Tom Muxlow

1
The statistical properties of the very weak radio
source population in the GOODS/ACS HDF-N region

• Tom Muxlow
• At the Edge of the Universe Sintra
  Oct 12th 2006
• The JBO group

Rob Beswick, Anita Richards, Hannah Thrall,
Simon Garrington, Alan Pedlar
2
Star-formation at High Redshift

• Deep galaxy studies indicate that early galaxies
  merge to form larger systems in a bottom-up
  scenario of galaxy assembly
• This implies that galaxy-galaxy interactions were
  common at early epochs
• Such interactions are likely to trigger major
  star-formation activity.

HDF (N)
Multi-wavelength studies of a number of fields
including deep radio observations involving VLA,
ATCA, MERLIN, EVN have shown that at flux
densities lt1mJy (1.4GHz) there is new population
of faint radio sources that are associated with
distant star-forming galaxies
Richards, Fomalont, Kellermann, Windhorst,
Condon, Norris, Garrett, Muxlow.
3
Characterizing the radio properties of nearby
star-forming galaxies

• eg - Arp 220 and M82
• Used to calibrate our models ? move to high z

• Size of star-forming (starburst) region typically
  few kpc
• Radio synchrotron emission from plasma (SNR)
• Strong FIR emission from UV-heated dust (O stars)
• SFR can be estimated from FIR and radio
  luminosities found to be correlated over many
  orders of magnitude

77 Mpc
Arp 220 HST ACS
3.2 Mpc
M82 Spitzer
4
Deep HDF-N MERLIN VLA data

• Original investigation - covers 10 arcmin2
  centred on the HDF-N
• High angular resolution 0.2-0.5 arcsec (postage
  stamp images)
• rms 3.3µJy/bm - one of the most sensitive 1.4GHz
  images made
• Precis of results
• 92 radio sources with flux densities gt40µJy.
• Angular sizes of 0.23
• 85 are associated with galaxies brighter than
  25th mag
• Remaining 15 are optically faint EROs at high
  redshift (some seen at sub-mm)
• Below 60?Jy sources are dominated by starburst
  systems
• Some high redshift starbursts show evidence for
  embedded AGN
• VLA Richards et al 2000 VLBI Garrett
  et al 2001
• Seungyoup Chi
• MERLINVLA Muxlow et al 2005

Starburst systems are identified as sources with
steep radio spectral index and with radio
structures extended on (sub-)galactic scale sizes
overlaying the central region of the optical
galaxy
5
GOODS NORTH New ACS Spitzer data

• Latest developments utilising the new GOODS
  north data
• ? deeper wider field HST ACS images deep
  Spitzer images
• 8.5 x 8.5 contiguous MERLINVLA radio field
  centred on HDF-N intersects with 13030 galaxies
  brighter that 28.3mag in ACS z-band field
• Using these new data we can now extend the
  analysis, to investigate statistically the very
  weak radio source population below 20µJy.
• Rob Beswick, Hanna Thrall, Tom Muxlow, Anita
  Richards, Simon Garrington.

See related posters from co-workers Rob
Beswick Sub-arcsec radio properties of MIR
sources in GOODS HDF-N Hannah Thrall Faint radio
counterparts to optical galaxies in HDF-N Anita
Richards High-z radio starbursts hosting
obscured X-ray AGN
6
Radio emission from ACS galaxies

• Radio flux density within 0.75 arcsecond of all
  13030 z-band optical galaxies in the 8.5 field,
  binned by magnitude
• (Note excluding all bright radio sources with
  S1.4gt40µJy close galaxy pairs)
• Control incorporates a random 7 arcsecond
  shift

Of the 2700 galaxies brighter than Z24mag,
around 1400 will have radio flux densities of
4µJy or greater
(8s for a deep e-MERLIN/EVLA image)
7
Radio source sizesvery weak (sub-20µJy) radio
sources
Average radio source sizes in each magnitude bin
can be derived from flux densities found in
annuli over radii of 0.25-2 arcsec
For detected systems (brighter than 25mag)
average radio source radii are in the range 0.6
0.8 arcsec Sub-arcsec angular resolution
required to image such systems
8
Starburst Luminosities
For those weak sources which we can at present
only study statistically... e-MERLIN, EVLA and
e-VLBI should image gt1000 starburst systems to
4µJy with perhaps 150-200 at high redshift in a
single field. Many thousands of systems with
radio flux densities lt1µJy will be studied
statistically
SKA and ALMA will ultimately extend this by an
additional order of magnitude. With more
redshifts, improved SED templates, and
extinction-free SFR indicators ? solve for cosmic
star-formation history Madau diagram
9
Summary

• Weak radio emission detected statistically in
  galaxies brighter than 26th magnitude (z-band).
• Radio properties reflect those of the less
  luminous starburst population detected
  individually with S1.4GHz gt40 µJy.
• Typical luminosities are several times that of
  M82
• Average source sizes lie in the range 1.2 1.6
  arcseconds. Marginal trend for
  fainter sources to be slightly smaller.
• No dramatic dependence on galaxy colour but
  some trends are detected (work in progress).
• Of the 2700 galaxies in the 8.5 field brighter
  than 24th magnitude, around half will have radio
  flux densities of 4µJy or greater.
• An e-MERLIN/EVLA deep integration will image
  1500 galaxies with perhaps 150-200 at high
  redshift in a single field.

10
(No Transcript)
11
Summary
At present Can show that the SFR density
increases dramatically to z1 and then
flattens At higher redshifts, the position of
the turnover point remains uncertain !! The next
few years will be very exciting