Radio galaxies: timescales and triggers Elaine Sadler, University of Sydney

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
(ngt10,000) radio/optical samples in the local
(zlt0.5) 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
  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).

6
Radio imaging surveys in the southern hemisphere
NVSS (VLA) n1.4 GHz, north of -40o
Data www.cv.nrao.edu/nvss
SUMSS (Molonglo) n843 MHz, south of -30o
85 complete at present
Datawww.physics.usyd.edu.au/astrop/SUMSS
7
Spectroscopic redshift surveys in the southern
hemisphere
2dFGRS (AAT) 220,000 galaxies to z0.3, 1500
deg2. 6dFGS (AAO Schmidt) 150,000 galaxies to
z0.15, all-sky.
Final goal is analysis of 10,000 radio-detected
AGN with zlt0.3.
8
Images of the optical and radio sky
180 galaxies per square degree to B19.4 mag
40 sources per square degree to S5 mJy
Optical DSS B median z0.1
Radio 843 MHz median z1
Overlap 2 objects per square degree in 2dFGRS
9
Typical 2dFGRS radio-source spectra

• 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

Ha
Hb
OIII
(Sadler et al. 1999)
10
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.
(Sadler et al. 2002)
Spectra essential!
11
Radio emission from star-forming galaxies (most
are IRAS 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)
12
Local (zlt0.1) 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 (gt 30 Msun/yr),
otherwise agree well.
Ha
Radio
13
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
Radio
ULIRGs
Normal galaxy line
IRAS
14
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
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 1020 W/Hz to avoid exceeding the space
density of early-type galaxies.
Much broader than optical LF!
Power-law F(P) a P-0.62
(Sadler et al. 2002)
16
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)
17
Linear size as an estimate of source age
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.
(Parma et al. 2002)
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? -gt (empirical)
evolutionary tracks!
(Sadler et al. 2002)
19
The P-d diagram for resolved 2dFGRS radio galaxies
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
Many unresolved?
LSB sources
20
Survey limits in radio surface-brightness
(R.Subrahmanyan)
NVSS/SUMSS limits 4mJy/arcmin2
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)
MR -24.0 to -24.9
MR -23.0 to -23.9
23
P-d diagram binned by optical luminosity (2)
MR -22.0 to -22.9
MR -21.0 to -21.9
24
Do FRII radio galaxies evolve to FR Is?
Galaxies shouldnt 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.
(Ledlow Owen 1996)
25
How many radio galaxies are triggered by
starbursts?
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)
26
PKS 0019-338, a post-starburst radio galaxy at
z0.128
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
27
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?
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 gt10,000
  radio detections.
• In the local universe (zlt0.3), 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. gt source lifetime?). Unlike QSOs,
  optical spectra are dominated by the stellar
  population, not the AGN.

29
Results so far

• Galaxy luminosity (-gt 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 ltlt statistical lifetime) and
  they are unlikely to evolve to classical radio
  galaxies. Fuelling could be by infall of HI
  clouds, rather than whole galaxies.

30
(No Transcript)
31
Redshift distribution of 2dFGRS radio sources
All Radio
All 2dFGRS galaxies
Radio AGN
Radio starburst
(Colless 2001)
32
Cosmic evolution of radio galaxies

The space density of powerful radio galaxies and
quasars was 1000 times higher at z2 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 z5, 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
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.
ATCA total HI map
35
(No Transcript)