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Deep Extragalactic Surveys
One of the most important applications of LOFAR will be to carry
out large-sky surveys. Such surveys are well suited to the characteristics
of LOFAR and have been designated as one of the key projects that
have driven LOFAR since its inception. Such deep LOFAR surveys of
the accessible sky at several frequencies will provide unique catalogues
of radio sources for investigating several fundamental areas of
astrophysics, including the formation of massive black holes, galaxies
and clusters of galaxies. Because the LOFAR surveys will probe unexplored
parameter space, it is likely that they will discover new phenomena.
The following known classes of extragalactic objects are of particular
interest:
Most Distant Radio Galaxies
The most efficient method for finding extremely
distant radio galaxies uses an empirical correlation between radio
spectral steepness and distance. Using this method, LOFAR will efficiently
pick out radio galaxies at larger distances than possible with present
radio telescopes. Detecting such objects before the epoch of reionisation
would be particularly important. Not only would this yield important
constraints on how and when massive black holes are formed, but
this would also allow detailed studies of the ISM at these high
redshifts through redshifted 21 cm absorption studies. Study of
these distant radio galaxies at other wavelengths will provide information
about the formation of massive galaxies and links between nuclear
activity and star formation. Since distant radio galaxies pinpoint
proto-clusters, studying the environments of these distant galaxies
will constrain the formation of clusters at the earliest epochs.
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Distant radio galaxies
are massive galaxies undergoing formation. This picture
taken with the Hubble Space Telescope shows that the distant
radio galaxy 1138–262 (z=2.2) is extremely clumpy. The
structure bears a remarkable resemblance to the predicted images
from computer models depicting the birth of massive galaxies
in clusters, through the merging together of large number of
small galaxies (from Pentericci et al 1999). LOFAR will locate
and study the most distant forming massive galaxies. |
Diffuse Emission in Galaxy Clusters
Clusters often contain diffuse radio sources that are shaped by
the dynamics of the gas in which they are embedded. Their large
extent, low surface brightness, and steep spectra makes these cluster
sources difficult to study with conventional facilities, such as
Westerbork and the VLA. Because of their steep spectra, LOFAR will
be able to detect and study such radio sources in the many tens
of thousands of clusters up to redshifts of two that will be detected
using the XMM-Newton X-ray telescope, the Planck satellite, and
the Sloan Digital Sky Survey. Such studies will be very relevant
for (i) understanding the effects on the dynamics of cluster gas
due to shock waves produced by cluster mergers, (ii) determining
the origin of cluster magnetic fields, and (iii) measuring the occurrence
and characteristics of diffuse cluster radio sources as a function
of redshift with the aim of constraining physical models for the
origin of these sources.
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Examples of cluster radio
sources showing interaction with the intra-cluster gas.
Small panels - left. Radio images of 4 diffuse radio sources
in nearby clusters showing the filamentary structure of the
synchrotron emitting plasma shaped by shocks in the cluster
gas (Slee et al 2001).
Larger panel - right. Simulation of interaction between a
cluster radio source and the intracluster medium. This simulation
takes account of the appropriate gas dynamics and magnetic
field configuration during the evolution of shocks in the
cluster gas (Ensslin and Brüggen 2002).
The large numbers of cluster radio halos that will be observable
with LOFAR will be used to probe evolution in the intra-cluster
gas due to e.g. merger-induced shocks out to a redshift of
z ~1.
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Star-forming Galaxies
Because of the large fields of view, surveys at the higher LOFAR
frequencies will detect unprecedented numbers of star-forming galaxies
with star formation rates of tens of solar masses per year at an
epoch at which the bulk of galaxy formation is believed to occur.
Since the ratio of radio flux to sub-mm flux is a sensitive redshift
indicator, LOFAR surveys, in combination with data from new far-IR
and mili-meter facilities such as the SCUBA-2 sub-mm imaging array,
the Spitzer and Herschel satellites and the ALMA (sub) mm array,
will also provide distances and thus facilitate a census of the
cosmic star-formation history, unhindered by the effects of dust
obscuration.
For nearby galaxies, LOFAR will enable spectral mapping at low frequencies,
thereby providing unique information about absorption and delineating
the spatial distribution of the warm ISM and galactic magnetic fields.
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Radio emission from
M82, the best studied "local" starburst galaxy
(from Muxlow et al 1995). This galaxy is forming new stars at
a rate of about 10 solar masses per year. The many clumps are
due to the remnants of exploding stars ("supernovae”).
LOFAR will locate and study an unprecedented number of distant
starburst galaxies. |
The survey key project will be optimised for obtaining
large samples of the various categories of extragalactic radio emitting
objects listed above. The provisional plan is to survey the entire
accessible sky at a number of “key frequencies”, 30,
75, 120, and 200 MHz (initially). An additional key frequency of
15 MHz will be added after the 10-30 MHz optimized LOFAR upgrade
has been implemented. The surveys will be carried out over several
epochs, enabling variable sources to be recognised.
More information:
see NL
science case for LOFAR (pdf)
<
In case of questions
or comments regarding LOFAR, or about these web pages, please contact
lofar@astron.nl
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