Source measurement technique
Point-like sources
All the parameters provided in this database have been obtained
using the the MIRIAD (Sault et al. 1995)
task MBSPECT.
MBSPECT makes a spectrum of the velocity (or frequency) axis of a
cube. The spectrum is beam-weighted, over the width
specified, for the other image axes. Robust spectral
baselines of order one are fitted, and the profile parameterised (velocities,
widths, moments). All the parameters are obtained for cubes hanning smoothed to a
resolution of 10 km/s.
For each source the input parameter list and the output provided by MIRIAD are given in the
ASCII file version of the final spectrum.
Tipically, for unresolved sources (the great majorirty of the sources detected by
AGES) we use the following parameters:
width=5,5 (i.e. 5'x5') being the spatial width of the box in pixels (in RA
and DEC) within which the spectrum is integrated.
xaxis=FELO: the x-axis is plotted as 'optical' velocity'.
yaxis=point: the pixels enclosed in the x-y area specified
are optimally weighted according to
the beam parameters, assuming that the source is unresolved.
xrange=V-1000,V+1000: velocity range plotted in the spectrum
and used to determine the bandpass (V is the galaxy's reccessional velocity).
hann=1: no additional smoothing.
order=1: order of optional robust (clipped polynomial) fit (0-10) to be applied
to the spectral axis. The fit is always subtracted from any
output data written.
profile= the velocity range over which the HI profile fit is performed, and the parameters estimated.
options=posfit,measure. Posfit: a source position is
estimated from a Gaussian fit to the moment map.
The moment map is formed using the velocity range specified
by the profile parameter. This new position is used when forming
the spectrum. Measure various spectral parameters on plotted spectrum.
Robust parameters are calculated by using an
algorithm which minimises the mean absolute deviation of
the flux-weighted velocities.
mask=This specifies the x-axis ranges to be excluded from any
continuum fit. We usually exclude the region in which the profile is computed and
any RFI or strong feature present in the spectrum.
Guide to the parameters provided in this database
Object ID
HI Source designation
(approved by the IAU "Clearing House" of Commission 5 Working
Group on Designations):
AGES JHHMMSS+/-DDMMSS
R.A.
Celestial Right Ascension precessed to the year 2000.
Coordinates have been determined using the best-fit position algorithm
within MBSPECT.
σ(R.A.)
Relative R.A. error (in minutes) as estimated by MBSPECT.
Dec.
Celestial Declination precessed to the year 2000.
Coordinates have been determined using the best-fit position algorithm
within MBSPECT.
σ(Dec.)
Relative R.A. error (in arcseconds) as estimated by MBSPECT.
Velocity
Heliocentric velocity (cz, in km/s), measured as the midpoint between the channels
at which the flux density drops to 50%.
σ(V)
Error on the recessional velocity estimated following Koribalski et al. (2004):
σ(V)= 3 x SNpeak-1(P δ(v))1/2
where SNpeak = Fpeak / (rms2+0.05xFpeak2)1/2,
P = 0.5x(W20 - W50), Speak is the peak flux, rms is the
rms of the spectrum, W20 and W50 are the velocity widths at the 20% and 50% level respectively and
δ(v) is the spectral velocity resolution.
W50
Velocity width of the source line profile measured at the 50\% level.
σ(W50)
Error on the 50% velocity width: σ(W50)=2xσ(V)
W20
Velocity width of the source line profile measured at the 20\% level.
σ(W20)
Error on the 20% velocity width: σ(W20)=3xσ(V)
Peak Flux
Peak flux of the source.
σ(Peak)
Error on the estimate of the peak flux: σ(Peak)=(rms2+0.05xSpeak2)1/2
Total Flux
Integrated flux of the source.
σ(Tot)
Error on the Total flux:
&sigma(Ftot)=4 x SNpeak-1(Fpeak x Ftot x δ(v))1/2
log(MHI)
HI mass (Log solar masses) obtained using:
MHI=2.36 x 105 x D2 x Ftot
where Ftot is the flux in the HI line (in Jy km/sec) and D is the distance in Mpc.
The distance is computed from the recessional velocity assuming D=Velocity/H0 where
H0=71 km/s Mpc-1
Peak S/N
Peak signal to noise ratio.
S/N mean
Mean signal to noise ratio defined as Ftot / (W x rms)
where W is the spectral width of the source in km/s.
rms
Rms along the spectrum in mJy.
Flag
Object flag, defined as follows: Flag 0 indicates sources detected by all the three independent
methods used for source extraction making them reliable sources or sources confirmed
by follow up observations. Flag 1 indicates objects detected only by one or
two different methods for source extraction that would not be considered as reliable sources since
they need follow up observations. Flag 2 indicates objects which are contaminated by radio
frequency interference: they are in general sure detections
but the HI parameters can be strongly affected and should not be used.
AGES field
AGES field in which the object has been detected.
Human-readable ID
Alternative ID used in some papers for greater readability.
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