The celestial clouds- GALAXY

SARAH JANE LAGATA
GUEST PROFESSORIAL LECTURER
Republic of the Philippines
BICOL UNIVERSITY GUBAT CAMPUS
Gubat, Sorsogon
BEED CONT : ASTRONOMY

•Trace the galaxy formation
process
•Use a tabular form classifying
types of galaxy
•Describe our own galaxy, the
Milky Way Galaxy.

History of Galactic (& Extragalactic) Astronomy
1610 - Galileo discovered the Milky Way is comprised of many stars
1755 - Immanuel Kant theorized that the galaxy has a planar structure,
some nebulae might actually be entire other galaxies or island universes
1774 -1781 - Messier catalog compiled including Andromeda galaxy as
M31
1781-1802 - William and Caroline Herschel conducted first “all-sky
survey” and cataloged 5000 nebulae, resolving some into their individual
stars
1845 - William Parsons (Lord Rosse), using a 72-inch telescope,
classified the nebulae into featureless ellipticals and whirlpool-like spiral
nebulae

Herschel could not account well for the effects of dust.
More dust along the disk causes the distribution of stars to drop-off artificially –
objects more than a few kpc from the Sun are obscured by dust.
1785 - Herschel attempted to determine the shape and size of Galaxy
Assumptions:
All stars have same intrinsic brightness
Star are arranged uniformly throughout the MW
He could see to the edge of the MW
SUN

•Kapteyn (early 1900s) used stellar parallax to estimate the true size of the Galaxy  Kapteyn
Universe
•10kpc diameter and 2kpc thick with the Sun less than a kpc from the center (rather
heliocentric)
•Tried to estimate scattering due to ISM gas but determined it to be insignificant (most
obscuration is due to dust absorption which has a smaller wavelength dependence)
•Shapley (1919) observed that globular clusters are distributed asymmetrically in the sky and that
if one assumes they are distributed about the center of the galaxy, this implies the Sun in not near
the center of the Galaxy
•Estimated distances to globular clusters using
variable stars and P-M relationship
•Concluded size to be 100kpc with Sun 15kpc from
center
Still wrong…didn’t account for dust absorption
which makes things look further away

Shapley realized that the globular clusters are all orbiting the center of
our Galaxy and map out the true extent of the Galaxy.

In 1920, the National Academy of Science hosted the Great Debate concerning the nature of
the Spiral Nebulae: were they island universes outside of the Milky Way?
•Shapley had MW size too big and therefore argued “NO”, they are part of the
Milky Way
•Others at that time believed the Kapteyn model of a much smaller MW and
argued “YES”, they are separate galaxies.
In 1922-1924 Edwin Hubble resolved the controversy using the superior 100-
inch telescope at Mount Wilson. He observed Cepheid variables in Andromeda
and, using the P-M relation (distance method), determined its distance to be
300kpc -- well outside of the MW (still off by a factor of 2 due to poor Cepheid calibrations)

GALAXY EVOLUTION - COSMIC HISTORY
• Galaxies change with redshift, reflecting
development in their stellar content, gas
content, and dynamical structure. These
changes are most pronounced at large redshift
and short wavelengths.
• New techniques allow us to approach the time
when galaxies took shape and the first stars
were formed.

STELLAR SPECTRA – THE FOSSIL RECORD
•Chemistry of stellar surface reflects initial
chemical makeup until late in its lifespan
•Stars’ orbits change only very slowly over time
•Makeup and motions of stars preserve a
detailed record of our galaxy’s history
•Early stars formed with low heavy-element
abundances and in a nearly spherical system

GALAXIES TODAY
• Spiral, elliptical, irregular
• Stellar and gas content linked to morphology
• Dwarf galaxies most common
• Dark matter dominates overall dynamics
• Disk and bulge components; differ in motions
and stellar properties

TRACING GALAXY EVOLUTION
• Galaxy types in clusters (Butcher-Oemler)
• Stellar populations from spectra
• Size/luminosity
• Hubble types
• Star-formation rate (UV,far-IR, radio, Ha)
• Nuclear activity (quasar density,luminosity)

THE EARLY GALAXY BESTIARY
• Lyman-break galaxies
• Extremely red objects (EROs) - the oldest young galaxies
and dusty environments
• Star-forming subgalactic objects
• Submillimeter galaxies
• Quasars and radio galaxies
• Absorption-line systems
• These often occur in combination (groupings)

LYMAN-BREAK GALAXIES (LBGS)
• Galaxy spectra show a cutoff at 912 A due
to absorption by neutral hydrogen
• This allows a straightforward multicolor
selection (blue in two bands, missing
shortward of that)
• Thousands of galaxies at z>2.7 have now
been found in this way

THE LYMAN BREAK
The Lyman break from satellite UV observations
of a star-forming region in the nearby spiral M33
The brightest LBG in the Hubble Deep
Field, a clumpy galaxy at z=3.21.
300 nm 450 nm
606 nm 814 nm

SUBMILLIMETER-BRIGHT
GALAXIES
• Found at z=2-3
• Most powerful early star-forming sites?
• Key on dust emission, not stars
• Clump with other high-redshift objects
• Many have buried quasar cores
Background: ionized-gas plume in submm galaxy ELAIS N2 850.4
at z=2.4, from NASA Infrared Telescope Facility, April 2003)

Background: ionized-gas plume in submm galaxy ELAIS N2 850.4
at z=2.4, from NASA Infrared Telescope Facility, April 2003)

EXTREMELY RED OBJECTS
(EROS)
• May be either intrinsically red or
reddened by dust absorption; both
kinds exist
• A way to seek the oldest galaxies at a
particular redshift, a sensitive probe of
when galaxy formation begn in earnest

SUBGALACTIC CLUMPS
• Small size, blue color, Lyman a emission
• Active star formation, low metallicity
• Evidence for global winds escaping systems
• Exist in groupings with bright galaxies/AGN
• Are these the early units predicted by
hierarchical schemes (and fitting dark-matter
simulations)?

Blue subgalactic objects versus nearby spiral M101
at the same ultraviolet emitted wavelength
Size:
1 kpc~3000
light-yr
Many are double
Comparable UV
luminosity to
bright galaxies
now

Supermassive Black Hole in the Galactic Center
Radio image (80 pc
across) shows
feature SgrA and
radio filaments
Radio image (10
pc across) shows
feature known as
SgrA West – center
of this is SgrA*
Investigate IR
stellar motions in
region about 1pc
across (a few
lightyears) to
estimate BH mass

The early Universe could be crowded
(a group at z=2.4)
Subgalactic objects
Quasars
Radio galaxy
ERO

INGREDIENTS OF A COSMIC
HISTORY
• Gravitational collapse and gas infall
• Star formation (a feedback process)
• Heavy-element production
• Winds and the intergalactic medium
• Growth of supermassive black holes
• The first stars – a breed apart

THE FIRST STARS (POPULATION III)
• Formed of pure hydrogen/helium
• Very massive (80-300 solar masses)
• Hot, short-lived
• Energetic supernova explosions
• Enriched surrounding gas, disrupted parental gas clouds
• Enrichment led to “normal” star formation
• Enriched intergalactic gas as well

GALAXIES
Image at
http://hubblesite.org/newscenter/archive/releases/galaxy/spiral/2005/
01/results/50/

WHAT IS A GALAXY?
• A galaxy is a large grouping of stars, gas, and dust
in space that are held together by gravity.
• The largest galaxies contain more than a trillion
stars. Smaller galaxies may have only a few
million.
• Scientists estimate the number of stars from the
size and brightness of the galaxy.

CONTENTS OF GALAXIES
Galaxies are made of
stars, planetary systems,
gas clouds, and star
clusters. Nebulas are
giant clouds of gas and
dust where stars may be
forming.
Nebula are found in
spiral galaxies but not
elliptical galaxies.

DYNAMICS OF GALAXIES
•Slowly Rotating Systems.
•Rotation Curves.
•Spiral Structure

STELLAR AGES AND ELEMENT ABUNDANCES IN GALAXIES
• Population synthesis of E galaxies show that
practically all their stars were formed
simultaneously about 15×109 years ago.
• The indicators of composition most easily
measured are the variations of colour indices
inside galaxies and between different galaxies.

OBSERVATIONAL PROPERTIES OF
GALAXIES

Galaxy Colors (Tracing Different Types of Stars)
• Galaxy colors thus indicate the different stellar types (the stellar
populations) that are present within different regions.
• Blue colors indicate the presence of hot O-type and B-type stars, while
redder colors tell us that no such stars are present. uniform red and gold
colors, indicating that they have experienced little or no recent star
formation. Other galaxies can be divided into reddish regions (made up of
old stars) and bluish regions (where star formation is on-going). blue light,
,they have rich reserves of gas clouds which are being turned into new stars
throughout the galaxy.

• Galaxy Luminosities and Sizes
• Galaxy Concentration and Asymmetry Indices (CI, AI)
• concentration index (CI), which tells us how concentrated
the light from the galaxy is within the central regions. The
concentration index is just the fraction of light emitted by
the galaxy within the inner 30% of the ellipse.
• asymmetry index (AI), which tells us whether the galaxy
appears smooth and symmetric, or displays substantial
amounts of substructure (such as the spiral arms in the
Milky Way disk).

SYSTEMS OF GALAXIES
•Groups.
•Clusters.
•Superclusters.

• The most common type of galaxy systems are
small, irregular groups of a few tens of galaxies.
• A typical example is the Local Group, which
contains two larger galaxies in addition to the Milky
Way – the Andromeda Galaxy M31, an Sb spiral of
about the same size as the Milky Way with two
dwarf companions, and the smaller Sc spiral M33.
The rest of the about 35 members of the Local
Group are dwarfs; about 20 are of type dE and 10
of type Irr I.

• A system of galaxies may be defined to be a
cluster if it contains a larger number (at least 50)
of bright galaxies. The number of members and
the size of a cluster depend on how they are
defined. One way of doing this is to fit the
observed distribution of galaxies within a cluster
with an expression of the form (18.8). In this way
a characteristic cluster radius of about 2–5Mpc is
obtained.

GALAXY CLUSTERS
• the Local Group
• includes the Milky Way, the Andromeda, and over 30 other
smaller galaxies
• the Virgo Cluster
• hundreds to thousands of galaxies, 60 million light-years away
• giant elliptical at center, formed by galactic cannibalism
• the Local Group is “falling” toward the Virgo Cluster at 60 to 250
miles per second!

SUPERCLUSTERS!
• clusters are bound together in larger structures, called
superclusters
• these superclusters have been mapped, and are
grouped into long strings
• 300 million to a billion light-years long
• 100 to 300 million light-years wide
• and only 10 to 30 million light-years thick
• in between these strings are huge voids of galaxies,
although some astronomers may have detected hot
gas

• Groups and clusters of galaxies may form even
larger systems, superclusters.
• For example, the Local Group belongs to the
Local Supercluster, a flattened system whose
centre is the Virgo Cluster, containing tens of
smaller groups and clouds of galaxies.

43
The local group of galaxies
Andromeda is
the nearest big
galaxy to the
Milky Way
Milky
Way

44
Light from Andromeda takes
2 million years to reach us.
Milky
Way
Andromeda

45
Local
group
The Local Supercluster
Here the local group has shrunk to the size of the
Milky Way in the last slide.

46
It left when the dinosaurs
were on Earth.
Light from Virgo has taken 65 million
years to reach us.
Virgo
Cluster

MORPHOLOGICAL
TYPES FOR GALAXIES

Fig. 18.3. The classification of normal spiral and S0 galaxies. (Mt. Wilson Observatory)

• come in different sizes (dwarf, large, giant)
• come in different shapes and classifications
• Ellipticals
• Spirals
• Lenticulars
• Irregulars
• are fairly close together, relative to their sizes

PROPERTIES OF ELLIPTICAL GALAXIES
• Round or elliptical in shape (spherical to football
shaped)
• Contain very little gas or dust.
• Because of little gas, no new stars are forming
thus mostly contain old stars
• The largest and smallest galaxies are elliptical
galaxies.
• About 1/3 of all galaxies.
• Simply massive blobs of stars.
• similar to the central bulge of a spiral galaxy

• these galaxies are characterized by stars with a
reddish-gold cast, indicating that there has
been little or no recent star formation and
suggesting that these galaxies are gas-poor.
• are more massive and more luminous on
average than their spiral cousins; they are the
giants of the Universe.

FORMATION OF AN ELLIPTICAL GALAXY

FORMATION OF AN ELLIPTICAL GALAXY
Movie

PROPERTIES OF SPIRAL
GALAXIES
• Shaped like flattened disks with one or more
spiral arms.
• have flat disk, spiral arms, central bulge, and a
surrounding halo
• some have a “barred” bulge
• are fairly large (no dwarf spirals)
• have lots of gas and dust and younger
stars in their arms, but older stars and
little gas or dust in their halos and central
bulges

• divided into sub-classes (Sa – Sd) based on
their bulge and disk component
characteristics.
• they have a central bar or not – a literal barlike
structure stretching across the central regions,
often serving as a conduit to funnel gas (and
encourage central star bursts) in the nucleus.

• A spiral galaxy has a central bulge surrounded
by the galaxy disk, a thin pancake which
rotates rapidly around the galaxy center.
(There is also a stellar halo, a large, diffuse
cloud of older stars which surrounds the entire
galaxy at large radii)

SPIRAL GALAXY--ANDROMEDA
NOAO/AURA/NSF Images at http://www.noao.edu/image_gallery/html/im0606.html and
http://www.noao.edu/image_gallery/html/im0685.html

SPIRAL GALAXY
ON EDGE
Image at
http://hubblesite.org/newscenter/archive/releases/galaxy/spiral/2006/24/image/a/results/50/

Image at
http://hubblesite.org
/newscenter/archive/
releases/galaxy/spiral
/2007/41/results/50/

ELLIPTICAL GALAXIES
Images at http://hubblesite.org/newscenter/archive/releases/galaxy/elliptical/2007/08/image/a/format/large_web/results/50/
and http://hubblesite.org/newscenter/archive/releases/galaxy/elliptical/1995/07/results/50/

PROPERTIES OF
IRREGULAR GALAXIES
• Do not fit into any other category.
• Chaotic mix of young stars, gas and
dust
• Usually found near large spiral
galaxies who may be distorting their
shape.
• may have a distorted shape from
interaction with another galaxy

IRREGULAR GALAXIES
NASA and NOAO/AURA/NSF Images at
http://hubblesite.org/newscenter/archive/releases/galaxy/irregular/2005/09/results/50/ ,
http://www.noao.edu/image_gallery/html/im0560.html , and http://www.noao.edu/image_gallery/html/im0993.html

LENTICULAR
• have a disk but no arms
• have little or no excess
gas and dust
• Are bats of the galaxy
world, sharing
characteristics of both
elliptical and spiral
galaxies,
Image at

PECULIAR
(those which display anomalous structures)
have a basic morphology tied to the elliptical, lenticular, or spiral classes, but
display unusual shapes, structures, or colors on top of that structure which
indicate that the galaxy was gravitationally perturbed by another galaxy in
the past

GALAXY GROWTH VIA INTERACTIONS
• Galaxies initially form from mergers of several gas
clouds
• Galaxies then are changed by interactions
• Galaxies grow gradually by galactic cannibalism
• Interactions disturb gas leading to starbursts
• Collisions can randomize stellar orbits leading to
the formation of elliptical galaxies

COLLISIONS!
• galaxies in groups and clusters often collide
• The Milky Way is moving at 300,000 mph toward the
Andromeda Galaxy
• They may collide in about 5 billion years
• Stars don’t usually collide
• New orbits, gas piles up to form new stars

THE ANTENNAE OR MICE
Information at http://hubblesite.org/newscenter/archive/releases/galaxy/interacting/1997/34/results/50/

Images from http://hubblesite.org/newscenter/archive/releases/galaxy/spiral/2002/21/image/a/results/50/ and
http://hubblesite.org/newscenter/archive/releases/galaxy/spiral/1999/16/image/a/results/50/
The
occasional
results of
two
galaxies
colliding:
ringed
galaxies

INTERACTING
Image from
http://hubblesite.org/newscenter/archive/releases/galaxy/interacting/20
00/34/results/50/

INTERACTING GALAXIES
• are involved in a short-lived transfer of stars
and gas between galaxies, where the two
galaxies are being disturbed by each other’s
gravitational attraction.

STARBURST GALAXY – M82
• Stars form
at a fast
rate

SUPERMASSIVE BLACK HOLES
• Found in almost every medium to large galaxy at the
center
• the larger the galaxy, the more massive the black hole
• are responsible for some of the galaxies with jets and
lobes which give off radio waves, x-rays, etc.

ACTIVE GALAXY
Image at http://hubblesite.org/newscenter/archive/releases/galaxy/spiral/2000/37/results/50/

AT THE CENTER OF A LARGE GALAXY
Image at
http://hubblesite.org/newscenter/archive/relea
ses/exotic/black-hole/1998/22/results/20/ and
http://hubblesite.org/newscenter/archive/relea
ses/exotic/black%20hole/2000/21/image/a/for
mat/web_print/results/20/

EVOLUTION
OF GALAXIES
Image at http://www.galex.caltech.edu/media/glx2007-
05f_img01.html

SEYFERT GALAXIES
• The Seyfert galaxies are named after Carl Seyfert, who
discovered them in 1943. Their most important characteristics
are a bright, pointlike central nucleus and a spectrum showing
broad emission lines.
• Almost all are spirals; the possible exceptions are of type 2.
They are strong infrared sources. Type 1 galaxies often show
strong X-ray emission.
• It is estimated that about 1% of all bright spiral galaxies are
Seyfert galaxies. The luminosities of their nuclei are about
1036–1038 W

R
A
D
I
O
G
A
L
A
X
I
E
S
• are galaxies that are powerful radio sources.
The radio emission of a radio galaxy is non-
thermal synchrotron radiation. The radio
luminosity of radio galaxies is typically 1033–
1038 W, and may thus be as large as the total
luminosity of a normal galaxy
• “Tailed” radio sources also exist. Their radio
emission mainly comes from one side of the
galaxy, forming a curved tail, which is often
tens of times longer than the diameter of the
galaxy.

QUASARS.
• The first quasar was
discovered in 1963,
when Maarten
Schmidt interpreted
the optical emission
lines of the known
radio source

GRAVITATIONAL LENSES
• The first example of this effect
was discovered in 1979, when
it was found that two quasars,
5.7 apart in the sky, had
essentially identical spectra. It
was concluded that the “pair”
was really a double image of a
single quasar.
• Gravitational lenses have also
been discovered in clusters of
galaxies.

OUR GALAXY: THE MILKY WAY
• has about 200 billion stars, and lots of gas and dust
• is a barred-spiral (we think)
• about 100,000 light-years wide
• our Sun is halfway to the edge, revolving at half a
million miles per hour around the center of the
Galaxy
• takes our Solar System about 200 million years to
revolve once around our galaxy

DESCRIBE THE MILKY WAY GALAXY
• Flattened Disk
• Bulge in Middle
• Fried Egg
• Not solid from edge to edge
• Huge
• 1000-3000 Light Years Thick
• 100,000 Light Years across (Diameter)

THE MILKY WAYImage at http://news.nationalgeographic.com/news/bigphotos/1945371.html

WHERE IS OUR SOLAR SYSTEM IN OUR GALAXY?
• The Sun and our solar system is located in one of the
outer arms (Orion’s Arm) of the galaxy. The distance
from the Sun to the center of the galaxy is about
30,000 light years.
• All objects in the galaxy revolve around its center.
• The sun and our solar system take about 240 million
years to make one trip around the center of our
galaxy.

The celestial clouds- GALAXY

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