Transcript The Milky Way and Its Neighbors

Galaxy Morphology
The Tuning Fork that Blossomed into a Lemon
Lance Simms
MASS Talk
9/8/08
Hubble’s Tuning Fork
 Tuning Fork Diagram used by Hubble from 1925-1935
 Irregular class was later added to right hand side
 Hubble originally thought evolution was from left to right
Irregulars would fall
over here
Lenticulars
S0 galaxies with large central bulge
No spiral arms, gas, or dust
Flattened disc of stars
Ellipticals – En
n=10(1-b/a)
b: semi-minor axis
a: semi-major axis
Bulge/Disc Ratio
Loose Arms
Gas and Dust
Lemon Classification of Vaucouleurs
A=‘Normal’
B=‘Barred’
Image: Mod. Phys Rev, G. De Vucouleurs, Large-Scale Structure and Direction of Rotation in Galaxies
Rotational Velocity Curves
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Differential rotation can be observed through
spectra
Useful for Spiral Galaxies that are viewed edge-on
Difficult to use for Ellipticals
Overall shift in spectral lines gives velocity and with
Hubble Law, approximate distance away
Away from us
Towards us
Note: Galaxy should be edge-on
Image for illustrative purposes
N II-658.53 nm (in rest frame)
Hα-656.28 nm (in rest frame)
Velocity Dispersions
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Profile width gives velocity dispersion σ
– Spectral fitting methods vary
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Mass is obtained via the virial theorem
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Very useful for elliptical galaxies
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Virial Theorem
2K U  0
K – Kinetic Energy
U – Potential Energy
Rv 2
M
G
α – Constant that depends on
distribution of mass within galaxy
Increasing dispersion
Irregular Galaxies
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Small percentage of known galaxies are irregulars (~3%)
Galaxies that do not show spiral or elliptical structure
• No nuclear bulge
• No spiral arms
Divided into two main types
• Irr-I : some structure
• Irr-II : chaotic mess
Some are Starburst Galaxies
• Very high rate of star formation
IC 1613 – Cetus
Mass range:
Size range:
Magnitudes:
Composition:
Color:
108 −1010 solar masses
1 − >10 kiloparsecs
−13 to −20 in B bandpass
Varied
Young Stars
HII regions
Varied, toward blue
IC 10 - Cassiopea
Spiral Galaxies
 We think about 66% of galaxies are spirals
 Most have active Star Formation (SF) occurring
in spiral arms
 Appearance depends on angle relative to our
line of sight
 Consist of 4 Distinct Components
4 MAIN COMPONENTS OF
SPIRAL
1) Flattened, rotating disc of stars and
1
4
2
3
gas
− Arms are in plane of disc
2) Central bulge with mainly old
stars
− Brightest component of
galaxy
3) Nearly spherical halo of stars
− Globular Clusters
− Dark Matter
4) Supermassive black hole at
Spiral Galaxies: A Slice of the Lemon
A – Normal spiral
-- no bar
r – internal ring around
nucleus
-- spiral arms begin
on ring
s – no internal ring
-- spiral arms begin
directly at nucleus
B – Barred Spiral
Spiral Galaxies
Mass range:
Size range:
Magnitudes:
Composition:
109 −1012 solar masses
5 − >100 kiloparsecs
−16 to −23 in B bandpass
Young and Old Stars
 Active Star Formation (SF) occurring
in spiral arms is very bright in UV
 Young stars emit towards UV
 Several types shown below
Spiral Galaxies
Mapping the Milky Way
Our Spiral – The Milky Way
 In past, mostly done with 2 methods:
1)Mapping HI regions with radio observations
- 21 cm line measurements
2)Mapping HII regions via Hα emission lines
- HII regions trace active star formation
 Old data showed that there were 4 arms
 New data from Spitzer indicates that there
are only 2 major spiral arms:
-Scutum and Perseus Arms
10,000 ly
Our Sun
Elliptical Galaxies
 Ellipticals appear to have very little gas or dust
 Approximately 10% of known galaxies are elliptical
 Stars orbit the galaxy center in all different planes
 Circular orbital velocity measurements do not work very well
 Sometimes a preferred direction of very slow rotation
 Luminosity decreases quickly from center so measurements are always made within 10 kpc.
 Detailed kinematic observations ( σ(r) and Vsys(r) ) only exist for some 10s of galaxies
 Usually limited to σo and Vsys at center
Before 1977
Theorists thought they understood ellipticals well in 1970s
= axially symmetric isothermal ensembles
= increasingly flattened the more rapidly they rotate
about center
M32
After 1977
Observations proved them wrong
= Spectroscopic data (stellar absorption lines) showed
that ellipticals do not rotate globally
= Not isothermal
= Velocity dispersion is anisotropic
= Now strong evidence that they are triaxial ellipsoids
http://www.astr.ua.edu/
Elliptical Galaxies
Mass range:
Size range:
Smallest:
Composition:
Color:
107 −1013 solar masses
0.1 − >100 kiloparsecs
Dwarf Ellipticals
Mostly old, red stars
Towards the red end
Luminosity Profiles:
Hubble’s Law (1930)
I /Io  [(r /a)  1]2
I is intensity emitted per unit area at r from center
a is core radius; Io is intensity per unit area at center
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De Vaucouleurs’s Law (1948)
log(I /Ie )  3.33[(r /re )1/ 4 1]
M87 –Largest Galaxy in Virgo Cluster
re is radius containing half of total luminosity
Ie is intensity at a distance re from center
Dwarf Spheroidal Galaxies
• Low luminosity galaxies
• More spherical than elliptical
• Companions to Milky Way or
other galaxies such as M31
• Little or no gas or dust
• No recent star formation
• Approximately spheroidal in shape
NGC 147 – Dwarf Spheroidal in Local Group
Spheroids:
A spheroid is basically an ellipsoid with to of its axes equal
Saturn is an oblate spheroid, flattened near equator
Equation in 3-d:
Oblate Spheroid
Globular Clusters
 Large, gravitationally bound groups of stars
 10,000 – 1,000,000 stars
 Not galaxies; considered a part of our galaxy
 Orbit center of our galaxy in elliptical orbits
 Some orbits are highly extended
 Some contain “Tidal Tails”
 Highly concentrated in Galactic Longitude
(337°)
NGC 5466
Tidal Tails
When globulars pass by
bulge of Milky Way, gravity is
strong enough to rip stars away
Trail of stars left behind is
called a Tidal Tail
Dwarf Spheroidal or Globular Cluster?
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Distinction between globulars (GCs)
and Dwarf Spheroidal Galaxies
(dSphs) is ambiguous
– Globular clusters are generally more
compact, but some dwarf galaxies
are also
– Small galaxies have about same mass
as globulars
– Galaxies are more “isolated”, but
there are intergalactic ‘tramp’
globulars
– Color Magnitude Diagrams (CMD)
look similar
• As of 2003, there were
– ~150 GCs
– ~9 dSphs
• Now, there are ~20 dSphs
Globulars and DSphs
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There is significant overlap in
i)
Mass
iii) Luminosity
iii) Size
ii)
Mass-to-light ratio
iv) Spread in Metallicity
• Apparently, ellipticity may be a distinguishing factor
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only 20 galaxies in plot, 1.4 data points per plot point
Taken from van den Bergh
Dwarf Spheroidal or Globular?
• Carina Low Surface
Brightness (LSB) dSph
Dwarf Spheroidal or Globular?
NGC 288 Globular Cluster