A1 21 Galaxies

Hubble Tuning Fork Diagram
LACC: §25.2, 3, 5

• Ellipticals
• Spirals (including Barred Spirals)
• Irregular

An attempt to answer the “big questions”: where are we?
what is the universe made of?

Thursday, May 13, 2010 1

Galaxies like colorful pieces of candy ﬁll the Hubble Deep Field - humanity's most distant yet optical view of the
Universe. The dimmest, some as faint as 30th magnitude (about four billion times fainter than stars visible to the
unaided eye), are the most distant galaxies and represent what the Universe looked like in the extreme past,
perhaps less than one billion years after the Big Bang. To make the Deep Field image, astronomers selected an
uncluttered area of the sky [about 2.5 arcmin across] in the constellation Ursa Major (the Big Bear) and pointed the
Hubble Space Telescope at a single spot for 10 days accumulating and combining many separate exposures. With
each additional exposure, fainter objects were revealed. The ﬁnal result can be used to explore the mysteries of
galaxy evolution and the infant Universe.

http://apod.nasa.gov/apod/ap980607.html


Hubble Tuning-Fork Diagram

http://ifa.hawaii.edu/~barnes/ast110_06/trotn.html


Types of Galaxies

All bright galaxies fall into one of three
broad classes according to their shape:
• Spiral Galaxies (~75%)
• Elliptical Galaxies (20%)
• Irregular Galaxies (5%)
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit4/types.html


Types of Galaxies

Despite the fact that the Hubble Sequence is based only on the
appearance of galaxies (morphology of galaxies), several physical
properties of galaxies vary smoothly along the sequence. We have,
little gas and dust <----------------------> lots of gas and dust
mainly Pop II stars <----------------------> Pop I & II stars
Reddish <----------------------------------> Bluish
little ongoing star formation <------------> star formation
large bulge <------------------------------> small bulge
tight,smooth arms <---------> open, loose arms
Mass: 108-1013 MSun (Ellipticals) <--> 1012-109 MSun (Spirals)
http://zebu.uoregon.edu/~imamura/123/lecture-3/lecture-3.html


Types of Galaxies

Spiral Galaxies Elliptical Galaxies Irregular Galaxies

Properties: Properties: Properties:
• Mass: 109 - 1012 Msun • Mass: 105 - 1013 Msun • Mass: 106 - 1011 Msun
• Diameter: 5 - 50 kpc • Diameter: 1 - 200 kpc • Diameter: 1 - 10 kpc
• Luminosity: 108 - 1011 • Luminosity: 106 - 1012 • Luminosity: 106 - few
Lsun Lsun x 109 Lsun

Structure & Dynamics: Structure & Dynamics: Structure & Dynamics:
• Disk + Spheroid • Spheroid of old stars • Chaotic structure, lots
• Supported by with little gas or dust of young blue stars
relatively rapid rotation, • Supported by random • Moderate rotation in
but spheroid is puffed motions of stars with Irregulars, but very
up by random motions. some very slow rotation chaotic motions as well.


Spiral Galaxies

Classiﬁcation Description
Sa Bright core, tightly wound spiral arms
Core dimmer than Sa, spiral arms more
Sb
loose
Core dimmer than Sb, open spiral
Sc
structure, more dust and gas
Dim core, loosely wrapped spiral structure,
Sd
lots of dust and gas and new star growth
http://astronomyonline.org/Astrophotography/GalaxyMorphology.asp

Barred Spiral Galaxies

Classiﬁcation Description
SBa Bright core, tightly wound spiral arms
Core dimmer than SBa, spiral arms more
SBb
loose
Core dimmer than SBb, open spiral
SBc
structure, more dust and gas
Dim core, loosely wrapped spiral structure,
SBd
lots of dust and gas and new star growth

Spiral Galaxies
Some other important properties of spiral galaxies include
(Sparke and Gallagher, page 172 – 214):
• Spiral galaxies produce most of the luminous light in the
Universe due to new star birth in the spiral arms
• Majority of galaxies in the Universe are spiral galaxies
• Half of all spiral galaxies are in the bared spiral class
• Spiral galaxies obey the Tully-Fisher relation – brighter
galaxies rotate faster
• Spiral galaxy rotation curves are dominated by Dark
Matter
• Rotation of stars in the spiral arms are organized while
the rotation of stars in the bulge are not (random rotation
orbits about the nucleus)


Elliptical Galaxies

Type E: Ellipticals

Show little internal structure:

•
Elliptical in shape

•
No disks, spiral arms, or dust lanes

•
Brightest stars are red
Classified by the degree of apparent flatness:

•
E0 is circular

•
E7 is flattest (~3:1 aspect ratio)



Irregular Galaxies

http://updatecenter.britannica.com/art?assemblyId=17808&type=A

Irregular Galaxies

Type I: Irregulars

Show an irregular, often chaotic structure.
Little evidence of systematic rotation.

Catch-all class:

•
Proposed systematic subclasses, but many
irregulars defy classification.

Significant dwarf irregular population,
classified as "dI"


Galaxies
Relative Stellar & Gas Content [mass to light ratio]
Spirals:

•
Range is ~10-20% gas

•
On-going star formation in the disks

•
Mix of Pop I and Pop II stars
Ellipticals:

•
Very little or no gas or dust

•
Star formation ended billions of years ago

•
See only old Pop II stars
Irregulars:

•
Can range up to 90% gas

•
Often a great deal of on-going star formation

•
Dominated by young Pop I stars
Dwarf Irregulars:

•
Very metal poor (<1% solar)

•
Forming stars for the ﬁrst time only now.

Dwarf Galaxies

Dwarf Galaxies

Low-luminosity Ellipticals & Irregulars.

•
Signiﬁcant number of dwarfs

•
Most common type of galaxy by number

There are no (convincing) Dwarf Spirals.

Possibilities:

•
Small versions of their larger cousins

•
Different population of objects despite their
superﬁcial similarities to larger E's and Irr's


Hubble Tuning Fork Diagram
LACC: §25.2, 3, 5
• Ellipticals: 20% of galaxies, E0 (spherical) to E7
(elongated), smallest and largest, population II stars,
no dust and gas, highest mass to light ratio
• Spirals (including Barred Spirals): 75% of galaxies
(1/2 barred, 1/2 not barred), medium sized,
population I and II stars, dust and gas in disk, no
dwarfs
• Irregular: random shape, small, lots of star formation,
lots of dust and gas, lowest mass to light ratio

LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.

• Ch. 25, pp. 577-578: 3.

Due at the beginning of next week’s ﬁrst class
period (unless there is a test that week, in which
case it’s due the same period as the test).
Be working on your Distance Ladders.


Galaxies
LACC: §25.2, 3, 5

• Measuring the Distances to Galaxies
• Galaxy Evolution
• Distribution of Galaxies


Galaxy Formation

http://outreach.atnf.csiro.au/education/senior/cosmicengine/galaxy_formation.html

Spiral Galaxies:
Supermassive Black Holes

The results ... show a close relationship between the black
hole mass and the stars that comprise an elliptical galaxy or
the central bulge stars of a spiral galaxy.
http://spaceﬂightnow.com/news/n0006/05hstblackholes/

Elliptical Galaxies:
Supermassive Black Hole
Though much more
analysis remains, an initial
look at Hubble evidence
favors the idea that titanic
black holes did not
precede a galaxy's birth
but instead co-evolved
with the galaxy by
trapping a surprisingly
exact percentage of the
mass of the bulbous hub
of stars and gas in a
galaxy.
http://spaceﬂightnow.com/news/n0006/05hstblackholes/


Galaxy Mergers

The Milky Way and the Andromeda galaxy will likely fall together
and merge within a few billion years. In this speculative simulation,
the two galaxies ﬂyby one another, exciting tidal tails and bridges
and collide on a second pass ﬁnally merging after several
convulsions. The last remnants of the smashed spirals show up as
shells and ripples surrounding a newborn elliptical galaxy.
http://www.galaxydynamics.org/spiralmetamorphosis.html (6 min)
http://www.youtube.com/watch?v=dJRc37D2ZZY (1 min)


Galaxy Collisions

http://www.theage.com.au/news/national/on-a-
collision-course-or-uniting-as-an-intergalactic-super-
system/2008/04/24/1208743153745.html

Distant Ladder

http://universe-review.ca/R02-07-candle.htm


The Tully-Fisher Relation

http://physics.uoregon.edu/~jimbrau/astr123/Notes/Chapter24.html#dist

The Tully-Fisher Relation
An observed relation between the
luminosity of spiral galaxies and their
maximum rotation velocity. The Tully-
Fisher relation is used as a way of
estimating distances to spirals. The form
is a linear relation between the absolute
magnitude of a galaxy and the logarithm
of the velocity at the ﬂat part of the
rotation curve, although the slopes and
intercepts of these relations are different
for Sa, Sb, and Sc type galaxies.
Approximations made in deriving the
relation are that the mass-to-light ratios
are constant for all galaxies and that the
average surface brightness of all galaxies
is also equal.
http://www.daviddarling.info/encyclopedia/T/Tully-Fisher_relation.html

Hubble’s Law

http://www.astronomynotes.com/galaxy/s7.htm

The Local Group
http://www.astronomy.com/asy/objects/
images/local_group_0305_diagram_800.jpg


Galaxy Clusters

http://www.astro.washington.edu/larson/Astro101/
LecturesBennett/DistanceScale/expansion.html


CL0024+17 Galaxy Cluster

How do we know that dark matter isn't just normal matter exhibiting strange gravity? A new observation of
gravitationally magnified faint galaxies far in the distance behind a massive cluster of galaxies is shedding new
dark on the subject. The above detailed image from the Hubble Space Telescope indicates that a huge ring of
dark matter likely exists surrounding the center of CL0024+17 that has no normal matter counterpart. What
is visible in the above image, first and foremost, are many spectacular galaxies that are part of CL0024+17 itself,
typically appearing tan in color. Next, a close inspection of the cluster center shows several unusual and repeated
galaxy shapes, typically more blue. These are multiple images of a few distant galaxies, showing that the cluster is
a strong gravitational lens. It is the relatively weak distortions of the many distant faint blue galaxies all over the
image, however, that indicates the existence of the dark matter ring. The computationally modeled dark matter
ring spans about five million light years and been digitally superimposed to the image in diffuse blue. A
hypothesis for the formation of the huge dark matter ring holds that it is a transient feature formed when
galaxy cluster CL0024+17 collided with another cluster of galaxies about one billion years ago, leaving a ring
similar to when a rock is thrown in a pond.
http://apod.nasa.gov/apod/ap070516.html

The Local Supercluster

The Local
The Local Group
Supercluster
http://www.pas.rochester.edu/~afrank/A105/LectureXV/LectureXV.html

The Local Supercluster

This (somewhat blurred) map identiﬁes We jump now to a map that carries out to
galaxies and galaxy clusters across a 1,000,000,000 light years which includes the
ﬁeld of view 400 million light years supercluster the Milky Way lies within:
across

http://www.fas.org/irp/imint/docs/rst/Sect20/A2a.html


Galaxies
LACC: §25.2, 3, 5
• Measuring the Distances to Galaxies: Cepheid
Variable (our Local Group), Tully-Fisher Relation
(distant spirals), Hubble’s Law (most distant galaxies)
• Galaxy Evolution: mass of central supermassive
black holes match size of central bulge, mergers and
cannibalism (Milky Way will collide with Andromeda)
• Distribution of Galaxies: Galaxy Clusters (e.g. Local
Group); Galaxy Superclusters (e.g. Virgo
Supercluster)

LACC HW: Franknoi, Morrison, and
Wolff, Voyages Through the Universe,
3rd ed.
• Ch. 25, pp. 577-578: 5 (choose from the following: Cepheid
Variables, Cepheid Variables, Tully-Fisher Relation, Type Ia Supernova,
Brights Cluster Galaxy ).

• Ch 26: Tutorial Quizzes accessible from:
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
http://

ﬁd=M20b&product_isbn_issn=9780495017899&discipline_number=19

Due at the beginning of next week’s ﬁrst class
period (unless there is a test that week, in which
case it’s due the same period as the test).
Be working on your Distance Ladders.

A1 21 Galaxies

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