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Starbursts and the triggering of the
activity in low redshift radio galaxies
Clive Tadhunter
University of Sheffield
Collaborators: J. Holt, D. Dicken, C. Ramos Almeida,
R. Gonzalez Delgado, R. Morganti, J. Rodriguez-Zaurin,
K. Inskip, K. Wills
Triggering mechanisms
• Galaxy mergers and interactions (Heckman et al. 1986,
Smith & Heckman 1989)
• Accretion of gas from hot X-ray haloes
- Bondi accretion of hot gas (Allen et al. 2006,
Best et al. 2006, Hardcastle et al. 2007, Buttiglione et al. 2009)
- Accretion of cool gas from cooling flow
(e.g. Bremer et al. 1997)
• Cold accretion from large-scale filamentary
structures (e.g. Keres 2005, Dekel et al. 2009)
Evidence for galaxy interactions and
mergers in non-starburst radio galaxies
Deep Gemini+GMOS imaging observations of 2Jy sample
A large proportion of nearby radio galaxies show evidence for
morphological disturbance consistent with triggering in major galaxy
mergers and interactions (see talk by Cristina Ramos Almeida)
Triggering starbursts in major galaxy mergers
Cox et al. (2008)
Starbursts in radio galaxies: occurrence
• Starburst rate from optical spectroscopy I:
- 2Jy(0.15 < z < 0.7): 20 -- 35% (22 objects)
Tadhunter et al. (2002)
- 3CR(z<0.2): 33% (14 objects)
Aretxaga et al. (2001), Wills et al. (2002)
- 2Jy (z<0.08, FRIs): 25% (12 objects)
Wills et al. (2004)
• UV imaging with HST
- 3C (z<0.1): 100% HEG (6 objects); 5% LEG
Baldi et al. (2008)
• Detection of PAH at mid-IR wavelengths
- 3C (z<0.1, FRIs): 75% (24 objects)
UV and PAH techniques
particularly sensitive to
low levels of SF, especially
if level of AGN activity weak
Leipski et al. (2009)
• Combined optical+far-IR continuum excess+MFIR colours+PAH
- 2Jy(0.05 < z < 0.7): 15--35%
Dicken et al. (2009,2010)
The reddened nuclear starburst in 3C305
WHT/ISIS
Hd
CaII K
Starburst Properties
Age: 0.4 - 0.9 Gyr
E(B-V)=0.4 - 0.8 mag
Mass:1.5+/-0.5x1010 Msun
(16 - 40% of total stellar mass)
Bruzual & Charlot(1996) models
Salpeter IMF (0.1-125 Msun)
3C305 (z=0.042) Heckman et al. 1986
Starburst dominated Objects (z>0.15)
3C459 (z=0.22) NTT+EMMI
YSP Properties
Age: 0.05 Gyr
Mass:4x109 Msun
(>5% of total stellar mass in slit)
Objects with v.young starburst
components
PKS0023-26 (z=0.340) - VLT/FORS2
PKS0409-75 (z=0.69) - VLT/FORS2
YSP age: 30Myr
Reddening: E(B-V)=0.8
YSP mass proportion: 9%
YSP age: 10Myr
Reddening: E(B-V)=0.9
YSP mass proportion: 4%
Holt et al. (2007)
These objects have:
- Low UV polarization
- Relatively weak narrow lines
- No broad lines detected
The Ages of the YSP in ULIRG and PRG
Tadhunter et al. (2005)
Holt et al. (2006,2007)
Wills et al. (2008)
Tadhunter et al. (2010)
Rodriguez-Zaurin et al.
(2007,2008,2009,2010)
Typical maximum age of radio source
Two main groups of starburst radio galaxies
• LIRG/ULIRG-like systems (tysp < 0.1Gyr)
- Most have:
35
11
L[OIII]  10 W; Lir  10 Lsun
- Radio source triggered quasi-simultaneously
with starburst
• Post-starburst systems (tysp > 0.2 Gyr)
- Most have:
L[OIII]  1035W; Lir  1011Lsun
- Radio source triggered (or retriggered) a significant
period after the starburst episode

Starburst radio galaxies general properties
Based on a detailed spectrosynthesis modelling of a sample of
22 radio galaxies with good evidence for YSP:
• 95% of starburst radio galaxies show signs of
morphological disturbance (tidal tails, fans, shells, dust
lanes, double nuclei etc.)
• Young stellar populations (YSP) contribute a significant
proportion of the total stellar masses (5-40%)
• The YSP are spatially extended -- they generally detected
across the full extents of the host galaxies over which
accurate measurements can be made (although brightest in
the nuclei)
Overall, the results are consistent with the triggering
of the activity in major, gas-rich galaxy mergers/interactions
Merger sequence for starburst radio galaxies
3C459
Cooling flow driven activity in Hydra A?
Gemini+GMOS
Hydra A, z=0.054, FRI
tysp  0.05 Gyr
Spitzer+IRS
The similarity between the
SRF rate and hot X-ray
cooling rate is consistent with
triggering by a cooling flow
SRF  4  16 M sun yr
1
CRX  ray  35  20 M sun yr1
Rafferty et al. (2006)
PKS0023-26: a compact radio source at
the centre of a cluster
PKS0023-26
z=0.322, CSS
tysp  0.04 Gyr
Spitzer+IRS
PAH
Star Formation in 4C41.17 at z=3.8?
La
Dunlop et al. 1994
Star formation rate:
2,000 - 10,000 M0/yr
Dey et al. 1997
3C Radio Galaxies at 60mm with IRAS
Only ~30% of 3CR radio galaxies at z<0.5
were detected by IRAS at 60mm
Spitzer/MIPS observations of complete
samples of radio-loud AGN
2Jy sample
• S2.7GHz > 2.0 Jy
• Intermediate redshifts
(0.05 < z < 0.7)
• Steep radio spectra
(a1.4-5GHz < -0.5)
• 46 objects
3CRR sample
• S178MHz > 10.0 Jy
• Low redshifts
(z < 0.1)
• FRII only
• 19 objects
• Spitzer/MIPS detection rates: 100% at 24mm and 90% at 70mm
• All objects have deep optical spectra, allowing accurate
spectral classification, measurements of emission line
luminosities, and assessment of stellar population mix
Correlations between
MFIR and optical
properties
• The 24mm luminosity is
strongly correlated with the
[OIII]5007 emission line
luminosity
• The 70mm luminosity is
also strongly correlated with
the [OIII] luminosity, but
with increased scatter
0.79 0.07
L24 mm  L[OIII]

• The slopes of the 24mm and
70mm correlations are similar
Tadhunter et al. (2007),
Dicken et al. (2008,2009)
0.83 0.09
L70mm  L[OIII]
The starburst contribution to the far-IR
• The far-IR emitting dust is predominantly heated by AGN illumination
• Starburst heating only significant in a minority of objects (~17 -- 35%)
Tadhunter et al. (2007)
Dicken et al. (2009, 2010)
A simple model for the dust/emission
line structures
Assume that both the emission
lines and MFIR emission
produced by AGN illumination
• Covering factors of mid-IR
and far-IR emitting dust
structures, and NLR:
Cmir, Cfir and Cnlr
L 24 mm  1  1012 L[OIII]
L 70mm  9  1012 L[OIII]
C mir
C nlr
C fir
C nlr
Energetic feasibility of AGN illumination
Dicken et al. (2009)
• Allowing forC mir
a modest
intrinsic
~ 12.5,amount
0.02  of
C nlr
 0.08extinction, it is
plausible thatCmuch
of the far-IR continuum is in most radio
nlr
galaxies is produced
AGN
illumination of the NLR clouds
 0.25 by
C mir
 1.0
Triggering non-starburst radio galaxies
• Non-starburst radio galaxies make up >60% of the
population of powerful radio galaxies
• Most non-starburst RG belong to the class of strong-lined
objects that are thought to be powered by cold accretion
• Triggering possibilities include:
- Gas accretion in a tidal encounter, or around the time of
first pass of nuclei in a merger
- Re-triggering the activity a substantial period
(>1Gyr) after the major merger-induced starburst
- Minor mergers (>3:1 mass ratio)
Triggering starbursts in major galaxy mergers
Cox et al. (2008)
Evidence for galaxy interactions and
mergers in non-starburst radio galaxies
Deep Gemini+GMOS imaging observations of 2Jy sample
0.05 < z < 0.7, 46 objects, r’ or i’ band
85% of the non-starburst radio galaxies in 2Jy sample show
signs of morphological disturbance (Ramos Almeida et al. 2009)
How do the WLRG fit in?
• Commonly proposed that weak line radio galaxies (WLRG)
are triggered/fuelled by Bondi accretion of the hot ISM in the
host galaxies/clusters
• But significant proportion of the optical starburst radio
galaxies (~40%) -- particularly those with older young stellar
populations -- are WLRG (e.g. Fornax A, Cen A, Hydra A, PKS0347+09,
PKS0620-52, 3C213.1, 3C236, 3C292, NGC612)
• The presence of SF provides evidence of cold accretion into
the circum-nuclear regions of some WLRG
• Most such objects show evidence for a rich ISM in the form
of dust lanes; many also show PAH features in their mid-IR
spectra
Star formation in Fornax A
Evidence for young stellar
populations:
- Diffuse stellar light has
luminosity weighted age 2-3 Gyr
(Kuntschner 2002)
- Globular clusters have
ages 3+/-0.5 Gyr
(Goudfrooij et al. 2003)
----> current galaxy formed from
a major merger of gas-rich
galaxies 2-3 Gyr ago
HST+ACS Goudfrooij et al. (2005)
The double AGN in PKS0347+05
Spitzer+IRS
WLRG
PAH
Sy 1
z=0.339, FRII
This WLRG is
clearly in an
interacting
system with a
rich ISM and
plenty of star
formation.
A torus in the WLRG PKS0043-42?
PKS0043-42
Spitzer+IRS
z=0.116, FRII
Silicate
absorption
Dicken et al. (2010)
Detection of weak PAH in nearby FRI galaxies
• 75% of the sample of 24
nearby FRI radio galaxies
presented by Leipski et
al.(2009) show PAH
features
• Possible evidence for lowlevel star formation in the
circum-nuclear regions
Evidence for a rich ISM in the nuclear
regions of WLRG
• Dust lanes (e.g. Fornax A, Cen A, Hydra A)
• Circum-nuclear (warm) gas disks (e.g. M87), and
cool molecular disks
• Detection of hot dust emission at mid-IR
wavelengths (e.g. PKS0043-42, Leipski et al.
2009)
• Detection of PAH features in mid-IR spectra of a
large proportion of nearby FRI sources (Leipski et
al. 2009)
• Detection of compact emission line regions
associated with the compact optical cores of FRI
radio sources (e.g. Capetti et al. 2005)
Conclusions
• Most SB radio galaxies consistent with triggering in major
gas-rich mergers; but triggering can occur before, around
or a long time after the coalescence of the merging nuclei
• Non-starburst radio galaxies also likely to be triggered in
galaxy mergers and interactions (but much earlier or later
in sequence, or by more minor mergers)
• Radio-loud AGN activity is not solely associated with a
particular phase of unique type of gas accretion event
• The heating of the far-IR emitting dust is likely to be
dominated by AGN illumination in the majority of PRG;
the far-IR doesn’t always provide a good diagnostic of SF
• Evidence for cool/warm gas accretion into the circumnuclear regions of WLRG
Correlation analysis -- 2Jy sample
L24 vs L[OIII]
L24 vs z
L24 vs Lrad
L[OIII] vs Lrad
An evolutionary link with ULIRGs?
• Some powerful radio galaxies are ULIRGs (3C48,
PKS1345+12, PKS1549-79, PKS2135-20, 3C459)
• ULIRG radio galaxies have stellar populations
properties similar to the general population of
ULIRGs (tysp < 0.1 Gyr)
• The stellar masses of the intermediate age, poststarburst stellar populations in some radio galaxies
are consistent with the idea that they have evolved
from ULIRG/LIRGs
ULIRG/starburst radio galaxy stellar mass
comparison
Nearby 1Jy ULIRGs (z<0.18)
(Rodriguez Zaurin et al.
2009, 2010)
Not all starburst radio
galaxies can have evolved
from ULIRGs
Starburst radio galaxies (z<0.7)
(Tadhunter et al. 2010)
Star formation in major mergers
Springel et al. (2005)
Activity and galaxy evolution
Evolution of activity and star formation
Black hole vs. galaxy bulge properties
z
Dunlop & Peacock 1990, Madau 1987
Tremaine et al. (2002)
Triggering?
Feedback?
Outstanding questions