Gustaaf van Moorsel

A new, kinematically anomalous HI component in the spiral galaxy NGC 2403

We discuss new, high-sensitivity H I observations of the spiral galaxy NGC 2403, which show extended emission at anomalous velocities with respect to the cold disk. This anomalous gas component (~1/10 of the total H I mass) is probably located in the region of the halo and rotates more slowly (~20-50 km s-1) than the gas in the disk. Moreover, it shows a distortion in the velocity field that we interpret as a large-scale radial motion (10-20 km s-1 inflow) toward the center of the galaxy. The most likely explanation for its origin and kinematics seems to be that of a galactic fountain. There is, however, a significant part of the anomalous gas that seems to be moving contrary to rotation and is difficult to understand in such a picture. These anomalous gas complexes discovered in NGC 2403 may be analogous to the high-velocity clouds of our Galaxy. They may be rather common in spiral galaxies and not have been detected yet for lack of sensitivity.

APERTURE SYNTHESIS OBSERVATIONS OF THE NEARBY SPIRAL NGC 6503: MODELING THE THIN AND THICK H I DISKS

We present sensitive aperture synthesis observations of the nearby, late-type spiral galaxy NGC 6503, and produce H I maps of considerably higher quality than previous observations by van Moorsel and Wells. We find that the velocity field, while remarkably regular, contains clear evidence for irregularities. The H I is distributed over an area much larger than the optical image of the galaxy, with spiral features in the outer parts and localized holes within the H I distribution. The absence of absorption toward the nearby quasar 1748+700 yields an upper limit of 5 × 1017 cm–2 for the column density of cold H I gas along a line of sight which should intersect the disk at a radius of 29 kpc. This suggests that the radial extent of the H I disk is not much larger than that which we trace in H I emission (23 kpc). The observed H I distribution is inconsistent with models of a single thin or thick disk. Instead, the data require a model containing a thin disk plus a thicker low column-density H I layer that rotates more slowly than the thin disk and that extends only to approximately the optical radius. This suggests that the presence of extraplanar gas in this galaxy is largely the result of star formation in the disk rather than cold gas accretion. Improved techniques for interferometric imaging including multi-scale Clean that were used in this work are also described.

The hi halos of spiral galaxies

A deep H I survey with the VLA of the spiral galaxy NGC 2403 has revealed the existence of a thick, low density layer of neutral gas surrounding the thin ‘cold’ disk. This layer has a mean rotation velocity 25–50 km s-1 lower than that of the disk and a 10–20 km s-1inflow towards the centre of the galaxy. In the central parts there are velocity differences from rotation of up to 150 km s-1.Chandra observations of NGC 2403 show a diffuse, hot X-ray emitting gas component with a temperature of a few 106 K. These results point at galactic fountain type of flows between disk and halo. ‘Halo’ gas with similar characteristics has also been observed in other spiral galaxies(e.g. NGC 6946, NGC 891). Such gas is probably similar to the IVCs and to some of the HVCs of the Milky Way.

Software for the EVLA: an update

The Expanded Very Large Array (EVLA) project is the next generation instrument for high resolution and high sensitivity long-millimeter to short-meter wavelength radio astronomy. It is currently funded by NSF, with completion scheduled for 2012. The EVLA will upgrade the VLA with new feeds, receivers, data transmission hardware, correlator, and a new software system to enable the instrument to achieve its full potential. This software includes both that required for controlling and monitoring the instrument and that with emphasis on the scientific functions related to the telescope. The primary goals of the software are: to maximize the scientific return of the EVLA; provide ease of use, for both novices and experts; exploit commonality amongst all NRAO telescopes where possible. The software design methodology includes detailed initial use-cases and requirements from the scientists, intimate interaction between the scientists and the programmers during design and implementation, and rapid prototyping and development cycles (as short as a week). This manuscript is an update of a similar description published for the SPIE meeting two years ago; a more full description and background can be found there, while this document concentrates on the changes since then, specifically in the area of the high level design and pre-observing software (user authentication; proposal preparation, submission, and handling; observation preparation; and observation scheduling).

Software for the EVLA

The Expanded Very Large Array (EVLA) project is the next generation instrument for high resolution long-millimeter to short-meter wavelength radio astronomy. It is currently funded by NSF, with completion scheduled for 2012. The EVLA will upgrade the VLA with new feeds, receivers, data transmission hardware, correlator, and a new software system to enable the instrument to achieve its full potential. This software includes both that required for controlling and monitoring the instrument and that involved with the scientific dataflow. We concentrate here on a portion of the dataflow software, including: proposal preparation, submission, and handling; observation preparation, scheduling, and remote monitoring; data archiving; and data post-processing, including both automated (pipeline) and manual processing. The primary goals of the software are: to maximize the scientific return of the EVLA; provide ease of use, for both novices and experts; exploit commonality amongst all NRAO telescopes where possible. This last point is both a bane and a blessing: we are not at liberty to do whatever we want in the software, but on the other hand we may borrow from other projects (notably ALMA and GBT) where appropriate. The software design methodology includes detailed initial use-cases and requirements from the scientists, intimate interaction between the scientists and the programmers during design and implementation, and a thorough testing and acceptance plan.

An Automated Method for Velocity Field Analysis

This contribution concerns velocity fields of galaxies and other objects in which the kinematics can be described by a rotating disk with radial symmetry around the center. It should be emphasized, however, that the method presented here is probably also applicable in a number of different astronomical situations, but it was primarily developed for disk-like spiral galaxies.