Download Chapter15- A Universe of Galaxies

Chapter 15
A Universe of Galaxies
The Hubble Deep Field
10 day exposure –field located in the Big Dipper
Edwin
Hubble
Edwin
Hubbl
e
Hubble’s Galaxy
Classification
Spiral Galaxies
• galaxies like the Milky Way with arcing
structures lying in a plane and emanating
from the nuclear bulge
M51
M81
Lenticular Galaxies
• Galaxies that have disks but no spiral
arms.
Barred Spiral Galaxies
• galaxies with a bar of stars running
through the nuclear bulge
NGC 1097
NGC 4123
M91
Elliptical Galaxies
• galaxies with an elliptical shape, no spiral
arms, and little interstellar matter
M32 - E2
NGC 4125
M87 - E1
Irregular Galaxies
• galaxies that are asymmetrical and are
sometimes just two or more galaxies
colliding
Large Magellanic Cloud – a small irregular galaxy that
orbits the MMilky Way
Centaurus A
M87
Measuring Cosmic Distances
• Distances to other galaxies are measured using
Main Sequence Fitting.
• This entails the use of a light source of known,
standard luminosity called a standard candle.
• The distance can be found using the luminositydistance formula
– Apparent brightness = luminosity/4d2
Comparison of the apparent brightness of stars in the Hyades Cluster with those
of the Pleiades Cluster
The Pleiades Cluster is
2.75 times farther away
because
(2.75)2 = 7.5 times
dimmer
The same luminosities
are assumed for all main
sequence stars of the
same color.
RR Lyrae and Cepheid Variable
Stars
• These are both pulsating variable stars.
• Their pulsation periods are on the order of
a few days.
• Using the period-luminosity relationship,
distances to other galaxies can be
estimated
Cepheid Period-luminosity Relation.
Cepheids of a particular period have very nearly the
same luminosity.
Edwin
Hubble
Edwin
Hubbl
e
http://www.seds.org
Right Ascension
Declination
Distance
Visual Brightness
Apparent Dimension
00 : 42.7 (h:m)
+41 : 16 (deg:m)
2900 (kly)
3.4 (mag)
178x63 (arc min)
Andromeda Nebula
Nebula M31,
spiral
galaxy
Andromeda
M3, is
is actually
actuallyanother
another
spiral
galaxy
Tully-Fisher Relation –
using galaxies as standard candles
• Astronomers discovered that the faster a
spiral galaxy rotates, the more luminous it
is.
• This relationship is called the Tully-Fisher
relation, after its discoverers
The Tully-Fisher relation.
Galaxy Observations
• During the 1920's Edwin Hubble and
Milton Humason photographed the spectra
of many galaxies with the 100 inch
telescope at Mount Wilson.
• They found that most of the spectra
contained absorption lines with a large
redshift.
Red Shift and Distance
24 Mpc
1200 km/s
300 Mpc
15,000 km/s
780 Mpc
39,000 km/s
1220 Mpc
61,000 km/s
Galaxy Observations
• Using the Doppler effect, Hubble
calculated the velocity at which each
galaxy is receding from us.
• Using the period and brightness of
Cepheid variables in distant galaxies,
Hubble estimated the distances to each of
the galaxies.
The Tully-Fisher Relation
Hubble’s Law
• Hubble noticed that there was a linear
relationship between the recessional
velocity and the distance to the galaxies.
• This relationship is know as Hubble’s
Law:
V=HD
recessional velocity = Hubble’s Constant  Distance
Hubble’s Law
• H is known as the Hubble constant.
• It’s true value appears to be somewhere
between 55 to 75 km/s/Mpc.
• This means that a galaxy that is 1
megaparsec from Earth will be moving
away from us at a speed somewhere
between 55 to 75 km/s.
The Distance Chain
or Ladder
Measuring Cosmic Age
Measuring Cosmic Age
Raisin Cake Model
Like raisins in rising raisin cake, galaxies
move away from each other in our
expanding universe.
Typical Cube of Galaxies
Homogeneous, isotropic universe?
NO!
The Birth of The Universe“The Big Bang”
A very rough estimate for the age of
the universe
Vrec  H o  dis tan ce
Vrec
dis tan ce

time
dis tan ce
time 
Vrec
dis tan ce

H o  dis tan ce
1

Ho
1
time 
 15billion _ years
65km / s / Mpc
All of space and time were created in the Big
Bang, which then expands. Analogous to the
surface of a balloon.
Cosmological Red Shift
As the universe expands, photons of
radiation are stretched in wavelength, giving
rise to the cosmological redshift.
Elliptical, Spiral and irregular galaxies at different ages.
Modeling Galaxy Birth
• The most successful models are based on
the following assumptions:
• Hydrogen and helium gas filled all of
space fairly uniformly early in the universe.
• The near uniformity had small
perturbations which allowed for dense
regions to exist.
Galactic Collisions
NGC 4038/4039 are a pair of
colliding spiral galaxies
Hubble Space Telescope Photos Of Distorted Young Galaxies.
The larger number of distorted galaxies in the past suggests that collisions
between galaxies were common during the first few billion years.
Star- Burst galaxies
While the Milky Way forms a new star
about once per year, starburst galaxies
can form over 100 new stars per year
Quasars and Active Galactic Nuclei
• Active galaxies are galaxies which are
much more luminous than normal galaxies
and have spectra that are nonstellar in
nature.
•This indicates that the energy they
emit is not simply the accumulated
light of many stars.
•Most of the energy from active
galaxies is in the radio and infrared
portions of the spectrum.
Planck curves for Active and Normal
Galaxies
Seyfert Galaxies
• Look like normal spiral galaxies except
with extremely bright central galactic
nucleus.
•The luminosity of the nucleus can
exceed that of the rest of the galaxy.
•Spectral lines are very broad,
indicating rapid rotation.
•Luminosities can vary by large
amounts in fractions of a year.
Active galactic
nucleus in the
elliptical galaxy
M 87.
Jet of particles
shooting outward
from the nucleus at
nearly the speed of
light
Radio Galaxies
• Active galaxies that emit most of their
energy in the radio part of the spectrum.
•Comparable to Seyferts in total
energy output.
•Usually associated with
elliptical galaxies.
Two Types Of Radio Galaxy
• Core- Halo Radio Galaxy: Energy is
emitted from a small central nucleus, as
with a Sayfert Galaxy.
•Lobe Radio Galaxy: Energy is emitted
from enormous radio lobes. These lobes
usually lie far beyond the galactic
nucleus and are usually much larger
than the visible part of the galaxy.
Radio image of the radio galaxy Cygnus A taken with the VLA.
Active Galaxies show some or all of the
following properties.
• High Luminosities.
• Energy emission is nonstellar.
• Energy output can be highly variable.
• Often exhibit “jets” and other signs of explosive
activity.
• Spectra show broad emission lines - indicate rapid
internal motions.
Central Engine of Active Galaxy
NGC 1461 in Virgo Cluster
Energy Emission
• Although the rotating supermassive black
hole model is now widely accepted, the
actual mechanism for the energy production
is uncertain.
• One popular model which explains some
features is the synchrotron radiation model.
Synchrotron Radiation
• A type of nonthermal radiation produced by
high-speed charged particles, such as
electrons, as they are accelerated in a strong
magnetic field.
Synchrotron Radiation
Quasi-stellar Objects (QUASARS)
• Circa 1960, astronomers observe what
appear to be faint blue stars identified with
radio sources.
• These objects had odd spectral lines which
appeared broadened and extremely
redshifted.
• Quasars are :
• believed to be some of the oldest
objects in the universe.
• some of the most distant objects
from us.
• the most luminous objects known.
Radio Jet in the
Quasar 3C 345,
shows a blob of
plasma moving
away from the
core at nearly
the speed of
light
Active Galaxy Formation
• Possible evolutionary track for
galaxies may be as follows:
–Quasars --->
–Radio/Sayfert Galaxies --->
–Normal spiral and elliptical
galaxies.
• Black holes are always present, but
reduce over time as they run out of
fuel.
Artist’s conception of an accretion
disk surrounding a super-massive
black hole.
Doppler Shift of the emission
lines in the nucleus of the
elliptical galaxy M 87 indicates
a 2-3 billion solar mass black
hole
End of Section