Spiral Galaxies
(Jason Ware, 1999)
Outline
1. Introduction
2. Basic Structure
3. Composition
4. Morphology
5. How It’s Made
6. Evolution
7. Age/Location
8. Size/Distance
9. Brightness
10.How they are studied
11. Research
(willard L, NGC 891 Galaxy)
Introduction
Galaxy
A collection of luminous stars, non-luminous
dark matter, in some cases gas and dust, that are
separated from other similar structures by
distances of 10 kps or more
Spiral Galaxy
Any member of the Hubble class of galaxies
characterized by having a disk and spiral arms
(Jones and Lambourne 2003)
Basic Structure:
•The central bulge
• The disk
• Stellar halo
(Jones and Lambourne 2003)
Composition
Overall composition
• Mass of gas to mass of stars ratio is about 5%-15%
• Stars thin out rapidly from the bulge
• interstellar medium
• mostly HI – neutral atomic hydrogen
• HII – ionized molecular hydrogen
• interstellar gas is 5% of the total mass
• Ratio of strength of hydrogen emission lines to others
decreases towards the center
(bertin and Lin, 1996)
The Bulge
• Very dense
population of old red
stars (Pop II)
•takes on the
characteristics of an
elliptical galaxy
• little to no star
formation
(NASA, M81 Spiral)
• Very little traces of gas
• low metal content
• postulated to have black holes at the center
(Inside Galaxies, 1972)
The Disk
• dominated by
young stars and
gas (pop I)
• star types O-B
• Very blue
• Contains both HI
and HII gas, dust
(NASA, NGC 2841)
• HII region – about 1/3 distance from center to
edge
• Neutral hydrogen (HI) peaks farther from the
center
• Star formation happens in the spiral arms
(Inside Galaxies, 1972)
Stellar Halo
• no star formation
• contains very old
Population II stars
• Spherical in shape
• no dust
(Andrew Cooper, stellar halo around the Milky Way)
• poor metal content
• stars form globular clusters and are more sparsely
separated than in the bulge
• contains dark matter that dominates the mass of the
galaxy
(Jones and Lambourne 2003)
Morphology
Different types of Galaxies
• ellipticals
• spirals
• irregulars
How to classify a spiral galaxy
• size of the bulge relative to the disk
• the tightness of the winding of the
spiral arms
(elmegreen, 1998)
(Jones and Lambourne 2003)
Sa and SBa – large bulge with tightly
wound spiral arms
(Jones and Lambourne 2003)
Sb and SBb – moderately small bulge
and looser wound arms
(Jones and Lambourne 2003)
Sc and SBc – small bulge and very loose,
fragmented arms
(Jones and Lambourne 2003)
How it’s made...
• Big bang
• 1 million years to cool down and expand
• matter clumps together (H and He)
• protogalaxies form
• type of galaxy is determined by the initial
conditions of the protogalaxy
• protogalaxy contracts under gravity
• critical limits – condensation and rotation
• Spirals form when the contraction is
stopped by the rotational radius limit
(NASA,
Big Bang)
(hodge,
1986)
Evolution
•Once a galaxy has taken
shape, action is pretty much
done
• stars form, they die, and
eject heavier materials
• gradually dims and reddens
•Far off spiral galaxies – looking at the
past
• younger, bluer, more active, explosive
(hodge, 1986)
Age
• Oldest stars in the galaxies tend to be about
13 billion years old
• typical age ~ 12 billion years old
• Milky Way ~ 13.6 billion yrs old (+- 800
million) (Pasquini, 2004)
• 1 of the first galaxies to be created
Location
• Galaxies tend to group together into clusters or
superclusters
• Local group – 30 or more galaxies
(Jones and Lambourne 2003)
Size/Mass/Distance
•10 – 10 solar masses
9
12
• typical diameter - 0.02-1.5 dmw
•the thickness of the disk is 1/10 the thickness
of the bulge
(Jones and Lambourne 2003)
Galaxy
Andromeda
Triangulum
Bode’s
Sculptor
Pinwheel
Whirlpool
Apparent size
(SIMBAD)
190′ × 60′
70.8’ × 41.7’
26.9’ × 14.1’
27′.5 × 6’.8
28′.8 × 26′.9
11′.2 × 6′.9
Distance
(SIMBAD)
778 ± 17kpc
730 to 940kpc
3.6 ± 0.12Mpc
3.5 ± 0.2Mpc
6.4 ± 0.5 Mpc
7.1 ± 1.2Mpc
Brightness
Absolute Magnitude
• 109 – 1011 solar luminosities
• absolute blue brightness is -16 to -23
Galaxy
Andromeda
Triangulum
Bode’s
Sculptor
Pinwheel
Whirlpool
Apparent Magnitude
(SIMBAD)
3.44
5.72
6.94
8.04
8.31
8.36
(Jones and Lambourne, 2003)
How to study
Isophotes – closed curves connecting points of equal apparent
surface brightness
Can be used to detect shape and luminosity of the galaxy
Stellar Spectroscopy - by looking at the thermal spectrum of
the galaxy, can determine what a galaxy is made of and it’s
temperature
Rotation curves – rotational speed vs radial distance to
determine the mass
Computer modelling – evolution and kinematics
(Jones and Lambourne 2003)
Research
Astronomy magazine – Astrophysicists
reports first simulation to create a
milky way-like galaxy
• results support the “cold dark matter”
theory
• The evolution of structure in the
universe is driven by the
gravitational interactions of dark
matter
• previously, simulations had bulges too
big compared to disk
• After the big bang, gravity pulled dark
matter together which grew into
clumps that made “gravitational wells”
to capture the matter that form galaxies
(University of California, Simulation)
Astronomy Magazine - Milky Way's spiral
arms are the product of an intergalactic
collision course
• Dwarf galaxy Sagittarius loaded
with dark matter collides with
the Milky Way
• force of impact sends stars
streaming in long loops
• stars tugged outward and
become ringed arms
• spiral arms emerged 2 billion
years ago due to this collision
• another collision due in 10
million years
(University of California, Spiral Arms)
References
Aller, L. 1972. ‘Galactic Explorations’. In: Harlow Shapley,
Cecilia Payne-Gaposchkin. Galaxies. 3rd ed. Cambridge:
Harvard University Press.
Aller, L. 1972. ‘Inside Galaxies’. In: Harlow Shapley, Cecilia
Payne-Gaposchkin. Galaxies. 3rd ed. Cambridge: Harvard
University
Bertin, G and C. C. Lin. 1996. Spiral Structure in Galaxies.
Massachusetts: MIT Press.
Elmegreen, D (1998). Galaxies and Galactic Structure. New
Jersey: Prentice Hall.
Hodge, P. (1986). Galaxies. London: Harvard University Press.
Jones, Mark H. Lambourne, R. (2003). An Introduction to
Galaxies and Cosmology. Cambridge: Cambridge
University Press.
Cooper, A. P., Cole, S., Frenk. (2010), Galactic stellar haloes in the CDM
model. Monthly Notices of the Royal Astronomical Society, 406: 744–766.
doi: 10.1111/j.13652966.2010.16740.x. Viewed at: Oct. 4, 2011
<http://www.physorg.com/news197044028.html>
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<www.daviddarling.info/encyclopedia/B/Big_Bang.html>
NASA. Fuzzy Wuzzy Galaxy. Viewed at: Oct 4, 2011
<ww.news.nationalgeographic.com/news/2011/02/pictures/NGC-2841>.
Last Accessed: 4th Oct 2011.
Pasquini, L, Bonifacio, P, Randich, S, Galli, D, Gratton, RG. 2004. ‘Beryllium in
turnoff stars of NGC6397:early Galaxy spallation, cosmochronology and
cluster formation’. Astron.Astrophys. 426 (2004) 651-657. viewed Oct 4,
2011. <arXiv:astr-ph/0407524v1>
SIMBAD. M-33, M-31, M-81, M-51a, Caldwell-65, M-101. Viewed at: Oct 4,
2011. <www.simbad.u-strasbg.fr/simbad/>
University of California. (2011). Astrophysicists report first simulation to
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<http://www.astronomy.com/en/NewsObserving/News/2011/08/Astrophysicists%20report%20first%20simulati
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University of California. (2011). Milky Way's spiral arms are the product
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<http://www.astronomy.com/NewsObserving/News/2011/09/Milky%20Ways%20spiral%20arms%20ar
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Ware, J. (1999). M31: The Andromeda Galaxy. Last accessed: Oct 4,
2011 <www.apod.nasa.gov/apod/ap991114.html>. Last accessed
4th Oct 2011.
Willard L. NGC 891 Galaxy. Last accessed 4th Oct 2011
.<www.unews.utah.edu/news_releases/u-celebratestelescope039s-039first-light039/>.