Post on 04-Jan-2016
Extragalactic Absorption Lines Observed from Arecibo
Chris Salter(National Astronomy & Ionosphere Center,
Arecibo Observatory,
Puerto Rico)
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)
The Arecibo 305-m Radio Telescope
Diameter = 305m; Platform Height = 137 m; Frequency Range = 327 MHz – 10 GHzPointing 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
Observing with the 305-m Telescope
Drive Configuration = Simple Azimuth/ElevationHighly Blocked Aperture (Platform, Support cables, Tie-downs, Dome, Dish centre panels, etc.)Accessible Zenith Angle Range 19.7 degreesSky Coverage: 0h R.A. 24h; –1.4 deg Dec 38.0 deg
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.
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,
(ONT – OFFT) ------------------- (ONC – OFFC)
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
18.80
15.90 (rising)
18.80
12.90 (transit)13.20
14.80 (setting)
17.30
Double Position Switching II
Integration time ~ 20 hrRMS 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.
HI and OH Absorption in CSS/GPS Sources
J0119+3210 (4C+31.04: HI)HI: N(HI) = 1.84 1018 Ts(K) 1420(v) dv / fc cm-2
OH: N(OH) = 2.25 1014 Tex(K) 1667(v) dv / fc cm-2
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)
HI and OH Absorption in CSS/GPS Sources III
N(HI) = 5.0 1020 cm-2, for Ts = 100 K
N(OH) 1.1 1014 cm-2, for Tex = 10 K
N(HI) / N(OH) 4.4 106
HST image of 3C258 (de Vries et al. 1997), overlaid by 5-GHzcontours (Akujor et al. (1991).