Minutes of the talk on:
      Galactic ALFA Surveys: What might Happen in a Week

Chris Salter (NAIC)

GALFA is really a consortium of 3 separate sub-consortia centered around different science topics: Galactic HI, Continuum and Recombination lines.

So far, interest in GALFA has been expressed through web-sign up by 56 individuals. This include 26 post-upgrade users and 30 potential new users. The breakdown includes: 33 US vs 23 non-US. 36 staff/faculty, 14 postdocs and 6 grad students.

The GALFA workshop will be held next week. There will be 29 participants (18 post-upgrade users, 11 potential new users), very heavily US-based.

Potential key projects

• HI Low Latitude survey:
• Mapping method: OTF scans, drive rate 12'/min, 4kHz resolution, rms=150 mK per pol on cold sky
• For Nyquist sampling, mapping speed = 2sq deg/hr
• Coverage of AO visible galactic plane, 250 hours, 7sec sampling.
• Could cover same area with VLA in >1000 hr with 10 times higher rms, and no low spatial freq
• Cold HI/narrow line self absorptions (Paul Goldsmith & Di Li)
• Features might be expected to extend all over a cloud like TMC
• 7 sec, drift scan, Nyquist, 3-sigma HINSA column density cm
• 12 < dec < 40, 3< RA< 6,
• need 450 hours, prefer 900 hours to oversample
• better than 1 kHz resolution over 8 MHz, need 8000 or 16000 channels. Therefore need new backend; can't do with WAPPs
• Continuum survey
• Using pulsar spectrometer in slow dump mode, 3 Jy (receiver SEFD)
• 1.33 sec, rms noise = 100 Jy on cold sky
• But confusion rms = 2 mJy so weakest believable detections 10 mJy
• For such a source rmsnoise/flux density = 1%
• Could get Faraday rotation, del(theta) = 0.004 * RM deg/channel
• Biggest problems:
  • a) baseline drifts;
  • b) 8 dB coma lobes for outer horns
• Thus, we'll need to scan fast and take the data as systematically as possible. A possible approach is to do nodding scans on the meridian at slew speed (2.5 deg/min). Then, sidelobe structure, Tsys and telescope gain will be functions only of declination. The method would be zigzag by meridian scans at elevation slew rate (2.5 deg/min), with each zig crossing many zags.
• Recombination lines
• 12 H lines are found in the ALFA band, from n=163 to 174.
Some not observable due to RFI, but there are plenty to chose from.
• Typical widths of Hn-alpha lines are 30 km/s or 150 kHz at L-band. The Carbon lines typically are 5 km/s.
• The need to observe many lines simultaneously creates significant challenges for the backend.
• Recombination line studies might include
  • Survey of HII regions
  • Study of recombination lines in the warm ionized medium WIM
  • Studies of the trans carbon lines (S, Mg) too and Beta lines as well as alpha

Of course, part of the idea would be to piggyback these surveys where possible. Note that the time to do the whole Arecibo sky would be 1000 hours. At the same time, you could make an L-band map of SNR in OTF mode, basketweaving scans in RA and in Dec (the HI map would require more time than the continuum data).

A map made with the current system was presented showing the distribution of HI coincident with continuum emission. A total power continuum map from Jan 03 data by Giovanelli et al is confusion-limited with all the strong NVSS sources clearly visible in the map.

Questions/Discussion: