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Lecture 2: The Milky Way Galaxy 1 Star Counts and Structure of the Galaxy ● current astronomical events ● stellar distances ● star counts ● structure of the Galaxy
Lecture 2: The Milky Way Galaxy 1Star Counts and Structure of the Galaxy
● current astronomical events● stellar distances● star counts● structure of the Galaxy
Current Astronomical Events2005 January 11th
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 4th.
'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.
Stellar Distances
(0) radar
Stellar Distances
sun
*E1
E2
d
π
90°
(1) (trigonometric or direct) “parallax”
r/d = tan ?π ~ π (rad), for d » rr
π (arcsec) = 206,265 π (rad)
d = 206,265 a.u. / π (“) = pc / π (“)
(pc ≡ 3.26 lyr)
(2) “spectroscopic parallax method”
m1 – m2 = -2.5 log (f1 / f2)
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
Stellar Distances
(3) main sequence fitting
Stellar Distances
(4) variable stars, esp. Cepheids & RR Lyrae
Stellar Distances
(5) supernovae● standard candles (esp. SN Ia's)● expanding photospheres method (“EPM”)
SN 1987A in theMagellanic Clouds
Star Counts
Sir William Herschel (1738-1822)
● discovered Uranus (1781)● with son, John, created “The General Catalog of Nebulae”; which becomes the New General Catalog (“NGC”)● believed nebulae = island universes● father of infrared astronomy● 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)
● makes Herschel's model more quantitative● “Kapetyn Universe”● basically heliocentric, with flattened, oblong distribution of stars
Harlow Shapley (1885-1972)
● using RR Lyrae variable stars measured distances to 93 globular clusters● globular clusters predominantly in the direction of Sagittarius, with centroid of distribution estimated to be ~15 kpc distant (assuming no dust)
Number Counts
● not very happy with treatment in the text, especially the games of changing between log and ln● dust will make things look fainter; if you ignore the possibility of dust, you will assume things are farther than they actually are● note difference between “differential star counts” (# stars with apparent mag m to m+dm) vs. “integrated star counts” (# stars brighter than m)● 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. disk2. bulge3. halo
Structure of the Galaxy:the Galactic Center
R0 = “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
● region of low extinction near the Galactic center
Structure of the Galaxy:the thin disk(s) + the thick disk
● young and old stars (“young thin disk” and “old thin disk” in the text)● exponential profile in height (z) and Galactocentric radius (R):
n(z,R) = n0 (e-z/z(thin)+ 0.2e-z/z(thick)) e-R/h(R)
n0 ~ 0.02 stars / pc³ (for 4.5 < MV < 9.5)
z(thin) ~ 0.325 kpc (“vertical scale height”) z(thick)~ 1.4 kpc h(R) ~ 3.5 kpc (“disk scale length”)
● Sun in (young) thin disk, with z=+30 pc● disks differ in stellar composition and kinematics
(kinematics to be covered next week)
Structure of the Galaxy:the thin disk(s) + the thick disk
● Population I stars: metal-rich, Z~0.02● Population II stars: metal-poor, Z~0.001
where Z = mass fraction in “metals”
● Population III stars = the first stars
simulation by T. Abel
Structure of the Galaxy:the thin disk(s) + the thick disk
● metallicity:
[Fe/H] ≡ log (NFe /NH) – log (NFe /NH)sun
● metals take time to form in Universe, created by SNe● younger/newer stars have larger value of [Fe/H] & Z● range is - 4 ≤ [Fe/H] ≤ +1
thin disk: - 0.5 < [Fe/H] < +0.3thick disk: - 0.6 < [Fe/H] < - 0.4
● Fe predominantly from Type Ia SNe, which take 109 yr● similar [O/H], more sensitive to Type II SNe (107 yr)
Structure of the Galaxy:the thin disk(s) + the thick disk
● thin disk: M ~ 6 x 1010
M(sun)
LB~ 1.8 x 1010
L(sun)
mass-to-light ratio:
M/LB ~ 3 (M/L)sun
● recall from last semester:
L/Lsun = (M/Msun)α
where α ~ 4 for M > 0.5 M(sun)α ~ 2.3 for M < 0.5 M(sun)
● can solve for <M(thin)> ~ 0.7 M(sun)
Structure of the Galaxy:the thin disk(s) + the thick disk
● thick disk: M ~ 3 x 109 M(sun)
LB ~ 2 x 108 L(sun)
mass-to-light ratio:
M/LB ~ 15 (M/L)sun
● lower mass stars, on average, than thin disk
● 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● M(HI)~4 x 109 M(sun)
Structure of the Galaxy:the thin disk(s) + the thick disk
NGC 2997
● neutral hydrogen, OB stars, HII regions, open clusters – all form spiral structure● older stars more evenly distributed in the disk(s) ● Sun close to “Orion-Cygnus arm”
Structure of the Galaxy:high-velocity clouds (HVCs)
seen at 21cm; velocities of 400 km/s or moresome high metallicity, some low metallicity
Structure of the Galaxy:Magellanic Stream
● HI emission● 180° across● tidal debris tail
Structure of the Galaxy:coronal gas
observed in absorption of highly-ionized species, such as far-UV OVI transition (by FUSE and Hubble's STIS)
Structure of the Galaxy:Galactic bulge
the Milky Way Galaxy, as viewed by COBE(1.2 to 3.4 µm)
Structure of the Galaxy:Galactic bulge
● ratio of minor to major axis ~0.6● (text says vertical scale height ~0.4 kpc)
● de Vaucouleurs profile (1948):
I(r) = I(re) exp{-7.67[(r/re)1/4
– 1]}
I(r) = surface brightness [Lsun/pc²]re = “effective radius”
Structure of the Galaxy:Galactic bulge
● large range in metallicity: -1 < [Fe/H] < +1● mean is near +0.3 (e.g., twice solar)● implies youth● mass-to-light ratio similar to thin disk
● some old stars too
Structure of the Galaxy:stellar bar
NGC 1365
● initially thought of as “3 kpc expanding arm”● preferred model today is that MWG is a barred spiral galaxy
Structure of the Galaxy:Galactic halo
● high-velocity stars, with large spatial extent (~50 kpc)● metal-poor, [Fe/H] < -0.8● old● low-metallicity globular clusters, as seen by Shapley, represent ~1% of the halo● (metal-rich globulars in thick disk)● M~1x109 M(sun)● n(r) ~ r -3.5
Structure of the Galaxy:magnetic fields
● Zeeman splitting of atomic lines● polarization of light ● 0.4 to 10,000 µG, weakest in halo, strongest at Galactic center● weak compared to terrestrial magnetic fields, but an important part of Galactic energetics
Structure of the Galaxy:cosmic rays
● bombarding us constantly● atmosphere does an OK job at stopping, especially at lower altititudes ● thought to be due to SNe in the Milky Way galaxy● large, separate branch of (astro)physics
Structure of the Galaxy:dark matter
● 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
● 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
“MACHOProject”(and her cousins)
12 million stars surveyedseveral 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 theMagellanic 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
● 17 gravitational lensing events identified● lensing sources are ~0.6 M(sun) ● contribute 20% of the halo mass
Structure of the Galaxy:dark matter
● need distances to fully understand the astrophysics● Spitzer image shows faint, red lensing source● colors give spectral type● distance of lensing source using “spectroscopic parallax”● M5 dwarf at 600 pc● only solved MACHO event
Nguyen, Kallivayalil, Werner,Alcock, Patten, & DS 2004,ApJS, 154, 266
THE END
scenes for next class:● kinematics of the Galaxy● the Galactic center
