Interferometric Radio Science
Tiziana Venturi
INAF, Istituto di Radioastronomia
4th ERIS, Rimini, 5 September 2011
Radio Astronomy at the cutting-edge of astrophysical
research
Roughly 70% of what we know today about the Universe and its
dynamics is due to radio astronomy observations, rather than
optical observations (from a presentation of Marcus Leech)
Outline
Very general introduction to Radio Astronomy & introduction to
the 4th ERIS
 Radio waves
 Angular resolution and need for interferometry
 Phase of the visibility function
 The u-v plane
 Mechanisms for radio emission in astrophysics
 The syncrotron radio spectrum
 New and upcoming facilities in the Northern and Southern
Hemispere
 The 4th ERIS
Radio Astronomy: wavelengths from a few mm to tens of meters
Visible light:
wavelengths in the region of 500nm, (5.0x10-7 m)
From a physics standpoint, there's no difference between visible light,
and microwave/radio-wave “light”.
Optical and Radio can be done from the ground
NRAO/AUI/NSF
5
Why radio interferometry
θ ~λ/ D
Ability to resolve fine detail highly dependent on wavelength
A 10cm optical telescope can resolve details that would require a
radio telescope over 42km in diameter at 21cm wavelength!
+
Earth rotation
synthesis
Sensitivity, however, is proportional to collecting area of the reflector,
regardless of wavelength
Angular resolutions at 20 cm (1.4 GHz)
Effelsberg
Connected elements
GMRT
EVLA D-array
D=100m θ ~ 9.4’
D=1km
θ ~ 44”
D=28km
θ ~ 1”
EVN
D=217 km θ ~ 150 mas
D~10000 km
θ ~ 5 mas
θ≈fraction of mas
HST
θ ~ 50 mas
(angular
resolution of
eMERLIN at
5 GHz)
Chandra
θ~1”
(angular
resolution of
the EVLA
Array A and
of the GMRT
at 1.4 GHz)
Overlay of the radio-optical & X-ray emission in a cluster of galaxies
Green
GMRT at
610 MHz
Optical
DSS-2
Red
Chandra
Overlay of the radio-optical & X-ray emission in Centaurus A
• Imaging in astronomy implies ‘making
a picture’ of celestial emission.
• We design instruments to make a
map of the brightness of the sky, at
some frequency, as a function of RA
and Dec.
• In astronomy, brightness (or specific
intensity) is denoted In,t(s).
• Brightness is defined as the power
received per unit frequency dn at a
particular frequency n, per unit solid
angle dW from direction s, per unit
collecting area dA.
• The units of brightness are in terms of
(spectral flux density)/(solid angle):
e.g:
•
watt/(m2 Hz Ster)
•
•
•
•
Image of Cygnus A at l = 6cm.
The units are in Jy/beam.
1Jy = 10-26 watt/(m2 Hz)
Here, 1 beam = 0.16 arcsec2
From R. Perley 2010
Main Issues with interferometric observations
Each pair of antennas
in an interferometer
is a baseline
Phase corruption
Calibration
u-v coverage
Deconvolution & Imaging
Amplitude carries
information on the
source intensity
Phase carries
information on
the source
absolute position
Amplitude uncertainties
and errors depend on the
individual antennas and
receivers
Phase errors depend on the
electronics, and on the different
propagation paths of the radio
signal through the atmosphere,
which introduce an unknown
quantity in the phase, which differs
from telescope to telescope
Changed positions
&
distorted sources
Ionosphere at low frequencies (and man-generated RFI !!!)
Absorption bands at high frequency in the troposhpere
The u-v plane
A radio interferometer array can be considered as a partially filled
aperture
- each pair of antennas gives a u-v point at a given time;
- the point source function (PSF, or beam) has a complicated structure, which
depends on the array, source declination and u-v coverage;
- the u-v plane shows what part of the aperture is filled by a telescope, and this
changes with time as the object rises and sets;
- a long exposure will have a better PSF/beam because there is better u-v plane
coverage (closer to a filled aperture)
The u-v plane is a plane tangential to the source in the celestial sphere. Each point on
that plane is the projection of a baseline at a given time.
Each pair of radio telescopes produces a track in the u-v plane.
The number of tracks is equivalent to N(N-1)/2, where N is the number of radio
telescopes in the interferometer.
ATCA – 1.4 GHz
Res. ~ 10”x5”
rms ~0.15 mJy/b
Southern Cluster of galaxies A3562
GMRT – 610 MHz
Res. ~ 8”x6”
rms ~ 0.08 mJy/b
Southern Cluster of galaxies A3562
GMRT – 610 MHz
Res. ~ 8”x6”
rms ~ 0.08 mJy/b
GMRT – 610 MHz
Res. ~ 30”x20”
rms ~ 0.14 mJy/b
Dirty Beams:
A snapshot (few min)
Full 10 hrs
VLA+VLBA+GBT
Sequence from Amy Mioduszewski (NRAO’s 2010 Synthesis Imaging Workshop)
What do we look at when we observe at radio frequencies?
Main mechanisms for radio emission
- Blackbody radiation
Cosmic Microwave
Background
- Thermal Bremsstrahlung
- Spectral lines from molecular
and atomic gas clouds
- Synchrotron radiation
- Thermal properties
- Ionized medium (T and ρ)
- Composition and properties (T and ρ)
of the ISM/IGM
- Relativistic electrons and magnetic
fields
Black body & Bremsstrahlung
radiation
• Emission from warm
bodies
– “Blackbody” radiation
– Bodies with temperatures of
~ 3-30 K emit in the mm &
submm bands
• Emission from
accelerating charged
particles
– “Bremsstrahlung” or freefree emission from ionized
plasmas
Neutral hydrogen (HI) line emission
Emits photon with a
wavelength of 21 cm
(frequency of 1.42
GHz)
Transition probability=3x10-15 s-1 = once in 11 Myr
Line emission
Molecular vibrational and rotational modes
• Commonly observed
molecules in space:
– Carbon Monoxide (CO)
– Water (H2O), OH, HCN,
HCO+, CS
– Ammonia (NH3),
Formaldehyde (H2CO)
• Less common molecules:
– Sugar, Alcohol, Antifreeze
(Ethylene Glycol), …
malondialdyde
Synchrotron radiation
Polarized emission provides information on the magnetic field
Spectrum of the synchrotron radiation
Turnover
Optically thin S α ν-α
Optically thick/Selfabsorbed
S α ν2.5
Aged part of the spectrum
due to radiative losses
S α ν-(α+k)
Different parts of the synchrotron spectrum provide different information on
the radio source and on the population of the radiating relativistic electrons
Example: an extragalactic radio source - 3C317
Steep spectrum
dominated by the
diffuse emission
Concave component
dominated by the
VLBI active nucleus
Synchrotron radio sources and their spectra
Radio galaxies on the kpc scale
WNB1127.5+4927
3C296
3C452
From M . Murgia 2008
Synchrotron radio sources and their spectra
Radio galaxies on the parsec scale
Synchrotron radio sources and their spectra
Diffuse cluster sources
Present and future radio facilities
Wide fields and the “weak” Universe
New and upgraded observational facilities over the whole radio window
are ready operational
ALMA
10 bands
from 35 to
850 GHz
Present and future radio facilities
Wide fields and the “weak” Universe
New and upgraded observational facilities over the whole radio window
are ready operational
ALMA
10 bands
from 35 to
850 GHz
EVLA
Complete
frequency
coverage from
1 to 50 GHz
Present and future radio facilities
Wide fields and the “weak” Universe
New and upgraded observational facilities over the whole radio window
are ready operational
ALMA
10 bands
from 35 to
850 GHz
EVLA
Complete
frequency
coverage from
1 to 50 GHz
ATCA from
2 to 86 GHz
Present and future radio facilities
Wide fields and the “weak” Universe
New and upgraded observational facilities over the whole radio window
are ready operational
ALMA
10 bands
from 35 to
850 GHz
EVLA
Complete
frequency
coverage from
1 to 50 GHz
eVLBI and
eMERLIN
from 1.6 to
22 GHz
ATCA from
2 to 86 GHz
Present and future radio facilities
Wide fields and the “weak” Universe
New and upgraded observational facilities over the whole radio window
are ready operational
ALMA
10 bands
from 35 to
850 GHz
EVLA
Complete
frequency
coverage from
1 to 50 GHz
eVLBI and
eMERLIN
from 1.6 to
22 GHz
ATCA from
2 to 86 GHz
GMRT
1.4 GHz –
240 MHZ
Present and future radio facilities
Wide fields and the “weak” Universe
New and upgraded observational facilities over the whole radio window
are ready operational
LOFAR
ALMA
10 bands
from 35 to
850 GHz
EVLA
Complete
frequency
coverage from
1 to 50 GHz
eVLBI and
eMERLIN
from 1.6 to
22 GHz
ATCA from
2 to 86 GHz
30-80 MHz
120-240 MHz
GMRT
1.4 GHz –
240 MHZ
Enjoy the Fourth ERIS!
Radio Astronomy: wavelengths from a few mm to tens of meters
Visible light:
wavelengths in the region of 500nm, (5.0x10-7 m)
From a physics standpoint, there's no difference between visible light, and
microwave/radio-wave “light”.
Living things have receptors for only a tiny part of the EM spectrum
s
s
Geometric
Time Delay
b
The path lengths
from sensors
to multiplier are
assumed equal!
A Sensor
X
multiply
average
Unchanging
Rapidly varying,
with zero mean
UV Plane Coverage and PSF
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
UV Plane Coverage and PSF
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
a snapshot of the U-V plane
(VLBA)
U-V coverage in a
horizon to horizon exposure
Long Baseline
Long
Baseline
Short Baseline
Short
Baseline
Antenna
fringe rates
Antenna
delays
active
phase
dying
phase
LOFAR
WENSS
NVSS
From M . Murgia 2008
GMRT-LOFAR
domain
EVLA
Domain
100-300 MHz
νobs < νs
Rare events
νs
More common
events
νs
Frequency