Explosions in galaxies

Vista8 in Astronomy, Vol. 22, pp. 307-319 © Pergamon Press Ltd. 1978.

0083-6656/78/0801-0307 $05.00/0

Printed in Great Britain

EXPLOSIONS IN GALAXIES* Sidney van den Bergh Dominion Astrophysical Observatory, Herzbcrg Institute of Astrophysics National Research Council of Canada, Victoria, B.C., Canada

I. INTRODUCTION In retrospect,

it is now clear that the most exciting astronomical discoveries of the

past century resulted from attempts to identify sources of celestial radio radiation with optical

objects.

In their pioneering papers,

strong radio sources consist of two main types: objects

in

which

violent

events

are

taking

Baade and Minkowski

(1953a,b)

showed that

I) Supernova remnants and 2) Extragalactic place.

The

study

of

supernova

remnants

ultimately led to the discovery of pulsars and neutron stars and forms the underpinning of ~ur

present

radio

ideas

sources

on the chemical evolution of galaxies.

resulted

in

the

identification

of

ever

Observations

more

distant

of extragalactic

and

more

powerful

emitters ultimately culminating in the discovery of quasars (Schmidt 1963).

The first comprehensive review of the evidence for the occurrence of violent events in galaxies was given by Burbidge, Burbldge and Sandage (1963).

The evidence marshalled by

these

ultimate

authors

established

beyond

reasonable

doubt

that

the

source

of

energy

release in explosive events was located in the nuclei of galaxies.

The cast of characters (M 82, M 87, NGC 1275, NGC 5128, Cygnus A) introduced by Baade and Minkowskl and by Burbldge et al. still dominates our thinking on explosive phenomena in galaxies. role

Furthermore,

in our

attempts

the first known quasars 3C48 and 3C273 continue to play a crucial to understand

these enigmatic

objects.

Although a great

observational effort is being devoted to the study of quasars relatively h a s been made towards understanding

them in detail.

little

deal of progress

A strong consensus among workers in

this field is, however, that the energy source in the nuclei of galaxies and in quasars is probably gravitational. II. NATURE OF THE CENTRAL ENERGY SOURCE All theories for the release of massive amounts of energy from the nuclei of galaxies ultimately rely on the fact that the density of matter in galactic nuclei high.

is exceedingly

Direct evidence for such a high mass density has now, apparently, been obtained for

the nucleus of the active galaxy M 87 (Young et al. 1978, Sargent et al. 1978).

In hlgh-denslty nuclei stellar collisions (cf. Saslaw 1 9 7 5 ) w i l l a large energy release from massive supernovae is expected.

occur frequently and

Alternatively, by analogy with

*Dominion Astrophysical Observatory Contribution No. 360 ffiNRC. No. 16299. 307

308

S. van den Bergh

galactic

X-ray

sources,

one

might

imagine

(Lynden-Bell

1969)

place by accretion of matter onto a massive black hole. own

galaxy,

existence ~

1033

of

nucleus

the

by

source Ne

of

II

active

Lo

with

a

galaxies

It should

by

<

energy

70

gives AU

Wollman

et

direct

massive

disks

a

el.

(1977),

rapidly

be emphasized

("spinars") that

such

evidence

and

radio

(in analogy with pulsars)

contain

release

takes

In the case of the nucleus of our

(1977)

if their

magnetoids

for

the

luminosity

of

the mass

in

the

it has been proposed

rotating

if their thermal energy is large

or they could be very flattened 1969).

al.

radius

Finally

will be almost spherical

(Morrison

et

observations

is < 4 x 106 M@.

nuclei

magnetoids 1976)

a compact _i s From

ergs

galactic that

radio-interferometry

that

magnetoids.

Such

(Ginzburg and Ozernoy

thermal might

energy

is small

themselves

contain

massive black holes.

Due

to

tidal

galaxies.

friction

As a result

(Ostriker

1977)

the cores of central cluster galaxies. that

small

of such cannibalism

are

to be expected

proximity

to each other.

from

galaxies

the nuclei

No detailed

the resulting

will

be

of captured

gobbled

objects

up

will

by

large

sink into

study has yet been made of the effects

interaction

of black

holes

that are in close

normal

galaxies

III. THE NATURE OF NUCLEI

From a morphological active nuclei - Seyfert

point of view one may consider

galaxies

of type I - and quasars

which the degree of activity

in the nucleus is the only variable.

provided

that spectra

by the observation

Ne, Si and Fe. These are elements observation

strongly

This speculation Williams

1976)

suggests

receives that

Large radio

numbers

sources

"classical

support

from those observed

associated

with

of various

This

in a stellar environment. (cf.

Strlttmatter

heavy elements

and

in quasars

are

show that

the

in the sun.

sources

elliptical

type whereas

occurs

from the observation

of the abundances

of maps of radio

double"

lines of C, N, O, Mg,

that are known to be produced by stellar evolution.

considerable

with

family in

Support for this view is

contain emission

that the quasar phenomenon

the ratios

not markedly different

of quasars

- galaxies

to form a one parameter

are now available.

galaxies

not a single

and

spiral

These maps

quasars

are

predominantly

that has so far been studied

of

the

is this

type (de Bruyn 1978).

Objects morphology Ekers

such as NGC

similar

(1978a)

to

points

1316

that

The

observation

fundamental energy

explosions

that

way.

in

S

though

galaxies.

that

It is unlikely in

5128

ellipticals,

(Centaurus

have

the

nuclei.

This

of is

(E)

and

quasars

E so

this and

difference Q

nuclei

because

(Ekers and Kotanye

they are intrinsically

classified

SO.

are seen edge-on suggesting

that

(Q) produce

been

classical

the E and q nuclei must differ

that

outbursts

A), which have a radio

sometimes

that have been misclasslfled.

ellipticals

NGC 4472 in the Virgo cluster even

objects

(S) do not, shows

involved

A) and NGC

in

out that none of these "SO" galaxies

they are, in fact, spheroidal

that spirals

(Fornax

observed

much

is due is

intrinsically 1978),

less luminous

to the fact

greater

doubles

from S nuclei

than

faint

that

that

have a classical

double

than are the nuclei

the mean

generated

E galaxies,

but

in some

such

by as

structure

of some spiral

Explosions in Galaxies

309

The similarity between active nuclei of ellipticals and quasars is emphasized by the fact that radio galaxies and QSO's can not be distinguished from each other on the basis of their radio observations

alone

(Mackay 1971, Kellermann

1972).

The observed differences

between the outbursts in E and Q nuclei on the one hand and in S nuclei on the other might be

due to environmental

factors,

i.e.,

low angular momentum gas in ellipticals might be

collected onto the nuclei more efficiently than the high angular momentum gas in spirals. Alternatively,

initial differences

between

E and S nuclei might account

differences between elliptical and spiral ~alaxies.

for the present

If the nuclei of E galaxies obtained a

significant fraction of their angular momentum from the interstellar gas, one might expect a correlation major

axis

between

of

the position

the optical

angle

isophotes

of a radio double and

of the parent

galaxy.

the

orientation

Available

of

observational

the data

(Bridle and Brandie 1973, Gibson 1975, and Sullivan and Sinn 1975) give no clear indication that such a correlation actually exists. type is predestined

This result slightly favours the idea that galaxy

by the nature of the nucleus

on which

the bulk of galactic matter

condenses.

As has been pointed out above, quasars and elliptical galaxies have indistinguishable radio morphology.

Furthermore, both E galaxies

(M 87), and quasars (3C273), are sometimes

observed to have synchrotron radiating Jets emanating from their nuclei. not,

so far, been observed

in any spirals.

This suggests

Such Jets have

that the quasar phenomenon is

simply a more violent form of the same type of event that is observed in radio ellipticals. An argument a~ainst this view is, however, provided by the fact (van den Bergh 1975) that strong

radio

concentrated

sources

occur

in

giant

in clusters whereas

elliptical

there

E

and

is no evidence

cD

galaxies

which

that quasars

are

strongly

are located in rich

clusters of galaxies (Roberts, O'Dell and Burbldge 1977).

point of view

(Khachkian and Weedman 1977) quasars and Seyfert

galaxies of type I are indistinguishable.

From a spectroscopic

The fact that Seyfert galaxies and quasars tend

to occur outside rich clusters emphasizes this similarity. that Seyfert galaxies are single radio sources

A major difference is, however,

(van der Kruit 1971a) whereas the majority

of quasars are classical doubles.

Due to scattered light problems, envelopes

(Kristian

it is exceedingly difficult

As a result, the true nature of these objects remains in doubt. prepared profoundly envelopes.

for

the possibility

affected This

to observe

the diffuse

1973) in which some relatively nearby quasars appear to be embedded.

both

that violent

the morphology

suspicion

activity

and

is strengthened

stellar by

In any case one should be

in the nuclei +

of

these objects

interstellar

the observation

content

has

of

quasar

that almost half

of all

Seyfert galaxies (cf. van den Bergh 1960) exhibit morphological peculiarities.

The relationship between quasars and BL Lac objects remains obscure.

Possibly objects

of the latter type occur in a relatively gas-free environment.

If this is the case it

does,

could

however,

become

difficult

to understand

flowing into a compact central object.

how

such

nuclei

are in fact powered by infall of gas is provided by the observation gas-rlch ellipticals,

be

fueled

gas

that only the rare

such as NGC 4278, contain small flat-spectrum radio sources

1978b) in their nuclei.

by

Strong support for the notion that galactic nuclei

(Ekers

310

S. van den Bergh

The

spectroscopic

differences

between

galaxies

on the other might

quasars

are being "fed" interstellar

in clusters.

possibly

Such a difference

swept clean of interstellar and Gott

for

the fact

notion

result

the central

objects

are located

such

1976).

are

compact

single

radio

and

1975)

Wardle

sources.

E-type

contain

compact

double

IV.

radio

holes

In

elllpticals

BL

apparently

Lacertae

are

of elliptical

however,

are presumably

do not become quasars.

quasar-like

environment

gas (Gunn

not,

(which

can be

might

galaxies.

sources

in which

the

lead one to suspect

It is therefore

that

surprising

that have so far been examined wlth hlgh spacial resolution The sole exception

nuclei

cores.

is 1400+162

at low frequencies.

with S-type

nuclei

that

If the latter conjecture

using long-basellne

as

sources.

are associated

black

would

in rlch clusters

medium)

in a gas-free

double structure

Lac objects

Thls explanation

cD galaxies

in the nuclei

that 9 out of I0 BL Lac objects

which has a classical

the central

from the fact that cluster

1976, Glsler

central energy source is situated "Lacertids"

that

and elliptical

gas by the ram pressure produced by the Intra-cluster

that

that

the one hand

be due to the fact

almost at rest relative to the Intra-cluster

The

on

gas at a higher rate than in the case for elllptlcals

might

1972, Lea and DeYoung

account

quasars

have

or

not

is correct,

(Baldwin,

It follows

(2) BL Lac objects

yet

had

a

chance

one might, however,

It would be worthwhile

et al.

1977)

that either

are young to

eject

(I) BL

(Altscbuler

double

radio

expect BL Lac objects

to investigate

to

this possibility

by

interferometry.

THE STRUCTURE OF RADIO SOURCES

Hlgh

resolution

themselves,

al___~.1975, Fomalont sources. which

radio

observations

often double.

and Miley

A particularly

1975)

to ~ 2 ° •

striking

Such observation

which differ

shows

3C274 - M 87 (Turland

that

3C31

(Simkln 1978) contain Jets.

source")

these

centered

are

on each parent

embedded galaxy.

that Jets are the blue-shifted component

is too faint

1977)

are

similar

in size

by 3CIII

galaxy

are e_~t

of the outer double

(Paullny-Toth

core can retain

1977),

3C66

etal.

1976

in

its preferential

(Northover

1973),

3C219

(except NGC 7385 which

(and

aligned

with)

It has been suggested

the

classical

doubles

much

that

1978 and

is

§ V)

to be lle along

double are powered by such Jets.

and Napier

greater

is a "head-tail

In which the red-shlfted

The fact that Jets are observed that classical

(Turland

1977), and

double

(cf. van den Bergh

of relativistic

(Guthrie

have

sources

In all cases in

3C273

radio

(e.~ Kellermann

(Waggett, Warner and Baldwin

components

but

cores

and NGC 6251

to be observable.

in the quasars

of double

these

periods of time.

the axis of double radio sources suggests

The Jets

of

In slze by a factor of ~ 3 x 105 , line up

an active

(Burch

1975a),

NGC 7385

jets

the cores

case Is provided

(spin axis?) for significant

A number of radio galaxies: 1975b),

that

the components

tend to llne up with the components

the inner and outer doubles,

orientation

show

Surprisingly

1975)

luminosities

and

3C147

(Wllklnson

than do those

etal.

observed

in

radio 8alaxles.

A final two

different

steep-spectrum

complication populations. (u ~ -0.2)

those with flat (a > -0.2)

is provided According quasars

by the observation to

are most

Settl

and

that quasars

WoltJer

common near

radio spectra are distributed

(1973)

the edges

appear and

to belong to

Schmldt

of the Universe

more or less unlformly

(1976) whereas

throughout

Explosions

space.

On the basis of their spectral

in Galaxies

indices

(Stein, O'Dell and Strlttmatter

objects might possibly be related to the flat-spectrum

V.

311

1976) BL Lac

type of quasar.

M 87 AND ITS JET

Observations

by Sargent et al. 1978 and by Young et al.

nucleus

of M

nucleus

is a Jet-llke string of unresolved knots.

87 contains

the outermost

of these

a massive

six knots

black

hole.

lie along

(Graham

1970,

Turland

1975a)

show

Extending

that M 87

outwards

some

20"

from

this

Within the accuracy of the data, all but

a straight

indicate that these knots emit highly polarised

(1978) appear to show that the

lie.

synchrotron (3C274)

Photographs radiation.

by Baade

(1956)

Radio observations

is a classical

double.

almost exactly aligned with the axis of symmetry of this double source.

The

Jet

is

In this respect it

resembles all other Jets that have so far been discovered.

Recently M

87

is

blastwave

Mathews

produced which was

interpretation

(1978)

by

generated

places

must be

larger

then it might,

source

in which

Faulkner,

the

that

that

the explosive

limit

the compact

is

in fact, redshifted

component

(-)

emitted

event

hot

source gas

that produced

in the core

situated

the M

behind

87 Jet.

If the Jet in M 87 represents

a relativistic

be

of a fast-expanding

the blue-shlfted

component

is

too

component faint

the

receding

component

to be

of a

This

From this age

finds that the ejection velocity

(1966) and Ryle and Longair and

X-ray

by

of 2 x 104 yr on the age of the Jet.

length of the Jet, Mathews

Gunn and Peterson

approaching

by

than 0.3c.

matter

suggested

radiation

an upper

limit and the observed Jet

has

thermal

observable.

of the

ejection

According

41967)

the brightness

4+) of

such

of

double to

ratio of the

a relativistic

double

source is given by

s

[II ÷- v/c oosq3 cos~

~+

(I)

In this relation v is the ejection velocity of the Jet, a is the spectral the sources and 8 is the angle between should be emphasized

that eqn

index of each of

the direction of ejection and the line of sight.

(I) neglects

possible

evolutionary

might result from the fact that (cf Idackay 1973) the approaching

brightness component

It

changes which

of the double is

seen at a time when it Is.older than the receding component.

According Jet has estimated

B = that

to de Vaucouleurs, 16.77. S-/S +

that the counterJet v/c ~ 0.56 if

No k

Angione

and Fraser

true counterJet~

I00.

With

this

(1968)

is observed

value

and

a

the brightest

on deep plates

= - 0.75

can only be invisible if 0.5 < v/c < 1.0.

8 ffi I0 °, v/c > 0.63 for

8 - 30 ° , v/c > 0.86 if

(Felten

knot in the M 87 from which 1970)

is

due

to

the

is

it follows

From eqn 41) it is seen that e= 50 ° and v = c at 8 = 57 °.

* the feature observed by Arp 41967) is not a true counterJet since it emits Ha and and because it is not located exactly opposite the non-thermal continuum Jet. t the main uncertainty in this estimate clusters with B ~ 22 surrounding M 87.

it

presence

of

numerous

[0 II]

globular

312

S. van den Bergh

Radio observations a result set

generally have a smaller dynamic range than do optical observations.

radio observations

brightness

limits

of jets associated

S-/S +

>

I0

with

compared

radio

to

the

galaxies

value

and quasars

S-/S +

~

i00

As

typically

found

from

direct plates of M 87.

From the frequency of Jets among bright high-latitude Bergh

(1978)

estimates

This observation energy

source

that

suggests

in active

show it to consist

~

10%

of

all

classical

that such double sources galaxies.

of a short

Observations

3CR (V~ron 1977) sources van den

double

radio

sources

are "fed" intermittently

contain

jets.

by the nuclear

of the M 87 Jet by Arp and Lorre

train of stellar

or semi-stellar

knots.

(1976)

This indicates

that

the nuclei of active galaxies behave somewhat

like bursters which also emit bursts of short

pulses.

One

that

bursts

correlate

major

difference

in bursters

is,

however,

whereas

burst

energy

an anti-correlation

and

between

time

these

interval

between

quantities

may be

indicated by the very scanty data on the M 87 Jet.

VI. THE NGC 1275 PHENOMENON

NGC 1275 is a supergiant cluster.

< V o • = 5460 ~ as

an object

200 km s -I

located

radial brightness young

stellar

Furthermore, lines

galaxy that is located at (or near)

Its radial velocity

velocity

(Chincarlni

the bottom

profile population

1943).

with

(which

of NGC

indicates

emits

of

well.

On

an

early-type

spectrum)

both

synchrotron

radiation

the

~ 300 km gas

cluster velocity

that it may reasonably

potential

(Lynds 1970)

only

that

the centre of the Perseus

to the average

1978) NGC 1275 is an elliptical

galaxy

range

1971)

the cluster

1275 emits

The entire

a velocity

dispersion

and Rood

of

(Rubin et al.

the nucleus

(Seyfert

nebulosity

at

of 5361 km s -I is so close

is embedded s -I

(Rubin

enveloping

NGC

he regarded

the basis galaxy is

emission

in filamentary

1275

emlsslon

1978).

is

a

superimposed.

and broad

et al.

of its

on which

not

This

being

low

ejected

explosively.

Furthermore with

radial

NGC

1275

Minkowski

velocities

itself.

reported

by

(1977b).

that

More

Burbidge

(1957) discovered are

and

and

by

Ekers,

Burbldge

van

and

Burbidge

NGC

1275

(1965)

during

colors

of

der

that

the

a violent

Hulst

this

Rubin

than high

et

Miley

NGC

1275

admit nebula

sprlal,

arms

has,

high

whose

however,

of

(1976)

event. 1953b). one

al.

noted

the

(1977,

found

was

This

lead

I

interpretation

-

type pentrating

distorted

following

galaxies.

cm

and

llne

from

to revival

by

that

by

been Adams

absorption

3C84, which is system is seen

the of

the

nucleus

difficulties

the

this

object

of

colliding

shows

and an

from the south a late-type,

strong

young associations

Sc

21

ejected

Neither

of

have

with the suggestion by Burbidge

material has

enveloping

Roberts and Saslaw

that the high velocity

I) The total diameter of the region containing characteristic

1978)

by DeYoung,

who

gas

(Oort 1976) show that it is

size

is

the

component

These authors state that "the structure

of the sprial are entirely

of

emission knots

in the core of the radio source

velocity

only

that

velocity

system

is confirmed

The observation

explosive

tightly wound

(1977)

and

component

(Baade and Minkowski

apparently

Bergh

higher

of

by

This conclusion

very

loose

s -I

region containing

in front of NGC 1275 seems to be incompatible

galaxy hypothesis the

(1965),

in the nucleus of NGC 1275.

in absorption

km

with the high velocity

associated with the high velocity situated

3000

observations

Dust patches associated

located in front of NGC 1275. (1973)

~

detailed

an extensive

tidal with

forces".

this

Van

is 33 (IO0/H) kpc.

morphology

nor

den

interpretation:

the

This

integrated

Explosions

luminosity

of

the

classification.

high

still

stripped 1275.

object

2) The chaotic appearance

due to tidal damage. are

velocity

This

approaching

is so because

each

galaxy core

in Galaxies

other,

b)

313

associated

with

NGC

1275

supports

of the high velocity

component

a) the hlgh-veloclty

and low velocity

their

relative

velocity

(which would survive a catastrophic

is

is unlikely

- 3000 km

tidal encounter)

that NGC 1275

an elliptical

represents

galaxy is therefore

a chance superposltlon

low.

account

of an active

for a) the presence by

Lynds,

c)

the

c)

no

The a priori

of a spiral and

4) The assumption that the high velocity component

in (but not interacting with) the elliptical

discovered

to be

is seen near NGC

(or collision)

of NGC 1275 is superimposed

shell

a

components s -1,

3) The core of the Perseus cluster contains only one (anaemic) spiral.

probability

such

component

Seyfert nucleus b) the peculiar

presence

of

recently

formed

stars

does not

filamentary and

d)

H II

the

X-ray

emission and the radio emission of NGC 1275.

Circumstantial

evidence

not nececessarily MGC

9-13-66

represent

(Burbidge,

the supergiant

in support

of the view that objects

a collision

Smith

between

and Burbidge

galaxies

1975,

double radio source DA 240.

is provided

1978), which

the

galaxy

by

in MGC

9-13-66

emission

-

lines are observed

to the axis

of symmetry

Since

MGC

motion

of

two

difficult

s -I.

It

should

perhaps

is not a Jet of the M 87 variety.

of

be

emphasized

This

that

is so because

in such Jets and 2) Jets in doube radio galaxies

is

not

located

in a

rich

cluster

and the gas in the protuberance

physically

to escape

of

the I) no

are parallel

the Jet in MGC 9-13-66

to the axis of symmetry of the double radio source DA 240.

9-13-66

this galaxy

km

observations at the centre

that differs from that of

defined by the double radio source whereas

is almost perpendicular

between

3000

1275 need

This object resembles NGC 1275 in that it has a

in which emission lines are observed with a velocity

parent

by

is located

protuberance

protuberance

resembling NGC

adjacent

the conclusion

galaxies that

relative

large

the

large

velocity

cannot be understood to

quantltltes

each

other.

of material

difference

in terms of the Hence

have

it

appears

been

ejected

from MCC 9-13-66 with a velocity of ~ 3000 km s "I.

V. NGC 5128 = CENTAURUS

NGC

5128

photographs

A

is an ellipclal

show

that

active

edges of this dust band. confirmed

obeys

immediately

raises

in clusters.

(1974) With

estimates

I00 ° < T s

According 0.15

cm

Me

to yr -I

is

taking

by

a dark

place, b o t h

that NGC 5128 is basically (van den 8ergh law which

The

most

by ram pressure

absorption the mass (K)

Paber for

line

hydrogen

< 1000 ° this yields and each

Gallagher I0 I0

L@.

and along

the

galaxy

is

that the main body

ellipitcal

explanation

Deep

galaxies.

This

from the giant ellipticals would

appear

to

be

that

such gas is swept out

(Gunn and Gott 1972).

measurements

of neutral

within

band.

an elliptical

1976) which shows

characterises

straightforward

absorbing

the gas that was ejected by evolving stars whereas

of cluster elliptlcals

21

photometry

crossed

the question why NGC 5128 looks so different

NGC 5128 retained

Prom

is

formation

the ~ = r -¼

that

occur

star

that

The suspicion

by photoelectric

of this galaxy

galaxy

an

assumed

gas

in NGC

a total gas mass

(1976) It

and

evolving

follows

from

distance

of

5128 to be ~H = in the range

stars their

in

Wright

7 x 105 T s M e •

108 Me to 109 M e •

elliptical

estimate

5 Mpc,

that

galaxies the

loose

stars

in

314

NGC

S. van den Bergh

5128

are

loosing

gas

at

a

rate

of

~

1.5

Me

yr -I

so

that

the

presently

observed

gas mass in this galaxy could have been built up in less than 109 yr.

The

attractive

devastating

blow

Graham

(1977).

of NGC

5128.

this

dust

These observations

indicate

whereas

band

dominant

is

that

3

km

In either

above

obtained

with

of stellar

velocity

gradient of

arcsec -I. in NGC

have

momentum

it follows

that

absorption is

<

the emission This

These originally

made

by

a giant

present

situation

companion

suggest

Baade

to NGC 5128.

I) Observations to survive and

equatorial

that

the gas

that

observed

should

(1954b) galaxy.

arose

with

shows

are

the following

violent

ffi -41°44"

(epoch 1950).

2)

5128

If NGC

Blanco

et el.

has,

no

Before

(1975)

encounters. for

I)

the

in NGC

orthogonal

5128

cannot

to each

have

been

The high angular momentum

of

by NCC southern

5128.

collided

5128

NGC

the

represents

5128

wlth

Since

the of

a

suggestion

the

is a field

collision

galaxy,

capture

the

of a spiral

companion

"high

photographs

candidate

nucleus nucleus

such in

recently,

Possibly

of

are

those

behind

found on a Palomar a ffi 13h26.2 m,

then one might

galaxy which

absorbing

by the

also

the "Jet" discovered

of the spiral

it appears

as

or

galaxy MGC 7-28-3 at

latitude"

5128,

relnterpretatlon

show that galaxy nuclei

second

a spiral

remnant

NGC

1963)

Infrared

of this galaxy.

the optlcal

hemisphere

associated wlth the hypothesized

re-examlne

need to be overcome:

such

The only possible

component

represents

projected

the

either

than does the gas, or 2)

the colllslonal

(cf. Sandage

evidence

in fact,

the stellar

and was disrupted on

every

in the equatorial

that

decay and subsequent

two dlfflcultles

galaxies

tidal

show

NGC

Since

Schmldt plate centred on NGC 5128 is the 14th magnitude

to observe

in

(1974) estimate of the total amount of hydrogen gas in NGC

dust band of NGC 5128.

expect

arcsec -I

almost

perhaps

that

from orbital

such a picture.

of colliding

(1971)

one

Minkowskl

Wrlght's

quite

Bradt

a by

in the main body of

s -I

much more slowly

vectors

and a spiral

presumably

NGC 5128 can be accepted

Kunkel

and

elliptical

5128 is not inconsistent

able

received

4-m telescope

an origin in or near the nucleus of NGC 5128.

difficulties

between

km

lines produced

e~ected by the stars that dominate the light in this ~alaxy. the gas also excludes

recently

lines

0.5

observations

5128 rotates

angular

has

the CTIO

to show no evldence for rotation of the main body

gradient

s -I

case,

outlined

appear

the

population

the stars

picture

observations

observations

the velocity

~

stellar

the gas and other.

self-consistent

Specifically,

galaxy

direction

and

from spectroscopic

patches

likely

that

by

ran into,

only appear any

material

to NGC 5128 lles in front of it in the south and

behind it in the north.

The strength

of

[NI1]

Blanco et el. observe

~ 6584 and the low radial velocity

in the "Jet" is consistent

the debrls of a splral galaxy correct

(relative

with the interpretation

that was torn apart by tidal forces.

then the explosive activity

to NGC 5128) which of this object as

If this conclusion

is

in the nucleus of NGC 5128 might have been trlggered by

gas dumped Into the nucleus of thls object by the spiral that collided with it.

Vlll. THE CHAOTIC GALAXY M 82

M 82 = NGC 3034 is a late-type source

3C231

object

are given

(Lynds by

1961).

galaxy of chaotic

Extensive

Solingerj

Morrlson

bibliographies and

Markert

appearance

which

contains

the radio

of previous work on this interesting (1977)

and

by O'Connell

and Mangano

Explosions

(1978).

From an optical

point

of view,

galaxy is that it has such an unusually

in Galaxies

the single

315

most

of its stars are deeply embedded in dense dust clouds. true surface brightness 20

times

higher

disk of spiral and Sandage has

integrated

O*Connell

surface

brightness

The relatively

early

there

is no

a

phase

direct

of a red/Infrared

the lack of Pn

O'Connell spiral

of

spectral

the conclusion

violent

star

type

in the

(Humason,

Mayall

that this chaotic galaxy

formation.

evidence

for

the

presence

of

Although

the

of

superglants.

its

extremely

A-type

young

objects.

shows

that the

exhibited

Palomar

close

by spectrum

scans

This conclusion

of

Schmldt plates show no resolved

companion

M

81

are

seen

to

is confirmed

the disk of M 82 obtained

be

stars

by

in M 82 whereas

teeming

with

OB

the

stars

and

Even plates taken in good selng with the 5-m telescope show only a sprinkling

of resolved objects; most of which are probably star clusters. of the disk of M 82 by O'Connell

only be fitted by models significantly

stars

formation

being

formed

than

MV =

a very

high

observations

-5.

in the main body at

Finally,

stellar population

that the observed

Balmer

Jump can

type earlier than B5 do not contribute

light.

of the optical

brighter

star

(1970) shows

in which stars of spectral

to the integrated

In summary, all lacks

encountered

plate pair obtained with the Hale 5-m telescope

emission

and Mangano.

arms

synthesis

and Mangano find that the

is normally

integrated

1970) of M 82 supports

through

that

outer disk of M 82 does not contain any bright HII regions. by

of this

the fact that most

spectrum of the disk of M 82 shows that the light of this galaxy is dominated by

stars,

Blinking

galaxies.

passed

characteristic

despite

in the disk of M 82 is V o = 16.0 mag arcsec -2, which is more than

the maximum

1956, O*Connell

recently

young

than

striking

high surface brightness

This

indicate

implies

of M 82 stopped

rate

but

the

that

that

the disk population

either

I)

an

o f M 82

intense

burst

of

~ 3 x 107 yr ago or 2) stars are still

mass

spectrum

of

star

formation

in M

82 is

truncated above ~ 6 M e •

The situation galaxy

in

which

MV = - 1 5 . required M 82

Many o f

~

probably

located

was

might, that

has

morphology of this

that

event

radiation

be

pressure

exerted

(1971) shows that

an order

or magnitude

hydrogen

energy

clouds

output

that

of

event

have

regions

Possibly in

b y Woodward

the

in this the

the

nucleus to

(1976)

which

by v i o l e t

bursts

than

that

required

been

mapped

clouds

of star

to account recently

and

Peimbert

1970)

near

in

M 82

of

such

Such

star

induced

star

and Assousa with

an

origin.

clouds

support

above

were

and

to the notion

Alternatively nudged

(Cottrell out

by

the

The work o f S a n d e r s

and

event must have been at least

the velocities Crutcher,

of

of

young stars

by L y n d s a n d S a n d a g e . for

nucleus

M 82.

and by Herbst

formation.

by

the

formation

of huge dust

in any violent

estimated

with

supernova

consists

distribution

gas

clusters

of

the

galaxy not so long ago.

dusty

star

the

(Lynds a n d S a n d a g e 1963) g i v e s t r o n g

the energy involved

is

Eli

analogous

o f NGC 5 1 2 8 ,

in the core of this

super

(Recillas-Cruz

giant

explosive

M 82 a n d

that

smaller

the

of as being

prevailing

(van den Bergh 1976), has a similar

object

assumed

O stars

of

clusters.

discussed

took place

Balamore

considerable

and

that

discovered

super

thought

MV = - 1 8

below the nucleus

might

by

flux

with has

sequence

compact nucleus

luminosity

an explosive

these

been

1971)

line

in

be

recently

the

chaotic

104 m a i n

triggered perhaps,

Perhaps

it

(1969,

emission

integrated

1977)

o f M 82 c o n t r a s t s

Bergh

the

formation

The

disk

the

superclusters

(1977).

the den

to maintain

are

formation

in van

of

Rogstad

Nevertheless, the high and

Chu

a

latitude (1977)o

b

316

S. van den Bergh

single

cloud

has

mass

a

complex MH

nucleus.

=

The

situated

2

x

I07

at ~ 3 arcmln M@

and

corresponding

a

(3 kpc)

radial

minimum

from

velocity

kinetic

the fundamental

of

60

energy

of

km

s -I

this

plane

of M 82

relative

cloud

to

the

complex

is

~ 7 x 1053 ergs.

The distribution observations doubt

that

the

fundmental rather

of hydrogen gas on the sky, which is derived from the high resolution

by Crutcher,

plane

of

than with

seems

quite

1972,

Solonger

Rogstad

reflecting

dust

and Chu and by Cottrell which

this galaxy,

the extended

possible

that

et al.

Lynds

is embedded

hydrogen

the present

1977)

by

and

in gas

envelope activity

inflow

(1977)

Sandage

clouds

of M 81

from

beyond

above

associated

(Roberts

in M 82 was

of matter

shows

discovered

with

1972).

below

M

82

the

itself

Nevertheless,

originally

the gaseous

reasonable

and

triggered

halo

it

(Elvius

of M 81 into

the

nucleus of M 82.

Support the

work

for the notion

of

interacting discussed

Adams

that infall of gas can activate

(1977a)

pairs.

who

Perhaps

in Adams"

finds

somewhat

paper belong

that

Seyfert

surprisingly,

galaxies

(with approximately

equal

Other well-known

examples

swept

1510

Pottasch

gas

recently

Bertola and di Tulllo (Arp and Bertola could have

within inner

NGC

which

exhibits

1977).

some

4258 is a luminous

late-type

faint outer spiral structure, arms.

interpretation shock

connection,

An Pa

recently

high

this galaxy filter

contains

Krult

1974)

ne =

gas

filamentary estimated

of

galaxies

the

type

I

that may have

3077

(Barbleri,

that NGC 1808

similarities

to M 82,

with

an

internal

thin and very bright

by Deharveng

filter photographs

in

the

that

they

the mean smoothed

3 cm -3

from which

have

the

disc

of

(1970)

is that an intense

this

galaxy.

of NGC 4258 exhibits

velocity

dispersion

of NGC 4258 published

anomalous The

arms total

been

electron

a total

1972) is obtained.

fashion.

and Pellet

The most stralght-forward

of this observation

within

Embedded

of

In

this

emission ~

350

lines

km

s -I

1962).

suggests

Oort and Mathewson the

NGC

by Deharveng

of

excited

density

ionized

by

shocks.

[SIll emission From

in these anomalous

gas mass

NGC

4258 were mass

to be ~ 1.5 x I08 M e by Oort (1974).

distributed

associated

with

their

arms

of 3 x 106 M@

A lower value for this mass would,

hydrogen

and Pellet

between the bright spiral arms discussed

The fact that these arms contain no OB stars and exhibit strong

emission measure,

if

and

two short,

these arms.

and Shu 1975)

generated

show two faint smooth Ha arms that are interleaved

be

1977)

morphological

to note that the nucleus

excitation

The Ha interference

to

to both

of active galaxies

photograph

strung out along

Roberts been

it is of interest

(Burbidge and Burbldge

der

frequency)

Finally one might speculate

striking

members

spiral with a highly peculiar morphology.

Inteference

(of. Roberts, has

moderately

above.

frequently

and "disturbed"

SPIRAL NGC 4258

shows unusually bright HII regions

of

and

is provided by

picked up gas during a recent encounter with its companion NGC 1792.

spiral

spiral

(Disney

1974, van der Hulst

1970),

IX. THE REMARKABLE

NGC

are

are

the interacting

and type II Seyfert subclasses. up

galactic nuclei

is estimated

(van der Krult, of course,

result

in a non-homogeneous the

(van

observed

anomalous

arms

or is

Explosions

Radio thermal from

continuum

emission

shock

observations

associated

amplification

Calculations

by

with of

in Galaxies

van

der

Kruit

the anomalous

a

pre-existing

317

et

arms.

al.

This

magnetic

by van der Kruit

et al. show

that

by assuming

that the nucleus

clouds

in

velocities other

spiral

amount

of

present

quite

~

galaxies

the

s -I.

(but

matter

the

compared

spirals.

clouds

observed

non-

might

arise

NGC

4258.

of

of the anomalous

of

radio that

problem

Way system

have been

disk

shape

plane

with double

latter

emission

the

the

this hypothesis

to the masses

The

in the case of our own Milky

high velocity

with

strong

of this galaxy ejected massive

fundamental

difficulties

large

typical

in

the observed

to be associated

appears

of

in

Obvious

are observed

nuclei

guise

massive

direction

I000 km

ejected

in

different

opposite

of

relatively

enhanced

field

spiral arms might be explained gas

that

show

that

sources are

also

with I) no

and 2) the

expected

arises

(van der Kruit

in and above

galaxy)

are

to be

in somewhat

1971b)

in which

the galactic

nuclear

bulge.

X. SUMMARY

The most violent known explosive the

spectra

ratios

that

suggests

these not

that

quasars that

of are

quasars

and cD/E

are

galaxies

such Classical

ejected

objects

in opposite

are

markedly

produced

different

produced produce

doubles

events

The emission

by

occur

from

by

heavy

those

explosive

double

are powered

directions

take place in quasars.

radio by

elements

that

events

are

at relativistic

Available bursts

velocities.

double structure.

central

galaxies

This

as

sources

yet

Alternatively

not

in spiral and elliptical

understood

difference

might

differ

result

with

This

environment.

Both

data

the

abundance sun.

on M 87 suggest

of short

The radio

with splral/SO galaxies do not exhibit classical energy

in

in a stellar

sources.

intermittent

that

observed

lines in

pulses

sources

that are associated

This indicates in some

from

E and S galaxies might have condensed onto differing

that the

fundamental

environmental

way.

factors.

types of nuclei.

TABLE I S,-,m~ry of morphological and environmental data on different types of objects in which violent events occur.

Type of object

Classical double radio source

cD galaxy

Occurs in rich cluster galaxies

yes

Exhibits Syl Spectrum

yes

no

Quasar

yes

no

yes

BL Lac

no

no

no

Seyfert I

no

no

yes

Observations observations

of NGC

of

NGC

4258,

1275) suggest

from galaxies with velocitles J.P.V.A. 22/3---0

the

parent

galaxy

that significant

of

DA

240

(and

perhaps

also

amounts of matter are sometimes

of a few thousand km s -I.

the

ejected

318

S. van den Bergh

In other cases such as M 82, some Seyfert galaxies and perhaps NGC 5128 the evidence possslbly implicates infall of material into a galactic nucleus as the sources of violent activity.

The occurrence of differing types of violent events in different kinds of objects and in various environments is summarized in table I. or

consistent

pattern•

Clearly

much

work

The data in this table exhibit no simple

remains

to

be

done

before

our

dimly

held

suspicions can be converted into a clear understanding of the explosive events that occur in galactic nuclei• REFERENCES Adams, T.F. (1977a) Ap. J. Suppl. 33, 19. • (1977b) Pub. A.S.P. 89, 488• Altschuler, D.R. and Wardle, J.F.C. (1975) Nature 255, 306. Arp, H.C. (1967) Ap. Letters I, I. Arp, H.C. and Bertola, F. (1970) Ap. Letters 6, 65. Arp, H.C. and Lorre, J. (1976) Ap. J. 210, 58. Baade, W. (1956) Ap. J. 123, 550. Baade, W. and Minkowskl, R. (1953a) Ap. J. I19, 206. • (1953b) Ap. J. 119, 215. Baldwln, J.A., Wampler, E.J., Burbldge, E.M., O'Dell, S.L., Smith, H.E., Hazard, C., Nordsleck, K.H., Pooley, G. and Stein, W.A. (1977) Ap. J. 215, 408. Barblerl, C. Bertola, F., and dl Tulllo, G. (1974) Astr. Ap. 35, 463. Blanco, "V.M., Graham, J.A., Lasker, B.M. and Osmer, P.S. (1975) Ap. J. (Letters) 198, L63. Bridle, A.H. and Brandle, G.W. (1973) Ap. Letters 15, 21. Burbldge, E.M. and Burbldge, G.R. (1962) Ap. J. 135, 694. • (1965) Ap. J. 142, 1351. Burbldge, E.M., Smith, H.E. and Burbldge, G.R. (1975) Ap. J.(Letters) 199, L137. • (1978) Ap. J. 219, 400. Burbldge, G.R., Burbldge, E.M. and Sandage, A.R. (1963) Rev. Mod. Phys. 35, 947. Butch, S.F. (1977) M.N.R.A.S. 181, 599. Chlncarlnl, G., and Rood, H.J. (1971) Ap. J. 168, 321. Cottrell, G.A. (1977) M.N.R.A.S. 178, 577• Crutcher, R.M. Rogstad, D.H., and Chu, K. (1977) preprlnt. de Bruyn, A.G. (1978) Stucture and Properties of Nearby Galaxies = I.A.U. Symposium No. 77 Reldel Publishing Co., Dordrecht - Holland• Deharveng, J.M. and Pellet, A. (1970) Astr. Ap. 9, 181. de Vaucouleurs, G., Anglone, R. and Fraser, C.W. (1968) Ap. Letters 2, 141. DeYoung, D.S., Roberts, M.S. and Saslaw, W.C. (1973) Ap. J. 185, 809. Disney, M.J., and Pottasch, S.R. (1977) Astr. Ap. 60, 43. Ekers, R.D. (1978a) Structure and Properties of Nearby Galaxies = I.A.U. Symposium No. 77, Reldel Publishing Co., Dordrecht - Holland• Ekers, R.D. (1978b) Physlca Scrlpta, in press• Ekers, R.D. and Kotanyl, C.G. (1978) Astr. Ap., in press• Ekers, R.D., van der Hulst, J.M. and Miley, G.K. (1976) Nature 262, 369. Elvlus, A. (1972) Astr. Ap. 19, 193. Faber, S.M. and Gallagher, J.S. (1976) Ap. J. 204, 365. Faulkner, J., Gunn, J.E. and Peterson, B.A. (1966) Nature 211, 502. Felton, J.E. (1970) Non-Solar X-ray and Gamma-ray Astronomy = I.A.U. Symposium No. 37., p. 216, Ed. L• Gratton, Reldel Publishing Co., Dordrecht - Holland Formalont, E.G. and bliley, G.K. (1975) Nature 257, 99. Gibson, D.M. (1975) Astr. Ap. 39, 377. Ginzburg, V.L., and Ozernoy, L•M. (1976) paper read at Joint Meetln~ of Commissions 40 and 48 at the XVI General Assembly of the IAU, Grenoble, 27 August 1976. Glsler, G.R. (1976) Astr. Ap. 51, 137. Graham, I. (1970) M.N.R.A.S. 149, 319. Graham, J.A. (1977) Bull. A.A.S. 9, 630. Gunn, J.E. and Gott, J.R. (1972) Ap. J. 176, I. Guthrle, B.N.G. and Napier, W.M. (1975) M.N.R.A.S. 172, 85. Herbst, W. and Assousa, G.E. (1977) Ap. J. 217, 473. Humason, M.L., Mayall, N.U., and Sandage, A.R. (1956) A.J. 61, 97. Kellermann, K.I. (1972) External Galaxles and quasi-Stellar Objects, I.A.U. Symposium No. 44, p. 190, Reldel Publishing Co., Dordrecht - Holland.

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Kellerman, K.I., Clark, B.G., Niell, A.E. and Shaffer, D.B. (1975) Ap. J. (Letters) 197, L113. Khachiklan, E. Ye. and Weedman, D.W. (1977) I.A.U. Colloquium No. 37, p. 411, eds. C• Balkowski and B.E. Westerlund, CNRS, Paris. Krlstlan, J. (1973) Ap. J. (Letters) 179, L61. Kunkel, W.E. and Brandt, H.V. (1971) Ap. J. (Letters) 170, L7. Lea, S.M. and DeYoung, D.S. (1976) Ap. J. 210, 647. Lo, K.Y., Cohen, M.H. , Schilizzl, R.T., and Ross, H.N. (1977) Ap. J. 218, 668. Lynden-Bell, D. (1969) Nature 223, 690. Lynds, R. (1970) Ap. J. (Letters) 159, LI51. Lynds, C.R. (1961) Ap. J. 134, 659. Mackay, C.D. (1973) M.N.R.A.S. 162, I. Mathews, W.G. (1978) Ap. J. 219, 408. M_inkowskl, R. (1957) Radio Astronomy ffiI.A.U. Symposium No. 4, ed. H.C. van de Hulst, p. 107, Cambridge University Press, Cambridge. Morrison, P. (1969) Ap. J. (Letters) 157, L73. Northover, K.J.E. (1973) M.N.R.A.S. 165, 369. O'Connell, R.W. (1970) The Stellar Content of M 82, unpublished C.I.T. thesis. O'Connell, R.W. and Mangano, J.J. (1978) Ap. J., in press• Oort, J.H. (1974) The Formation and Dynamics of Galaxies = I.A.U. Symposium No. 58, p. 375, Reldel Publishing Co., Dordreeht - Holland. • (1976) Pub. A . S . P . 88, 591. ostriker, J.P. (1977) The Evolution of Galaxies and Stellar Populations, Ed. B.M. Tinsley, p. 369, Yale University Observatory, New Haven• Paullny-Toth, I.K.K., Preuss, E., Witzel, A., Kellermann, K.I., and Shaffer, D.B. (1976) Astr. Ap. 52, 471. Recillas-Cruz, E. and Peimbert, M. (1970) Bol. Obs. Tonantzlntla Tacuaya 5, 247. Roberts, D.H., O'Dell, S.L. and Burbidge, G.R. (1977) Ap. J. 216, 227. Roberts, M.S. (1972) External Galaxies and quasi-Stellar Objects, ed. D.S. Evans, p. 12, Reldel Publishing Co., Dordrecht - Holland. Roberts, W.W., Roberts, M.S., and Shu, F. H. (1975) Ap. J. 196, 381. Rubln, V.C., Ford, W.K., Peterson, C.J., and Oort, J.H. (1977) Ap. J~ 221, 693. Rubin, V.C., Ford, W.K., Peterson, C.J. and Lynds, C.R. (1978) Ap. J. Suppl., in press. Ryle, M. and Longair, M.S. (1967) M.N.R.A.S. 136, 123. Sandage, A.R. (1963) Ap. J. 138, 863. Sanders, R.H. and Balamore, D.S. (1971) Ap. J. 166, 7. Sargent, W.L.W., Young, P.J., Boxenberg, A., Shortridge, K., Lynds, C.R. and Hartwlck, F.D.A. (1978) AP" J', in press. Saslaw, W.C. (1975) Dynamics of Stellar Systems = I.A.U. Symposium No. 69, ed. A. Hayli, p. 379, Reidel Publishing Co., Dordrecht - Holland. Sehmldt, M. (1963) Nature 197, 1040. Schmldt, M. (1976) Ap. J. (Letters) 209, L55. Settl, G. and WoltJer, L. (1973) Ann. N.Y. Acad. Sci. 224, 8. Seyfert, C.K. (1943) Ap~ J. 97, 28. Simkln, S.M. (1978) Bull A.A.S. 9, 586. Sollnger, A., Morrlson P., and Markert, T. (1977) Ap. J. 211, 707• Stein, W.A., O'Dell, S.L. and Strlttmatter, P.A. (1976) A~9" Rev. Astr. Ap. 14, 173. Strlttmatter, P.A. and Williams, R.E. (1976) Ann. Rev. Astr. Ap. 14, 307. Sullivan, W.T. and Sinn, L.A. (1975) Ap. Letters 16, 173. Turland, B.D. (1975a) M.N.R.A.S. 170, 281. . (1975b) M.N.R.A.S. 172, 181. van den Bergh, S. (1960) Pub. David Dunlap Obs. 2, 159. . (1969) Ap. J. (Letters) 156, LI9. . (1971) Astr. Ap. 12, 474. • (1975) Ap• J. (Betters) 198, LI. • (1976) Ap. J. 208, 673. • (1977) Astr. Nachr., in press• • (1978) preprint. van der Hulst, J.N. (1977) unpublished Gronin~en Ph.D. thesis. Van der Krult, P.C. (1971a) Astr. Ap. 15, II0. . (1971b) Astr. Ap. 13, 405. • (1974) Ap. J . 192, 1. Van d e r K r u i t , P . C . , O o r t , J . H . , and Mathewson, D.S. (1972) A s t r . Ap. 21, 169. Veron, P. (1977) A s t r . Ap. S u p p l . 30, 131. W a g g e t t , P . C . , Warner, P . J . , and B a l d w i n , J . E . (1977) M.N.R.A.S. 181, 465. W i l k i n s o n , P . N . , Readhead, A . C . S . , P u r c e l l , G.H., and A n d e r s o n , B. (1977) N a t u r e 269, 764. Wollman, E•R•, G e b a l l e , T . R . , Lacy, J • H . , Townes, C.H., and Rank. D.M. (1977) A p . J . (Letters) 218, LI03. Woodward, P.R. (1976) Ap. J. 207, 484. Wright, M.C.H. (1974) Astr. Ap. 31, 283. Young, P.J., Westphal, J.A., Kristlan, J., and Wilson, C.F. (1978) Ap. J., in press,