Transcript NORMAL GALAXIES - TCNJ | The College of New Jersey

NORMAL GALAXIES
COLLECTIONS OF STARS,
GAS (and DARK MATTER):
HUGE VARIETY OF TYPES
What are the three major
types of galaxies?
Hubble
Ultra
Deep
Field
Hubble
Ultra
Deep
Field
Hubble
Ultra
Deep
Field
Spiral Galaxy
Hubble
Ultra
Deep
Field
Spiral Galaxy
Hubble
Ultra
Deep
Field
EllipticalGalaxy
Galaxy
Elliptical
Spiral Galaxy
Hubble
Ultra
Deep
Field
EllipticalGalaxy
Galaxy
Elliptical
Spiral Galaxy
Hubble
Ultra
Deep
Field
EllipticalGalaxy
Galaxy
Elliptical
Irregular Galaxies
Spiral Galaxy
Basic Galactic Facts
 SIZES
1 kpc -- 100 kpc across
109 M -- 1013 M dwarf galaxies down to 107 M
 BASIC STRUCTURES
SPIRAL -- Milky Way & most overall
ELLIPTICAL -- most dwarfs and giants
IRREGULARS -- often ``satellites'’
 LOCATIONS
Isolated (Field)
Groups (Local group includes ~50)
Clusters (100s to 1000s of galaxies)
Superclusters (clusters of clusters!)
Voids (galaxies not there)
ELLIPTICAL GALAXIES
• Ellipticity: E0 -- E7 (round to flatest)
Projected on the sky:
• # = (1 - b/a) x 10
• Can be more elliptical than they are
seen to be (Projection Effect)
• Often really tri-axial
• Some (e.g. Centaurus A) include dust
disk -- big elliptical swallowed a spiral!
Elliptical Galaxy Shapes
• M49 is close to a circle: E2
• M84 is an “average” E3
• M110 is a dwarf E (Andromeda satellite): E5
SPIRAL GALAXIES
• Classify by Hubble Type: S0; Sa--Sc;
SBa--SBc (tuning fork diagram)
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•
•
•
•
S0: disk seen, but no spiral arms
Sa: prominent nucleus, tightly wound arms
Sb: significant nucleus, moderate arms
Sc: small nucleus, patchy loose arms
SBa, SBb, SBc: central bar from which arms
emerge
• Milky Way is a SBb (weak bar though, and
between SBb and SBc)
“Regular” Spiral Types
M101-pinwheel galaxy 51 HST images
Barred Spiral Types
S0 (lenticular) Galaxies
S0 and SB0 have disk and bulge but no visible spiral arms
Irregular Galaxies: Magellanic Clouds
Irregular Galaxies:
Interactions and Starbursts
Key Info on Galaxy Types
• Colors: E's red, S's various, Irr's, blue
• Populations:
E: Pop II
Irr: Pop I
S: disk, Pops I+II; halo, Pop II
• Sizes:
E's 1-100 kpc, S's: 3-50 kpc; Irr's 1-15 kpc
• # of Stars:
Ellipicals: 107 (dwarfs) to 1013 (cD central dominant)
Spirals: 1010 - 1012
Irregulars: 107 - 1011
Gas Content and Star Formation
• Gas Content:
E: up to 30% of ordinary matter (baryonic) mass,
but very hot (T > 107 K)
S: typically 5-15% of baryonic mass, in many
ISM
phases (10 K < T < 106 K)
Irr: typically 20-50% of baryonic mass, in many
ISM phases
• Star Formation:
E: very little, if any, currently (so RED)
S: moderate amount in disk (some BLUE with
many YELLOW and RED)
Irr: often lots currently (so BLUE)
Thought Question
Why does ongoing star formation lead to a bluewhite appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light
Thought Question
Why does ongoing star formation lead to a bluewhite appearance?
A. There aren’t any red or yellow stars
B. Short-lived blue stars outshine others
C. Gas in the disk scatters blue light
What Determines Galactic Shapes?
• The quasi-spherical shapes of Ellipticals as
well as halo and bulge stars in spirals arise
from their stars' original RANDOM
VELOCITIES.
• The orbits of stars in spiral galaxies come
from stars mainly forming in a flattened disk,
supported by ROTATION.
• The odd shapes of Irregulars are not well
understood but many probably arise from
tidal distortion by bigger galaxies.
Formation of a Spiral Galaxy
Summary: The Hubble Sequence
Like stars on the Main Sequence galaxies are born into a
type in the Hubble Sequence and they DO NOT usually
move along the Hubble Sequence.
Mergers and Cannabalism
• The biggest E and S
galaxies have almost
certainly MERGED WITH
comparable sized
galaxies, or
CANNABALIZED several
smaller galaxies over
billions of years.
• Typically S+S  E,
• S+E  E, E+E  E
• So as the universe ages
the fractions of: S's goes
down, E's up.
• BUT sometimes mergers
induce more star
formation and spiral disks
Cen A: new dust disk from
S swallowed by big E
First Project: Galaxy
Classification
• Use the Sloan
Digital Sky
Survey to learn
how to classify
galaxies
• Then use the
Galaxy Zoo to do
some new
classifications
and thereby
contribute to
actual research
Second Project:
Photometric Redshifts
Part of U Alaska
• Use real data and
“Research Based
astronomers’
Science Education”
tools to discover
project
distant galaxies
and to measure
their distances
and ages.
Where are they found?
• Most Es (except dwarfs) near CENTERS OF CLUSTERS.
• Most S's: in the FIELD or toward EDGES OF CLUSTERS.
• Irr's locations are less well known; probably like Spirals.
Coma
Cluster
about
100
Mpc
away
DISTRIBUTION OF GALAXIES
• Most galaxies are in clusters;
• Most clusters are part of superclusters.
• Our Local Group has about 50 members.
MW + LMC, SMC, Draco, Fornax,
Sculptor, Leo etc is one sub-group;
Andromeda (M31) + M32, M33, NGC 147
and more is another sub-group
• Total extent about 1 Mpc
(M31 is 700 kpc from MW)
The Local Group
Some Properties of CLUSTERS
• The nearest
CLUSTER is the
Virgo Cluster, about
15 Mpc away;
• Clusters vary in size
and richness, from
100 up to over 5000
galaxies.
• Within clusters, E's
and S0's dominate
the central parts
(90% or so) but S's
and SB's dominate
the outskirts of
clusters.
Cluster Merger Movie
• Many clusters
grow through
mergers of
smaller clusters
• Some clusters
are still growing
today
• Collisions can
heat gas in
clusters
(intracluster
medium) to
~108K, giving
off X-rays
THE COSMIC DISTANCE LADDER
•
•
•
•
SPECIAL VARIABLE STARS
Giants and supergiants will PULSATE in the
INSTABILITY STRIP; above the MS for A and F
stars, where variations in He opacity drive
increases and decreases in R and T.
Some variable stars calibrate distances
All RR LYRAE stars are nearly the same
luminosity, some 70 times the Sun's.
Periods between 2 and 24 hours.
CEPHEID VARIABLES have luminosites proportional
to their periods from ~200 L (for 1 day)
to ~10000 L (for 50 days)
Both are "STANDARD CANDLES" that allow
DISTANCE DETERMINATIONS to NEARBY
CLUSTERS and many, relatively nearby GALAXIES.
Next Step: Tully-Fisher Relation
• There is a very strong correlation between rotational speeds and
luminosities for Sc galaxies. Why?
• Roughly: rotation speed ~ mass ~ luminosity
• Measure brightness and estimate luminosity, then distance.
• The 21 cm H I line is broader in the faster rotating galaxies;
IR magnitudes give better estimates of total brightness
• This Tully-Fisher relation is good out to 200 Mpc!
Cosmic
Distance
Ladder,
Illustrated
Type 1a SNe
take us out
beyond 1 Gpc:
discussed last
week
White-dwarf
supernovae
can also be
used as
standard
candles
Type Ia SNe as Standard Candle
Apparent
brightness of
white-dwarf
supernova
tells us the
distance to its
galaxy
(up to 10
billion lightyears)
HUBBLE's LAW
• Back in 1920's Edwin Hubble found that nearly all
galaxies showed REDSHIFTS!
• Even more interesting, the fainter the galaxy,
therefore, probably the more distant the galaxy, the
greater the redshift.
• When distances (r) were calibrated using Cepheid
variables, Hubble found:
• v = H0 r
where v = c (/) is the “expansion velocity”
and z = / is the redshift. (So v = cz)
Galaxy Spectra and Hubble’s Law
• Discover Hubble's Law
Using Hubble’s Law
• If one knows enough galaxy distances from
independent measurements and has z's for all of
those galaxies then one gets a value for
• H0 = average of all (v/r) measurements.
• This yields: 50 < H0 < 100 km/s/Mpc and most likely,
H0 = 72 km/s/Mpc (with an error of 3 km/s/Mpc)
• An example: say a line of 5000 Å is seen at 5500 Å
• z = / = (5500 Å -5000 Å)/5000 Å = 0.10
So v = cz = 0.10 x 3.00 x 105 km/s = 3.00 x 104
km/s If H0 = 75 km/s/Mpc, then
• r = v / H0 = (30,000 km/s)/(75 km/s/Mpc) = 400 Mpc
Cause of Hubble's Law
Distances
between faraway
galaxies change
while light
travels
distance?
Astronomers
think in terms of
lookback time
rather than
distance
Copernican Principle, Expanded
• VERY IMPORTANT
POINT: The expansion
of the Universe shown by
the Hubble Law should
be independent of
location in the Universe.
EVERYONE WOULD
SEE AN EQUIVALENT
EXPANSION AWAY
FROM THEM.
• In other words, we do not
believe we are at a
“special” place in the
universe.