Lecture 27: Ingredients for a
Galaxy
Readings: Sections 25-2, 25-3
and 25-6
Key Ideas: The Parts of a
Galaxy
Stars
Gas
Ionized –
optical/UV emission lines
Neutral – 21
cm emission for H
Molecular –
radio and mm emission
Dust
Central Supermassive Black Holes
Dark Matter
What are ÒGalaxiesÓ?
Large assemblies of stars,
gas, dust and dark matter, held together by gravity
Sizes:
Largest ~ 1 Trillion
stars (or more)
Smallest ~ 10
Million stars
Milky Way &
Andromeda ~ 200 Billion stars
By way of comparison, Nabisco
has baked ~ 400 billion Oreos since 1913É.
Stars
Arranged in different shapes:
Spiral, Elliptical, & Irregular
Hubble Classification 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%)
Basic classification system
was developed by Edwin Hubble in the 1930s, and refined in later decades.
Detecting Gas
Gas can be detected by its
emission lines
Hot gas has emission
lines in the visible
Cool gas has
emission lines in the millimeter and radio
Different atoms will emit
different wavelengths/frequencies of light
Neutral hydrogen at
21-cm
Molecules are found in
coolest gas and they also have distinctive spectra.
KirchoffÕs Laws/Atomic
Fingerprint Reminders
Hot Gas
Hot gas (10,000K) radiates in
the visible. We see ionized H as a red color.
Cool Gas
In cool regions, the hydrogen
is neutral.
Electrons do not jump between
orbits.
Change of spin changes the energy.
Photon with a wavelength of 21-cm produced.
Neutral Hydrogen Gas in the
Galaxy
Radio waves are not affected
by dust.
We can view 21-cm emission
from neutral hydrogen from across the Galaxy.
Distance to H0
clouds a problem (see Section 25-3 and Figure 25-11)
Spiral arms clearly visible
We see neutral hydrogen gas
in other galaxies as well. Example: M 83
Cold Gas
The coldest gas is in the
form of molecules.
H2 is difficult to
detect
Other molecules, such as CO
and NH3, detected more easily.
Changing vibration or
rotation of the molecules=emission/absorption lines.
Not much energy is
needed=millimeter/radio wavelengths
Molecules can be
distinguished by their spectra.
Molecules in Space
There are lots of interesting
molecules out there, from the simple to the complex.
Examples:
CO: carbon monoxide
NH3: ammonia
C2H4O2:
glycine
CH3CH2OH:
ethanol
c-C2H4O:
ethylene oxide
CH2CHOH: vinyl
alcohol
HC11N
The Pillars of Creation
Hot & cold gas are often
close to each other.
Stars born out of cold gas
and start shining.
Photons ionize and heat some
of the surrounding gas.
Example: Eagle Nebula from
Hubble
Dust
Small particles (like smoke
or soot)
Found mixed with gas
Dust has MUCH less effect at
infrared wavelengths
Hearts of Darkness
Deep in the centers of the
Milky Way and Andromeda are supermassive black holes.
Masses > 1
Million MSun!
Found by the effects of their
gravity on the innermost stars:
Stars
orbiting much faster than expected from the number of stars present.
Evidence of excess
X-ray and radio emission
Use
the orbital speeds and sizes to estimate the mass of the central dark object.
Presence of Black Hole in the
Center of the Milky Way detected by motions of stars.
See the movie at
boojum.as.arizona.edu/~jill/EPO/Movies/MilkyWayBlackHole.swf
Supermassive Black Holes
Such black holes are
extremely large:
Stellar-mass black
holes are expected to be at most a few x 10 MSun
Questions
What are such large
black holes doing at the centers of our Galaxy?
How could such large
black holes form?
Do other galaxies
harbor similarly large black holes in their centers?
Detecting Black Holes in
Other Galaxies
In the Milky Way, we detected
the black hole by the motions of stars near the Galactic Center.
In the nearest galaxies, we
can measure the motions of individual stars as well, but for the vast majority
of galaxies we cannot distinguish individual stars.
We have to rely on the integrated
light.
Integrated Light
For most galaxies outside the
Milky way, we need to study their integrated properties.
The light from many stars is
blended together, so that the color and spectrum that we observe are average
properties.
More luminous stars
contribute more light to the color/spectrum. A few O&B stars can emit more
light than thousands of M dwarfs.
Black HoleÕs Effect on
Stellar Motions
Stars in the centers of
galaxies move faster when thereÕs a black hole. We measure this as a velocity
dispersion. This broadens the lines
in the integrated spectrum as some starsÕ lines are Doppler-shifted to the blue
and some to the red.
Many black hole masses in the
centers of galaxies have been measured. Almost all galaxies have them in their
centers.
Black Holes—Bulge
Relation
The larger the bulge, the
larger the black hole
Why? Related to how both
bulges and supermassive black holes form?