1) The study compares X-ray properties of 14 radio galaxies divided into high-excitation (HEG) and low-excitation (LEG) galaxies based on optical emission lines. 2) HEGs have higher average intrinsic neutral hydrogen column densities and X-ray luminosities than LEGs. They also have stronger optical [OIII] emission lines. 3) The distributions of column densities and X-ray luminosities between HEGs and LEGs are bimodal. HEGs also show evidence for accretion disks from detected iron K-alpha lines, which are absent in LEGs. 4) The results suggest LEGs have a less efficient accretion regime than HEGs, possibly only accre

1. High-Excitation and Low-Excitation
Radio Galaxies: an X-ray
comparison
ALMA Mater Studiorum - Università di Bologna
Presentata da: Relatore:
Chiar.mo Prof.
Valentina Scipione Giorgio G. C. Palumbo
Co-relatore:
Dott.ssa Paola Grandi

2. AGN Overview
• A small fraction (15-20%) of
AGN are Radio Loud (RL)
• An AGN is Radio Loud when
F5GHz/FB >10
(controversial classification)
• otherwise is Radio Quiet (RQ)
Spiral Elliptical
RL => Elliptical
RQ => Elliptical and Spiral

3. Structural components:
• Core (difficult to be resolved)
• Jets (up to kpc or Mpc scales)
• Hot Spots (at the end of jets)
• Lobes (up to Mpc scales)
Radio Galaxies

4. FR I & FR II
Two morphological types, according to where the most of the
luminosity is radiated ↔ radio power division at
L178 MHz ≈ 1025 Watt Hz-1 sr -1 (Fanaroff & Riley 1974):
 FR I: edge darkened, with
extended twin lobe
structure
 FR II: edge brightened,
with bright hot spots

5. Optical Classification
 Narrow-Line Radio Galaxies (NLRGs)
 Broad-Line Radio Galaxies (BLRGs)

6. NLRGs: Optical Classification
Based on optical oxygen lines ratios (Laing et al. 1994):
Low-Excitation Radio Galaxies
(LEGs):
• [O II]/[O III] > 1
(Jackson & Rawlings 1997)
• both FR I and FR II
morphology
High-Excitation Radio Galaxies
(HEGs):
• exclusively FR II
morphology

7. NLRGs: Optical Classification
FR II LEGs share the same radio properties of HEGs, host similar
massive black holes, but…
are less bright than HEGs in the [O III] line by a factor of 10!
Strong obscuration or different accretion modes?
Low-Excitation Radio Galaxies
(LEGs):
• [O II]/[O III] > 1
(Jackson & Rawlings 1997)
• both FR I and FR II
morphology
High-Excitation Radio Galaxies
(HEGs):
• exclusively FR II
morphology
Based on optical oxygen lines ratios (Laing et al. 1994):

8. Outline
• Definition of the sample
• Chandra analysis of the objects in the sample
• Comparison between HEG and LEG on the basis of their X-ray
nuclear properties:
• Measure of the X-ray unabsorbed luminosities in the energy
range 2 – 10 keV
• Measure of the intrinsic neutral hydrogen column densities
• Characterization of the Fe Kα line (~ 6.4 keV)  indicator of
efficient accretion disk
• Comparison between [O III] line luminosities (taken from
Buttiglione et al. 2010) and X-ray luminosities

9. Our Sample
All FR II NLRGs of the 3CR
Catalogue:
• with z < 0.1
• observed with Chandra
X-ray Observatory satellite
14 sources
6 LEG 8 HEG

10. 3CR Catalogue
• Third (revised) Cambridge Catalogue (3CR) of radio sources
(Bennet 1972) using extended emission at 178 MHz
• Survey of all sources North of δ = - 05 degrees with a Fr > 9 Jy:
unbiased and homogeneous
• Largely used catalogue
• Huge set of multiwavelenghts observations (VLA, HST, Spitzer,
Chandra, Fermi…)
• Complete optical spectroscopy (Buttiglione et al. 2010)
• …but systematic X-ray comparison between LEGs and HEGs
still missing

11. X-ray Data Analysis: LEG
Observation:
• 3 of 6 in cluster
• one in a merging system
An example: 3C 388
• At the center of a cluster
• X-shaped X-ray
morphology, typical of radio
plasma/ICM interactions
• Clear cavities in the ICM
correspond to the radio lobes

12. X-ray Data Analysis: LEG
An example of merging system: 3C 442
• At the center of a group of
galaxies
• Double-lobed radio structure
with no jet
• Merging system: host galaxy
NGC 7237 interacting with
NGC 7236
• Extended soft X-ray emission
• X-ray cavities correspond to radio cavities

13. X-ray Data Analysis: LEG
Spectra:
Name Model NH (atoms cm-2) Γh
3C 388 PLh + Th - 1.7
3C 442 NH (PLh) + Th 7 ⨯ 1021 1.8

14. X-ray Data Analysis: HEG
Observation:
• one in group
• one in a merging system
An example: 3C 403
• X-shaped morphology
• Radio “wings”
• Emission from hot gas of
the host galaxy

15. X-ray Data Analysis: HEG
An example of merging system between two AGN: 3C 321
• Merging system with a
companion galaxy
hosting an AGN
• Both embedded in
extended soft X-ray
emission
• One-sided jet
• X-ray emission associated with radio hot spots

16. X-ray Data Analysis: HEG
Spectra:
Name Model
3C 321 NH (PLh + Fe Kα) + PLs+Th 1.2 ⨯ 1024 1.3
3C 403 NH (PLh + Fe Kα) + PLs+Th 4.4 ⨯ 1023 1.7

17. Results: NH
• Bimodal distribution
Class < NH > σ
LEG 22.06 0.90
HEG 23.77 0.17
LEGs obscuration: dust lanes
crossing the galaxies (Martel et
al. 2000)

18. Results: Lx
• LEGs are less bright than
HEGs by a factor of 10 also
in the X-ray band!
Class < Lₓ > σ
LEG 42.20 0.96
HEG 43.64 0.58

19. X-ray – optical correlation
• LEGs and HEGs occupy
different regions of the
L[O III] – Lₓ plane
• HEGs have higher Lₓ and
L[O III]
• Lₓ and L[O III] correlated
(r ~ 0.6 and Pcorrelation ~ 98%)
• Lₓ and L[O III] good tracers
of accretion rate
[O III] line luminosity vs X-ray luminosity:

20. A possible interpretation
HEGs
• Higher intrinsic neutral
hydrogen column densities
• High X-ray luminosities
• Fe Kα line in 6 out of 8
• High [O III] line luminosities
LEGs
• Lower intrinsic neutral
hydrogen column densities
• Low X-ray luminosities
• No Fe Kα line
• Low [O III] line luminosities
 Accretion less efficient in LEGs
 LEGs probably only accrete hot gas
 FR II LEGs could be in a quiescent phase

21. Conclusions
• We analyzed X-ray spectra of 14 3CR FR II NLRGs with z < 0.1
• The sample is composed by two main spectroscopic sub-
populations: 6 sources are LEG, 8 HEG
HEGs
• Complex spectra
• Rich of cold material
• < NH > ≈ 1024 atoms cm-2
• Fe Kα line detected in 75%
of sources
LEGs
• < NH > ≈ 1022 atoms cm-2
• No detection of Fe Kα line

22. Conclusions
• Intrinsic neutral hydrogen column densities of LEG are lower
than HEG: bimodal distribution
• HEG are 10 times brighter than LEG in X-ray luminosity
• HEG and LEG occupy different regions of the L[O III] – Lₓ plane
• Hypothesis of different accretion regime is preferred: LEG
accretion is less efficient than HEG
• LEG could be in a quiescent phase