Transcript Galaxies and Their Structure

Slides which are important are marked by
Galaxies:
Their Structure and
Evolution
Physics 360/Geology 360
Astronomy
J. Swez
Hubble Deep Field
Island Universes or
Nebula?
• Galaxies appear as either nearby gaseous nebula
or else ‘island universes’ with stars like our galaxy
• Finding their distances was required to tell the
difference
• Edwin Hubble found the distance to the
Andromeda galaxy with a Cepheid Variable
• Nearest Galaxy Large Magellanic Cloud, 150,000
ly from the Sun
When did the study of galaxies
begin?
*Charles Messier discovered galaxies as early as the 28th
century. He began the Messier Catalog (e.g. M31)
*Sir William Hershel (19th century) began the
classification system known today as NGC (New General
Catalog). Sir Hershel was also known for his advances in
photography.
*Edwin Hubble (1899-1853) classified galaxies in the 20th
century in terms of Spiral, Elliptical and Irregular.
*The key to galactic classification was the measurement
of distances.
*The key to the measurement of distances was Cephid
Varibles
How we found distances for remote stars.
Remember this only works up to a 100 parsecs
(or 250 parsecs from apace). Certainly not
galactic distances!
Review Slide. This you should know from
Chapter 12.
Variable Star Light Curves
Remember!
Cephids are
classed as
Type I and
Type II
depending on
the shape of
their period
curve.
Pulsating Star HR Diagram
Review
Slide! You
should
already
know
about HR
diagrams.
Cepheid Period-Luminosity
Relationship
• Cepheid variables are luminous variable stars
• The period of their variation is regular
• The period corresponds to luminosity (Period
Luminosity Law)
 Cepheids are good distance indicators (standard
candles)
 More Slowly they Pulsate—More Luminous
 RR Lyrae 40 L, Cephids up to 106 L (an L is a solar
mass unit)
Distance From Cepheids
Hubble Tuning Fork
Diagram
From Thomas Arny (text) page 485
An organizational tool, not necessarily tied to
structure or evolution of galaxies.
The Hubble “Tuning Fork” Further Explanation
1.
This page was copied from Nick Strobel's Astronomy Notes. Go to his site at www.astronomynotes.com for the
updated and corrected version.
Spiral Galaxies
Barred Spiral Galaxies
The giant elliptical galaxy
M87, also called Virgo A, is
one of the most remarkable
objects in the sky. It is perhaps
the dominant galaxy in the
closest big cluster to us, the
famous Virgo Cluster of
galaxies
M87's diameter of apparently about
7' corresponds to a linear extension
of 120,000 light years, more than
the diameter of our Milky Way's
disk. However, as M87 is of type E1
or E0, it fills a much larger volume,
and thus contains much more stars
(and mass) than our galaxy,
certainly several trillion (10^12)
solar masses (J.C. Brandt and R.G.
Roosen have estimated 2.7 trillion).
This galaxy is also of extreme
luminosity, with an absolute
magnitude of about -22.
•This is a 2048x2048 CCD image of
the Coma Cluster. This cluster has a
recessional velocity of about 7000
km/s and is the densest cluster in
our local region of the Universe. In
contrast to the Hercules cluster,
Coma has almost no spiral galaxies
in its central regions. The cluster is
strongly virialized and has a hot
intracluster medium which
generates strong X-ray emission.
This image shows the central few
100 kpc of the Coma cluster. At a
slightly larger radius , images reveal
that Coma is still rather devoid of
spiral galaxies. It is generally
believed that the cold hydrogen gas
in the disks of spiral galaxies is
swept out of them as they orbit
through the intracluster medium of
Coma.
An example of clusters of galaxies
Elliptical Galaxies
•Malin 1 is an excellent
example of a very Low
Surface Brightness Spiral
Galaxy. This galaxy was
only recently discovered and
can barely be seen in this 30
minute exposure using a
100-inch telescope in Chile.
The galaxy disk covers
about half of the image
Frame yet is barely visible.
There are lots of these out
there.
http://zebu.uoregon.edu/images/m
alin1.gif
M77 Spiral Galaxy M77
(NGC 1068), type Sb, in
Cetus Discovered 1780 by
Pierre Méchain.
This magnificient galaxy is one of the
biggest galaxies in Messier's catalog,
its bright part measuring about
120,000 light years, but its faint
extensions (which are well visible e.g.
in the DSSM image) going perhaps
out to nearly 170,000 light years. Its
appearance is that of a magnificient
spiral with broad structured arms,
which in the inner region show a quite
young stellar population, but more
away from the center, are dominated
by a smooth yellowish older stellar
population.
SO Galaxy
Irregular Galaxies
•M101 is a High Surface
Brightness Galaxy: It has lots of
active star formation and many
H II regions. This exposure was
through a blue filter with an
exposure time of 30 seconds
with a 52-inch telescope using
4.85:1 re-imaging optics in front
of the CCD
http://zebu.uoregon.edu/i
mages/m101big.gif
Can you classify M101
according to the Hubble
Classification?
•This is a 2048x2048 CCD image of the Hercules cluster. This cluster has a
recessional velocity of 11,000 km/s. It is noteworthy in that it contains a number
of spiral galaxies, many of which are intereacting. This is a sure sign that the
cluster still has substructure and is not fully virialized yet.
Andromeda Galaxy M31 “a color picture”
Spiral
Galaxy
M31
(NGC
224), type
Sb,
Androme
da
Galaxy
among
the
brightest
of the
Messier
Galaxies
M32 is the small yet
bright companion of the
Great Andromeda
Galaxy, M31, and as
such a member of the
Local Group of galaxies.
It can be easily found
when observing the
Andromeda Galaxy, as it
is situated 22 arc
minutes exactly south of
M31's central region,
overlaid over the
outskirts of the spiral
arms.
Elliptical Galaxy M32 (NGC 221), type
E2, in Andromeda A Satellite of the
Andromeda Galaxy, M31
How would
you classify
this galaxy
(lower left
of picture)?
•M31+N205 from McDonald Observatory 10/27/95
http://www.seds.org/messier/galaxy.html
Discovered in 1773 by Charles Messier as the famous
Whirlpool Galaxy. Its companion, NGC 5195, was
discovered in 1781 by his friend, Pierre Méchain
M51 (NGC 5194),
type Sc, in Canes
Venatici The
distance to this
galaxy is thought to
be 37 Mly (million
light years) but a
recent (2001) STScI
Press Release gave
31 million light
years. According to
our present
understanding, the
pronounced spiral
structure is a result of
M51's current
encounter with its
neighbor, NGC 5195
(the fainter one in
Messier's
description).
Galaxy Types
Percent of all
Galaxies
(probable)
Percent of all
Galaxies
(observed)
Dust
Percent of mass as
Hydrogen clouds
Stellar Populations
Dominant Color
Spectrum of
Centers
Luminosity (in
solar luminosities)
Diameter (kilolight years, kly)
Mass (in solar
masses)
Mass/Luminosity
Ratio
Dwarf
Elliptical
Giant
Elliptical
Lenticular
(S0)
~60
<1
…
~30
~10
~10
<1
<1
~80
~10
None
None
Some
Some
Some
0
0
0
II
II
II
Red
Red
Red
K
K
K
105
1011
108-1010
107-109
>1
500
10-150
1-50
106-109
109-1012
1-5
5-80
50
Spiral
(Sa and SBa)
Spiral
(Sb and SBb)
1
3
Spiral
(Sc and SBc)
9
I and II
I and II
I and II
Red (Halo, Core) Red (Halo, Core) Red (Halo, Core)
Blue (arms)
Blue (arms)
Blue (arms)
K-G
G
F
Irregular (Ir)
20
I
Blue
F-A
~5x1011
~3x1011
~1011
109
20
10
5
3
Galactic Evolution
• The age of the oldest stars in each
galaxy is about the same
All galaxies are about the same age
• Galaxies are distinguished by the amount
of star-forming material
• Different galaxy types correspond to:
• different star formation rates, or
• large scale changes in gas/dust content
Galaxy Collision - core
Galaxy Collision
Galaxy Collisions
Ring Galaxies
A result of galactic collisions. The central gas is disrupted.
Galactic Collisions /
Mergers/Cannibals
• Galaxies may collide
• Collisions occur between gas, not stars
• Collisions -> rapid star formation / gas
loss
• This produces ring galaxies and ellipticals
• A few mergers -> giant galaxy
Galactic cannibals and giant ellipticals
Galaxy Clusters
• Poor Clusters
• 10-100 galaxies
• Spirals, Irregulars and lastly dwarf ellipticals
• Local Group - our galaxy cluster
• Rich Clusters
• Thousand + galaxies
• Ellipticals, SO, with spirals at edges only
• Giant Ellipticals at the center
Local Group
• Our ‘poor’ cluster of galaxies
• Dominated by Milky Way and Andromeda
• Probable additional large galaxy near Milky
Way
Rich Galaxy Clusters and
Local Supergroup
• Rich clusters have
thousands of galaxies
• Hercules cluster
• Galaxy clusters
organize into groups
• Local Supergroup
Large Scale Structure
Large scales
show a
bubble and
void structure
Active Galaxies / Radio
Galaxies/Quasars
• Read about them in the news!
• Active Galaxies
• Galaxies that have bright, varying cores. Abnormally lare
radiation from a tiny region in the galactic core
• Radio Galaxies
• Galaxies with massive radio emissions from jets
• Quasars
• Distant active/radio galaxies
• Seyfert Galaxies a spiral galaxy with abnormally luminous core
from a region less than a ly year across
• Jets in Quasars sometimes move with superluminal motion
• Most distant Quasar about 10 billion ly
Radio Galaxies
Some galaxies show massive, radio emissions
These are tied to massive jets in the core
Galactic Accretion Disk
Core!
Jet from
core of M87
Accretion Disk and Jet
Formation
The Hubble Law
In 1914 Vesto Slipher (lived 1870--1963) announced his results
from the spectra of over 40 spiral galaxies (at his time people
thought the ``spiral nebulae'' were inside the Milky Way). He found
that over 90% of the spectra showed redshifts which meant that
they were moving away from us. Edwin Hubble and Milton
Humason found distances to the spiral nebulae. When Hubble
plotted the redshift vs. the distance of the galaxies, he found a
surprising relation: more distant galaxies are moving faster away
from us. Hubble and Humason announced their result in 1931: the
recession speed = H × distance, where H is a number now called
the Hubble constant. This relation is called the Hubble Law and
the Hubble constant is the slope of the line.
The Redshift and the Hubble Law
In 1920’s astronomers discovered galaxies are
moving away from one another
The spectrum of a galaxy is the spectrum of all
its component stars added together
If a galaxy is moving toward or away from us
its spectral lines will be Doppler-Shifted-e.g.
motion away from us lengthens the wavelength
Nearly all galaxies are moving away
Hubble discovered in 1920 that V = HD where
H is the Hubble Constant = 65 km/s per Mpc
Thus by measuring the red shift we can find
the galactic distance (See Problem 16.2)
Left: Red Shift for a galaxy nearer; Right Red
Shift for a more distant Galaxy
The Hubble Law as applied to the recession of galaxies
Gravitational Lenses
How do they Work?
Einstein—matter bends light, see Fig.
16.26
Why do we think they Exist
Multiple quasar images but idential
spectral history
Existence of Black Holes and Dark Matter
verified?
Figure from: Arny, an Introduction to Astronomy p
495
Why is there believed Dark Matter to
Exist
• The line with the dots is the
theoretical curve for a galaxy
consisting only of stars
• The observed line is with the
x’s
• Only the material between the
star’s orbit and the galaxy’s
center contributes to the
gravity force
• Something must keep the
outer stars from flying out
• Answer: Dark Matter