Minutes of the talk on :
      HIDEEP and HIJASS Results

Erwin de Blok (Cardiff)

HIDEEP

• Specifications:
  • 4 x 8 deg region centered in Centaurus (13h40m, -30d00')
  • Velocity range from -1280 to 12,000 km/s
  • 1024 channels at velocity resolution of 18 km/s
  • Beam size of 14' with pixel size 4'
  • Rms noise ~ 3.2 +/- 0.2 mJy.
• Detections:
  • 173 detections in total after 3 passes of cube: 2 humans and 1 computer.
  • The profiles were extracted and quantified.
  • Detections are peak flux and velocity width limited with the limits at 9.6 mJy and 4 channels (52.8 km/s).
  • Completeness limit is ~ 18 mJy.
• HIDEEP optical follow up:
  • Imaged with UK Schmidt R-band Tech-Pan plates, then digitized.
  • 6 x 4 degrees overlap. Limiting magnitude of 26.5.
  • 96 HI sources in the field: none without optical counterpart.
    • 59% had previously known redshift while 24% where known objects but had no redshift previously.
• HIDEEP galaxy properties:
  • HIDEEP lies in Supergalactic plane. For cz < 10,000 km/s, there is good correlation between the optical large scale structure and that derived from the HI detections. Beyond 10,000 km/s, the HI seems to lie in voids, but we're not sure if that is real or due to selection effects because of optical redshift availability.
  • A large fraction of the galaxies detected in HI are LSB. A K-S test shows that late type ESO-LV and HIDEEP differ at 99% level.
• HI column densities:
  • HIDEEP should be sensitive to N_H > 2.1 x 10¹⁶ times the velocity width.
  • To figure out the actual HIDEEP sensitivity, you have to decide how to derive the limits on the column density when the beam is large (unresolved sources). We adopt an average ratio D_HI/D_opt of galaxies in the literature to estimate a hybrid column density.
  • Most of HIDEEP populate the high column densities.
  • We do not see low density ones.
  • The problem may be with the adopted ratios, but we would need to systematically underestimate the HI size by a factor of 3 to explain the discrepancy.
  • Most galaxies in HIDEEP have

Therefore, there appears to be a lack of low column density galaxies. This result needs to be explored further. It could be due to ionization, frozen disks (T_spin ~ T_cmb), or the effects of environment.

Questions/Discussion:

Riccardo	What is the source of the optical redshifts you use to determine the large scale structure at the high velocity end?
Erwin	NED.
Riccardo	That may explain your finding a "void". Most of those are probably from the SSRS which is very shallow.
DJ	When you go from 64 MHz to 100 MHz, don't you think there will be lots more interference?
Erwin	We took all data at night. That helped a lot because there was no solar interference. You would want to tailor the survey strategy to minimize rfi.
Steve	In terms of your column density, you are assuming filled beam and then adopting an optical size. I think you need to use the optical sizes for the assumption. used, not equal to one. That would fill in the gap.
Erwin	That's probably true. It would bring up the line a bit but maybe not the whole way.