| Arecibo Galaxy
Environment Survey (AGES)
M74 (NGC 628) GROUP
OBSERVATIONS: FIRST RESULTS
The precursor run is finally complete. Here are some initial results
that illustrate the success of the observing technique and also areas
where we had problems.
The points in the graph above were calculated by taking the reduced
datacubes and finding the mean value for a large region of sky that
didn't contain sources over the whole channel range. The channel range
selected was the inner 3696 channels, thus avoiding the increase in
noise at the edge of the bandpass. The noise appears to reduce as t-0.49
+/- 0.06 which is in very good agreement with the t-0.5
dependence as theoretically predicted. Following this predicted
decrease in noise with integration time we could have reached a final
noise level of
0.95 mJy if all 25 nights' observing had been successful.
New member of the group?
One of the mose exciting results is a new HI detection within the group
(circled above). There are a number of possibilities for the source of
this emission: it is a member of the group previously undetected; it is
a tidal plume from the interaction of UGC 1171 & UGC 1176; it is an
artefact (the result of emission from UGC 1171 being picked up in the
coma lobes). The velocity profiles of UGC 1171 and the new detection
have different shapes and peak in different channels, suggesting that
this is indeed a new detection and not the result of emission being
picked up in the sidelobes. Opitcal observations of this new object are
still being completed and more HI analysis is ongoing to get the mass
of the object.
and GRIDZILLA do continuum too!
Shown above is a continuum map that was produced from 16 nights
observing. Data was reduced using the LIVEDATA
packages. The continuum reduction is a relatively new feature of
LIVEDATA and GRIDZILLA and the early results are promising. This image
was compared with the NVSS image of the same region and the positional
values are correct to within 1 arcmin (1 pixel in the final ALFA data).
On some nights we experienced an unusual source of RFI. While the RFI
was quite narrow, usually occupying a few channels, it would shift
frequencies in a quasi-periodic fashion contaminating up to 60 MHz of
the band. If one wasn't bad enough there was a harmonoic that would
also rear its ugly head. Below are two graphs showing the noise in each
channel of the spectrometer, for two identical regions of sky taken
over two consecutive nights. The first night's data contained the
birdy, the second night we were birdie-free. The effect on the quality
of the final data is quite astonishing.
Click here to see a
movie of the 'wandering birdie'. It manifests
itself as a ghostly image of the ALFA footprint projected on to the
sky. Each frame of the
movie is an image of the observing region from each individual velocity
(or frequency) channel in the datacube. As we progress through the
movie frequency is increasing. The footprint appears to move across the
image, reflecting the sky's movement over ALFA.
Since the end of the observations we have been informed that the birdie
has been traced to some equipment in the focus cabin. Once this RFI has
been shielded this should not prove to be a significant problem for
subsequent observing runs.