Gas in absorption versus gas in emission

Gas in galaxies is the fuel for the formation of stars. In the local Universe we can study gas in galaxies in detail by using radio telescopes that are sensitive to emission lines such as those of neutral hydrogen at a wavelength of 21 cm and of carbon-monoxide at 3 mm. Unfortunately, these lines are very weak and cannot be detected - with present technology - in normal galaxies in the earlier universe. This high redshift gas can only be observed in absorption against bright background quasars. One of my aims is to reconcile the observations of gas seen in emission and absorption. 

Neutral hydrogen gas surveys in the local universe

Blind surveys for neutral hydrogen (HI) in the local universe  provide us with a unique view of the local galaxy population. These surveys,

conducted with telescopes such as Arecibo and Parkes, result in catalogues of galaxies purely selected on their gas content. As the HI 21-cm line does not suffer from extinction effects that hamper optical surveys, such catalogues give a clear view of the 3-dimensional distribution and clustering properties of galaxies in the local universe. Furthermore, galaxies that optically faint but gas rich, are easily identified in these surveys while they could be easily missed in optical surveys.

High velocity clouds as building blocks

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Star formation in gas-rich galaxies

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Properties of Damped Ly α absorbers

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The Tully-Fisher relation

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Reconciling the Local Galaxy Population with Damped Ly α Cross Sections and Metal Abundances

Where is the Molecular Hydrogen in Damped Ly α  Absorbers?

Selected recent presentations

Evolution in the neutral gas - Invited review at “Astronomy in the next decade: synergies with the SKA”, Bonn, April 2008

Cold gas in galaxies and QSO absorption lines - invited colloquium Bologna, March 2008

What we learn from comparing DLAs with 21-cm-selected galaxies - contributed talk at “The evolution of galaxies through the HI window”, Arecibo, February 2008

The local HI Mass function - invited talk “Gas accretion and star for-mation”, Garching, September 2007

ALMA and APEX -  invited talk “The Astro-physical Science Cases at Dome C”, Potsdam, September 2007

Comparing gas in galaxies and DLAs - invited talk “HI Survival through Cosmic Times”, Sarteano, Italy, June 2007

The HIPASS Catalogue: II - Completeness, Reliability and Parameter Accuracy

How much gas in the local universe

These blind 21-cm surveys can be used to measure exactly how much neutral hydrogen (HI) there is in the local universe and how it is distributed over galaxies of different masses.

This diagram shows the HI mass function, the number density of galaxies as a function of their HI mass. The results from the HI Parkes All Sky Survey (HIPASS) were used to derive this diagram. A summation over all masses gives the total HI mass density, which turns out to be only a factor of two lower than that at a time when the Universe was just a quarter of its present age.

The HI mass function of galaxies from a deep survey in the 21-cm line

The HIPASS Catalogue: II - Data Presentation

The HIPASS Catalogue: ΩHI and Environmental Effects on the HI Mass Function of Galaxies

The 1000 Brightest HIPASS Galaxies: HI Mass Function and ΩHI

Last update: 26 July 2006 15:34