AGN classifiction

velodromeryeΠολεοδομικά Έργα

15 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

70 εμφανίσεις

High Energy
Astrophysics

Active Galactic Nuclei

Introduction



Active Galactic Nuclei (AGN) are powerful sources of
radiation which exist in the centre of 1
-
10% of all
galaxies



Galaxies which host an AGN are known as active
galaxies



The span of observed AGN luminosities is huge L~10
40



10
47

erg/s



The more luminous AGN outshine their host galaxies by
factors of 1000 or more



AGN are the most luminous long
-
lived objects in the
universe

Introduction: Historical discovery



Fath 1908 at Lick took a spectrum of NGC1068 and
found emission lines



Slipher 1917



Hubble 1926 study more systems



Seyfert 1941,1943: galaxies whose nuclei spectra show
high
-
ionization emission lines. Broad emission lines
arising from small, bright nucleus and covering a wide
range of ionization => Seyfert galaxies



Introduction: Historical discovery



After II World War with the development of
radioastronomy, radiogalaxies were discovered



Quasars were discovered in 1963 when Maarten
Schmidt interpreted the optical emission lines of the
known radio source 2C273 as redshifted hydrogen
Balmer lines.



AGN Zoology



Radio galaxies



Radio Quasars



BL Lac Objects



Optically Violent Variables (OVV’s)



Radio Quiet Quasars (QSOs)



Seyfert I galaxies (SyI)



Seyfert II galaxies (SyII)



Low Ionization Nuclear Emission
-
Line Regions (LINERS)

AGN classification



There is a large diversity in the nature of the observed
radiation from different AGN



The classification scheme has varied with time



A fundamental division in the classification scheme is
based on the radio properties of AGN: radio
-
loud or
radio
-
quiet



AGN classifiction



Radio

loud AGN emit collimated jets of plasma which
feed energy and high
-
energy particles into an extended
halo or lobe. In extreme cases the halo can be in excess of
1Mpc in extent



Both the jets and radio halo are observed to be sources
of continuum radio emission, presumably due to
synchrotron emission from relativistic electrons in the
jet/halo plasma

Radio
-
loud and radio
-
quiet AGN

AGN classifiction



There are many galaxies where radio imaging reveals
well
-
collimated jets linking the AGN with the halo



Outflowing blobs have been observed within the jets.
Sometimes, the apparent proper motion of these blobs
exceeds the speed of light (superluminal motion). This
probably results from relativistic bulk motion in a
direction that is somewhat towards the observer



Some radio
-
loud AGN present only one observable jet

Radio
-
loud and radio
-
quiet AGN

AGN classifiction



Inferred relativistic jet motion are in the range of
Lorentz factors of 3
-
10



The physical nature of these jets, along with the
mechanisms responsible for their acceleration and
collimation are very poorly understood

Radio
-
loud and radio
-
quiet AGN

AGN classifiction



Radio
-
quiet AGN do not show large scale collimated
jets



However, bipolar radio emission, typically with and
extent less than 0.5 kpc, is often seen and attributed to an
outflow from the AGN

Radio
-
loud and radio
-
quiet AGN

AGN classifiction



Within the class of radio
-
quiet AGN classification is
performed primarily on the basis of optical/UV spectral
properties

Radio
-
quiet AGN classification

Seyfert I galaxies/ radio
-
quiet quasars



Spectrum is complex and multicomponent



Called broad
-
line AGN due to the presence of broad
(FWHM~2000
-
20000 km/s) permitted optical/UV emission
lines

AGN classifiction

Seyfert I galaxies/ radio
-
quiet quasars



Both low
-
ionization broad lines (MgII
λ
2798 and
hydrogen lines) and high
-
ionization broad lines
(CIV
λ
1549, OVI
λ
1035 and NeVIII
λ
774) are common



The region from which these emission line arise is known
as broad line region (BLR)



The optical/UV emission lines arise from relatively dense
(n
e

~ 10
9

cm
-
3
) photoionized clouds with a small volume
filling factor

AGN classifiction

Seyfert I galaxies/ radio
-
quiet quasars



The presence of low
-
ionization lines, coupled with
photoionization modeling, suggests that at least some of the
material responsible for the lines is optically thick to
Lyman continuum radiation



The line widths are due to cloud motions



Collissional de
-
excitation in this relatively high density
plasma dominates over forbidden line emission



The optical/UV spectra also present narrow (~500 km/s)
permitted and forbidden emission lines

AGN classifiction

Seyfert I galaxies/ radio
-
quiet quasars



Commonly observed narrow lines are the hydrogen lines,
[OII]
λ
3727, [OIII]
λ
5007, [NII]
λ
6583, [SII]
λ
6716/6731.



the material responsible for these lines, which resides in
the narrow line region (NLR), is more teneous than that in
the BLR and lies at greater distances from the putative
black hole



The NLR can be spatially resolved in nearby AGN and is
found to have typical sizes of 0.1 kpc

AGN classifiction

Seyfert I galaxies/ radio
-
quiet quasars



The distinction between a Seyfert I nucleus and a radio
-
quiet quasar (RQQ or quasi
-
stellar object QSO) was
traditionally made on the basis of whether the galaxy or
AGN was discovered first



SyI nuclei are generally lower
-
luminosity than radio
-
quiet
quasars: the dividing luminosity is approx L
bol

~ 10
45

erg/s
L
X

~ 10
42

erg/s



In practice, SyI nuclei and radio
-
quiet quasars seem to
form a continous sequence in luminosity

AGN classifiction

Seyfert I galaxies/ radio
-
quiet quasars



They display strong continuum radiation from IR through
gamma
-
ray wavelengths



The higher
-
energy emission is often observed to vary
rapidly

AGN classifiction

Seyfert II galaxies



The spectra of Seyfert II galaxies display narrow
permitted and forbidden emission lines



They differ from SyI galaxies because no broad lines are
observable and the optical/UV radiation is much weaker



The fact that broad optical/UV lines can often be
observed at sufficiently long wavelengths suggests that at
least some SyII nuclei are simply reddened SyI nuclei and
the BLR would have to lie behind the reddening material



X
-
ray observations reveal photoelectric absorption from
cold gas presumably associated with the dust

AGN classifiction

Narrow line Seyfert I galaxies (NLS1)



They differ from Seyfert I in that their permitted
optical/UV lines are significantly narrower than SyI (but
still broader that the lines from the NLR)



The ratio of [OIII]
λ
5007/H
β

< 3, a criterion that seems to
differentiate SyI from SyII



They display stronger than typical optical line complexes
due to FeII emission



X
-
ray continuum is softer and more variable than normal
SyI

AGN classifiction

Narrow Emission Line galaxies (NELGs)



They show narrow (FWHM<1000 km/s) emission lines,
typically of H
α
, H
β
, [OIII]
λ
5007



X
-
ray luminosities L
x

~ 10
42

erg/s (two orders of
magnitude brighter than normal galaxies)



NELGs are often classified as AGN but part of the
emission may be due to star formation



The number of sources and their hard spectra make them
good candidates for producing the X
-
ray background

AGN classifiction



Within the class of radio
-
loud AGN, classification is
performed on the basis of both radio morphology and
optical/UV spectral properties

Radio
-
Loud AGN classification

FR
-
I (Fanaroff
-
Riley
-
I) radio galaxies



Radio images of low luminosity (L
radio

< 10
42

erg/s) radio
-
loud AGN usually reveal radio surface brightness profiles
that fall continuously as one proceeds from the nucleus to
the edge of the radio lobe (edge
-
darkened morphology)



They display narrow emission lines from a NLR, but
normally not broad emission lines

AGN classifiction

FR
-
II radio galaxies



Powerful radio
-
loud AGN (L
radio

> 10
42

erg/s) display
different radio morphology. Apart from the actual nucleus
itself, the radio surface brightness can be low in the central
regions of the source and dramatically brighten towards the
edge of the halo as the jet(s) are seen to terminate at strong
shocks (edge
-
brightened morphology)



The optical properties can be either SyI
-
like or SyII
-
like
(known as broad line radio galaxies BLRG and narrow line
radio galaxies NLRG, respectively)

AGN classifiction

Radio
-
loud quasars



Distinction between BLRG and radio
-
loud quasars (RLQ
or quasars) is blurred



A BLRG type object in which the continuum emission of
the AGN dominates over the radio emission is known as a
RLQ



RLQ often posses one
-
sided jets which show
superluminal motion

AGN classifiction

Blazars



Radio
-
loud objects displaying very strong and variable
continuum at all wavelengths



the emission lines are either very weak or absent from the
optical/UV spectrum



The presence or absence of lines leads to the
subclassification: optically
-
violent variable (OVV) or BL
Lac object

The supermassive black hole model



The current paradigm is that the fundamental power
source of all AGN is accretion onto a supermassive black
hole



Some theoretical models predict that there is a massive
black hole at the center of most galaxies due to processes
probably related to galaxy formation



At some level the interstellar medium (ISM) of the host
galaxy must accrete onto a central black hole



Non
-
axisymmmetric potentials resulting from either
galactic bar formation or a nearby companion galaxy can
further increase the mass flow rate into the central regions
of the galaxy

The supermassive black hole model



Together with any stellar mass loss associated with a
nuclear star cluster, significant quantities of gas can be
deposited in the central parts of a galaxy and enter the
region where the black hole mass dominates the
gravitational potential



Gas inevitably posses a non
-
zero angular momentum
when it enters the region dominated by the potential of
the black hole. As a result, the inflowing gas will form a
flattened, rotationally supported structure, an accretion
disk

The supermassive black hole model



Accretion disks are the engine via which some fraction
of the gravitational potential energy of the infalling
material is transformed into observable radiation and
kinetic energy of jets/outflows



Accretion disks posses some form of “viscosity” in
order to transport angular momentum outwards and allow
matter to flow inwards, thereby liberating energy



It is probably a self
-
sustaining magnetic dynamo which
probably provides the required angular momentum
transport

The supermassive black hole model



In the standard model, the accretion disk is
geometrically thin and radiatively efficient

The supermassive black hole model



Data supporting the existence of supermassive black
holes



The width of the broad optical/UV emission lines is
interpreted as being due to material motions under the
influence of only gravity, then a deep gravitational
potential is needed



The velocity dispersions of stars in the central regions of
many galaxies appear to increase as one proceeds inwards
to the core of the galaxy. These observations imply a mass
concentration at the galactic nucleus of the sort expected if
a black hole were present

The supermassive black hole model



Data supporting the existence of supermassive black
holes



The large amplitude variability of X
-
ray emission
implies that, at any one time, the emission is dominated by
a very small number of sources



The efficiency in which matter is converted into energy
exceeds that possible from any thermonuclear process.
The only process (apart from particle anihiliation) this
efficient is accretion of matter onto some compact object
with R < 100 R
sch

The supermassive black hole model



Data supporting the existence of supermassive black
holes



In order that the accretion flow does not halt due to the
pressure of the outgoing radiation, the accreting object
must posses a minimum mass of M ~ 10
6

Msolar



The only object known with this mass 10
6

Msolar in that
radius 100 R
sch

is a black hole

The supermassive black hole model



Data supporting the existence of supermassive black
holes



Imaging and spectroscopy from HST reveal rotating
disks indicating large masses in small radii therefore
arguing in favor of black holes

The supermassive black hole model



Data supporting the existence of supermassive black
holes



Many AGN produce H
2
O and OH masers. The
intrinsically narrow width of the maser emission line
allows accurate velocities to be determined from a
measurement of the line frequency. High spatial VLBI
observations map the velocity field in very small radius



The M101 maser finds almost perfect Keplerian motion
demonstrating that, at these scales, 0.5 pc from the source,
the potential is dominated by a point
-
like mass with
M~10
7

Msolar

The supermassive black hole model



Data supporting the existence of supermassive black
holes



The X
-
ray fluorescent K
α

emisison line is thought to
originate from the innermost regions of the accretion disk
and thus probes a regime where orbital velocities are
mildly
-
relativistic and gravitational redshifts are
becoming large



The profile of the iron line is in agreement with that
expected from material in a thin accretion disk near a
black hole

AGN unification



The central feature of the unification schemes is that the
observed properties, and thus the classification, of a given
AGN depend upon its orientation



The main ingredients of this scheme are:



A supermassive black hole 10
6
-
10

Msolar



An accretion disk and corona, heated by magnetic and/or
viscous processes so that it radiates at optical through soft
X
-
ray energies



high velocity gas, often referred as the BLR



lower velocity gas in the NLR

AGN unification



The main ingredients of this scheme are:



an obscuring torus (or other geometrical form) of gas
and dust, hiding the BLR from some directions



a relativistic jet, formed within ~ 100 Rsch of the black
hole, and extending outwards for tens of kpc, and in some
cases as much as a Mpc

AGN unification



The strongest case for unification can be made for SyI
and SyII galaxies



Some SyII appear to be SyI apart from the fact that the
BLR and continuum radiation are heavily reddened



The prototypical SyII galaxy NGC1068 shows broad
optical emission lines when observed in polarized light.
Interpreting polarization due to scattering, this shows that
there are directions from which NGC1068 displays a SyI
spectrum

Radio
-
Quiet AGN unification

AGN unification



Antonucci & Miller 1985 proposed that all Seyfert
galaxies posses a dusty torus of gas at distances
intermediate between the BLR and NLR. And observer
whose line of sight to the black hole intercepts this torus
would see a heavily reddened (or completely
extinguished) BLR and central continuum radiation but
an unreddened NLR. This would be identified as a SyII
galaxy. If the line
-
of
-
sight does not intercept the torus, the
central regions can be observed leading to a SyI
classification

Radio
-
Quiet AGN unification

AGN unification



Powerful radio
-
quiet quasars may be more luminous
versions of the same basic central engine, probably due to
a higher accretion rate



It is unclear whether most radio
-
quiet quasars have
dusty tori as not many type
-
II quasars have been found



NLS1 have unusual properties hard to accommodate in
this scheme

Radio
-
Quiet AGN unification

AGN unification



NELGs appear to be simply low
-
luminosity obscured
Seyfert nuclei



However, there may be intrinsically narrow
-
line object
(without BLR) amongst this population. These objects
may have an advection
-
dominated accretion disks rather
than the standard thin disks. Within this model, the low
luminosity of these objects results from the small
radiative efficiency of the accretion disk (most of the
energy is advection through the event horizon). The
resulting lack of a strong photoionizing continuum could
explain the lack of BLR

Radio
-
Quiet AGN unification

AGN unification



Blazars, radio
-
loud quasars and FR
-
II galaxies have
unified under a similar scheme to the Seyfert one. The
only difference is the inclusion of relativistic jets of
synchrotron emitting plasma. The relativistic beaming
and Doppler shifts associated with this jet are a source of
further anisotropy in addition to the obscuration of the
torus

Radio
-
Loud AGN unification

AGN unification



Blazars are objects in which our line
-
of
-
sight lies within
the jet cone of the source. The strong relativistic
aberration and Doppler shifting of the jet emission
produces the highly variable and continuum dominated
emission



RLQ are objects in which the line
-
of
-
sight is close to,
but not within, the jet cone. The continuum is strongly
beamed but not as strong as to swamp the emission from
the BLR

Radio
-
Loud AGN unification

AGN unification



In objects with larger orientation angles, the central
continuum flux falls resulting in an FR
-
II BLRG



For yet larger angles, the dusty torus obscures the
central continuum source and BLR, resulting in an FR
-
II
NLRG



It is unclear how FR
-
I radio galaxies fit into this
scheme. Clearly some mechanism produces different
radio morphologies for FR
-
I and FR
-
II and this
mechanism is related to source power

Radio
-
Loud AGN unification

AGN unification



An FR
-
I unification model as above is not straight
forward. Some blazars can be beamed FR
-
I, but there is a
lack of BL FR
-
I. This suggests either a torus with a small
opening angle or the absence of the BLR in FR
-
I sources

Radio
-
Loud AGN unification

The AGN Unified Model

blazars,
Type 1 Sy/QSO

broad lines

Urry & Padovani, 1995

Type 1 AGN SED

Manners, 2002

mm

far
-
IR

near
-
IR

Optical
-
UV

X
-
rays

The AGN Unified Model

radio galaxies,

Type 2 Sy/QSO

narrow lines

Urry & Padovani, 1995

Type 2 AGN SED

Norman et al, 2002

Radio

far
-
IR

optical
-
UV

X
-
rays

Type 1 AGN SED

Manners, 2002

mm

far
-
IR

near
-
IR

Optical
-
UV

X
-
rays