Spiral galaxies must acquire gas to maintain their observed level of star formation beyond the next few billion years. A source of this material may be the gas that resides between galaxies, but our understanding of the state and distribution of this gas is incomplete. Radio observations of the Local Group of galaxies have revealed hydrogen gas extending from the disk of the galaxy M31 at least halfway to M33. This feature has been interpreted to be the neutral component of a condensing intergalactic filament, which would be able to fuel star formation in M31 and M33, but simulations suggest that such a feature could also result from an interaction between both galaxies within the past few billion years (ref. 5). Here we report radio observations showing that about 50 per cent of this gas is composed of clouds, with the rest distributed in an extended, diffuse component. The clouds have velocities comparable to those of M31 and M33, and have properties suggesting that they are unrelated to other Local Group objects. We conclude that the clouds are likely to be transient condensations of gas embedded in an intergalactic filament and are therefore a potential source of fuel for future star formation in M31 and M33.

Studies of binary pulsars provide insight into various theories of physics.Detection of such systems is challenging due to the Doppler modulation of the pulsed signal caused by the orbital motion of the pulsar. We investigated the loss of sensitivity in eccentric binary systems for different types of companions. This reduction of sensitivity should be considered in future population synthesis models for binary pulsars. This loss can be recovered partially by employing the 'acceleration search' algorithm and even better by using the 'acceleration-jerk search' algorithm.

Very sensitive 21cm H I measurements have been made at several locations around the Local Group galaxy M31 using the Green Bank Telescope (GBT) at an angular resolution of 9. 1, with a 5σ detection level of N H I = 3.9 × 10 17 cm −2 for a 30 km s −1 line. Most of the H I in a 12 square degree area almost equidistant between M31 and M33 is contained in nine discrete clouds that have a typical size of a few kpc and H I mass of 10 5 M . Their velocities in the Local Group Standard of Rest lie between −100 and +40 km s −1 , comparable to the systemic velocities of M31 and M33. The clouds appear to be isolated kinematically and spatially from each other. The total H I mass of all nine clouds is 1.4 × 10 6 M for an adopted distance of 800 kpc with perhaps another 0.2 × 10 6 M in smaller clouds or more diffuse emission. The H I mass of each cloud is typically three orders of magnitude less than the dynamical (virial) mass needed to bind the cloud gravitationally. Although they have the size and H I mass of dwarf galaxies, the clouds are unlikely to be part of the satellite system of the Local Group as they lack stars. To the north of M31, sensitive H I measurements on a coarse grid find emission that may be associated with an extension of the M31 high-velocity cloud population to projected distances of ∼ 100 kpc. An extension of the M31 high-velocity cloud population at a similar distance to the south-east, toward M33, is not observed.

We present a deep search for H I 21-cm emission from the gaseous halo of Messier 31 as part of Project AMIGA, a large Hubble Space Telescope program to study the circumgalactic medium of the Andromeda galaxy. Our observations with the Robert C. Byrd Green Bank Telesope target sight lines to 48 background AGNs, more than half of which have been observed in the ultraviolet with the Cosmic Origins Spectrograph, with impact parameters 25 ρ 330 kpc (0.1 ρ/R vir 1.1). We do not detect any 21-cm emission toward these AGNs to limits of N (H I) ≈ 4 × 10 17 cm −2 (5σ; per 2 kpc diameter beam). This column density corresponds to an optical depth of ∼ 2.5 at the Lyman limit; thus our observations overlap with absorption line studies of Lyman limit systems at higher redshift. Our non-detections place a limit on the covering factor of such optically-thick gas around M31 to f c < 0.051 (at 90% confidence) for ρ ≤ R vir . While individual clouds have previously been found in the region between M31 and M33, the covering factor of strongly optically-thick gas is quite small. Our upper limits on the covering factor are consistent with expectations from recent cosmological "zoom" simulations. Recent COS-Halos ultraviolet measurements of H I absorption about an ensemble of galaxies at z ≈ 0.2 show significantly higher covering factors within ρ 0.5R vir at the same N (H I), although the metal ion-to-H I ratios appear to be consistent with those seen in M31.

While bright, blue, compact galaxies are common at z ∼ 1, they are relatively rare in the local universe, and their evolutionary paths are uncertain. We have obtained resolved H I observations of nine z ∼ 0 luminous compact blue galaxies (LCBGs) using the Giant Metrewave Radio Telescope and Very Large Array in order to measure their kinematic and dynamical properties and better constrain their evolutionary possibilities. We find that the LCBGs in our sample are rotating galaxies that tend to have nearby companions, relatively high central velocity dispersions, and can have disturbed velocity fields. We calculate rotation velocities for each galaxy by measuring half of the velocity gradient along their major axes and correcting for inclination using axis ratios derived from SDSS images of each galaxy. We compare our measurements to those previously made with single dishes and find that single dish measurements tend to overestimate LCBGs' rotation velocities and H I masses. We also compare the ratio of LCBGs' rotation velocities and velocity dispersions to those of other types of galaxies and find that LCBGs are strongly rotationally supported at large radii, similar to other disk galaxies, though within their half-light radii the V rot /σ values of their H I are comparable to stellar V rot /σ values of dwarf elliptical galaxies. We find that LCBGs' disks on average are gravitationally stable, though conditions may be conducive to local gravitational instabilities at the largest radii. Such instabilities could lead to the formation of star-forming gas clumps in the disk, resulting eventually in a small central bulge or bar.