The Atlas Project

Introduction: Why compile an Atlas of DRAGNs?

Most of what we know about DRAGNs has come from interpretation of images made with radio telescopes. For many purposes it is important to have good images of a sample of DRAGNs selected in a well defined way, so that it is clear what biases have been imposed on the DRAGN population as a whole (there are always some biases!). The simplest type is the complete sample, which contains all the DRAGNs brighter than a given flux density in a certain region of sky. The best-studied complete sample consists of the brightest DRAGNs in the accessible regions of the northern hemisphere, and was originally based on the 3CR catalogue (though it has been revised several times since; see the history of the sample). In this Atlas we present high-quality images of the nearest half of this sample, together with other data either derived from the images or taken from the astronomical literature. (Most of the data from the literature was collected by Laing & Riley, although we give the original references).

We have put the Atlas together because modern radio images of the 3C sample are scattered over many papers, making it very difficult to use for systematic analysis. This is in striking contrast to weaker samples (e.g. the B2 nearby galaxies, Fanti et al. 1987; high-redshift quasars, Barthel et al. 1988, the deep B3 survey, Vigotti et al. 1989). In combination with the 3C sample, these surveys provide a wide range in redshift, linear size, and radio power, and by quantifying the variation of structure with such parameters we should be able to understand better both the dynamics and the cosmological evolution of DRAGNs.

The Atlas images

The images in the Atlas have been chosen to given a good overview of the total structure of each DRAGN. We hoped to obtain images meeting the following criteria: (See the note on image quality).

Because of the second criterion, we could not always use the highest resolution image available, since these often fail to detect the faintest regions. At present a few of our images fall marginally below these requirements, and there are also a few for which we are still finalising the image. Check the status page for the latest details. In a few cases (e.g. the Steep-spectrum cores), we could not capture all the important structure in a single image, and so we present two images at different resolutions.

In most cases the best images satisfying our criteria are at frequencies near 1.5 GHz (20 cm wavelength). We have tried to make the Atlas more uniform by selecting data taken in this waveband when we had a choice, but we include images at frequencies ranging from 0.3 to 8.4 GHz.

As well as the full resolution images, we have made images at a resolution of exactly 1/20th the LAS, so that we have images of strictly comparable quality for all the DRAGNs. These "C20" images are mostly made from the same data as the full resolution images, either by directly smoothing the images or in some cases by re-imaging the visibility data using a taper. In a few cases we use a different dataset for the C20 images because it shows the large-scale structure more clearly. These cases are noted in the comments on the individual DRAGNs.

For a few objects we also have "supplementary" higher-resolution images which are not suitable for the main Atlas image (they often lack sensitivity).

For those who wish to do quantitative work with the Atlas images, both the full resolution and C20 images are being made available in FITS format via FTP.

Please Note If images from this Atlas are useful to you, please acknowledge the original contributors, who are noted in the Credits: entry of the relevant Image table on the Main or "Other images" pages.

A wish list

There are various data we would like to include, but so far have not had the time to collect. They include:
Optical Images
In fact HST has now imaged nearly all the Atlas identifications...
Both optical and radio.
X-ray data
e.g. from the ROSAT All-Sky Survey. (Would mostly be upper limits, though).

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Last modified: 1996 November 26
J. P. Leahy