As an example of typical results, Fig. 1 displays the images ("cleaned'' and Hanning-smoothed in velocity and R.A.) for 3 adjacent declination strips of our survey The velocity range is indicated on the horizontal axis and the R.A. range (800 spectra) on the vertical axis. The strong positive and negative signals around 0 kms-1 are due to residuals of the Galactic HI emission. Some standing waves due to this emission entering the receiver are visible in the lower velocity band.
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On the three images displayed here, eleven extragalactic signals from
9 galaxies can be identified. We inspected the images of the 17 strips
of the survey carefully by eye and listed all candidates
as well as all detections above the 4 threshold.
The galaxy candidates thus identified were run through the NED and LEDA databases for crossidentifications. The match in positions, velocity if available, morphology and orientation were taken into account.
The positions of detections identified in the driftscan
survey were determined according to the line number: sequence in R.A.
of the 800 contiguous spectra (cf., the 11 detections
in Fig. 1) as ).
The 16 s reflect the integration time per spectrum
plus the 0.33 s per read-out period. The positions (R.A. and Dec)
were interpolated if the signal appears in various lines or
strips according to the strength of the signal. The positional accuracy
turned out to be remarkably precise in R.A. over the whole R.A. range
(cf., Table 2a) and reasonable in Dec if a candidate was evident
in more than one strip.
If no clear-cut crossidentification could be made, the POSS I and II sky surveys were inspected. If this did not yield a likely counterpart, the five scans leading to the final images were inspected individually to decide whether the signal was produced by radio interference in an individual scan.
In this manner, 53 galaxy candidates were retained from the 17 Nançay strips. Of these galaxy candidates, 33 could be identified with an optical counterpart, 30 of which have published HI velocities in agreement with our driftscan measurements.
For the 20 galaxy candidates without a clear optical counterpart,
pointed follow-up observations were made with the Nançay telescope.
These follow-up observations are
described in Sect. 3.2.2 and allow the
establishment of a database of blind HI detections in the
volume sampled, which is reliable down to the 4 level.
Interestingly, none of the candidates could be
confirmed. But, note that for the number of independent measurements
that were obtained in this survey at, say 50 kms-1 resolution,
one expects to obtain of the order of 15 positive and 15 negative
deviations exceeding
purely by chance.
The observing technique recorded the integrations
every 16.3 s, during which time the beam drifted through the
sky by a full Half-Power-Beam-Width. In comparison,
a "pointed'' integration, with the
source of interest located on the beam axis, would reach a
sensitivity of about
Jy for
a given velocity resolution
after
s
integration with dual polarization. This would lead to a
detection limit for
integral line fluxes
Jy kms-1.
However, sources that pass through the beam do not appear in the
data at the full strength they have when observed at the
peak of the beam for the full integration time. Instead, a source
that traverses the HPBW from one half-power point to the other
suffers a loss in signal-to-noise ratio of a factor of 0.81,
and a source that chances to pass the centre of the beam at the boundary
of two integrations suffers a factor of 0.74. (Had samples been taken
at 8 s intervals, these factors would be 0.84 and 0.80.)
An additional loss of S/N occurs because the survey declination strips
were spaced by a full beamwidth (22'). A source falling 11' from the
beam centre is observed at half power. Some of this loss in sensitivity
is recovered by averaging adjacent strips, so that these sources
midway between the two strip centres then experience a net loss in S/N
of relative to the strip centre.
Combining the loss factors incurred due to R.A. sampling and declination
coverage shows that the S/N is reduced relative to pointed observations
by factors ranging from 0.81 to . Thus there is a range
to the
detection limit.
It is clear from the distribution of non-confirmable signals
(cf., Fig. 2)
that there is also non-Gaussian noise at work, such as that resulting
from radio interference, leading to a few spurious signals with
apparent significance of as much as
.
The HI line flux and the profile velocity width are two
measured quantities. Figure 2 shows the integral
line flux for each galaxy plotted as a function of its velocity width.
The detection limit (delineated in Fig. 2) rises as
, since the signal I from a
galaxy with flux density
grows as
,
while the noise
in measuring a flux density is distributed
over
is
, so
that the signal-to-noise ratio
drops as a signal of constant I is spread over increasing width.
The detection boundary would be a band for true fluxes due to the
lower survey sensitivity for detections offset from the centre of
the survey strip.
The dashed line in the figure marks a region to the lower
right that is not
expected to be populated, since galaxies of large velocity width typically
have large HI masses, and, to have such a low measured flux, these galaxies
would lie beyond the end of the survey volume, which is limited by the
bandwidth of the spectrometer.
Within a declination strip, the HI parameters were measured
from the sum of the lines in which a candidate was detected
using the Spectral Line Analysis Package by
Staveley-Smith (1985).
We list here the heliocentric central line velocity using
the optical convention ,
, the profile FWHM, W50, the profile width measured at 20%
of the peak flux density level, W20, the integrated line flux, I, and the
logarithm of the HI mass,
. The latter should be
regarded as preliminary as the HI mass was determined
for the strongest appearance of the signal and is uncorrected
for the reduction in observed flux (described in Sect. 3.1) due
to offset from the centre of the beam. Moreover,
the adopted distances were taken straight from the heliocentric
HI velocity
using a Hubble constant H0=100 kms-1 Mpc.
For each HI detection a possible optical identification is given.
Optical redshifts are known for 17 optical candidates, though 30 do
have independent HI velocities (cf., Table 4). The listed
optical data
are mainly from the NED and LEDA databases; D and d are the
optical major and
minor axis diameters, respectively, and is the
heliocentric optical systemic velocity.
These follow-up observations have an rms noise of about 6 mJy on
average (compared to 10 mJy for the survey).
We obtained our observations in total power (position-switching) mode using
consecutive pairs of two-minute on- and two-minute
off-source integrations. The autocorrelator was divided into two pairs
of cross-polarized (H and V) receiver banks, each with 512 channels
and a 6.4 MHz wide
bandpass. This yielded a velocity resolution of kms-1, which was
smoothed to 10.5 kms-1, when required, during the data analysis.
The centre frequencies of the two banks were tuned to the radial
velocity of the galaxy candidate (cf., Table 2b).
The follow-up observations first were repeated on the optimized
position from the driftscan detection. If not confirmed, a second round
of pointed observations was made, offset by half a beam (11)to the north and south of the original position while the original
R.A. position which can be determined to higher accuracy was kept
constant. In a few cases, a further search at quarter beam offsets
was done as well. The follow-up observations consisted of a number of
cycles of 2 min on- and 2 min off-source integration.
Off-source integrations were taken at
approximately 25
east of the target position.
We reduced our follow-up pointed HI spectra using
the standard Nançay spectral line reduction
packages available at the Nançay site. With this software
we subtracted baselines (generally third order polynomials), averaged
the two receiver polarizations, and applied a declination-dependent
conversion factor to convert from units of to flux density
in mJy. The
-to-mJy conversion factor is determined via
a standard calibration relation established by the Nançay staff
through regular monitoring of strong continuum sources. This
procedure yields a calibration accuracy of
.In addition, we applied a flux scaling factor of 1.25 to our spectra based on
a statistical comparison (Matthews et al. 1998) of recent Nançay data with
past observations. The derivation of this scaling factor
was necessitated by the a posteriori discovery that a number
of line calibration sources monitored at Nançay by other groups
as a calibration normalization check (see Theureau et al. 1998)
were quite extended compared with the telescope beam, and hence would be
subject to large flux uncertainties.
None of the candiates were confirmed, despite the fact that some of
them have a rather large mean HI flux density. Only 4 "candidates''
are below our threshold. But, as mentioned above, for the number of
independent measurements that were obtained in this survey
we expect of the order of 15 positive and 15 negative
deviations exceeding purely by chance. Many of the "false''
detections have very low linewidths (around 30 kms-1) and could
still be due to radio interference.
The most disappointing candidate was No. 50. This object was identified on two adajcent declination strips and on 2-3 adjacent spectra in R.A. with very consistent HI properties and a similar high S/N ratio (8 - 10) on both declination strips. It therefore seemed one of the most promising new gas-rich dwarf candidates in the CVn region.
Seven dwarf candidates of the optical BTS survey were not detected in the
blind HI line survey. With the exception of BTS 160, these dwarfs were
observed individually to a lower sensitivity as the driftscan survey,
in the same manner as described in Sect. 3.2.2.
We did not include BTS 160, because this dwarf had already been detected by
Hoffman et al. (1989) at Arecibo with 0.9 Jy kms-1. With an
average flux density of 26 mJy this dwarf was hence clearly below
the threshold of our driftscan survey. Of the 6 remaining dwarfs, none
were in fact detected, although the pointed observation of BTS 151
revealed a strong signal with a flux of 20 Jy kms-1 (cf., values in
brackets in Table 3), i.e., a detection which should have popped up as
about a 10 detection in the driftscan mode. However, this
signal matches exactly the velocity of the nearby
large spiral galaxy NGC 4656 (cf., Tables 2a and 4, object
No. 32) which is 45
and 17
away in R.A.
and Dec respectively, hence less than a beamwidth from the pointed observation.
The detected signal thus clearly originates from the spiral NGC 4656, and not
from the dwarf elliptical BTS 151.
The data of these observatons are summarized in Table 3. Most of these possible dwarf members of the CVn group were not detected in HI. This supports their classification as early type dwarfs.
Of the 33 reliable detections, 30 galaxies had HI velocities published in the literature. For the 3 galaxies without prior HI data we obtained follow-up observations with the Nançay radiotelescope. We also obtained pointed follow-up observations for 8 galaxies already detected before in the HI line, which seemed to merit an independent, new observation. The observations followed the procedures as described in Sect. 3.2.2.
The results from the driftscan images are summarized in Table 4 together with our new pointed observations and HI parameters from independent observations.
Overall, the agreement between the measurements obtained from the driftscan survey and independent pointed observations are very satisfactory, with a few discrepancies discussed below:
No. 12 (NGC 4173): The HI line parameters of this 5' diameter edge-on system measured at Arecibo by Williams & Rood (1986) and independently by Bothun et al. (1985) are quite different from the other available values.
No. 13 (NGC 4203): This lenticular galaxy has an optical inclination of
about 20. Assuming that the gas rotates in the plane of the
stellar disc, very high HI rotational velocities would be derived.
However, radio synthesis imaging (van Driel et al. 1988) has shown
that the outer gas rotates in a highly inclined ring at an inclination
of about 60
, the value adopted in correcting the profile
widths (see Table 5).
For 4 galaxies (No. 33 = UGC 7916, No. 34 = UGCA 309, No. 35 = NGC 4861, No. 38 = CGCG 0999) the listed properties such as morphological type, diameters and magnitudes differ between the LEDA and NED. The values in Table 2b and Table 5 are from LEDA.
We have compared the systemic velocities, line widths,
integrated line fluxes and HI masses of the 33 reliable survey
detections with available pointed observations. It should be noted that the
comparison data represent in no way a homogeneous set of measurements, as they
were made with various radio telescopes throughout the years; especially, care
should be taken that no HI flux was missed in observations with the round Arecibo beam.
A comparison of the difference between systemic velocities (actually,
the centre velocities of the line profiles) measured at Nançay and
elsewhere shows no significant dependence on radial
velocity, with the exception of one data point for
NGC 4173 (No. 12). Here we consider the radial velocity of 1020 kms-1
measured at Arecibo by Williams & Rood (1986) as spurious, seen
the agreement between the 3 other measured values. The mean value
and its standard
deviation of the Nançay-others systemic velocity difference
is kms-1 (for a velocity resolution of 10.2 kms-1 at Nançay).
A comparison of the difference between the W50 and W20 HI line widths measured at Nançay and elsewhere shows a good correlation as well; the largest discrepancy is found between the survey values for the 5' diameter edge-on system NGC 4173 (No. 12) and the Arecibo W50 and W20 measurements by Williams & Rood (1986) as well as by Bothun et al. (1985), while the other pointed observations of this object are consistent with ours.
A comparison of the integrated HI line
fluxes measured in the Nançay survey and elsewhere shows
a reasonable correlation. This indicates that the assumption of
a constant (uncalibrated) system temperature of 50 K throughout the
survey is justified. Three larger discrepancies occur; this concerns
No. 13 = NGC 4203 for which
the Nançay survey flux is considerably higher than the flux measured at
Arecibo. As the galaxy's outer HI ring is much larger than the Arecibo beam
(see van Driel et al. 1988) the Nançay measurement will reflect the
total flux. For No. 06 = BTS59 and No. 14 = UGC 7300, the Nançay flux
is also considerably higher than a flux measured
at Arecibo by Hoffman et al. (1989), respectively
Schneider et al. (1990);
both are objects of about optical diameter only.
A comparison between the HI masses derived straight from the detections in the strip-images with pointed observations shows an astonishingly narrow correlation from the lowest to the highest HI masses detected in this survey. Overall, only a slight offset towards lower masses is noticeable for the driftscan results compared to pointed observations. This obsviously is due to the fact that the driftscan detects the galaxies at various offsets from the centre of the beam.
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