Radio sources in the 2dF Galaxy Redshift Survey (2dFGRS)

1
Radio sources in the 2dF Galaxy Redshift Survey
(2dFGRS)

• With Russell Cannon (AAO), Carole Jackson (ANU),
  Vince McIntyre (ATNF) and the 2DFGRS team (PIs
  Matthew Colless John Peacock)
• Cross-match the 2dF Galaxy Redshift Survey
  (spectra of 250,000 galaxies to bJ19.4 mag) with
  large-area radio continuum surveys (NVSS at 1.4
  GHz, SUMSS at 843 MHz)
• When 2dFGRS complete, will have good-quality
  spectra of 4000 radio-emitting galaxies to
  z0.3. Currently analysed 900 galaxies (20).
• Goal Accurate study of local radio source
  populations as benchmark for work at higher z

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Main themes of this talk

• Radio telescopes are highly efficient machines
  for probing the distant universe and measuring
  the cosmic evolution of galaxies.
• Developing a proper physical understanding of
  galaxy formation and evolution requires data sets
  much larger than those available in the past.
• The astronomy of the 21st century will be
  dominated by computer-based manipulation of huge
  homogeneous surveys of various types of
  astronomical objects. Van den Bergh
  (2000), PASP 112, 4.

3
Optical and radio views of the sky
DSS B band
SUMSS 843 MHz
Optical - Galactic stars and a few nearby galaxies
Radio - distant galaxies with median z1
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A brief history of the Universe
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Redshift and lookback time for a universe with
Ho50 km/s/Mpc, W1
Redshift Time Since Big Bang Fraction
of . z (in Gyr109 yr)
current age 1400 250,000
yr 0.0019
. 20 0.1 Gyr
1.0
. 10 0.3
2.7
. 5 0.9
6.8
. 3 1.6
13
. 2 2.5
19
. 1 4.6
35
. 0.5 7.1
54
. 0.3 8.8
67
. 0.2 9 .9
76
. 0.1 11.3
87
. 0 13.0
100
COBE
Peak of Galaxy formation?
. 2dF
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Nearby galaxies Hubble type is related to
star-formation history
Galaxy classification scheme first proposed by
Hubble (1936)
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The Milky Way Galaxy in far-IR (COBE)
Much of what we currently know about galaxy
formation comes from studies of the stellar
populations in our own Milky Way
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Galactic archaeology Stellar populations in
nearby galaxies

• Techniques Spectroscopy of resolved
  stars/clusters line-strength
  gradients, colour gradients.
• Spiral galaxies Wide range in stellar ages (0 to
  13 Gyr) and metalliciies. 10 (Sc) to 90(Sa) of
  available gas now converted to stars. Star
  formation continues to present day.
• Elliptical galaxies Age/metallicity degeneracy,
  but stellar population all old (?). Assembled
  from merger of subsystems, but Mg/Fe ratio
  implies rapid formation (lt1Gyr). Kinematics,
  metallicity, luminosity etc. closely linked
  (fundamental plane).

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Radio galaxies in the local universe

Radio galaxy PKS 2356-61 (ATCA image, radio
emission shown in red, optical light in
blue) Radio synchrotron emission, collimated
radio jets powered by accretion disk around
supermassive black hole (Blandford Rees 1978)
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Unified model for Active Galactic Nuclei (AGN)
(Urry Padovani)

• Ingredients of a model AGN
• Black hole
• Accretion disk
• Collimated jets

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M87 - a nearby radio galaxy with a central
supermassive black hole
(Harms et al. 1994)
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Correlation between bulge mass and black hole
mass (Kormendy Richstone 1995)
Black hole mass-bulge mass correlation implies
that formation of galaxy and central black hole
(AGN) are closely coupled (i.e. in mergers,
black holes also merge?) Explains how AGN know
what kind of galaxy they live in.
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Galactic time machines Direct observations at
high redshift (z0.1 to 5)

• Techniques Selection of candidates by colour or
  other criteria, spectroscopy with large
  optical/IR telescopes.
• Elliptical galaxies Ellipticals at z1
  (lookback time 8-9 Gyr) still look old, main
  epoch of formation probably earlier than z3.
• Cosmic evolution Powerful radio galaxies much
  more common at high redshifts - energy output
  implies supermassive black holes (109 solar
  masses) in nuclei of many ellipticals. Beyond
  z2, radio galaxies have disturbed optical
  morphology (Miley et al.), possibly implying that
  black hole formation precedes star formation?

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Galaxies in the Hubble Deep Field
Our deepest view of the Universe in optical
light Median redshift of z1 implies galaxies
typically appear as they were when the Universe
was a third of its current age.
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The star formation history of the Universe
(Baugh et al. 1998)
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The rise and fall of quasars -
evidence for an AGN/starburst link?
(Keel 2000)
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High-redshift radio galaxies - ancestors of
present day ellipticals?
Steep radio spectra efficiently select high-z
galaxies. Infrared K magnitude can be used as
initial redshift estimator.
(Keel 2000)
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The K-band Hubble diagram (van Breughel et
al. 1999)
Finding high-z galaxies 1) Radio filter
(e.g. spectral index) 2) IR (K-band) imaging -
estimate z 3) Optical/IR spectra (8m-class
telescopes)
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Cosmic evolution of active galaxies -
interpreting radio data

• First need to disentangle the following
• Orientation effects Relativistic beaming for
  sources oriented near line of sight. Differences
  in observed emission-line widths, projected
  source sizes.
• Source lifetime Typical AGN lifetime 108 years,
  expect correlation between age and source size
  and/or luminosity. Onset of active phase may be
  related to interaction/merger.
• Host galaxy luminosity On average, bigger
  galaxies have more massive BHs, stronger radio
  sources.
• Therefore need a large sample of nearby objects.

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How large a sample of active galaxies do we need
at z0?

• Need At least 50 galaxies/bin for lt15 error
  bars
• Radio power At least 10 bins to cover full
  range observed (at least 1021 to 1026 W/Hz).
• Host galaxy luminosity At least 4 bins to cover
  full range in optical luminosity. Plus, for
  sample as a whole
• Orientation effects Say 5 bins to cover full
  range in orientation.
• Source lifetime Say 5 bins to cover full age
  range and investigate AGN/starburst connection.
• i.e. Need spectra of 5,000-10,000 galaxies as
  local benchmark for studies of cosmic evolution.
  

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The 2dF Galaxy Redshift Survey
Goal 250,000 galaxy spectra in 1700 deg2 of sky
(completion end 2001)
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2dF corrector, robot positioner and fibre-fed
spectrographs on the AAT
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Typical 2dFGRS radio-source spectra (Sadler et
al. 1999)

• Star-forming galaxy, z0.14 (40)
• Emission-line AGN, z0.15 (10)
• Absorption-line AGN, z0.14 (50)

Ha
Hb
OIII
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2dFGRS radio sources - progress so far

• Have analysed data taken up to May 1999 (58,454
  spectra, 20 of final 2dFGRS data set)
• 757 confirmed radio-source IDs - 1.5 of 2dFGRS
  galaxies
• Spectra classified by eye (60 AGN, 40
  star-forming galaxies)
• Cross-matching with far-infrared (IRAS) and
  X-ray (ROSAT) catalogues
• 2dFGRS spectra cover the closest 5 of NVSS/SUMSS
  radio sources (flux limit 2-3 mJy)

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Redshift distribution of 2dFGRS radio sources
(and all galaxies)
(Colless 2001)
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Spatially-resolved 2dFGRS radio sources
Around 25 of 2dFGRS radio sources are spatially
resolved by the 45 arcsec radio beam, allowing us
to measure their projected linear sizes. In
star-forming galaxies, radio emission is usually
confined to the galactic disk (scales of a few
tens of kpc). In active galaxies, sources are
often several hundred kpc in size.
3 GRGs
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Local radio luminosity function (RLF) for 2dFGRS
radio sources (Sadler et al. 2001)
RLF measures space density of radio sources as a
function of luminosity. To account for greater
survey depth for luminous sources, use V/Vmax
method (Schmidt 1968)
Mixture of AGN and SF galaxies
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Radio emission from star-forming galaxies
UGC 09057 NGC 5257/5258
NGC 7252 z0.0054
z0.0223
z0.0161 Derived star formation rate

1.8 Msun/yr 120
Msun/yr 32 Msun/yr
(Radio emission is dominated by synchrotron
radiation from electrons accelerated by supernova
remnants)
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Far-infrared - radio correlation for 2dFGRS
galaxies
In star-forming galaxies, far-IR and radio
emission are tightly correlated. Above 1023 W/Hz
(i.e. implied star formation rates of 100
Msun/yr), many star-forming galaxies also have
active nuclei. Signs are Seyfert-like
emission-line ratios and (sometimes) excess radio
emission
AGN spectrum o SF spectrum
Normal galaxy line
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Local RLF for star-forming galaxies
RLF derived from 2dFGRS data fits onto values for
nearby bright (RSA) galaxies (Condon 1989). Star
formation rates derived from radio data are
typically 10-100 Msun/yr (vs 1
Msun/yr for Milky Way).
RSA
2dFGRS
NVSS radio limit (3mJy) biases towards high SFR
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Local star-formation density from radio and Ha
data
Local star formation density (zero-point of Madau
diagram) in Msun/yr/Mpc3 Ha 0.013 /-0.006
(Gallego et al. 1995) Radio 0.022 /-0.004
(Sadler et al. 2001) Radio data show more
galaxies with very high SFR (gt 30 Msun/yr),
otherwise very good agreement.
Ha
Radio
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What are the high SFR galaxies?

• Radio LF for star-forming galaxies implies that
  galaxies with SFR gt 30 Msun/yr are far more
  common than Ha surveys suggest, and may account
  for up to 40 of the local star-formation
  density.
• Dust obscuration in star-forming regions could
  lead to under-estimate of Ha line strength.
• Deep VLA studies of clusters at z0.4 (Smail et
  al. 1999) and local (z lt0.5) post-starburst
  galaxies (Miller Owen 2001) also suggest that
  star-forming regions can be hidden by dust.
• Important to study the 2dFGRS high-SFR galaxies
  in more detail (high-res. radio images, IR
  spectra...) - are the high star-formation
  rates real?

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Radio emission from active galaxies
TGN284Z051 TGN348Z183
TGS153Z214 z0.1065
z0.1790
z0.2079 1.4 GHz radio power and projected linear
size 1024.3 W/Hz
1025.0 W/Hz 1024.8 W/Hz
327 kpc 475 kpc
471 kpc
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Local radio LF for active galaxies
RLF derived from 2dFGRS data fits onto values for
nearby bright E/S0 galaxies derived by Sadler et
al. (1989). RLF must turn over not far below 1020
W/Hz to avoid exceeding the space density of
early-type galaxies.
Power-law F(P) a P-0.62
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Black hole mass spectrum for active galaxies in
the local universe
Can use radio LF for AGN to estimate the local
mass density of black holes (gt3x107 Msun)
following relation from Franchescini et al.
(1998 ADAF model) . Total min. BH density
rBH1.6x105 Msun/Mpc3 agrees with Choksi Turner
QSO estimate (rBH1.4-2.2 x105 Msun/Mpc3).
No turnover in BH density yet!
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Local radio luminosity function of active and
star-forming galaxies
Below 1025 W/Hz, the local radio source
population is always a mixture of AGN and
star-forming galaxies. i.e. There is probably
no observational regime where radio surveys
detect only star-forming galaxies.
Low-lum AGN are hard to find
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Summary Results so far

• The local radio source population is a mixture of
  star-forming galaxies and AGN, but 2dFGRS spectra
  usually allow us to distinguish them
  unambiguously.
• The local star-formation density derived from the
  radio continuum is higher than the value measured
  from Ha because we find more galaxies with SFR gt
  30-50 Msun/yr (possibly dust-obscured in optical
  light).
• The black-hole mass density in AGN agrees with
  the value derived for QSOs in the early Universe,
  suggesting that local radio galaxies are the
  direct descendants of high-z QSOs.

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The next steps...

• With the full 2dFGRS data set Evolution of the
  AGN and SF luminosity functions to z0.3, split
  by radio spectral index.
• With the 6dF Galaxy Survey From mid-2001, expect
  12,000 radio-source spectra to z0.1 (16
  detection rate!), define faint end of RLF,
  starburst/AGN connection, (Tom Mauch thesis).
• Going deeper Deep 2dF spectroscopy to z0.5,
  photometric redshifts to z1, steep-spectrum
  sources k-band imaging/8m spectroscopy to zgt3.