Australia Telescope National Facility

Science Goals

Outreach Summary:

We are using the Australia Telescope Compact Array, which is a large radio-telescope near Narrabri, NSW, Australia, to make a survey of all the radio emission from a large patch of the sky. By doing this, we will get a snapshot of thousands of galaxies, stetching in time from shortly after the Big Bang to the present day, in different stages of formation and evolution.We will then use this information, together with information from other telescopes such as the Spitzer Space Telescope, to work out how galaxies form and evolve.

Science Overview:

We are using the Australia Telescope Compact Array to image about seven square degrees surrounding the SWIRE Chandra Deep Field South and ELAIS-S1 regions, with the aim of producing the widest deep radio survey ever attempted, in fields with deep optical, infrared, and X-ray data. Our primary goal is to find out how galaxies formed and evolved through cosmic time, by penetrating the heavy dust extinction which is found in active galaxies at all redshifts, and studying the star formation activity and active galactic nuclei buried within.

Although we are only about half-way through the survey, our data are proving remarkably fruitful. For example, we have discovered a new and unexpected class of object (the Infrared-Faint Radio Sources), we have found that the radio-FIR correlation extends to low flux densities, and we have found powerful AGN-like radio objects in galaxies with star-forming SEDs.

We can break our primary goal into a number of overlapping projects as follows:

What is the evolutionary relationship between starbursts and AGN?

We know that both star-forming galaxies, and galaxies with AGN, are present at high redshifts. It is difficult to explain how the massive black holes necessary for the AGN can have formed so early in the lifetime of the universe. We also don't understand the relationship between AGN and normal galaxies - did every galaxy at some stage possess an AGN? Are AGN a cause or a consequence of galaxy formation?

Unlike the well-studied objects in the local Universe, we do not yet understand the evolution of radio sources in the early Universe. For example, in the local Universe, the mass of the massive black hole (MBH) in a galaxy is related to that of the bulge of the galaxy. We don't know whether this is true in the early Universe, nor how it is related to the star formation rate. Particularly interesting are those cases where the radio AGN lies buried within a host galaxy whose optical/infrared spectrum or SED appears to be that of a star-forming galaxy. It is likely that such sources represent an evolutionary stage in the development of AGNs. Understanding the relationship between the AGN activity and the star-forming activity in these galaxies is a primary goal of this project.

Can we find high-redshift AGNs?

Radio-loud sources like 3C273 are so strong that current radio-telescopes can observe them up to arbitratily high redshifts. Thus, the 1600 sources already detected with ATLAS almost certainly include a handful of quasars and radio galaxies at z>7. The problem is, which of the 1600 sources are they? We are addressing this using five approaches:

Does the radio-FIR correlation change with redshift or with galaxy properties?

The radio-FIR correlation is a powerful tool for measuring the star formation history of the Universe. So we really need to understand if and how it changes. Watch this space!

Can we trace the radio luminosity function to high z?

Yes. Probably. Watch this space!

Are there rare objects that are only found in a wide/deep survey like this?

We have already discovered the Infrared-Faint Radio Sources (see New Results), which were unknown and unexpected before this project, demonstrating the value of making wide-field surveys such as this. We do not know what other rare types of object may be found in our data.

This isn't really a project goal, as this question cannot be fully answered at radio wavelengths until we have the next generation radio telescopes (SKA etc). But we can lay the groundwork, and sort out the radio-FIR correlation, which will enable the SKA to tackle this job.

The global star formation rate is a measure of overall activity in the Universe, so here we are really asking how the matter in the Universe has evolved (aka the Madau diagram).

We know that the global star formation rate at z=1 was about a factor of ten (per co-moving volume) than it is now. In the 1990's, it was widely believed to have decreased monotonically from a maximum around z=1 to some low value at z~5. This is now known to have been seriously in error because the effect of extinction by dust had been underestimated. It is now thought the the global star formation rate is approximately constant, or perhaps rises slightly, from z=1 to z~4, and then decreases to z>6. But this is still very uncertain as many measures depend on correction for dust extinction.

Radio observations of star-forming activity can provide an independent way of measuring this, as the radio is unaffected by dust, and radio emission from a star-forming galaxy is an excellent measure of the star formation rate of the galaxy, provided no AGN is present.

Radio observations have their own challenges, however:

• We need to be sure that no AGN is present.

• We still need to measure a redshift, and at present that can only be done in the optical or IR

• Current surveys such as ATLAS can only see star-forming galaxies out to z~1. Thus, ATLAS itself is not going to solve the problem, but it will pioneer the techniques to enable next-generation telescopes like SKA to do it.