1. Clive Tadhunter University of Sheffield
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

2. 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)

3. 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)

4. Triggering starbursts in major galaxy mergers
Cox et al. (2008)

5. Starbursts in radio galaxies: occurrence
Starburst rate from optical spectroscopy I:
- 2Jy(0.15 < z < 0.7): % (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)
Leipski et al. (2009)
Combined optical+far-IR continuum excess+MFIR colours+PAH
- 2Jy(0.05 < z < 0.7): %
Dicken et al. (2009,2010)
UV and PAH techniques
particularly sensitive to
low levels of SF, especially
if level of AGN activity weak

6. The reddened nuclear starburst in 3C305
WHT/ISIS
Starburst Properties
Age: Gyr
E(B-V)= mag
Mass:1.5+/-0.5x1010 Msun
( % of total stellar mass) Hd
CaII K
Bruzual & Charlot(1996) models
Salpeter IMF ( Msun)
3C305 (z=0.042) Heckman et al. 1986

7. 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)

8. Objects with v.young starburst components
PKS (z=0.340) - VLT/FORS2
PKS (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%
These objects have:
- Low UV polarization
- Relatively weak narrow lines
- No broad lines detected
Holt et al. (2007)

9. 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

10. Two main groups of starburst radio galaxies
LIRG/ULIRG-like systems (tysp < 0.1Gyr)
- Most have:
- Radio source triggered quasi-simultaneously
with starburst
Post-starburst systems (tysp > 0.2 Gyr)
- Radio source triggered (or retriggered) a significant period after the starburst episode

11. 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

12. Merger sequence for starburst radio galaxies

13. Cooling flow driven activity in Hydra A?
Gemini+GMOS
Hydra A, z=0.054, FRI
Spitzer+IRS
The similarity between the
SRF rate and hot X-ray
cooling rate is consistent with
triggering by a cooling flow
Rafferty et al. (2006)

14. PKS0023-26: a compact radio source at the centre of a cluster
z=0.322, CSS
Spitzer+IRS
PAH

15. Star Formation in 4C41.17 at z=3.8?La
Dunlop et al. 1994
Star formation rate:
2, ,000 M0/yr
Dey et al. 1997

16. 3C Radio Galaxies at 60mm with IRAS
Only ~30% of 3CR radio galaxies at z<0.5
were detected by IRAS at 60mm

17. 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 24m and 90% at 70m
All objects have deep optical spectra, allowing accurate
spectral classification, measurements of emission line
luminosities, and assessment of stellar population mix

18. Correlations between MFIR and optical properties
The 24m luminosity is
strongly correlated with the
[OIII]5007 emission line
luminosity
The 70m luminosity is
also strongly correlated with
the [OIII] luminosity, but
with increased scatter
The slopes of the 24m and
70m correlations are similar
Tadhunter et al. (2007),
Dicken et al. (2008,2009)

19. The starburst contribution to the far-IR
SB Heating
AGN Heating
The far-IR emitting dust is predominantly heated by AGN illumination
Starburst heating only significant in a minority of objects (~ %)
Tadhunter et al. (2007)
Dicken et al. (2009, 2010)

20. 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

21. Energetic feasibility of AGN illumination
Dicken et al. (2009)
Allowing for a modest amount of intrinsic extinction, it is
plausible that much of the far-IR continuum is in most radio
galaxies is produced by AGN illumination of the NLR clouds

22. 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)

23. Triggering starbursts in major galaxy mergers
Cox et al. (2008)

24. 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)

25. 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, PKS , PKS , 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

26. 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)

27. The double AGN in PKS0347+05 WLRG Spitzer+IRS This WLRG is
clearly in an
interacting system with a rich ISM and plenty of star formation.
PAH
Sy 1
z=0.339, FRII

28. A torus in the WLRG PKS0043-42? PKS0043-42 Spitzer+IRS z=0.116, FRII
Silicate
absorption
Dicken et al. (2010)

29. 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 low-level star formation in the circum-nuclear regions

30. 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. PKS , 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)

31. 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 circum-nuclear regions of WLRG

32. Correlation analysis -- 2Jy sample
L24 vs Lrad
L24 vs L[OIII]
L[OIII] vs Lrad
L24 vs z

33. An evolutionary link with ULIRGs?
Some powerful radio galaxies are ULIRGs (3C48, PKS , PKS , PKS , 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, post-starburst stellar populations in some radio galaxies are consistent with the idea that they have evolved from ULIRG/LIRGs

34. 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)

35. Star formation in major mergers
Springel et al. (2005)

36. 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?

37. Outstanding questions