THE RADIO SPECTRAL INDEX OF SUB MILLIMETRE GALAXIES - Edo Ibar UK Astronomy Technology Centre

THE RADIO SPECTRAL INDEX OF SUB­MILLIMETRE GALAXIES

Edo Ibar ­­ UK Astronomy Technology Centre

In collaboration with: R.J. Ivison, P.N. Best, K. Coppin, A. Pope, Ian Smail & J.S. Dunlop.

                                                                   February 2010 – Heidelberg, Germany.

INTRODUCTION

 Sub­millimetre (submm) galaxies (SMGs) were first discovered by the Sub­mm Common
 User Bolometer Array (SCUBA) camera on top of the James Clerk Maxwell Telescope
 (JCMT) in the late 90's.

                               Massive star­formation rates >1000 Mo/yr.
                               Luminosities >1012Lo.
                               Masses: (DM)~5x1012Mo, (H2)~1010Mo, (Stellar)~1011Mo.
                               Mostly powered by star­formation rather than nuclear active
                                   galactic nuclei (AGN) activity.
                               They typically reside at high redshift (~2.2).
                               Obscuration and distance makes their detection extremely
                                   difficult.
                               The identification of SMG is not trivial (“Blobology”).
SCUBA

                                                                        SMG J02399­0136
                                                                        Ivison et al. (1999)

INTRODUCTION

 What can we learn about SMGs using radio observations?

 Radio observations are preferentially used to pinpoint SMGs thanks to their low source
 surface density and by the common nature provided by the far­IR/radio correlation.
 SMGs are mostly detected at faint flux densities, S1.4GHz < 150Jy.

 Based on the current Jy­sensitivity of deep radio surveys, star­forming galaxies are
 mostly detected up to z~3.
 Roughly ~50­70 per cent of SMGs are detected in deep radio surveys.

      However, little was known about their radio spectral energy distribution.

THE RADIO SPECTRAL INDEX OF SMGs

 We base our analysis on the Lockman Hole field which has one of the most extensive
 coverages at mm/submm wavelengths.

 The parent sub­millimetre sample:
 Instrument Wavelength (m) Area (arcmin2) FWHM (arcsec) R.M.S. (mJy/beam) No. sources (SNR>3.5)          Reference
  MAMBO          1200            197           11.0             0.6                 23                Greve et al (2004)
  SCUBA           850            484           14.8             2.0                 57               Coppin et al. (2006)
  AzTEC          1100           1115           18.0             1.0                 51             Austermann et al. (2009)

The combination of all these surveys resulted in an overall sample of 111 SMGs (20 of which
are detected in more than one sample).

 The radio data:
     Instrument Frequency (MHz) Area (deg2) FWHM (arcsec) R.M.S. (Jy/beam) No. sources (>5)         Reference
        VLA             1400           0.56        4.1              6.0             1452           Ibar et al. (2009)
       GMRT              610           0.98        6.5             15.0             1587           Ibar et al. (2009)

THE RADIO SPECTRAL INDEX OF SMGs

 Our criterion for cross­matching SMGs and radio sources is based on the probability
 for random association described by Downes et al. (1986).
                                                                   rs = 3 x RA(SNR)

 where,                                                            S
                                                                       *
                       (ST)                                                r
                                                                               *

                    (S )
                                                         ST

 The Poisson probability is therefore:

    Pstat
 Traditionally Pstat < 0.05 has been used to define detections, i.e. a probability lower than 5
 per cent that the submm galaxy is associated by random chance with a radio source.

THE RADIO SPECTRAL INDEX OF SMGs

                                                         Linear sizes between 1 and 8 kpc.

  Considering the redshift distribution of SMGs          MERLIN+VLA
  and their physical radio size, we have
  assumed sources to be unresolved at the
  resolutions of our work.

                                                                            Biggs & Ivison (2008)

                                                  Based on simulations, we find flux density
                                                  estimations to be a factor ~1.2 better in
                                                  scatter when sources are extracted as
                                                  being point­like.

THE RADIO SPECTRAL INDEX OF SMGs

 We find 44 SMGs (40 per cent) with single and reliable radio counterparts in at least
 one of the radio waveband ­­ 41 and 37 detections by the VLA and GMRT, respectively.

  The SMG radio spectral index is similar to the found locally in star­forming galaxies.

THE RADIO SPECTRAL INDEX OF SMGs

 The fact we do not observe a spectral steepening suggests that ageing effects or
 Inverse­Compton scattering off the CMB do not seem to affect the radio emission.

                         Continuous injection          Radiation losses:

 Carilli et al. (1991)

             Scenarios: A continuous injection mechanism? Fainter magnetic fields?

THE RADIO SPECTRAL INDEX OF SMGs

 What about the AGNs in the SMG sample?

 Using mid­IR colour­colour selection criteria,
 followed by IRS Spitzer spectroscopy, Coppin et al.
 were able to identify mid­IR AGN­dominated
 sources in SHADES fields.

                                                       Coppin et al., submitted
      M82
                            Mrk231

THE RADIO SPECTRAL INDEX OF SMGs

  We find that SMGs with a dominant mid­IR AGN component have peculiar radio spectral
  indices compared with the bulk of the sources. Steeper spectra are suggested.

THE RADIO SPECTRAL INDEX OF SMGs

There are interesting similarities between these mid­IR AGN dominated SMGs and
Ultra Steep Spectrum Sources (USS).

    High redshifts
    Large masses
    Proto­galaxy­clusters(?)

BUT!

    Radio identified SMGs are much fainter than USS
    Star­formation dominates the power, rather than AGN activity

Recently, a more extensive study for a sample of
USS has shown that these unusual spectral indices
do not show evidences for curvature in the SED,
suggesting an intrinsically different emission
mechanism or a product of the environment.

                                     Bryant et al. (2009)

FIN

Possible scenarios:

    Are these massive galaxies with an AGN component embedded in denser media?
    Do they have stronger magnetic fields that make synchrotron losses more efficient?
    Are AGNs evolving differently than star­forming galaxies as a function of redshift?
    Is this result actually significant?

Tackling scenarios:

    Radio­relic, compact core or extended starburst/radio­lobes can be constrained by
    using high resolution radio imaging, e.g. MERLIN, EVN, VLBI.

    Better statistics will be obtained soon with Herschel data currently being analysed.

FIN