The HI Nearby Galaxy Survey (THINGS) project
(Brinks, Bagetakos, Portas, Usero)
Figure 1a: A composite of the atomic hydrogen distribution of all THINGS galaxies. The images shown are reproduced to have the same linear resolution so that their HI morphologies can be compared directly. |
Elias Brinks is one of three leading members of the international THINGS collaboration. THINGS, short for "The HI Nearby Galaxy Survey", is a large program at the NRAO VLA to perform 21-cm HI observations of the highest quality ( <7'', <5 km s-1 resolution) of nearby galaxies. The goal of THINGS is to investigate key characteristics related to galaxy morphology, star formation and mass distribution across the Hubble sequence. A sample of 34 objects with distances between 3 and 10 Mpc have been observed, covering a wide range of properties.
Figure 1b: VLA HI data (blue in all panels) overlaid with various combinations of Spitzer Space Telescope near- and mid-infrared maps, highlighting mainly old stars (3.6µm), PAH emission (8 µm) and warm dust (24 µm), respectively (for colour coding, see legend in each panel). |
THINGS is designed to complement SINGS, the Spitzer Infrared Nearby Galaxies Survey. For the THINGS sample, high-quality observations at comparable resolution will thus be available from the X-ray regime through to the radio part of the spectrum. THINGS data can be used to investigate issues such as the small-scale structure of the ISM, its three-dimensional structure, the (dark) matter distribution and processes leading to star formation. Figures 1a and 1b give an impression of the enormous wealth of information that has been assembled and an indication how combining THINGS with SINGS will lead to new insights into the structure of the ISM, its energy balance, and the relation and feedback mechanisms that exist between star formation and the interstellar medium.
In what follows a presentation is given of the sub-projects which being undertaken by Brinks and his collaborators at the University of Hertfordshire.
Structure in the neutral ISM
One of the key scientific drivers for embarking on the THINGS project is the desire to understand the fine scale structure of the interstellar medium (ISM) and how this varies as a function of galaxy (Hubble) type, metallicity, star formation rate, etc. Previous studies have shown that massive stars, via their stellar winds and as a result of them exploding subsequently as a supernova, can shape their immediate surrounding. Because massive stars form usually in clusters, these effects accumulate and large, coherent structures can form in the ISM. These structures, in principle expanding bubbles of coronal gas, reveal themselves in 21-cm maps of neutral, atomic gas as expanding shells or holes. One of the aims of THINGS is to relate porosity of the ISM as traced by the HI structures to current and past star formation, to determine the energy balance in the ISM, to link this to the observed velocity dispersion and characteristics of turbulence in spiral galaxies, and lastly how swept up material in these expanding shells can trigger subsequent generations of star formation. Figure 2 is an illustration of the rich structure in the form of HI shells and bubbles in the ISM of two THINGS targets, NGC 6946 and NGC 3184.
Figure 2: Comparison of VLA HI (red) and Spitzer Space Telescope 8 µ maps (green) showing a stunning correlation between the structure in the ISM seen at these two different wavelengths. |
PhD thesis: Ioannis Bagetakos (October 2006 - September 2009)
Supershell induced star formation
THINGS clearly demonstrates that the ISM is dominated by giant shells and supershells, ranging in size from the resolution limit to up to kpc size. The most widely accepted view on how these shells form is that the combined effects of stellar winds and supernovae originating in an OB association or super star cluster (SSC) deposit vast amounts of energies within a small volume over a short time interval resulting in a pressure-driven, expanding shell. GMCs can form within the accumulation front of shells and supershells. From recent CO maps (LMC, SMC, IC10) there is now growing evidence that GMCs do indeed form on the expanding rims of shells, or within the interface where two expanding shells run into each other (see Figure 3).
IRAM Plateau de Bure observations have been allocated to study a few small areas in some THINGS targets in detail in CO to investigate the relative location of GMCs with respect to the HI shells and supershells.
Figure 3: CO column density map (blue) overlaid on a VLA HI map (red). The CO traces cold, molecular hydrogen, i.e. regions which are likely to collapse under self-gravity and produce stars. Notice how those regions seem to concentrate on the rims of HI shells. |
Postdoctoral Fellowship: Antonio Usero (October 2006 - September 2009)
The outer edges of galaxies
Spiral galaxies are thought of as having a well-defined boundary. Observations at 21-cm wavelength of the atomic neutral hydrogen gas in the disks reveal two important facts: the HI disks extend far beyond their optical counterparts and they are sharply truncated. In the literature this has been ascribed to the extragalactic radiation field acting as an ionising agent of the outermost reaches of a neutral gas disk.
As part of his MSc thesis, AP calibrated and mapped 6 galaxies for which ultra-deep (>20hr) VLA D-array data were available in the archive and analysed data for 2 of them, NGC3198 and NGC1517. A preliminary analysis of THINGS data of NGC3198 seems to show that THINGS maps are sensitive enough, i.e., the detection threshold falls below the canonical 1019 at cm-2 below which the extragalactic radiation field starts to ionise a substantial fraction of the HI disks. This is illustrated in Figure 4.
Figure 4: Left: Integrated HI surface density map of NGC 3198 overlaid with neutral HI column densities contours extending to very low levels of 4 x 1018 and 5 x 1019 atoms cm−2. Right: HI column density profile along the north semi-major axis at 10" resolution, based on the new THINGS data, as compared to the older low resolution VLA data (solid line with circles). |
PhD thesis: Antonio Portas (October 2006 - September 2009)
Extraplanar HI
The study of extraplanar gas has in recent years moved away from dealing with a purely local phenomenon, mainly related to High Velocity Clouds (HVCs) belonging to the Galaxy, to something much more general, essentially due to increases in the resolution and sensitivity of telescopes (VLA, GBT, WSRT, GMRT) as well as to their innovative use.
HVC-like HI clouds and extraplanar gas (the distinction is often semantic) has been found in a variety of (nearby-ish) galaxies, e.g. throughout the circumgalactic environment of M31 and in the form of extraplanar gas in NGC253, NGC891 and NGC2403. Various origins have been proposed for gas at the "wrong" place and/or "wrong" (or rather, unexpected) velocity, among them: a galactic fountain, gas stripped and then captured from dIrr companions, infall of tidal material after a major interaction, like the Magellanic Stream, and accretion from the intergalactic medium or from a possible population of dark-matter dominated mini-halos.
It is proposed to search for and characterise this extraplanar gas in galaxies of the THINGS sample.
PhD thesis: Valeria Buenrostro (October 2005 - September 2008)