LOFAR
Low Frequency Radio Array
Veres Péter
Structure of the presentation
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Introduction
Technical data
Low Frequency Sky
Surveys
Z>>6 Radio Galaxies
Mpc Radio Galaxies
Radio Haloes
Low Frequency Variables
Large –Scale Structure
Global Reionization of the
Universe
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Thermal & Nonthermal
Emission
Compact Sources in Nearby
Galaxies
Polarization @ Low
Frequencies
HII Regions with Low
Frequecy
Supernova Remnants
(Extrasolar) Planets
Introduction
Netherlands Foundation for Research in Astronomy
Inventory of scientific programmes using a LOFAR
Hierarchical array of fixed elements operating at:10 - 300MHz
Existing LFRTs: GMRT in India and VLA in th US
LOFAR will provide:
Greater sensitivity
Greater spatial resolution
Greater frequency range
Greater freq. coverage & agility
In general
Multi beaming capability at full strength
 High speed data processing
 Extreme agility in frequency & pointing
 Ionospheric compensation
 Advanced computational techniques
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Low Frequency Sky
Surveys
At 20-100 MHz the sky is virtually unexplored
Expected from LOFAR:
• distant (z>6) radio galaxies
• extended (Mpc sized) and powerful galaxies @ z>1
• radio halos associated with X-ray clusters
• Low Frequency Variables & constraints on LSS of
the Universe with TPCF
• new galaxies in the neighborhood
Mpc-Sized Radio Galaxies @ z>1
Giant Radio Galaxies (GRG)
•projected linear dimensions >1 Mpc
•3C 236>5.7 Mpc
•expand well out of their clusters into the
IGM
probe of IGM
•verify AGN endstage theories
•many of these large sources are quasars ()
WNB 2147+816
z=0.15
linear size : 3.7 Mpc
Radio Haloes
Things to know about RH
 extended radio emitting plasma ~1 Mpc
Mostly present in clusters that:
 are extremely rich
 few spiral galaxies (~10%)
 large velocity dispersion (~1000 km/s)
 large X-ray luminosity
 Large core radii (>0.3 Mpc)
Explanation: a massive merger going on in the cluster, distorting its
morphology
Things sought:
Few radio haloes are catalogued (~10) because of low surface density.
LOFAR will provide large samples: study of merging activity associated
with still forming clustres
LOFAR is expected to find a few hundreds galaxies with radio haloes
Low-Frequency Variables
Things to know about LFR:
• deeply burried inside the host galaxy
• size less than a few tenths of arcsecs
• the cause: Refractive Interstellar Scintilation
• only present at low frequencies
Things to expect
• if the LFVs turn out to be high redshift objects they
define a clean sample of galaxies
• if they are as small as we think
they’re very
young, we can obtain data abut how strong radio
sources form
• perhaps they’re confined in dense ISM
might explain why is ISM so dense
Large-Scale Structure
Things known:
 at z~1 there were a hundred times more
galaxies than is today
 evidence of anisotropies of radio sources
e.g. bright sources seem to be more
concentrated than fainter ones (but we don’t
know why)
Expected from LOFAR:
• cleaner sample of intrinsically bright, not
because of Doppler boosting
Global Reionization of the Universe
What we know:
• the epopch of galaxy formation lies somewhere between
z=5-10
• protogalaxy contenders are very abundent (one per
square minute) (search for high –z quasars takes very
long optical time)
• at z>5 (today’s record) the 21cm line is shifted below
230MHz
What is thought, expected:
• Individual structures around the reionization era aren’t
massive enough to detect with e.g. SKA
• Reionization edge may be detectable (Shaver)
Consequence: direct measurement of th e baryon
content of the Univese. For this a full range (110250MHz) is needed
• LOFAR is perfectly suitable for this job. Expected
integration time is one day
Thermal & Nonthermal Emission in
Nearby Galaxies
Basics:
 Continuum emission from galaxies give off information
about their ISM
 @ low frequencies synchrotron is th dominating
mechanism
 In the 20-300 MHz range we expect to see th e old
population of relativistic electrons coming from SNRs
 With LOFAR separation of thermal and non-thermal
emission will be possible
 Spectral index variation across galaxies clues to
relativistic electron production & evolution
 Thermal component dominates above 300 MHz
Importance: least biased view of star formation
Compact Sources in Nearby
Galaxies
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LOFAR may be used to search for
compact non-thermal sources in the
neighbouring galaxies
Possibly SNRs but there’s a chance for
detexting pulsars as well
Search for low frequency nuclear emission
is also an issue (e.g. in the center of the
MW & M31). Their nature is uncertain
Radio Polarization @ Low
Frequencies
The polarization offers insight into:
 Galactic magnetic field
 plasma turbulence
 ionized components at any temperature
Mechanism: Faraday rotation
Low Frequency Recombination Line
Observations of Galactic HII Regions
What we know about the ionized ISM:
 from optical and radio samples: T=5000 to 15000K
What we would like to see:
 cool ionized gas sampled by low frequecy radio
recombination lines (20-100MHz)
 n=400-700(alpha)
 n=500-750(beta)
 atoms are ~20 microns and very sensitive to their
surroundings – probes of ambient physical conditions
 probably carbon, not hydrogen
Supernova Remnants and Pulsars
Continuum imaging and spectral indices
What we know:
 SNR are responsable for the
Galactic cosmic rays and most
of the synchrotron radiation
 models constructed agree with
observations alpha=-0.5 (-0.3
thru -0.8)
 same spectral index
everywhere (ENIGMA)
What is needed:
 high resolution SNR images
over a wide Freq. range (wide
range = more precise alpha)
 filaments vs. smooth
component problem
 IC 443
Jupiter
Things known:
 Jupiter produces decametric bursts at
frequencies up to 40 MHz (cause: rotation,
placement & Io )
 size of source : less than 500 km
 process: unknown
 highly circularly polarized suggesting cyclotron
emission
What is expected?
 the resolution of 1/10 D (Jup) enables to track
the source
Extrasolar Planets
What is known about exoplanets?
 Jupier sized (if we’re lucky)
 d<1 A.U.
 their observation is possibly selection bias?
How does Radio Astronomy fit in?
• as seen at Jupiter (Jupiter has Io within its
magnetosphere)
• detection of decameter bursts offers lower mass planets
at any distances form their parent star
• Sun’s interference: sporadic bursts can be detached
from the signal
• chances: 0.4 mJ @ 1pc or 4 microJ @ 10 pc (marginal)
Summary
Science Taxonomy
LOFAR Science is being categorized according to the Science Taxonomy. The
main categories, together with the initial point of contact (SCB Member) are:
 100 : Cosmological Studies (Reionization) - Ger de Bruyn (ASTRON)
 200 : Extragalactic Surveys - Huub Röttgering (Leiden)
 300 : Acceleration, Turbulence & Propagation in the ISM - Jim Cordes
(Cornell)
 400 : Targeted Extragalactic Observations - Frazer Owen (NRAO)
 500 : Galactic Surveys - Bryan Gaensler (Harvard)
 600 : Transients - Rob Fender (Amsterdam) and Colin Lonsdale (Haystack)
 700 : Solar System - Namir Kassim (NRL)
 800 : Ionosphere - Namir Kassim (NRL)
 900 : Active Observations - Namir Kassim (NRL)
THE END