An Atlas of DRAGNs

Description of the tabular data

This page gives a brief description of the data tabulated in this atlas.

Radio data


The 3C or 4C name (if there is one).

Source: Laing & Riley, except where their "common name" was the NGC number.


Flux density at 178 MHz, in Jansky.

It is not easy to convert from the voltage recorded by a radio telescope to absolute units. The usual technique is to compare the output with measurements of a few "flux density standards", bright radio sources whose absolute flux density is assumed to be known. From time to time, the standards are measured more accurately, forcing a revision of all the derived flux densities. The current best values for the standards were derived by Baars et al. (1977), so our flux densities are "on the Baars et al scale". Baars et al. listed conversion factors to bring measurements in various surveys onto their scale, but for 3C and 4C these are not accurate; and we use the conversion factor of Roger, Bridle & Costain (1973). Thus it is more correct to say that we are on the RBC scale. This is a factor of 1.09 brighter than the scale of Conway, Kellerman & Long (1963) used in 4C, and a factor of 1.18 brighter than the original 3CR scale.

Source: Laing & Riley.


Spectral Index, generally evaluated between 178 & 750 MHz. We use the convention:


Source: Laing & Riley.


Structural class on the system of Fanaroff & Riley (1974). See the description of classification schemes.

Source: The Atlas C20 maps.


More detailed structural class, as described on the classification schemes page. Source: this Atlas.


The flux density at 5 GHz (in mJy) of the compact core (or upper limit if none detected). Since most cores have roughly flat spectra, we used measurements at other frequencies if no 5 GHz data was available. This is not very accurate, but compact cores tend to be strongly variable, so these numbers are meaningful only at the factor-of-two level.

Ref 1

Reference for core flux density.

Right Ascension, Declination

Celestial position of the radio core or optical nucleus (whichever more accurate). Equinox B1950.0 (see the note on coordinates).


Position is of radio core
Position is of optical nucleus or galaxy.

Ref 2

Reference for position.


Faraday Rotation Measure. In rad m-2. In nearly all cases this will be due to Faraday rotation in the interstellar medium of our Galaxy.

Ref 3

Reference for RM.


Wavelength (in centimetres) at which the integrated polarization over the DRAGN is depolarized to half its initial fractional polarization.

Ref 4

Reference for Lambda½.


Largest Angular Size, in arcsec. This is the angular distance between the most widely-separated regions in the DRAGN showing detectable emission.

Source: this Atlas.

lg P178

Logarithm (base 10) of the radio power at 178 MHz emitted (not received) frequency, in units of Watt Hertz-1steradian-1.

Calculated from the flux density, spectral index, and redshift (of cluster, if available), and with our standard cosmological assumptions.

Size, D

Projected linear size, in kiloparsec.

Calculated from the largest angular size and redshift (of cluster, if available), and with our standard cosmological assumptions.

Optical data

IAU Name

IAU-format name, of the form BHHMM±DDF, where B stands for Besselian Equinox 1950, HH are the hours of Right Ascension, MM are the minutes of Right Ascension, ±DD is the signed integer part of the Declination, and F is the first decimal place of the Declination (unrounded). Thus an object at

RA = 01h 04m 39s, Dec = +32° 08' 43",

would have IAU name B0104+321.

Optical Name

The name of the optical identification, if there is one.


Optical identification, i.e. the optical object whose centre coincides with the compact radio core (if a core is detected), or which appears to be the centre of activity (otherwise).
Quasar (QSO)
Various definitions of "quasar" or "quasi-stellar object" have been proposed. In the Atlas, "quasar" and "QSO" are used interchangeably to mean objects which appear stellar (i.e. point-like) on the photographic plates of the Palomar Sky Survey. All quasars so defined have broad emission lines in their optical spectra (see below), and are at least several times more luminous than typical radio galaxies. Quasars are just luminous active galactic nuclei.
Galaxy (Gal)
The galaxies all appear to be luminous ellipticals, although many are abnormal, showing large-scale dust features, extended emission-line nebulae, and/or disturbed structure including tails which resemble those produced in tidal interactions.

With electronic detectors and high resolution (especially on the Hubble Space Telescope), galaxies have been detected surrounding many quasars, and faint point-like nuclei are often found in the centres of radio galaxies, especially those with broad emission lines. Thus the difference between galaxy and quasar is a matter of degree. On some definitions,galaxies with bright nuclei like 3C 109 and 3C 390.3 would qualify as quasars.

Source: Laing & Riley; description of galaxies based on HST images by de Koff et al. (1996).


Description of the optical spectrum:
Absorption--- Include Picture ---
Only absorption features are detected. These are the blended absorption lines of the stars in the galaxy. Many, if not all, of the spectra classified as "absorption" would show emission lines if observed with more sensitivity.
Fluorescent line emission, produced by numerous "clouds" in and around the active galactic nucleus.

Emission line spectra are classified according to their line width and excitation as follows:

LINER --- Include Picture ---
Low-excitation, very narrow emission lines (H alpha, [NII]). LINER stands for Low Ionization Nuclear Emission Region. This type of spectrum is characteristic of low-luminosity AGN, both radio-loud and radio-quiet.
Narrow --- Include Picture ---
Strong, high-excitation emission lines with widths of up to a few hundred kilometres per second. Often called a Seyfert 2 spectrum.
Broad --- Include Picture ---
Broad (> 1000 km/s) permitted lines such as H alpha, superimposed on a strong narrow-line spectrum. Often called a Seyfert 1 spectrum. Intermediate types with a relatively weak set of broad lines (so-called Seyfert 1.5, etc), are classified here simply as Broad. Broad lines are always associated with strong continuum emission from the nucleus, often bright enough for the identification to be classified as a quasar.

Source: Laing & Riley.


Redshift. For an explanation, see the page on cosmology.

Source: Spinrad et al. (1985).

Cluster z

Mean redshift of any cluster of galaxies in which the DRAGN is situated. This should be a more accurate estimate of the cosmological redshift, especially for head-tail DRAGNs which may be moving rapidly through the cluster.

In the main pages, the entry "Best z" gives the cluster redshift if available, otherwise the redshift of the identification.

Ref 5

Reference for cluster redshift.

Apparent magnitude

Apparent magnitude of optical identification. In the optical data table, we give magnitudes in the following bands:

Johnson Blue
Johnson Visual (roughly green).
Kron-Cousins Red.
Gunn or Spinrad red.
Near infra-red, centred on 2.2µm.
Typically data is only available in one or two bands for each object, and there is no band where we have magnitudes for all objects.

An asterisk indicates a rough estimate, good to 0.5-1.0 mag or so. Other values have rms accuracy of typically 0.05-0.1 mag. In the main pages, under mag., we quote a representative magnitude (preferably red) in the form "R=18.5" etc, where the first letter gives the band.

References for each value, together with details of the aperture used, are given on the Data page for each source.

Structural data


Geometric mean observed frequency of the data used to make the C20 image.


Total flux density in Jansky, measured on the C20 image.


Estimated flux density of the core (mJy) at the frequency of the C20 image (usually based on the full resolution image).


Flux density of the northern and southern lobes, respectively, in Jansky. Measured on the C20 image.


"Hotspot flux densities", i.e. peak flux density of the northern and southern lobes, in Jansky per beam, measured on the C20 image.


Compactness, i.e. the ratio of the sum of the hotspot flux densities to the total extended flux density:

(HN + HS)/(Stot - Score).

This has been corrected to a standard emitted frequency of 2 GHz, assuming an average spectral index difference of 0.3??? between hotspots and the rest of the lobes.


Largest angular size in arcsec.

lN, lS

Length of the lobes, i.e. the angular distance from the core or ID to the most distant part of each lobe, in arcsec.

wN, wS

Angular widths of the lobes, measured along a line perpendicular to the line defining the lobe length, and taken as

Axial Ratio

The length-to-width ratio of the DRAGN, defined as

(lN+ lS)/(wN + wS).

Image data listed on the main pages


Dimensions of the image (Right Ascension × Declination)


Look-Up Table (or Transfer Function): either Logarithmic or Linear. The look-up table has been further modified by conversion to pseudo-colour and further adjustment of the transfer function. For reasons which should become apparent if you read the note on displaying images, we use the following strategy. Ideally, we use a linear greyscale which reaches white at the brightest point in the extended structure (so that background sources and compact cores are burnt out). If this leaves the fainter parts of the structure invisible, we apply pseudocolour. If this is still not enough (the majority of cases) we use a logarithmic transfer function as well.


The size of the clean beam (full-width at half maximum). For some WSRT images the clean beam is elliptical with the major axis aligned north-south. For these objects we quote both dimensions, e.g. "29×52 arcsec".


The approximate frequency of the data used to form the image. Often the images are made by combining data at several frequencies with a range of up to 20%. The quoted frequency is roughly the geometric mean, and should not be taken too seriously.


The program(s) used to make the final images. VTESS is the AIPS Maximum Entropy Method implementation. CLEAN is the CLEAN algorithm, usually in the form of the AIPS tasks MX and APCLN.
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Last modified: 1997 March 14
J. P. Leahy