Tobias Westmeier

Westerbork H I observations of high-velocity clouds near M 31 and M 33

We have undertaken high-resolution follow-up of a sample of high velocity H  clouds apparently associated with M 31. Our sample was chosen from the population of high-velocity clouds (HVCs) detected out to 50 kpc projected radius of the Andromeda Galaxy by Thilker et al. (2004, ApJ, 601, L39) with the Green Bank Telescope. Nine pointings were observed with the Westerbork Synthesis Radio Telescope to determine the physical parameters of these objects and to find clues to their origin. One additional pointing was directed at a similar object near M 33. At 2 � resolution we detect 16 individual HVCs around M 31 and 1 HVC near M 33 with typical H  masses of a few times 10 5 Mand sizes of the order of 1 kpc. Estimates of the dynamical and virial masses of some of the HVCs indicate that they are likely gravitationally dominated by additional mass components such as dark matter or ionised gas. Twelve of the clouds are concentrated in an area of only 1 ◦ × 1 ◦ at a projected separation of less than 15 kpc from the disk of M 31. This HVC complex has a rather complicated morphological and kinematical structure and partly overlaps with the giant stellar stream of M 31, suggesting a tidal origin. Another detected feature is in close proximity, in both position and velocity, with NGC 205, perhaps also indicative of tidal processes. Other HVCs in our survey are isolated and might represent primordial, dark-matter dominated clouds.

The Australian Square Kilometre Array Pathfinder: System Architecture and Specifications of the Boolardy Engineering Test Array

This paper describes the system architecture of a newly constructed radio telescope - the Boolardy Engineering Test Array, which is a prototype of the Australian Square Kilometre Array Pathfinder telescope. Phased array feed technology is used to form multiple simultaneous beams per antenna, providing astronomers with unprecedented survey speed. The test array described here is a 6-antenna interferometer, fitted with prototype signal processing hardware capable of forming at least 9 dual-polarisation beams simultaneously, allowing several square degrees to be imaged in a single pointed observation. The main purpose of the test array is to develop beamforming and wide-field calibration methods for use with the full telescope, but it will also be capable of limited early science demonstrations.

Gas and dark matter in the Sculptor group: NGC 300

We used the Australia Telescope Compact Array to map a large field of approximately 2° x 2° around the Sculptor group galaxy NGC 300 in the 21-cm line emission of neutral hydrogen. We achieved a 5σ H I column density sensitivity of 10 19 cm -2 over a spectral channel width of 8 km s -1 for emission filling the 180 x 88 arcsec -1 synthesized beam. The corresponding H I mass sensitivity is 1.2 x 10 5 M ⊙ , assuming a distance of 1.9 Mpc. For the first time, the vast H I disc of NGC 300 has been mapped over its entire extent at a moderately high spatial resolution of about 1 kpc. NGC 300 is characterized by a dense inner H I disc, well aligned with the optical disc of 290° orientation angle, and an extended outer H I disc with a major axis of more than 1 ° on the sky (equivalent to a diameter of about 35 kpc) and a different orientation angle of 332°. A significant fraction (about 43 per cent) of the total detected H I mass of 1.5 × 10 9 M ⊙ resides within the extended outer disc. We fitted a tilted ring model to the velocity field of NGC 300 to derive the rotation curve out to a radius of 18.4 kpc, almost twice the range of previous rotation curve studies. The rotation curve rises to a maximum velocity of almost 100 km s -1 and then gently decreases again in the outer disc beyond a radius of about 10 kpc. Mass models fitted to the derived rotation curve yield good fits for Burkert and NFW dark matter halo models, whereas pseudo-isothermal halo models and Modified Newtonian Dynamics (MOND)-based models both struggle to cope with the declining rotation curve. We also observe significant asymmetries in the outer H I disc of NGC 300, in particular near the edge of the disc, which are possibly due to ram-pressure stripping of gas by the intergalactic medium (IGM) of the Sculptor group. Our estimates show that ram-pressure stripping can occur under reasonable assumptions on the density of the IGM and the relative velocity of NGC 300. The asymmetries in the gas disc suggest a proper motion of NGC 300 towards the south-east. At the same time, our data exclude IGM densities of significantly higher than 10 -5 cm -3 in the vicinity of NGC 300, as otherwise the outer gas disc would have been stripped.

Ca II and Na I absorption signatures from extraplanar gas in the halo of the Milky Way

Aims. We analyse absorption characteristics and physical conditions of extraplanar intermediate- and high-velocity gas to study the distribution of the neutral and weakly ionised Milky Way halo gas and its relevance for the evolution of the Milky Way and other spiral galaxies. Methods. We combine optical absorption line measurements of Ca ii/Na i and 21 cm emission line observations of H i along 103 extragalactic lines of sight towards quasars (QSOs) and active galactic nuclei (AGN). The archival optical spectra were obtained with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope, while the 21 cm H i observations were carried out using the 100-m radio telescope at Effelsberg. Results. The analysis of the UVES spectra shows that single and multi-component Ca ii/Na i absorbers at intermediate and high velocities are present in about 35 percent of the sight lines, indicating the presence of neutral extraplanar gas structures. In some cases the Ca ii/Na i absorption is connected with H i 21 cm intermediate- or high-velocity gas with H i column densities in the range of 10 18 to 10 20 cm −2 (i.e., the classical IVCs and HVCs), while other Ca ii/Na i absorbers show no associated H i emission. The observed H i line widths vary from ΔvFWHM = 3. 2k m s −1 to 32. 0k m s −1 indicating a range of upper gas temperature limits of 250 K up to about 22 500 K. Conclusions. Our study suggests that the Milky Way halo is filled with a large number of neutral gaseous structures whose high column density tail represents the population of common H i high-velocity clouds seen in 21 cm surveys. The Ca ii column density distribution follows a power-law f (N) = CN β with a slope of β ≈− 1.6, thus comparable to the distribution found for intervening metal-line systems toward QSOs. Many of the statistical and physical properties of the Ca ii absorbers resemble those of strong (Wλ2796 > 0. 3A ) Mgii absorbing systems observed in the circumgalactic environment of other galaxies, suggesting that both absorber populations may be closely related.

Effelsberg HI observations of compact high-velocity clouds

We have mapped 11 compact high-velocity clouds (CHVCs) in the 21-cm line emission of neutral, atomic hydrogen, using the Effelsberg 100-m radio telescope. The aim of our observations was to study the overall distribution of the warm neutral medium of CHVCs with high sensitivity. The achieved baseline rms of σrms ≈ 50 mK at the original 2. 6k m s −1 velocity resolution allows us to search for evidence of ram-pressure interaction with the ambient medium. In addition, we have obtained spectra along an appropriate axis across each CHVC with longer integration times and denser angular sampling. These deep slices with σrms ≈ 25 ... 35 mK allow us to determine the column density profile in greater detail as well as the velocity and line width gradient across each cloud. The most outstanding result of our observations is the complexity of the H  column density distribution and the line profiles of the investigated CHVCs. We have found only one cloud with a spherically-symmetric appearance. Among the remaining clouds we observe head-tail structures, bow-shock shapes, and objects with irregular shapes. These complex morphologies in combination with the obtained physical parameters suggest that ram-pressure interactions with an ambient medium may play a significant role in shaping some of the CHVCs from our sample. These results are consistent with a circumgalactic distribution of CHVCs with typical distances of the order of 100 kpc. The pressure of the ambient medium might also stabilise CHVCs in addition to their own gravitational potential.

Comparison of potential ASKAP Hi survey source finders

The large size of the ASKAP Hi surveys DINGO and WALLABY necessitates automated 3D source finding. A performance difference of a few percent corresponds to a significant number of galaxies being detected or undetected. As such, the performance of the automated source finding is of paramount importance to both of these surveys. We have analysed the performance of various source finders to determine which will allow us to meet our survey goals during the DINGO and WALLABY design studies. Here we present a comparison of the performance of five different methods of automated source finding. These source finders are duchamp, gamma-finder, a cnhi finder, a 2d–1d wavelet reconstruction finder and a sigma clipping method (s+c finder). Each source finder was applied to the same three-dimensional data cubes containing (a) point sources with a Gaussian velocity profile and (b) spatially extended model-galaxies with inclinations and rotation profiles. We focus on the completeness and reliability of each algorithm when comparing the performance of the different source finders.

Low-column density gas clumps in the halo of the Milky Way

We report on the detection of low-column density neutral hydrogen clumps in the halo of the Milky Way. Using high-resolution (FWHM ∼ 7 km s -1 ) optical spectra obtained with the VLT/UVES spectrograph we detect narrow interstellar absorption by Ca II and Na I at high radial velocities near u LSR -150 km s -1 toward the quasar PKS 1448-232 (/ = 335.4, b = +31.7). Follow-up Hi 21 cm observations with the VIA unveil the presence of a complex of small neutral hydrogen clumps with H 1 column densities <8 × 10 18 cm -2 . The measured H line widths imply that the gas is relatively cold with temperatures T ≤ 900 K. Although the line of sight towards PKS 1448-232 does not pass immediately through a known large high-velocity cloud (HVC). the sky position and the measured radial velocities suggest that these clumps arc associated with HVC cloud complex L. An inspection of other UVES quasar spectra shows that weak. narrow Ca II absorption at high velocities is a common phenomenon, even in directions where high-velocity H 1 21 cm emission is not detected. This suggests that the Milky Way halo contains a large number of high-velocity neutral gas clumps with low H I column densities. If such clumps are typical for halos of spiral galaxies, they should contribute significantly to the population of strong Mg II absorbers and Lyman-Limit Systems (LLS) seen in the circumgalactic environment of other galaxies.

SOFIA: A flexible source finder for 3D spectral line data

We introduce SOFIA, a flexible software application for the detection and parametrization of sources in 3D spectral line data sets. SOFIA combines for the first time in a single piece of software a set of new source-finding and parametrization algorithms developed on the way to future H I surveys with ASKAP (WALLABY, DINGO) and APERTIF. It is designed to enable the general use of these new algorithms by the community on a broad range of data sets. The key advantages of SOFIA are the ability to: search for line emission on multiple scales to detect 3D sources in a complete and reliable way, taking into account noise level variations and the presence of artefacts in a data cube; estimate the reliability of individual detections; look for signal in arbitrarily large data cubes using a catalogue of 3D coordinates as a prior; provide a wide range of source parameters and output products which facilitate further analysis by the user. We highlight the modularity of SOFIA, which makes it a flexible package allowing users to select and apply only the algorithms useful for their data and science questions. This modularity makes it also possible to easily expand SOFIA in order to include additional methods as they become available. The full SOFIA distribution, including a dedicated graphical user interface, is publicly available for download.

ASKAP HI imaging of the galaxy group IC 1459

We present HI imaging of the galaxy group IC 1459 carried out with six antennas of the Australian SKA Pathfinder equipped with phased-array feeds. We detect and resolve HI in eleven galaxies down to a column density of

GASKAP-The Galactic ASKAP Survey

\sim10^{20}