The Arecibo Radiotelescope. World s largest radio/radar telescope

Transcription

1 The Arecibo Radiotelescope World s largest radio/radar telescope

2 The Radio Telescope of the Arecibo Observatory National Astronomy and Ionosphere Center Reflector diameter of 305 meters (1000 ft) Built in 1963 by faculty and engineers Cornell University under contract with the DARPA and later the National Science Foundation, it underwent major upgrades in the 1970s and the 1990s. Located in a natural sinkhole in northwestern Puerto Rico.

3 The Arecibo telescope was built at this site in order to take advantage of the vicinity to the Equator and of the topography of the terrain, which provided a nearly spherical valley and minimized excavation.

4 History of Arecibo The karst limestone terrain characteristic of northwestern Puerto Rico is made up of small hills ( haystacks ) and sink holes, one of which is the site of the Arecibo telescope. Not much excavation was needed! June 1960 Dec 1960

5 History of Arecibo

6 While the primary reflector (the dish ) is suspended just above ground level over a network of steel cables, the focal structure with all the receiving equipment which weighs 900 tons - is suspended 150 meters above the ground, and can be steered to point at cosmic sources within 20 degrees of zenith.

7 The Arecibo Radiotelescope

8 Gregorian Optics The dome is referred to as the Gregorian. A Gregorian focus refers to one in which the secondary reflector is placed behind the focal point of the primary reflector. In this case, there are 3 reflectors: primary, secondary, tertiary.

9 Under the Arecibo dish, roads snake around among a network of support and tension cables, amidst lush tropical vegetation.

10 Feed Smaller antenna (waveguide horn) that transfers incoming signal to receiver Term originated with antennas used for radar transmissions Martha with the ALFA receiver array and its 7 feed horns The 430 MHz line feed, featured in the film Goldeneye

11 Receivers at Arecibo Receiver Freq Range Receiver Freq Range Name (GHz) Name (GHz) 327-MHz MHz MHz ALFA L-wide S-low S-narrow S-high 3-4 C C-high X L-band: GHz, cm 21 cm HI line at MHz (if at rest)

12 Resolution of the Arecibo Telescope Feed illuminates an annulus of 700 ft ~ 213 m = 2.1x10 5 cm L-band λ=21 cm Ө(rad) = 1.22 x x10 4 = 1.22 x 10-3 radians ~ 4 arcmin

13 Three Perspectives of the Sky Heliocentric Geocentric Local Centered on Sun with Sun fixed Centered on Earth with Earth fixed (not moving) Centered on observer (Your view of the sky) Celestial sphere: Imaginary sphere of huge radius centered on Earth and aligned with Earth s poles.

14 Celestial Sphere North Celestial Pole (NCP) Extension of Earth s axis northward to celestial sphere Celestial Equator (Cel. Eq.) Extension of Earth s equator out onto celestial sphere. South Celestial Pole (SCP) The stars have (essentially) fixed locations on the celestial sphere. The Sun, Moon and planets move (with respect to the stars)

15 Coordinates On Earth On Celestial Sphere Longitude Right Ascension Latitude Declination Equator (lat = 0 ) Celestial Equator (Dec. = 0 ) Greenwich meridian (long=0 ) Vernal Equinox (R.A.=0 ) By convention

16 Azimuth & Zenith Angle E Celestial equator Azimuth Angle Measured in degrees Tells how far east of north the source is located as measured along the horizon N 18 Arecibo W S Zenith Angle Measured in degrees Tells how far down from the zenith a source is located The Arecibo feeds can be steered to receive signals from the sky within 20 degrees of the Arecibo zenith.

17 Some Arecibo Facts Observatory located at +18 N Latitude; can observe within 20 of zenith (overhead) Main reflector is 305 feed in diameter, but feeds illuminate annuli of about 700 feet in diameter Receiver systems operate from 6m - 3cm (47 MHz - 10 GHz) Slew rate of 25 /min in azimuth and 2.5 /min in zenith angle Pointing accuracy of 5 arcseconds 3 pairs of cables that lead under dish for mm precision placement of platform Tie downs are controlled dynamically in real time, keeping platform level.

18 How do I get time on the telescope? Telescope operates 24 hours a day (although some experiments must run during night (or day!) Submit a proposal which is judged by a panel of referees Deadlines are March 1 st and September 1 st Oversubscription rate is 4:1 Open to qualified observers regardless of affiliation ALFALFA was allocated 4400 hours over 7.5 years APPSS has been allocated 110 hours in 2015 (we will propose for more time in 2016)

19 Research at Arecibo Atmospheric Science Measures composition, temperature, and density of upper atmosphere Measures the growth and decay of disturbances in the ionosphere Radar Astronomy Radio energy is transmitted, reflected and then collected. Studies surface features, composition, size, shape, rotation and path of target Studies objects within our solar system

20 Radio Astronomy at Arecibo Continuum Observations: detection, monitoring Thermal radiation (hot gas) Non-thermal radiation (synchrotron: electons in magnetic field) Pulsars: Discovery, timing, monitoring VLBI (Very Long Baseline Interferometry) Spectral Line Observations HI line MHz/ 21 cm line OH lines (1612, 1665, 1667, 1720 MHz) CH, H 2 CO, methanol, etc.

21 Continuum Observations Radio frequency observations over a wide range of frequencies (wide bandwidth) Example: studying synchrotron emission in our own galaxy or other galaxies

22 Pulsars Neutron stars were a purely theoretical concept until observations of the 33-ms pulsar in the Crab Nebula in 1968 Rapidly rotating neutron stars emit when synchrotron radiation in beam; we see it when beam sweeps past Earth Magnetic axis and rotation axis not aligned Crab Nebula

23 The Binary Pulsar PSR Discovered in 1974 by Russ Hulse and Joe Taylor at Arecibo Shrinking of orbit due to loss of energy by gravitational radiation as predicted by Einstein 1993 Nobel prize to Hulse and Taylor

24 PSR B and its Planets First detection of an extrasolar planet EVER Discovered by Alex Wolszczan & Dale Frail through pulsar timing At least 3 bodies of Earth-like masses around PSR B Figure from Alex Wolszczan

25 VLBI - Very Long Baseline Interferometry Joined the VLBI network in the late 1990s NAIC commits 4% of AO s telescope time to VLBI Broad bandwidth video recorders record signals and are then replayed later in the same location Now incorporated into High Sensitivity Array (HSA) Track time evolution/motion of sources with extreme precision (micro-arcseconds)

26 ALFALFA: A Census of Gas-bearing Galaxies A galaxy is a gravitationally bound object that consists of billions (and billions) of stars, gas clouds (of varying temperature and density = interstellar medium), dust clouds (mixed with the gas), and (so it seems), 90% dark matter. Optical surveys, like the Sloan Digital Sky Survey, detect the stellar component of galaxies. ALFALFA is designed to detect the cool (not hot; not cold) atomic gas in and near galaxies. ALFALFA is a blind survey; we observe the whole area of sky, whether or not we think/know there is an optical galaxy there. ALFALFA is a spectroscopic survey; not only do we detect the HI line flux, we also measure its frequency (velocity) and the width of the HI line (a measure of rotational velocity).

27 21-cm Line of Atomic HI Through Hydrogen maser measurements the frequency is: 1,420,405, ± Hz Energy hc/λ ~ 5 x 10-6 ev Compared to energy of a visible light photon which is about 2 ev. About 4.4% of the visible matter in our galaxy is HI =>4.8 x 10 9 M. The fraction of interstellar space filled with HI clouds is 20% to 90%. In the MW there are some HI atoms; At the rate A 10, about atoms per sec would emit a photon. In reality, the transition probability is 10 5 times larger than A 10 Hence the galactic HI emission is very easily detectable.

28 HI emission from galaxies Under most circumstances, the total H I mass can be derived from the integrated line profile; that is, the flux (integrated over all frequencies where there is signal) is proportional to the number of hydrogen atoms. The frequency (velocity) spread of the line reflects the velocities of the gas atoms, not quantum mechanics => hence the width of the line tells about the motions of the gas (rotation within the galaxy or turbulence, expansion, etc) SdV V V HI mass Distance Mass Rest frequency MHz

29 Clues from the HI line HI mass and distribution (for extended objects) Normal, star-forming disks Low mass, LSB dwarfs Potential for future star formation (HI content) HI deficiency in clusters History of tidal events Redshifts Rotational velocities Dark matter Redshift-independent distances via Tully-Fisher relation HI absorption: optical depth Link to Ly-α absorbers Fundamental constant evolution HI in M31 Credit: R. Braun SdV HI mass V Distance V Mass 29

30 Doppler Shifts and Hubble s Law Edwin Hubble (1927) The further away a galaxy is, the faster it is moving away from us At small velocities, where v<<c, this is approximated by v ~ cz.

31 The Arecibo radio telescope The National Astronomy and Ionosphere Center

32 Arecibo program A2982 The Arecibo Pisces-Perseus Supercluster Survey (APPSS) Targeted LBW observations of galaxies in the direction of the Pisces-Perseus Supercluster and predicted by their optical/uv appearance and brightness to lie within 120 Mpc of us. ALFALFA: effective integration time of 40 seconds/beam LBW: 5 minutes ON-source + 5 minutes OFF-source 32

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34 Finding optical counterparts and confusion The centroiding accuracy for a mapping survey like ALFALFA goes roughly as For HPFW(PSF)/(S/N) where S/N is the signal to noise ratio of the detection. 3 4 For ALFALFA that is about 18. 5

35 High S/N 35

36 Low S/N 36

37 Identifying Optical Counterparts ALFALFA source centroids good to ~18 (depends on S/N) ALFALFA catalogs include: the HI centroid position the position of the most probable OC OC s SDSS PhotoObjID and SpecObjID (where applicable) Of sources in α.40: 1013 have no OC 844 of those could be HVCs (or LG minihalos) 199 (<2%) extragalactic Of those, <50 are isolated

38 Finding optical counterparts and confusion The centroiding accuracy for a mapping survey like ALFALFA goes roughly as For HPFW(PSF)/(S/N) where S/N is the signal to noise ratio of the detection. 3 4 For ALFALFA that is about Distant sources may be confused : which galaxy is responsible for the HI signal?

39 Position-switched observing The signals we are trying to take are billions (and billions) of times weaker than the radio noise contributed by the receiver, electronics, antenna, cosmic microwave background and the sky overall. Somehow, we have to subtract off all those unwanted contributions to find our signal. We assume that a random position in the sky does not contain an HI line source at the exact same velocity as our target source. We observe such a position, but track it over the exact same Az, ZA as our observations of the target source. => ON-OFF pair 39

40 Position-switched observing Position telescope on target source Track ON-source for 5 minutes Move to same Az,ZA as at start of ONsource track but ~6 mins from now; this is the OFF-source position. Track OFF-source for 5 minutes. Record noise with CAL (noise diode) ON for 10 secs; then record noise for 10 sec with CAL-OFF. Go to next target. Repeat ON-OFF-CAL ON/OFF sequence This is what the command file (for a set of sources for the whole night) does. 40

41 Estimating how long we should integrate The radiometer equation for our observations For LBW, T~ 30K G ~ 11 K/Jy Δf : is the bandwidth per channel t s : is the effective integration time, in secs f t : accounts for the degree of smoothing, the technique applied for bandpass subtraction, clipping losses, etc. The factor of 2 under the square root comes from the fact that we average the two independent polarizations. See e.g., Giovanelli , AJ 130, 2598 Jess lecture 41

42 LBW followup: WAPP search mode 42