistent with earlier estimates, computed for the population of optically selected galaxies. This consistency occurs because, although the distribution of optical central surface brightnesses among galaxies is flat, we fail to find a population of galaxies with central surface brightnesses fainter than 24 B-mag/arcsec^2, even though there is no observational selection against them., Comment: 5 pages, including 3 encapsulated postscript figures. Presented at the workshop `HI in the Local Universe', Sydney, May 13-15 1996. Accepted for publication by PASA. Also available from http://www.atnf.csiro.au/Publications/HI_workshop/proceedings.html