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

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.

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)

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)

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 106-8 years (starbursts) or 107-9 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).

Radio imaging surveys in the southern hemisphere

NVSS (VLA): 1.4 GHz, north of -40o

SUMSS (Molonglo): 843 MHz, south of -30o ~85% complete at presentData:www.physics.usyd.edu.au/astrop/SUMSS

Data: www.cv.nrao.edu/nvss

Spectroscopic redshift surveys in the southern hemisphere

2dFGRS (AAT): ~220,000 galaxies to z~0.3, 1500 deg2.

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.

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

• 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

HH

[OIII]

(Sadler et al. 1999)

Local radio luminosity functions for 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.

Spectra essential!

(Sadler et al. 2002)

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 Msun/yr 120 Msun/yr 32 Msun/yr (Radio emission is dominated by synchrotron radiation from

electrons accelerated by supernova remnants)

Local (z<0.1) star-formation density

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. 2002)

Radio data show more galaxies with very high SFR (> 30 Msun/yr), otherwise agree well.

H

Radio

IRAS galaxies in the 2dFGRS

At star formation rates above ~100 Msun/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

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: 1024.3 W/Hz 1025.0 W/Hz 1024.8 W/Hz 327 kpc 475 kpc 471 kpc

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 1020 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)

Radio LF and statistical lifetimes

Statistical lifetime of a radio source of luminosity PR

is TR = (NR/NE). TE, for parent pop. NE, with lifetime TE

(Schmidt 1966, estimated TR~ 109 yr for radio galaxies)

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.

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)

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

Survey limits in radio surface-brightness

(R.Subrahmanyan)NVSS/SUMSS limits ~4mJy/arcmin2

Powerful radio galaxies are found mainly in the brightest ellipticals

Implies that black-hole mass is a key parameter!

P-d diagram binned by optical luminosity (1)

MR = -24.0 to -24.9

MR = -23.0 to -23.9

P-d diagram binned by optical luminosity (2)

MR = -22.0 to -22.9 MR = -21.0 to -21.9

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.

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?

PKS 0019-338, a post-starburst radio galaxy at z=0.128

Balmer abs. lines imply a massive (~1010 Msun) starburst occurred ~0.15 Gyr ago.

Compact, steep-spectrum radio source has P1.4 ~ 1025

W/Hz

Nearby active ellipticals with accreted HI and strong, compact

central radio sources

Moderate power (P1.4 ~ 1023W/Hz), flat spectrum compact radio AGN, recently triggered? Or episodic short bursts of activity? Gas infall without star formation?

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 108 yr (i.e. > source lifetime?). Unlike QSOs, optical spectra are dominated by the stellar population, not the AGN.

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 (~106 yr) are orders of magnitude smaller than the time since the starburst began (~ 108 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.

Redshift distribution of 2dFGRS radio sources

(Colless 2001)

All Radio

All 2dFGRS galaxies

Radio AGN

Radio starburst

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).

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)

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 1010 Msun.