Lecture 2: The Milky Way Galaxy 1 Star Counts and Structure of the ...

Lecture 2: The Milky Way Galaxy 1
Star Counts and Structure of the Galaxy
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current astronomical events
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stellar distances
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star counts
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structure of the Galaxy

Current Astronomical Events
2005 January 11 th
conditions on Mauna Kea for the past two nights

NASA's Deep Impact (P.I.
Michael A'Hearn; Univ. of MD)
launched at ~11 AM today,
from Cape Canaveral, Florida.
Scheduled to blast a 317 pound
copper probe into Comet
Tempel 1 at 23,000 m.p.h. for
collision on 2005 July 4 th .

'mysterious blobs' = ULIRGs

common proper motion
detected by Hubble
Space Telescope

Spitzer shows Vega to be dustier than expected;
dust perhaps from Pluto-sized objects colliding

Huygens probe scheduled to reach Titan (Saturn's largest moon)
on Friday; was released on Christmas Day.

(0) radar
Stellar Distances

Stellar Distances
(1) (trigonometric or direct) “parallax”
E 1
π
*
r
sun
90°
E 2
d
r/d = tan ?π
~ π (rad), for d » r
π (arcsec) = 206,265 π (rad)
d = 206,265 a.u. / π (“)
= pc / π (“)
(pc ≡ 3.26 lyr)

(2) “spectroscopic parallax method”
m 1 – m 2 = -2.5 log (f 1 / f 2 )
m = “apparent magnitude”
M = “absolute magnitude” (m at 10 pc)
since f ~ 1/(distance)²,
m – M = 5 log (d / 10 pc)
= 5 log d – 5
m – M = “distance modulus”
5 log d = (m – M) + 5

Hertzsprung-Russell Diagram
color

(3) main sequence fitting
Stellar Distances

Stellar Distances
(4) variable stars, esp. Cepheids & RR Lyrae

Stellar Distances
(5) supernovae
●
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standard candles (esp. SN Ia's)
expanding photospheres method (“EPM”)
SN 1987A in the
Magellanic Clouds

Star Counts
Sir William Herschel (1738-1822)
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discovered Uranus (1781)
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with son, John, created “The General
Catalog of Nebulae”; which becomes
the New General Catalog (“NGC”)
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believed nebulae = island universes
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father of infrared astronomy
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measured disk-light nature of
distribution of stars in the Milky Way

Thomas Wright (1711-1786): “A New Theory of the Universe”

assumptions:
(1) all stars have same absolute magnitude
(2) number density of stars is ~constant
(3) no dust/obscuring material
(4) can see to edge of the stellar distribution

Jacobus Kapetyn (1851-1922)
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makes Herschel's model
more quantitative
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“Kapetyn Universe”
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basically heliocentric, with
flattened, oblong
distribution of stars

Harlow Shapley (1885-1972)
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using RR Lyrae variable stars
measured distances to 93
globular clusters
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globular clusters predominantly
in the direction of Sagittarius,
with centroid of distribution
estimated to be ~15 kpc distant
(assuming no dust)

Number Counts
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not very happy with treatment in the text, especially the
games of changing between log and ln
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dust will make things look fainter; if you ignore the
possibility of dust, you will assume things are farther than
they actually are
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note difference between “differential star counts” (# stars
with apparent mag m to m+dm) vs. “integrated star
counts” (# stars brighter than m)
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Olbers' paradox – duh! If density is constant, then
integrate star counts goes as volume ~ d³, which is
divergent

Structure of the Galaxy
NGC 891

1. disk
2. bulge
3. halo

Structure of the Galaxy:
the Galactic Center
R 0 = “solar Galactocentric distance”
= 8.5 kpc
(will discuss next week both where this number
comes from and the interesting phenomena
associated with the Galactic center)

Structure of the Galaxy:
Baade's window
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region of low extinction
near the Galactic center

Structure of the Galaxy:
the thin disk(s) + the thick disk
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young and old stars (“young thin disk” and “old thin
disk” in the text)
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exponential profile in height (z) and Galactocentric
radius (R):
n(z,R) = n 0 (e -z/z(thin) + 0.2e -z/z(thick) ) e -R/h(R)
n 0
~ 0.02 stars / pc³ (for 4.5 < M V
< 9.5)
z(thin) ~ 0.325 kpc (“vertical scale height”)
z(thick)~ 1.4 kpc
h(R) ~ 3.5 kpc (“disk scale length”)
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Sun in (young) thin disk, with z=+30 pc
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disks differ in stellar composition and kinematics
(kinematics to be covered next week)

Structure of the Galaxy:
the thin disk(s) + the thick disk
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Population I stars: metal-rich, Z~0.02
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Population II stars: metal-poor, Z~0.001
where Z = mass fraction in “metals”
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Population III stars =
the first stars
simulation
by T. Abel

Structure of the Galaxy:
the thin disk(s) + the thick disk
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metallicity:
[Fe/H] ≡ log (N Fe /N H ) – log (N Fe /N H ) sun
● metals take time to form in Universe, created by SNe
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younger/newer stars have larger value of [Fe/H] & Z
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range is - 4 ≤ [Fe/H] ≤ +1
thin disk: - 0.5 < [Fe/H] < +0.3
thick disk: - 0.6 < [Fe/H] < - 0.4
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Fe predominantly from Type Ia SNe, which take 109 yr
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similar [O/H], more sensitive to Type II SNe (107 yr)

Structure of the Galaxy:
the thin disk(s) + the thick disk
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thin disk: M ~ 6 x 10 10 M(sun)
L B ~ 1.8 x 10 10 L(sun)
mass-to-light ratio:
M/L B ~ 3 (M/L) sun
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recall from last semester:
L/L sun = (M/M sun ) α
where α ~ 4 for M > 0.5 M(sun)
α ~ 2.3 for M < 0.5 M(sun)
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can solve for ~ 0.7 M(sun)

Structure of the Galaxy:
the thin disk(s) + the thick disk
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thick disk: M ~ 3 x 10 9 M(sun)
L B ~ 2 x 10 8 L(sun)
mass-to-light ratio:
M/L B ~ 15 (M/L) sun
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lower mass stars, on average, than thin disk
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neutral hydrogen (HI), seen at 21 cm, confined to a
very thin disk, with z(HI) ~ 90 pc near the Sun, but
puffing up at large Galactocentric radius: warp
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M(HI)~4 x 109 M(sun)

Structure of the Galaxy:
the thin disk(s) + the thick disk
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neutral hydrogen, OB stars, HII
regions, open clusters – all form
spiral structure
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older stars more evenly distributed
in the disk(s)
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Sun close to “Orion-Cygnus arm”
NGC 2997

Structure of the Galaxy:
high-velocity clouds (HVCs)
seen at 21cm; velocities of 400 km/s or more
some high metallicity, some low metallicity

Structure of the Galaxy:
Magellanic Stream
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HI emission
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180° across
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tidal debris tail

observed in
absorption of highlyionized
species, such
as far-UV OVI
transition (by FUSE
and Hubble's STIS)
Structure of the Galaxy:
coronal gas

Structure of the Galaxy:
Galactic bulge
the Milky Way Galaxy, as viewed by COBE
(1.2 to 3.4 µ m)

Structure of the Galaxy:
Galactic bulge
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ratio of minor to major axis ~0.6
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(text says vertical scale height ~0.4 kpc)
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de Vaucouleurs profile (1948):
I(r) = I(r e ) exp{-7.67[(r/r e ) 1/4 – 1]}
I(r) = surface brightness [L sun /pc²]
r e = “effective radius”

Structure of the Galaxy:
Galactic bulge
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large range in metallicity: -1 < [Fe/H] < +1
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mean is near +0.3 (e.g., twice solar)
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implies youth
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mass-to-light ratio similar to thin disk
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some old stars too

Structure of the Galaxy:
stellar bar
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initially thought
of as “3 kpc
expanding arm”
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preferred model
today is that
MWG is a
barred spiral
galaxy
NGC 1365

Structure of the Galaxy:
Galactic halo
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high-velocity stars, with large spatial extent (~50 kpc)
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metal-poor, [Fe/H] < -0.8
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old
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low-metallicity globular clusters, as seen by Shapley,
represent ~1% of the halo
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(metal-rich globulars in thick disk)
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M~1x109 M(sun)
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n(r) ~ r -3.5

Structure of the Galaxy:
magnetic fields
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Zeeman splitting of atomic lines
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polarization of light
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0.4 to 10,000 µG, weakest in halo, strongest at
Galactic center
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weak compared to terrestrial magnetic fields, but an
important part of Galactic energetics

Structure of the Galaxy:
cosmic rays
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bombarding us constantly
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atmosphere does an OK job at stopping, especially at
lower altititudes
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thought to be due to SNe in the Milky Way galaxy
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large, separate branch of (astro)physics

Structure of the Galaxy:
dark matter
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kinematics (next week) show us that mass density
ρ(r) ~ 1/(a² + r²)
where a = 2.8 kpc
and only 30% of Galaxy luminous (HI, stars), with
dark matter dominating at larger radii

Structure of the Galaxy:
dark matter
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kinematics (next week) show us that mass density
ρ(r) ~ 1/(a² + r²)
where a = 2.8 kpc
and only 30% of Galaxy luminous (HI, stars), with
dark matter dominating at larger radii

Structure of the Galaxy:
dark matter
Candidates for dark matter:
● MACHOs (massive compact halo
objects)
● brown dwarfs (e.g., low-mass stars)
● white dwarfs (e.g., burnt-out stars)
● neutron stars (e.g., more dead stars)
● Stellar black holes (e.g., yet more
dead stars)
● mini (primordial) black holes
● massive (primordial) black holes
● WIMPs (weakly-interacting massive
particles; e.g., neutrinos, axions, etc...)

Structure of the Galaxy:
dark matter
“MACHO
Project”
(and her
cousins)

12 million stars surveyed
several years – 17 events

Structure of the Galaxy:
dark matter

Structure of the Galaxy:
dark matter
identify quasars

Structure of the Galaxy:
dark matter
quasars behind the
Magellanic Clouds
Geha et al. (2003)

Structure of the Galaxy:
dark matter
huge numbers
of variable
stars (e.g.,
eclipsing
binaries, RR
Lyrae, etc...)

Structure of the Galaxy:
dark matter
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17 gravitational
lensing events
identified
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lensing sources
are ~0.6 M(sun)
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contribute 20%
of the halo mass

Structure of the Galaxy:
dark matter
Nguyen, Kallivayalil, Werner,
Alcock, Patten, & DS 2004,
ApJS, 154, 266
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need distances to fully
understand the astrophysics
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Spitzer image shows faint, red
lensing source
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colors give spectral type
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distance of lensing source
using “spectroscopic parallax”
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M5 dwarf at 600 pc
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only solved MACHO event

THE END
scenes for next class:
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kinematics of the Galaxy
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the Galactic center