radio and (sub)millimeter astronomy during the next 10 years or so… relevance for a cherenkov...
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Radio and (Sub)millimeter Astronomy During the Next 10 Years or So…
Relevance for a Cherenkov Telescope Array
Karl M. Menten
Max-Planck-Institut für Radioastronomie, Bonn
CTA Meeting, Paris March 1, 2007
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10-2
3
Radio Continuum Emission:
• non thermal (= synchrotron radiation)
• general ISM, SNRs
• AGN
• PSRs
• thermal (= Bremsstrahlung)
• HII regions
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Thermal emission can also be observed in spectral lines:
Radio: 21 cm line of neutral hydrogen HI (1421 MHz)
(Sub)mm: Rotational emission from CO: 115.5 GHz and multiples thereof
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Our milky way across the electromagnetic spectrum
CO
HI
60 – 100 m
2 – 4 m
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The 21-cm Neutral Hydrogen Line
All-sky map of emission in the 21-cm line
G a l a c t i c p l a n e
Hartmann & Burton
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Columbia/CfA CO survey (Dame/Thaddeus et al.)
1.2 m
Carbon monoxide (CO) emission
[CO/H2] 10-4
[all other molecules/H2] << [CO/H2]
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COBE FIRAS 7 resolution Fixsen et al. 1994
Millimeter
Submillimeter
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Galactic plane
Interstellar medium cartoon
very hot low density gas
diffuse cloud
Giant Molecular Cloud (GMC)
Dense cloud coresSupernova
new stars(IR
sources)
*
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Giant Molecular Clouds
Typical characteristics of GMCs:
– Mass = 104...106 M
– Distance to nearest GMC = 450 pc (Orion)– Typical size = 5...100 pc– Size on the sky of near GMCs = 5...dozens x full moon– Average temperature (in cold parts) = 20...30 K– Typical density = 102...106 molecules/cm3
– Contain ca. 1% dust (by mass)– Typical (estimated) life time = ~107 year– Star formation efficiency = ~1%...10%
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Half-power beamwidth
Full width at half maximum (FWHM) 1.22 /D
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Response of a radio telescope to radiation
Main beam B
Full width at half maximum FWHM=1.22/D
FWHM
“Error beam”
Error beam can pick up significant part of the signal, up to 50%
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B = 22 @ 380 GHz
APEX 12m
B = 22 @ 112 GHz
IRAM 30m
1.22 /D
(Telescopes are not reproduced on same scale)
B = 22 @ 44 GHz
Effelsberg 100m
B = 4’ @ 4.0 GHz
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beambeam efBI 1
1f 1f
is called the filling factorf
1f
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Our milky way across the electromagnetic spectrum
CO
HI Atomic Gas: H
Molecular Gas: H2
60 – 100 m
2 – 4 m
rays All interstellar matter
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2cm
1
1
)1(
N
fTT
fTT
efTT
L
L
L
1f
Empirical CO column density determination:
• HE (~100 MeV – few GeV) -ray emissivity number of nucleons
• CO emissivity WCO(K km s-1) -ray emissivity
N(cm-2) = XWCO or n(cm-3) = X/l WCO CO emission is always optically thick
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Moriguchi
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The Galactic Center Region as seen by SCUBA at 850 m
Pierce-Price et al. 2000
(Optically thin) (sub)millimeter continuum emission from interstellar dust is an excellent column density probe
Problem: Weakness of emission. Need N > a few 1022 cm-2 to make large-scale mapping practical.
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DSingle dish: = /D
B
Interferometer: = /B
Largest structure that can be imaged given by telescope diameter zero spacing problem
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desIbVbs
c
i
2
Interferometry
• combine signals from two antennas separated by baseline vector b in a correlator; each sample is one
“visibility”
• each visibility is a value of the spatial coherence function V (b) at
coordinates u and v
• obtain sky brightness distribution by Fourier inversion:
s
b
• Telescopes can be combined all over the world: Very Long Baseline Interferometry (VLBI) (sub)milliarcsecond
resolution
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4.9 GHz/instantaneous sampling of a source at = 30 and hour-angle 0 /VLA/A configuration.
More data points are filled in as the Earth rotates
ALMA snapshot
Central hole
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The Very Large Array (VLA)
• Built 1970’s, dedicated 1980
• 27 x 25m diameter antennas
• Two-dimensional 3-armed array design
• Four scaled configurations, maximum baselines 35, 10, 3.5, 1.0 Km.
• Eight bands centered at .074, .327, 1.4, 4.6, 8.4, 15, 23, 45 GHz
• 100 MHz total IF bandwidth per polarization
• Full polarization in continuum modes.
• Digital correlator provides up to 512 total channels – but only 16 at maximum bandwidth.
VLA in D-configuration(1 km maximum baseline)
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Angular Resolution
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DSingle dish: = /D
B
Interferometer: = /B
Largest structure that can be imaged given by telescope diameter zero spacing problem
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Largest Angular Scale
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The Australia Telescope Compact Array
Six 22m diameter antennas movable in E-W direction
Most interesting for CTA: L- and S-band (1350 and 2700 MHz)
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Radio void HESS peak
SNR RXJ713.7-3946 a.k.a. G347.3-0.5
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ROSAT
ATCA 40” beam
Lazendic et al. 2004
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Interferometer field of view = FWZP of unit telescope
“Mosaicing”
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1357 MHz 2495 MHz
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NVSS “offi
cial
ly” s
tops
here
ATCA NRAO VLA Sky Survey
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Aharonian et al. 2005
Brogan et al. 2005
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March 2007
Aharonian et al. 2006 Funk et al. et al. 2007
MOST 843 MHz B = ca. 2 arcmin
Whiteoak & Green 1996
ASCA Source
J1640-465
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Chandra
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ALMA Science Requirements• High Fidelity Imaging • Precise Imaging at 0.1” Resolution• Routine Sub-mJy Continuum Sensitivity• Routine mK Spectral Sensitivity• Wideband Frequency Coverage• Wide Field Imaging Mosaics• Submillimeter Receiver System• Full Polarization Capability• System Flexibility (Total Power capability on
ALL antennas)
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Chajnantor
SW from Cerro Chajnantor, 1994 May AUI/NRAO S. Radford
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Complete Frequency Access
Note: Band 1 (31.3-45 GHz) not shown
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ALMA Specifications• 50 12-m antennas, at 5000 m altitude site• Surface accuracy 25 m, 0.6” reference pointing
in 9m/s wind, 2” absolute pointing all-sky• Array configurations between 150m to ~15km• 10 bands in 31-950 GHz + 183 GHz WVR. Initially:
• 86-119 GHz “3”• 125-163 GHz “4”• 211-275 GHz “6”• 275-370 GHz “7”• 385-500 GHz “8”• 602-720 GHz “9”
• 8 GHz BW, dual polarization• Interferometry, mosaics, & total-power observing• Correlator: 4096 channels/IF (multi-IF), full Stokes• Data rate: 6Mb/s average; peak 60Mb/s
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150 m
Very small field of view: 20” FWHM at 300 GHz
ALMA – Extreme Configurations
Most compact:
10,000m
Most extended:
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The CTA will have an angular resolution of ca. 2 arcmin.
Most HESS sources are extended on 10’s of arcmin to ~1 degree scale
In radio and (sub)mm, want imaging capability that allows good fidelity multi-wavelength imaging that recovers these structures.
• Radio: Interferometer multi- (at least 2-), long wavelengths
• (Sub)mm: Single dish telescopes with spectral line receiver arrays
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The APEX telescopeBuilt and operated by• Max-Planck-Institut fur Radioastronomie• Onsala Space Observatory• European Southern ObservatoryonLlano de Chajnantor (Chile)Longitude: 67° 45’ 33.2” WLatitude: 23° 00’ 20.7” SAltitude: 5098.0 m
• 12 m• = 200 m – 2 mm• 15 m rms surface accuracy• In opertaion since September 2005• First facility instruments:
• 345 GHz heterodyne RX• 295 element 870 m Large Apex Bolo-
meter Camera (LABOCA) http://www.mpifr-bonn.mpg.de/div/mm/apex/
MPG45%
ESO24%
OSO21%
Chile10%
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To study larger-scale molecular cloud environments, degree-scale areas have to mapped.
CO lines are relatively strong.
• Still: 1 deg2 40000 APEX beam areas
Advantages of array receivers:
• Mapping speed
• Mapping homogeneity (map lage areas with similar weather conditions/elevation) minimize calibration uncertainties.
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Important:
• Uniform beams
• Uniform TRX
and
TRX not “much” worse than TRX of state-of-the-art single pixel RX
Common sense requirements:
Schuster et al. 2004
http://iram.fr/IRAMES/telescope/HERA/
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Columbia/CfA 1m CO J = 1 0 (115 GHz)
FWHM = 8.7 arcmin FWHMeff= 30 arcmin
IRAM 30m CO J = 2 1 (231 GHz)
HERA 9 x 11”
Factor ~160
in resolution!
Schuster et al. 2004
Ungerechts & Thaddeus 1987
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• 2 x 7 pixels
• frequency range 602 – 720 and 790 – 950 simultaneously
• beamsize 9" – 7" and 7" – 6"
• IF band 4 – 8 GHz
CHAMP+Carbon Heterodyne Array of the MPIfR
Philipp et al. 2005
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COBE FIRAS 7 resolution Fixsen et al. 1994
Covered now by CHAMP+@APEX 7 450 m/7 350 m array
Will be Covered by APEX 7 870 m/19 600 m array (to arrive in 2008)
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The APEX Galactic Plane survey
• Image continuum emission from interstellar dust over -80° < l < +20° ; | b | < 1°• Instrumentation: LABOCA (Large APEX BOlometer CAmera) = 295 bolometers for observing at 870 m
• APEX beam at 870 m:18"= MSX pixels = Herschel at 250 m
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Other Submillimeter Facilities in the high Atacama desert:• ASTE – The Atacama Submillimeter Telescope Experiment
• 10m
• NAO Japan, Tokyo U., Osaka Prefecture U., U. Chile
• Nanten-2
• 4m
• Nagoya U., Osaka Prefecture U., Seoul National U., Cologne U., Bonn U., U. Chile
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The Expanded Very Large Array
The EVLA Project:– builds on the existing infrastructure - antennas, array,
buildings, people - and, – implements new technologies to produce a new array
whose top-level goal is to provide
Ten Times the Astronomical Capability of the VLA. – Sensitivity, Frequency Access, Image Fidelity, Spectral
Capabilities, Spectral Fidelity, Spatial Resolution, User Access
– With a timescale and cost far less than that required to design, build, and implement a new facility.
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Frequency – Resolution Coverage
● A key EVLA requirement is continuous frequency coverage from 1 to 50 GHz.
● This will be met with 8 frequency bands:
– Two existing (K, Q)– Four replaced (L, C, X, U)– Two new (S, A)
● Existing meter-wavelength bands (P, 4) retained with no changes.
● Blue areas show existing coverage.
● Green areas show new coverage.
Current Frequency Coverage
Additional EVLA Coverage
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Sensitivity Improvement 1s, 12 hours
Red: Current VLA, Black: EVLA Goals
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This talk concentrated on observations of extended objects.
Needless to say, the greatly enhanced point source sensitivity of the EVLA will greatly enhance observing capabilities for compact sources (AGN, pulsars, GRBs)
• LSI+61303 is also a famous radio source!
• All the PKS objects are strong radio sources
Problem: No good VLBI capability in the southern hemisphere
Even greater sensitivity will be provided by the Square Kilometer Array (“A hundred times the VLA”)
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One part of the EVLA plan currently not funded is the “E”-configuration, which would give much better response to extended structure
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E configuration would allowhigh fidelity imaging of 10’ sized structures up to 5 GHz
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Some conclusions:Some conclusions:• Long wavelength radio continuum observations can give interesting complemenary data to the CTA
• Relation of radio continuum emission to VHE ray emission presently unclear (“What makes a VHE ray source radio=loud?”)
• Need targeted radio observations. Survey data not sufficient
• (Sub)millimeter spectral line observations show were the baryons are. Can provide information on the column densities and dynamics of molecular material in the vicinity of VHE ray sources
• Didjn’t talk about high resolution radio observations of pulsarsand extragalactic VHE ray sources
All of the above will greatly be enhanced by capabilities that come available within the next 3 – 4 years
It would be good to have an EVLA in the southern hemisphere