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

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

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

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

Optical Classification Narrow-Line Radio Galaxies (NLRGs)

Broad-Line Radio Galaxies (BLRGs)

NLRGs: Optical ClassificationBased 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

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):

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

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

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

X-ray Data Analysis: LEGObservation:

• 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

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

X-ray Data Analysis: LEGSpectra:

Name Model NH (atoms cm-2) Γh

3C 388 PLh + Th - 1.7

3C 442 NH (PLh) + Th 7 ⨯ 1021 1.8

X-ray Data Analysis: HEGObservation:

• 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

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

X-ray Data Analysis: HEGSpectra:

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

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)

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

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:

A possible interpretationHEGs

• 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

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

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