ALFALFA

The Arecibo Legacy Fast ALFA Survey

Comparison of E-ALFA Programs with Previous Blind HI Surveys:

HIPASS and HIJASS cover the same area of sky that is visible at Arecibo, HIPASS south of Dec.= +25°, and HIJASS further to the north. However, in addition to the large increase in sensitivity, ALFA surveys provide 2 direct benefits over the other two: improved angular and velocity resolution. The significant higher angular resolution (FWHM ~3.5arcmin for ALFA versus 12 arcmin for HIJASS and 15.5 arcmin for HIPASS) will help to limit the confusion of sources that plagued those other surveys. The HIPASS follow-up needed is enormous and therefore has been limited to the highest flux sources. It will be years before the sources are followed-up (if ever). An ALFA survey will be able to do science with the survey data directly, without time consuming interferometric follow-up. Additionally, the higher velocity resolution of ALFA will be useful in several ways: First, detecting edge-on galaxies with peak fluxes near the noise limit. The edge of a double peak spectrum is much sharper at higher velocity resolution which should make it easier to automatically detect these sources. Second, the higher velocity resolution will allow more accurate velocity and velocity width measurements, without the need for follow-up. Even the narrowest sources will be detected over several channels. Third, since most rfi is narrow band, the higher frequency resolution will be extremely useful in identifying and excising rfi.

The HIJASS survey has a further serious limitation. Very bad rfi in the frequency band corresponding to cz ~ 4500 - 7500 km/s (within the range of much of the interesting large scale structure e.g., Pisces-Perseus, A1367-Coma-Great Wall). In addition, HIJASS is not scheduled to do any more observing in the Arecibo range (a 4° × 4° region in Virgo and a few other areas have been covered at this point) for the next few years.

The principal advantage that an Arecibo survey will have over previous surveys is depth and the number of independent volumes surveyed. Table B.1 below includes a comparison of the major surveys, including those discussed here. For comparative purposes, the rms noise per beam quoted for each survey has been scaled to a velocity resolution of 18 km/s, the resolution of HIPASS.

Table B.1 Comparison of major blind HI surveys
Survey Area Beam Vmax Vresa ts rmsb Ndet min MHI c Ref
  (deg2) (arcmin) (km/s) (km/s) (s) (mJy)   (Msun)  
AHISS 65  3.3 -700 - 7400 16 var 0.7 65 1.9x106 1
ADBS 430  3.3 -650 - 7980 34 12 3.6 265 9.9x106 2
WSRT 1800 49.  -1000 - 6500 17 60 18 155 4.9x107 3
Nancay CVn 800 4 x 20 -350 - 2350 10 80 7.5 33 2.0x107 4
HIJASS 1115 12.  -1000 - 10000d 18 400 13 222 3.6x107 5
HIJASS-VIR 32 12.  500 - 2500 18 3500 4. 31 1.1x107 6
HIDEEP 60 15.5 -1280 - 12700 18 9000 3.2 173 8.8x106 7
HIZSS 1840 15.5 -1280 - 12700 27 200 15. 110 4.1x107 8
HICAT 21341 15.5 300 - 12700 18 450 13. 4315 3.6x107 9
HIPASS   15.5 300 - 12700 18 450 13. (6000) 3.6x107 10
AUDS 0.4  3.5 -960 - 47000e TBD 70 × 3600 0.02 (40) 0.6x10 6 11
AGES TBD  3.5 -960 - 47000e TBD 300 0.5 TBD 1.4x106 12
ALFALFA 7000  3.5 -2000 - 18000 11 28 1.6 (30000) 4.4x106
a after Hanning smoothing.
b per beam, for W = 18 km/s. Note: ADBS gives 3-4 mJy for 7s, scaled to 12s and 18 km/s.
c at 10 Mpc, for 5σ detection with W = 30 km/s.
d Gap in velocity coverage between 4500-7500 km/s caused by rfi.
e Assumes second generation backend.

References:
1: Zwaan et al. (1997)
2: Rosenberg & Schneider (2002)
3: Braun et al. (2003)
4: Kraan-Korteweg et al. (1999)
5: Lang et al. (2003)
6: Davies et al. (2004)
7: Minchin et al. (2003)
8: Henning et al. (2000)
9: Current HIPASS survey, to Decl. < +2°; Meyer et al. (2004), Zwaan et al. (2004)
10: Final HIPASS survey (including northern extension)
11: Freudling et al. AUDS precursor proposal
12: Davies et al. AGES precursor proposal

Last modified: Thu Jan 6 08:28:05 by haynes