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Stuff that Matters: Interstellar Matter in Nearby Galaxies

Final Activity Report Summary - VLASINGS (Stuff that Matters: Interstellar Matter in Nearby Galaxies)

Galaxies are the building blocks of the Universe. They consist of billions of stars and much of what we know about them has traditionally come from observations with optical telescopes, gathering visible light. However, there is more to galaxies than meets the eye, so to speak. There is abundant material occupying the space between the stars which is referred to as the Interstellar Medium or ISM for short. In fact, it is this material in the ISM from which stars formed in the first place. The ISM consists for 90% of hydrogen, mostly in neutral atomic or molecular form. Neutral atomic Hydrogen (referred to by astronomers as HI) also emits radiation but not in the visible part of the spectrum. It rather radiates at much longer wavelengths which fall in the radio part of the electromagnetic spectrum, specifically at a wavelength of around 21 cm.

The project which was given the acronym VLASINGS aimed to use this 21-cm emission to study the characteristics of galaxies in general, and their ISM in particular. Despite modern radio telescopes having been available for the better part of 25 years, astonishingly few detailed observations existed of galaxies. The aim of THINGS, The HI Nearby Galaxy Survey, was to radically change this. We selected 34 galaxies out of a larger sample of objects which had been observed with the Spitzer Space Telescope and for which ancillary data at other wavelengths, such as near- and far-UV, optical and near-IR, and (sub-)millimeter, was already available. Care was taken to select galaxies across a range of types in order to be able to ascertain if their HI characteristics depend on Hubble type, metallicity, star formation rate (SFR), etc.

The observations were made with the Very Large Array (VLA) of the National Astronomy Radio Observatory, arguably the world’s premier radio telescope. Some 150 hours of archival material was combined with about 300 hours of new observations, pushing the angular resolution down to that obtained with the Spitzer Space telescope, and with exquisite velocity resolution of 5 km s-1 or higher. The data are of unprecedented quality and allow us to probe the structure in the ISM down to the sizes of individual atomic cloud complexes.

The analysis of the data is focusing on three broad categories:
- Star formation and star formation thresholds.
- The fine-scale structure in the ISM.
- The shape of the Dark Matter haloes.

Star formation and star formation thresholds.
There have been several studies trying to link the density in the ISM to star formation (SF). The idea being that the higher the density, the more active SF will be. But SF can only take place where the gas density exceeds a threshold level. Previous studies have been hampered severely by the lack of high resolution maps of the gas distribution and resorted to annular averaging. With THINGS we are now in a position to make a region by region analysis (where a region can be as small as an individual atomic gas cloud, or ~100 pc). Several papers are being devoted to studying these aspects. In particular, a major effort has gone into comparing half a dozen theoretical and empirical predictions for the sufficient and necessary conditions for star formation to commence, leading to the conclusion that two competing theories, one basically promoting the idea of a constant threshold, the other a threshold which is regulated by ambient pressure, give similarly good fits to the observations.

The fine-scale structure in the ISM.
Once stars have formed, the most massive ones will photo-ionise their surroundings and blow clear an area surrounding them because of their strong stellar winds. These massive stars age rapidly and explode as supernovae. These massive explosions will further affect the surrounding ISM, enlarging the pre-existing cavity blown by the stellar winds. Because massive stars tend to form in groups, a lot of explosions will take place within a short time span and within a small volume. This leads to the formation of large cavities, known as (super-)giant shells. A major project is underway to make an inventory of these HI shells and to link their occurrence to recent (past) star formation, putting our knowledge about the small-scale structure of the ISM on a much firmer footing.

The shape of the Dark Matter haloes.
Because the HI emission is a spectral line, one can use the Doppler shift to map the radial velocity of the gas emitting the 21-cm transition. This allows us to study the kinematics of our target galaxies. It has been shown that the stellar light (and radiation emitted by material between the stars) corresponds to only a minor fraction of the matter which makes up a galaxy. In fact, more than 80% of the matter in a galaxy is in a form which to date has eluded direct detection and is referred to as Dark Matter. The gas in the ISM feels the gravitational influence of this Dark Matter (or DM) and can be used as a test particle to trace its distribution. Several theory groups have proposed how the DM should be distributed within a galaxy. THINGS will allow us to put these theories to the test. A first result is that the most popular theoretical model which predicts a sharply increasing DM density profile towards the centre of a galaxy (a so-called cusp) is incorrect and that DM haloes have a flat density distribution (a core).

Several other sub-projects have been started. One such study deals with the question where exactly a galaxy runs out of (visible) material. In general, the neutral gas disk of a galaxy extends well beyond the optical (stellar) material. We seem to confirm that the there is an edge to the gas disk, most likely caused by the all-pervading extragalactic radiation field and our observations can in principle be used to estimate its strength.

Other studies deal with the detailed temperature and opacity distribution of the neutral gas, with the time delay between gas compression due to spiral density waves and the onset of star formation, and with small scale perturbations in the thin gas disks from pure circular motion around the nucleus.

In summary, THINGS is turning into the gold standard for HI studies of nearby galaxies. It will likely remain the definitive database until the next generation of radio interferometers, such as the planned Square Kilometre Telescope will come on-line, a decade from now.