Transcript Document

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
10-23
Radio Continuum
Emission:
• non thermal
(= synchrotron
radiation)
• general ISM,
SNRs
• AGN
• PSRs
• thermal
(= Bremsstrahlung)
• HII regions
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
Our milky way across the electromagnetic spectrum
HI
CO
60 – 100 m
2 – 4 m
The 21-cm Neutral Hydrogen Line
G a l a c t i c
p l a n e
All-sky map of emission in the 21-cm line
Hartmann & Burton
Carbon monoxide (CO)
emission
[CO/H2] 10-4
[all other molecules/H2] << [CO/H2]
Columbia/CfA CO survey
(Dame/Thaddeus et al.)
Millimeter
Submillimeter
COBE FIRAS
7 resolution
Fixsen et al. 1994
Interstellar medium cartoon
Galactic plane
Dense cloud cores Supernova
*
very hot low density gas
diffuse cloud
Giant Molecular Cloud (GMC)
new stars
(IR sources)
Giant Molecular Clouds
Typical characteristics of GMCs:
= 104...106 M
–
Mass
–
Distance to nearest GMC
–
Typical size
–
Size on the sky of near GMCs = 5...dozens x full
moon
–
Average temperature (in cold parts) = 20...30 K
–
Typical density
–
Contain ca. 1% dust (by mass)
–
Typical (estimated) life time
–
Star formation efficiency
= 450 pc (Orion)
= 5...100 pc
= 102...106 molecules/cm3
= ~107 year
= ~1%...10%
Half-power beamwidth
Full width at half maximum
(FWHM) 1.22 /D
Response of a radio
telescope to radiation
Main beam B
FWHM
Full width at half maximum
FWHM=1.22/D
“Error beam”
Error beam can pick up
significant part of the signal,
up to 50%
1.22 /D
Effelsberg 100m
IRAM 30m
APEX 12m
B = 22 @ 44 GHz
B = 4’ @ 4.0 GHz
B = 22 @ 112 GHz
B = 22 @ 380 GHz
(Telescopes are not reproduced on same scale)
 I beam  fB  1  e
f 1
f 1
f
is called the filling factor

beam
f 1
Our milky way across the electromagnetic spectrum
HI
Atomic Gas: H
CO
Molecular Gas: H2
60 – 100 m
2 – 4 m
 rays
All interstellar matter
f 1

TL  fT (1  e )
Empirical CO column density determination:
  1  TL  fT
  1  TL  fT
• HE (~100 MeV – few GeV) -ray emissivity 
  N cm
2

CO emission is
always optically thick
number of nucleons
• CO emissivity WCO(K km s-1)  -ray emissivity
 N(cm-2) = XWCO or n(cm-3) = X/l WCO
Moriguchi
(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.
The Galactic Center Region as seen by SCUBA at 850 m
Pierce-Price et al. 2000
Single dish:  = /D
D
Interferometer:
 = /B
B
Largest structure that can be
imaged given by telescope
diameter  zero spacing
problem

s
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:



V b   I s e
2i  

sb
c

b
d
• Telescopes can be combined all over the world: Very Long
Baseline Interferometry (VLBI)  (sub)milliarcsecond
resolution
ALMA snapshot
4.9 GHz/instantaneous
sampling of a source at  = 30
and hour-angle 0 /VLA/A
configuration.
Central hole
More data points are filled in as the Earth rotates
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)
Angular Resolution
Single dish:  = /D
D
Interferometer:
 = /B
B
Largest structure that can be
imaged given by telescope
diameter  zero spacing
problem
Largest Angular Scale
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)
SNR RXJ713.7-3946 a.k.a. G347.3-0.5
Radio void
HESS peak
ATCA
40” beam
ROSAT
Lazendic et al. 2004
Interferometer field of view
= FWZP of unit telescope
“Mosaicing”
1357 MHz
2495 MHz
ATCA
NRAO VLA Sky Survey
Aharonian et al. 2005
Brogan et al. 2005
March
2007
J1640-465
ASCA Source
MOST 843 MHz
B = ca. 2 arcmin
Whiteoak & Green 1996
Aharonian et al. 2006
Funk et al. et al. 2007
Chandra
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)
Chajnantor
SW from Cerro Chajnantor, 1994 May
AUI/NRAO S. Radford
Complete Frequency
Access
Note: Band 1 (31.3-45 GHz) not shown
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
ALMA – Extreme Configurations
Most compact:
Most extended:
10,000m
150 m
Very small field of view: 20” FWHM at 300 GHz
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
The APEX telescope
Built and operated by
• Max-Planck-Institut fur Radioastronomie
• Onsala Space Observatory
• European Southern Observatory
on
Chile
10%
Llano de Chajnantor (Chile)
OSO
Longitude: 67° 45’ 33.2” W
MPG
21%
45%
Latitude: 23° 00’ 20.7” S
ESO
Altitude: 5098.0 m
24%
• 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 Bolometer Camera (LABOCA)
http://www.mpifr-bonn.mpg.de/div/mm/apex/
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.
Common sense
requirements:
Important:
Schuster et al. 2004
http://iram.fr/IRAMES/telescope/HERA/
• Uniform beams
• Uniform TRX
and
TRX not “much” worse than TRX of
state-of-the-art single pixel RX
Columbia/CfA 1m CO
J = 1  0 (115 GHz)
FWHM = 8.7 arcmin
FWHMeff= 30 arcmin
Ungerechts & Thaddeus 1987
IRAM 30m
CO J = 2  1 (231 GHz)
HERA 9 x 11”
Schuster et al. 2004
CHAMP+
Carbon Heterodyne Array
of the MPIfR
• 2 x 7 pixels
• frequency range 602 – 720 and
790 – 950 simultaneously
• beamsize 9" – 7" and 7" – 6"
• IF band 4 – 8 GHz
Philipp et al. 2005
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)
COBE FIRAS
7 resolution
Fixsen et al. 1994
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
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
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.
Frequency – Resolution Coverage
●
●
●
●
●
A key EVLA requirement is
continuous frequency coverage
from 1 to 50 GHz.
Additional EVLA
Coverage
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
Sensitivity Improvement 1s, 12 hours
Red: Current VLA,
Black: EVLA Goals
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”)
One part of the EVLA plan currently not funded
is the “E”-configuration, which would give much
better response to extended structure
E configuration would allow
high fidelity imaging of 10’
sized structures up to 5 GHz
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 pulsars
and 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