Hydrogen is not only the basic element in the periodic system and it is also the fundamental building block of galaxies and everything else in the Universe. A large density of hydrogen gas will trigger the formation of stars. These stars will eventually form the galaxies. Despite all the beautiful images we have from galaxies, we don't know how this gas enters into the galaxies in the first place. And we also don't know where it is coming from. Simulations predict a large and extended "Cosmic Web" of gas filaments between galaxies. Detection of this web is extremely difficult and one of the 'holy grails' of astronomy. Better understanding of the interaction between galaxies with their direct environment, will provide crucial insight in how galaxies are formed.
IMAGINE is a Legacy Project on the Australia Telescope Compact Array (ATCA) to make images of the extended region around 28 spiral galaxies in Neutral Hydrogen (HI). The survey will study how this gas is fuelling the galaxies. Furthermore it will help in understanding the interaction between galaxies and the surrounding material, the so called Inter-galactic Medium.
Imaging Galaxies Inter-galactic and Nearby Environment
The single biggest challenge in detecting neutral hydrogen gas in the Inter-galactic Medium is sensitivity. Extremely good surface brightness has to be reached in order to detect even the densest clumps of gas. Current radio telescopes can observe the inner parts of galaxies very well. Unfortunately there are not many observations beyond the edge of the galactic disk which is at a typical column density of NHI ~ 1019 cm-2. Although most observations have not typically reached column densities below this density, this is not a fundamental limitation. Long integration times with an (almost-)filled aperture can achieve the required sensitivity to permit direct imaging of faint gas between galaxies. The shown image highlights how observations of neutral hydrogen gas in galaxy NGC891 have improved over the past decades. More and more gas is being recovered when improving the sensitivity.
In this project we will use the Australia Telescope Compact Array (ATCA) to further improve the sensitivity by and order of magnitude. We will observe a sample of 28 galaxies down to a brightness sensitivity of NHI ~ 1018 cm-2. This is significantly more sensitive than other surveys and will help us to understand the link between the disk of galaxies and their extended environment.
ATCA Legacy Survey
The Australia Telescope Compact Array (ATCA) is a radio telescope consisting of an array of six identical 22-meter dishes. The inner five dishes can be moved along a three-kilometre railway track. The sixth antenna is placed three kilometres west of the rest of the array. By combining the signal of the different telescopes in aperture synthesis mode, radio images can be generated. Changing the distance between individual dishes, will result in images at a different resolution. ATCA has been operated by CSIRO Astronomy and Space Sciences (CASS) for the last decades.
Only recently a program of Legacy Projects has been implemented. This program will conduct very large scientific projects, not reproducible by any combination of smaller projects. The scope of this program is to generate data of general and lasting importance to the broad astronomy community. IMAGINE is one of two Legacy Projects that has been allocated full support and observation time.