1. Extragalactic Absorption Lines Observed from Arecibo Chris Salter (National Astronomy & Ionosphere Center, Arecibo Observatory, Puerto Rico)

2. Principal Collaborators Tapasi Ghosh (NAIC, Arecibo) — Especially Emmanuel Momjian (NAIC, Arecibo) D.J. Saikia (NCRA, Pune, India) Neeraj Gupta (NCRA, Pune, Arecibo) Karen O’Neil (NRAO, Green Bank, WV)

3. The Arecibo 305-m Radio Telescope Diameter = 305m; Platform Height = 137 m; Frequency Range = 327 MHz – 10 GHz Pointing Accuracy ~ 5 arcsec (rms) Sensitivity ~ 11 K/Jy @ 1.4 GHz, (1 Jy = 10 -26 W m -2 Hz -1 ) System Temperature ~ 25 K @ 1.4 GHz

4. Observing with the 305-m Telescope Drive Configuration = Simple Azimuth/Elevation Highly Blocked Aperture (Platform, Support cables, Tie-downs, Dome, Dish centre panels, etc.) Accessible Zenith Angle Range  19.7 degrees Sky Coverage: 0 h  R.A.  24 h ; –1.4 deg  Dec  38.0 deg

5. Observing Complications Position Switching: Track ON (Target) for t min, then track OFF (Reference) position for same length of time, tracking over the same pattern in (Az, El), i.e. track the ‘footprint’ over the same path on the dish. (ON – OFF) subtracts out standing waves (if they are the same for ON and OFF), while OFF provides a reference bandpass. Thus, (ON – OFF) / OFF normally supplies a “standing-wave free”, bandpass-corrected, spectrum. HOWEVER, if the target radiates continuum emission, as well as spectral lines, the quantity (ON – OFF) does not at all cancel the standing waves due to that continuum emission, and the (ON – OFF)/OFF spectrum is degraded by a “baseline ripple”. If the path difference between the direct and reflected rays is 2D, then the baseline ripple across the spectrum has a repetition frequency of 10 -6 * c/2D MHz/cycle ~ 1 MHz for Arecibo.

6. Double Position Switching (DPS) The residual “baseline” ripple from a source’s continuum emission is proportional to the intensity (flux density) of the target. Hence, if we also observe an ON/OFF pattern on another continuum source of different redshift (Calibrator), tracking over a similar (Az, El) range to the Target source, then the quantity, (ON T – OFF T ) ------------------- (ON C – OFF C ) cancels the standing-wave residual, leaving a baselined spectrum whose amplitude is proportional to the ratio of the flux densities of the Target and Calibrator sources. Zenith Angle 18.8 0 15.9 0 (rising) 18.8 0 12.9 0 (transit) 13.2 0 14.8 0 (setting) 17.3 0

7. Double Position Switching II Integration time ~ 20 hr RMS noise = 350  Jy/beam In theory, simple Position Switching results in about a factor of two better SNR for a given integration time than DPS. In practice, for a source emitting continuum radiation, the SNR soon ceases to improve by  t, and DPS quickly surpasses it. DPS continues to integrate down as  t for at least 20 hr.

8. HI and OH Absorption in CSS/GPS Sources J0119+3210 (4C+31.04: HI) HI: N(HI) = 1.84  10 18 T s (K)   1420 (v) dv / f c cm -2 OH: N(OH) = 2.25  10 14 T ex (K)   1667 (v) dv / f c cm -2

9. HI and OH Absorption in CSS/GPS Sources II J1347+1217 (4C12.50: HI) J1407+2827 (Mrk 668: HI) J2316+0405 (3C459: HI) J0901+2901 (3C213.1)

10. HI and OH Absorption in CSS/GPS Sources III N(HI) = 5.0  10 20 cm -2, for T s = 100 K N(OH)  1.1  10 14 cm -2, for T ex = 10 K N(HI) / N(OH)  4.4  10 6 HST image of 3C258 (de Vries et al. 1997), overlaid by 5-GHzcontours (Akujor et al. (1991).

11. HI and OH Absorption in CSS/GPS Sources IV

12. HI and OH Absorption in CSS/GPS Sources V

13. Cooling Flow Cluster, Abell 2390

14. OH Satellite Lines in Arp 220 2MASS image of Arp 200 (3 x 3) 1420 MHz 1665/1667 MHz

15. OH Satellite Lines in Arp 220: II 1720 MHz 1612 MHz

16. Damped Ly-  Absorber Search