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Jodrell Multi-beam Virgo Survey

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Minutes of the talk on:
The Jodrell Multi-beam Virgo Survey

Jonathan Davies (U. Cardiff)

A more detailed version of this talk by the speaker is available in HTML, PDF or PPT format.

The Jodrell Virgo Survey

The Jodrell Virgo survey is part of HIJASS and encompasses 4 X 8 degrees, fully sampled.

Used 4 beams with 12 arcmin resolution.
64 MHz bandpass with 1024 channels (12 km/s resolution).
So far we have used data only between 500 and 2500 km/s.
The area scanned covers a quarter of the cluster (top quadrant).
The RMS noise is 4 mJy/beam compared to HIJASS and HIPASS standard surveys of 13 mJy/beam.
The sensitivity is 5 x 10⁷ solar masses at the distance of Virgo.

Actually, it is very difficult, if not impossible, to find isolated HI clouds in the absence of star formation. HIPASS has already shown this to be the case.

The centers of clusters like Virgo are well known to be populated by HI deficient objects. This indicates that we should perhaps survey the outskirts of the cluster to look for low mass clouds.

In the survey, objects were detected using a combination of by-eye method and automated routine.

Results

28 detections of well known galaxies
4 new sources of which one is probably a normal galaxy behind M86
3 have no GOOD optical counterparts
As anticipated, the center of cluster finds no detections.

Four new detections were shown along with their comparable HIPASS spectra which are, as expected, much noisier. Weak detections were also confirmed by pointed Jodrell observations.

In HIPASS, every HI object has an optical counterpart.
- For 3 of the HIJASS/Virgo detections, there is no clear counterpart, though some smudges in images, not near center of pixel.
The derived HI mass function shows a precipitous drop off below 10 solar masses.
- Optically, Virgo has a steep luminosity function compared to field; perhaps the HIMF is likewise truncated because the galaxies get stripped by the ICM.
Furthermore, the distribution of radial velocities is not Gaussian; more galaxies are found at high radial velocity, e.g. cz > 1500 km/s. Do they represent an infalling population?

Conclusions

M $_{\rm HI}$ /L much lower than HI selected field galaxies
Velocity distribution suggests infall or extension along the line of sight.
Mean column density $\sim$ 10 $^{20}$ atoms/cm2
What is the relationship between the cluster/field LF and the HIMF?
What debris from the galaxy formation process has or is assembling itself into the cluster? Need to get below 10 solar masses to see if the mass function really turns over and to see if proposed high mass HVC's exist in the cluster.
A reduction in the ambiguity in optical identification (due to large beam size) is particularly important in the Virgo cluster.

Discussion/Questions:

Riccardo	The redshift distribution is of a region off-center from the cluster so the results may show what is expected just from the increase in the volume sampled behind the cluster
Jon	The targetted field doesn't cover any of the clouds with sysematically higher velocity.
Martha H.	Primary/secondary distances do indicate an elongation into the background for Virgo, along the line of sight.
Wim vD.	Some of us will be back in a few months to use AO to look at LSB galaxies in Virgo.
Noah B.	You should have gone beyond 2500 km/s to look at galaxies in void.
Jon	Yes, that was obvious when we looked at the redshift distribution.
Noah B.	How far can you go?
Jon	3500-4000 km/s with Jodrell Bank, which we are in the process of doing.
Peppo G.	Are you planning to cover more of the cluster?
Jon	We'd like to, but we have no more time yet.
Lister	You need to know distances to these objects.
Jon	We put all at 16 Mpc.
Lister	But the objects at 2500 km/s must be in the background, right?
Jon	Not according to Binggeli.
Lister	But he's looking in core and this region is off to one side.
Jon	It should be true for the area I'm looking at; he includes different galaxy "clouds".
Lister	Do you have any plan to get redshift independent distances?
Jon	Some of them should have them already.
Wolfram F.	Do the velocity widths suggest that there should be optical counterparts?
Jon	Yes. One is very narrow so I suspect it may be a face on galaxy.
Steve S.	The distribution of velocities is very interesting. You could also look at the distribution of widths to see whether they are likely to be at larger distances.