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Today in Astronomy 142: the Milky Way

The shape of the Galaxy Stellar populations and motions

! Scale height, velocities and the mass per area of the disk

! Rotation curve, spiral arms

Below, the 13CO(1-0) 5kpc molecular ring survey: Boston University-FCRAO Milky Way Galactic Ring Survey (GRS).

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Shapley and the location of the Galactic center

In 1917, Harlow Shapley applied Henrietta Leavitt ‘s pulsating-star distance-measurement discovery to the distribution of globular clusters. He reasoned that these clusters were so massive that they would accurately trace the Galaxy’s global gravitational potential. He found that these clusters were arranged spherically symmetrically about a point in the plane of the Milky Way, in the constellation Sagittarius, tens of kpc away. ! Modern measurements give 8.5 kpc for the distance.

Thus the Sun is quite far from the center of the Galaxy

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The Milky Way’s central regions, in starlight (near-infrared), from the NASA COBE DIRBE experiment.

NGC 891, also in starlight (near-infrared), from the 2MASS survey (U. Mass./NASA).

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Galactic fountain

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HI observations of the LMC Kim et al. ACTA images

showing HI shells and super shells

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More edge-on galaxies

NGC 4303, credit Chris Howk and HST heritage team

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More edge-on galaxies

NGC 4217 (HST image, Chris Howk)

WIYN Consortium

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Spitzer/IRAC and HST composite of the

Sombrero Galaxy

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Schematic structure of the Milky Way

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Astronomy 142 10 movie by David Law

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What was missing from that last slide?

• The Milky Way is a barred galaxy • Molecular ring outside the bar • The Milky Way is right now destroying a nearby diffuse galaxy called Sag A dwarf • In the halo there are stellar streams which are remnants of tidally disrupted dwarf galaxies • The outer part of the galaxy is probably flocculent rather than grand design • The outer part of the galaxy is warped and ringed? • The disk has two components (abundances and star counts):

thin disk (few hundred pc) and a thick disk (1kpc), The thick disk is older.

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Motions of stars in the Galaxy, and their use in determining its mass distribution

How much does the Galaxy weigh? Stars move about in response to the gravitational potential of the rest of the stars in the galaxy. Their systematic motions (rotation) can be used simply with Newton’s Laws to measure masses. ! This works even better with interstellar gas than with

stars. Stars collide in-elastically very rarely, so their random motions can be used with thermodynamics to measure masses - the stars in this sense can be thought of as particles in a gas. Let’s do some examples of the latter.

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Mass per unit area of the Galaxy’s disk, in the Sun’s neighborhood

Recall formula for pressure in terms of particle number density, speed and momentum (lecture, 9 February):

Consider a certain class of stars to be gas particles, and consider the component of their motion perpendicular to the Galactic plane. Suppose the distribution of these stars extends above and below the plane by some scale height H/2. Consider stars lying on the ends of a cylinder of Galactic matter that extends above and below the plane.

Number of particles that hit wall Typical momentum per particle Time interval in which they hit

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Mass per unit area of the Galaxy’s disk, in the Sun’s neighborhood (continued)

Weight of the cylinder, approximately:

If stellar “gas pressure” balances gravity, then

Compare to force on test mass m from infinite plane with mass per unit area µ : Force from gravity (next slides):

H/2

dA

dW

(All on RHS observable.)

dW

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Proof of the previous assertion

r

µ mass per unit area h

dr

dF

m

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An easier way to estimate the force from gravity from a thin sheet using Gaus’s law

A

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Mass per unit area of the Galaxy’s disk, in the Sun’s neighborhood (concluded)

Putting the numbers in, for the solar neighborhood:

Star counts in the solar neighborhood enable us to estimate the luminosity per unit area L (also called the surface brightness) of the disk locally. This leads to the mass to light ratio:

Thus the solar neighborhood on average emits light less efficiently than the Sun does – consistent with there being more lower-mass stars than higher-mass stars.

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Dark matter I: the Galactic disk in the solar neighborhood

When this procedure was first applied to observations by Oort in the late 1940s, the resulting value of µ was greater than that of visible stars and interstellar gas in the Solar neighborhood by a factor of about 2. ! Missing mass, or dark matter: mass that emits no light but

can be detected by its gravity? Since then, ! Better (less biased) samples of stars have led to smaller

estimates of the total µ. ! The discovery of neutral atomic and molecular gas in the

ISM has increased the “luminous” mass a bit. ! Better counting of the numbers of brown and white dwarfs ! Now the luminous and gravitating µ nearly match.

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Relation between dynamics and stellar population

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Dark matter II

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Adding up the components

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Spiral structure

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The winding problem for transient structure

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Density wave theory for spiral structure

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Spiral density waves,

HI, CO clusters and higher

stellar density all lie along the

spiral arms

Spiral structure

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Views of Spiral structure

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Summary

The Milky Way as an edge on Galaxy Galactic fountain Spiral density waves Components of the Milky Way The Milky Way’s rotation curve How to compute the vertical force from gravity from a thin sheet