1. Radio galaxies: timescales and triggers Elaine Sadler, University of Sydney Goal: To develop a self-consistent picture of how radio galaxies are triggered, and how they evolve over their lifetime. Tools: Very large (n>10,000) radio/optical samples in the local (z<0.5) universe

2. Outline of this talk: Why elliptical galaxies are interesting. Why studies of stellar populations and radio galaxies are intimately connected. Why timescales and triggers for radio galaxies are hard to understand, and how large surveys in the local universe can help. “Work in progress” - collaborators include: Russell Cannon (AAO), Carole Jackson, Ron Ekers, Ravi Subrahmanyan (ATNF), Dick Hunstead, Tom Mauch (Sydney), Warrick Couch (UNSW), Raffaella Morganti (ASTRON), John Peacock (ROE) and the 2dFGRS, 6dFGRS, 2dF/SDSS LRG and SUMSS teams.

3. Why elliptical galaxies are interesting (1) Powerful radio galaxies are (always) luminous ellipticals Radio synchrotron emission, collimated radio jets powered by accretion disk around supermassive black hole (Blandford & Rees 1978) ATCA image of PKS 2356-61 (red: radio continuum; blue: optical light)

4. Why elliptical galaxies are interesting (2) “Co-evolution” of galaxies and black holes Black hole mass and bulge mass are correlated; luminous ellipticals have the most massive central black holes. Implies that processes of galaxy evolution and AGN fuelling are closely related (i.e. a nucleus ‘knows’ what kind of galaxy it lives in). (Kormendy & Richstone 1995)

5. Timescales and triggers for radio galaxies AGN/black hole physics has been reasonably well understood for the past 30 years, but we still understand relatively little about the `life cycle’ of active galaxies, or why some galaxies are much more active than others. The trigger for radio galaxies/AGN is usually assumed to be an interaction or merger with another galaxy, but evidence for this is largely circumstantial. Problem is that evolution on timescales of 10 6-8 years (starbursts) or 10 7-9 years (radio galaxies) is not directly observable, so causality is difficult to test. Solution: study large, complete samples which span a small range in redshift (volume limited).

6. Radio imaging surveys in the southern hemisphere NVSS (VLA):  1.4 GHz, north of -40 o SUMSS (Molonglo):  843 MHz, south of -30 o ~ 85% complete at present Data:www.physics.usyd.edu. au/astrop/SUMSS Data: www.cv.nrao.edu/nvss

7. Spectroscopic redshift surveys in the southern hemisphere 2dFGRS (AAT): ~220,000 galaxies to z~0.3, 1500 deg 2. 6dFGS (AAO Schmidt): ~150,000 galaxies to z~0.15, all-sky. Final goal is analysis of 10,000+ radio-detected AGN with z<0.3.

8. Images of the optical and radio sky Optical DSS B: median z~0.1 Radio 843 MHz: median z~1 180 galaxies per square degree to B~19.4 mag 40 sources per square degree to S~5 mJy Overlap: ~2 objects per square degree in 2dFGRS

9. Star-forming galaxy, z =0.14 (40%) “Starburst” Emission-line AGN, z =0.15 (10%) “Seyfert” Absorption-line AGN, z =0.14 (50%) “Radio galaxy” Typical 2dFGRS radio-source spectra HH HH [OIII] (Sadler et al. 1999)

10. Local radio luminosity functions for active and star-forming galaxies Below 10 25 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. Spectra essential! (Sadler et al. 2002)

11. Radio emission from star-forming galaxies (most are IRAS galaxies) UGC 09057 NGC 5257/5258 NGC 7252 z=0.0054 z=0.0223 z=0.0161 Derived star formation rate: 1.8 M sun /yr 120 M sun /yr 32 M sun /yr (Radio emission is dominated by synchrotron radiation from electrons accelerated by supernova remnants)

12. Local (z<0.1) star-formation density Local star formation density (zero-point of Madau diagram) in M sun /yr/Mpc 3 : Ha: 0.013 +/-0.006 (Gallego et al. 1995) Radio: 0.022 +/-0.004 (Sadler et al. 2002) Radio data show more galaxies with very high SFR (> 30 M sun /yr), otherwise agree well. HH Radio

13. IRAS galaxies in the 2dFGRS At star formation rates above ~100 M sun /yr, many star- forming galaxies also have active nuclei. Signs are Seyfert- like emission-line ratios and (sometimes) excess radio emission Normal galaxy line IRAS Radio ULIRGs

14. Radio emission from active galaxies TGN284Z051 TGN348Z183 TGS153Z214 z=0.1065 z=0.1790 z=0.2079 1.4 GHz radio power and projected linear size: 10 24.3 W/Hz 10 25.0 W/Hz 10 24.8 W/Hz 327 kpc 475 kpc 471 kpc

15. The local radio LF for active galaxies RLF measured from 2dFGRS data fits onto values for nearby bright E/S0 galaxies. RLF must turn over below 10 20 W/Hz to avoid exceeding the space density of early-type galaxies. Power-law  ( P )   P -0.62 Much broader than optical LF! (Sadler et al. 2002)

16. Radio LF and statistical lifetimes Statistical lifetime of a radio source of luminosity P R is T R = (N R /N E ). T E, for parent pop. N E, with lifetime T E (Schmidt 1966, estimated T R ~ 10 9 yr for radio galaxies)

17. Linear size as an estimate of source age (Parma et al. 2002) The sizes of both FRI and FRII radio galaxies increase with time as jets propagate outwards (FRIIs, with relativistic jets, expand faster). Age estimates from spectral aging and dynamical methods are reasonably consistent.

18. The radio P-d diagram Plots radio power P against linear size d. But… P is related to statistical lifetime, d to source age. What happens if we require these to be consistent? -> (empirical) evolutionary tracks! (Sadler et al. 2002)

19. The P-d diagram for resolved 2dFGRS radio galaxies LSB sources Many unresolved? But need to beware of: 1) Cutoff in radio surface brightness 2) Radio power (and probablity of hosting a radio source) increases with optical luminosity

20. Survey limits in radio surface-brightness (R.Subrahmanyan) NVSS/SUMSS limits ~4mJy/arcmin 2

21. Powerful radio galaxies are found mainly in the brightest ellipticals Implies that black-hole mass is a key parameter!

22. P-d diagram binned by optical luminosity (1) M R = -24.0 to -24.9M R = -23.0 to -23.9

23. P-d diagram binned by optical luminosity (2) M R = -22.0 to -22.9 M R = -21.0 to -21.9

24. Do FRII radio galaxies evolve to FR Is? (Ledlow & Owen 1996) Galaxies shouldn’t change much in (red) optical luminosity on the timescale of a radio-source lifetime, so time evolution in this diagram should be mostly downward (and across the FRII /FRI line?) Test is to check for continuity of linear size across the break.

25. Radio-source populations in the 2dFGRS: In general, AGN and star-forming galaxies are almost disjoint in a colour- magnitude diagram. Evolution from one class to another (e.g. starburst followed by an AGN) must therefore be rare. k-corrected CMD for 3256 2dFGRS galaxies detected at 1.4 GHz (Red: AGN, Blue: Star-forming galaxies) How many radio galaxies are triggered by starbursts?

26. PKS 0019-338, a post-starburst radio galaxy at z=0.128 Balmer abs. lines imply a massive (~10 10 M sun ) starburst occurred ~0.15 Gyr ago. Compact, steep-spectrum radio source has P 1.4 ~ 10 25 W/Hz

27. Nearby active ellipticals with accreted HI and strong, compact central radio sources Moderate power (P 1.4 ~ 10 23 W/Hz), flat spectrum compact radio AGN, recently triggered? Or episodic short bursts of activity? Gas infall without star formation?

28. Main points: In a complete, volume-limited galaxy sample, all stages in radio-galaxy evolution should be present in numbers proportional to the typical lifetime in that stage. Need samples of >10,000 radio detections. In the local universe (z<0.3), can start to map out the time evolution of individual radio galaxies without the extra complication of cosmic evolution. If mergers/starbursts trigger radio galaxies, their spectral signature should remain detectable in 2dF/6dF spectra for at least several times 10 8 yr (i.e. > source lifetime?). Unlike QSOs, optical spectra are dominated by the stellar population, not the AGN.

29. Results so far: Galaxy luminosity (-> black hole mass) has a huge influence on the probability of hosting a powerful radio galaxy. Need to analyse data in small M bins. Some powerful, compact 2dFGRS radio galaxies show a post-starburst optical spectrum (e.g. PKS 0019-338), but typical radio-source ages (~10 6 yr) are orders of magnitude smaller than the time since the starburst began (~ 10 8 yr). Why? Large numbers of compact, low-luminosity radio sources probably imply that such activity is episodic (source age << statistical lifetime) and they are unlikely to evolve to classical radio galaxies. Fuelling could be by infall of HI clouds, rather than whole galaxies.

31. Redshift distribution of 2dFGRS radio sources (Colless 2001) All Radio All 2dFGRS galaxies Radio AGN Radio starburst

32. Cosmic evolution of radio galaxies The space density of powerful radio galaxies and quasars was ~1000 times higher at z~2 than it is now (Willott et al. 2002). The similar cosmic evolution of AGN and star formation density is often invoked to suggest that both are triggered by galaxy mergers (e.g. Dunlop 1997).

33. K-z relation for radio galaxies At all redshifts to at least z~5, radio-galaxy hosts are the most luminous galaxies - this implies a direct ‘line of descent’ between high- redshift AGN and local giant ellipticals. Poses some challenges for hierarchical models of galaxy `assembly’. (De Breuck et al. 2002)

34. NGC 1490 - a massive gas cloud in a galaxy group ATCA total HI map HIPASS detection looked unambiguous - the E1 galaxy NGC 1490 was the only catalogued galaxy in the 15 arcmin Parkes beam. But... later ATCA images show the HI is not in NGC 1490! Total HI mass ~1.1 x 10 10 M sun.