An Atlas of DRAGNs
Description of the tabular data
This page gives a brief description of the data tabulated in this
atlas.
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.
A NOTE ON FLUX DENSITY SCALES
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.
Reference for core flux density.
Celestial position of the radio core or optical nucleus
(whichever more accurate). Equinox B1950.0
(see the note on coordinates).
- R
- Position is of radio core
- O
- Position is of optical nucleus or galaxy.
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.
Reference for RM.
Wavelength (in centimetres) at which the integrated polarization over the
DRAGN is depolarized
to half its initial fractional polarization.
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.
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.
Projected linear size, in kiloparsec.
Calculated from the largest angular size and redshift
(of cluster, if available), and with our standard
cosmological assumptions.
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.
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.
- Emission
- 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).
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.
Reference for cluster redshift.
Apparent magnitude of optical identification.
In the optical data table, we give magnitudes in the following bands:
- B
- Johnson Blue
- V
- Johnson Visual (roughly green).
- R
- Kron-Cousins Red.
- r
- Gunn or Spinrad red.
- K
- 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.
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.
LAS
Largest angular size in arcsec.
Length of the lobes, i.e. the angular distance from the core or ID to the
most distant part of each lobe, in arcsec.
Angular widths of the lobes, measured along a line perpendicular to the
line defining the lobe length, and taken as
The length-to-width ratio of the DRAGN, defined as
(lN+ lS)/(wN + wS).
Dimensions of the image (Right Ascension × Declination)
LUT
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.
Beam
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".
Frequency
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.
Method
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
.
Last modified: 1997 March 14
J. P. Leahy
jpl@jb.man.ac.uk