Hugo van Woerden

Neutral hydrogen and star formation in the irregular galaxy NGC 2366

We present deep UBVJHK Hα images and H I maps of the irregular galaxy NGC 2366. Optically, NGC 2366 is a boxy-shaped exponential disk seen at high inclination angle. The scale length and central surface brightness of the disk are normal for late-type galaxies. Although NGC 2366 has been classified as a barred Im galaxy, we do not see any unambiguous observational signature of a bar. There is an asymmetrical extension of stars along one end of the major axis of the galaxy, and this is where the furthest star-forming regions are found, at a radius of 1.3 times the Holmberg radius. The star formation activity of the galaxy is dominated by the supergiant H II complex NGC 2363, but the global star formation rate for NGC 2366 is only moderately elevated relative to other Im galaxies. The star formation activity drops off with radius approximately as the starlight in the inner part of the galaxy but it drops faster in the outer part. There are some peculiar features of the H I distribution and kinematics. First, the integrated H I shows two ridges running parallel to the major axis that when deprojected appear as a large ring. Second, the velocity field exhibits several large-scale anomalies superposed on a rotating disk; some of these may be from a weak bar that has no inner Lindblad resonance. Third, the inclination and position angles derived from the kinematics differ from those derived from the optical and H I morphology. Fourth, there are regions in the H I of unusually high velocity dispersion, but these regions are not associated with the optical galaxy nor any obvious H I feature. Instead the velocity dispersion correlates with a deficit of H I emission in a manner suggestive of long-range, turbulent pressure equilibrium. In other respects the H I is fairly normal. The azimuthally averaged surface density of H I is comparable to that of other irregulars in the inner part of the galaxy but drops off slower and extends further in the outer parts. The H I around the star-forming complex NGC 2363 is fairly unremarkable. As in other disk galaxies, the gas in NGC 2366 is lumpy and star-forming regions are associated with these H I complexes. H II regions are found where the gas densities locally exceed 6 M☉ pc-2. This threshold is required to provide a cool phase of H I as a first step toward star formation. NGC 2366, like other irregulars, has low gas densities relative to the critical gas densities of gravitational instability models, so large-scale gravitational instabilities operate slowly or not at all. Considering the lack of shear in the optical part of this galaxy, the relative slowness of such instabilities may not be a problem—there is little competition to the slow gravitational contraction that follows energy dissipation. This differs from the situation in giant spiral disks where the shear time is short, comparable to the energy dissipation time, and strong self-gravity is required for a condensation to grow and dissipate its turbulent energy before it shears away. The subthreshold surface densities are also not unusual if they are viewed using the critical tidal density for gravitational self-binding of a rotating cloud, rather than the critical surface density from the usual disk instability condition. The peak densities in all regions of star formation are equal to the local tidal densities, giving an agreement between these two quantities that is much better than between the surface density and the critical value. Evidently the large-scale gas concentrations are all marginally bound against background galactic tidal forces. This condition for self-binding may be more fundamental than the instability condition because it is local, three-dimensional, and does not involve spiral arm generation as an intermediate step toward star formation.

Distances and Metallicities of HVCS

We review the methods for determining the distances and metallicities of high-velocity clouds. A short summary is given of several indirect methods, but we concentrate on the absorption-line method, which is based on the detection and non-detection of interstellar absorption lines in the spectra of background and foreground stars, as well as of extragalactic targets. The requirements and problems for determining both distances and metallicities are discussed, and summaries are given of current results. These include a distance bracket of 8 to 10 kpc for complex A, lower limits of ∼5 kpc for complexes C and H, and an upper limit of ∼4 kpc for part of complex M. We also find that several HVCs have low metallicity (∼0.1 to 0.2 solar for complexes A, C, WD, and cloud WW 84), while the Magellanic Stream shows near-Magellanic values, and some other HVCs (complexes M, WB, and HVC 100−7+110) have near-solar metallicity. These results clearly show that the HVC phenomenon is diverse.

Neutral hydrogen observations of the amorphous galaxy NGC 4670 at moderate spatial resolution

We present a moderate resolution H I map of the amorphous galaxy NGC 4670. Our previous lower resolution data had shown a symmetric H I distribution centered on the single supergiant H II region and highly concentrated to the center of the galaxy. We now resolve the central H I distribution into three clouds, with the eastern cloud located on the central H II region and 1.1 kpc from the westernmost cloud. Masses of the clouds are of order 10(8) M. and are associated with the intense central star formation that has produced a luminous, compact star cluster. The central gas surface density is very high at the new resolution, and the surface density drops off rapidly with radius. We determine a rotation curve for the galaxy; however, the low and uncertain value of the inclination (i similar to 28 degrees) and the possible presence of a warp make the derived rotation speed (V-R similar to 110 km s(-1)) quite uncertain. There is some correspondence between features in the H I distribution and locations of Ha loops and filaments that emanate from the central H II region.

Distribution and Motions of Atomic Hydrogen in Lenticular Galaxies

We report the results of HI observations of eleven gas-rich SO/SOa galaxies with the Westerbork Synthesis Telescope. The majority of these galaxies have most of their hydrogen outside the optical body, in annular configurations with diameters ∼2 times the optical. These outer gas rings are often clumped, incom plete, and in approximately circular motion. They may represent the remnants of primordial, often warped, gas disks; or they may have formed from gas (or dwarf galaxies) accreted recently. Optical spectra could discriminate between these possibilities.

History of HVC research — an Overview

The discovery of gas clouds at high velocity outside the disk of the Milky Way dates from the middle of the 20th century. Since their discovery, numerous new techniques and new instruments have allowed great strides in the understanding of the high-velocity cloud phenomenon. This chapter presents a review of these developments, organized by period (five sections each covering about one decade), and within each period summarizing the progress in each of several subfields, such as radio surveys, UV observations, and theories.

Diameters of HI Disks in Virgo Cluster — And Field Galaxies

Environmental effects may be of great importance to the structure and evolution of galaxies. Sullivan and collaborators (1981, AJ 86, 919) find that the ratio between HI content and luminosity of galaxies tends to be smaller in regions of high galaxy density than elsewhere. It is not quite clear what processes reduce the gas content (or enhance the luminosity!): collisions, mergers or tidal interactions of galaxies, encounters with gas clouds, stripping by ram pressure? Detailed comparison of gas distributions and motions in cluster and field galaxies may help answer this question.

The Gas Content of Early-Type Galaxies

This paper reviews recent observations of neutral hydrogen in elliptical and lenticular galaxies.

Properties of Neutral Hydrogen Clouds

In the region +19° <b< + 30°, 122° < l <143°, 21-cm line profiles were analyzed into Gaussian components, and these components combined into clouds, by means of computerized procedures. Maps of some of the clouds so defined are presented, and their properties discussed. Many clouds are elongated and/or irregular in shape. Histograms of cloud parameters are presented and subjected to statistical analysis. There is evidence for tenuous clouds of large angular extent but very low column density (N H < 1019 cm-2).

Division X, XII / Commission 40, 41 / Working Group Radio Astronomy

The IAU Working Group on Historical Radio Astronomy (WGHRA) was formed at the 2003 General Assembly of the IAU as a Joint Working Group of Commissions 40 (Radio Astronomy) and 41 (History of Astronomy), in order to: a) assemble a master list of surviving historically-significant radio telescopes and associated instrumentation found worldwide; b) document the technical specifications and scientific achievements of these instruments; c) maintain an on-going bibliography of publications on the history of radio astronomy; and d) monitor other developments relating to the history of radio astronomy (including the deaths of pioneering radio astronomers).

Division X Working Group on Historic Radio Astronomy

During the Rio General Assembly we held the following meetings of the Working Group: a Business Meeting, a Science Meeting on “The Development of Aperture Synthesis Imaging in Radio Astronomy”, and a Science Meeting on “Recent Research”.