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|Title: ||Deep multi-frequency radio observations of the SHADES fields and the nature of the faint radio populaton|
|Authors: ||Ibar, Eduardo|
|Supervisor(s): ||Ivison, Rob|
|Issue Date: ||2009|
|Publisher: ||The University of Edinburgh|
|Abstract: ||The two SCUBA HAlf-Degree Extragalactic Survey (SHADES) fields are amongst the
richest places in the sky in terms of multi-wavelength coverage. They comprise an
eastern section of the Lockman Hole (LH) and the central portion of the Subaru-
XMM/Newton Deep Field (SXDF).
In this thesis, I have obtained extremely deep, multi-frequency radio imaging of
the SHADES fields using the GiantMetre-wave Radio Telescope (GMRT) and the Very
Large Array (VLA), at 610MHz and 1.4GHz, respectively.
These data are used to analyse the nature of the sub-milliJansky (sub-mJy) radio
population, which has been hotly debated in the last few years: are they powered by
star-forming or nuclear activity? To tackle the problem, I employ different approaches
making use of the large variety of multi-wavelength data in the SHADES fields.
I begin by analysing the spectral index, α610MHz
1.4GHz , of radio sources detected in the
LH, to explore the dominant emission mechanism. Based on a robust 10 σ detection
criterion, I find a constantmedian spectral index of α610MHz
1.4GHz ≈ −0.6 to −0.7 for sources
between S1.4GHz ≈ 200 μJy and 10mJy. This result suggests that the galaxy population
in the sub-mJy regime is powered by optically-thin synchrotron emission – starforming
galaxies or lobe-dominated active galactic nuclei (AGN).
Making use of X-ray observations in the LH, I show that the fraction of radio
sources detected in the hard X-ray band (between 2 and 10 keV) decreases from 50
to 15 per cent between S1.4GHz ≈ 1mJy and . 100 μJy, which strongly suggests a
transition from AGN to star-forming galaxies.
Based on the deep, multi-wavelength coverage of the SXDF, I explore the behaviour
of the far-infrared (FIR)/radio correlation as a function of redshift. I combine
the q24 factor – the logarithmic flux density ratio between Spitzer 24-μm and VLA 1.4-
GHz flux densities – with available photometric redshifts and find strong evidence
that the correlation holds out to z ≈ 3.5. Based on M82-like k-corrections and using
a high-significance (S1.4GHz > 300 μJy) radio sub-sample, I find a mean and scatter
of q24 = 0.71 ± 0.47. Monte-Carlo simulations based on these findings show that
fewer sources deviate from the correlation at fainter flux densities (i.e. fewer radioloud
AGN). I predict that the radio-loud fraction drops from 50 per cent at ∼ 1mJy to
zero at . 100 μJy.
The validity of the FIR/radio correlation out to very high redshifts adds credibility
to identifications of sub-millimetre (submm) galaxies (SMGs) made at radio wavelengths.
Based on a sample of 45 radio-identified SMGs in the LH, I find a median
radio spectral index of α610MHz
1.4GHz = −0.72 ± 0.07, which suggests that optically-thin
synchrotron is the dominant radio emission mechanism.
Finally, as anAppendix I include a theoretical treatment that constrains the average
geometry of the dusty, torus-like structures believed to obscure a large fraction of the
AGN population. I use the distribution of column densities (NH) obtained from deep
∼ 1Msec X-ray observations in the Chandra Deep Field South. I find that to reproduce
the wide observed range of NH, the best torus model is given by a classical “donut”-
shaped distribution with an exponential angular dependency of the density profile.|
|Sponsor(s): ||Particle Physics and Astronomy Research Council – PPARC (now the Science and Technology Facilities Council – STFC) and Fundación Andes|
|Keywords: ||multi-wavelength coverage|
optically-thin synchrotron emission
active galactic nuclei
|Appears in Collections:||Physics thesis and dissertation collection|
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