Today in Astronomy 142: the Milky Way - Astro Pas Rochester

Today in Astronomy 142: the Milky WayThe shape of the GalaxyStellar populations and motions! Scale height, velocities and themass per area of the disk! Rotation curve, spiral armsBelow, the 13CO(1-0) 5kpcmolecular ring survey:Boston University-FCRAO Milky WayGalactic Ring Survey(GRS).Astronomy 142 1

Shapley and the location of the Galactic centerIn 1917, Harlow Shapley applied Henrietta Leavitt ‘spulsating-star distance-measurement discovery to thedistribution of globular clusters. He reasoned that theseclusters were so massive that they would accurately trace theGalaxy’s global gravitational potential.He found that these clusters were arranged sphericallysymmetrically about a point in the plane of the Milky Way, inthe 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 GalaxyAstronomy 142 2

The Milky Way’scentral regions, instarlight (nearinfrared),from theNASA COBEDIRBEexperiment.NGC 891, also instarlight (nearinfrared),from the2MASS survey (U.Mass./NASA).Astronomy 142 3

Galactic fountainAstronomy 142 4

HI observations of the LMCKim et al. ACTA imagesshowing HI shells and super shellsAstronomy 142 5

More edge-on galaxiesNGC 4303, credit Chris Howk and HST heritage teamAstronomy 142 6

More edge-on galaxiesNGC 4217 (HST image,Chris Howk)WIYN ConsortiumAstronomy 142 7

Spitzer/IRAC and HST composite of theSombrero GalaxyAstronomy 142 8

Schematic structure of the Milky WayFigure: Chaisson and McMillan, Astronomy TodayAstronomy 142 9

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 diffusegalaxy called Sag A dwarf• In the halo there are stellar streams which are remnants oftidally disrupted dwarf galaxies• The outer part of the galaxy is probably flocculent ratherthan 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 athick disk (1kpc),The thick disk is older.Astronomy 142 10

Motions of stars in the Galaxy, and their use indetermining its mass distributionHow much does the Galaxy weigh?Stars move about in response to the gravitational potential ofthe rest of the stars in the galaxy.Their systematic motions (rotation) can be used simply withNewton’s Laws to measure masses.! This works even better with interstellar gas than withstars.Stars collide in-elastically very rarely, so their randommotions can be used with thermodynamics to measuremasses - the stars in this sense can be thought of as particlesin a gas.Let’s do some examples of the latter.Astronomy 142 11

Mass per unit area of the Galaxy’s disk, in theSun’s neighborhoodRecall formula for pressure in terms of particle numberdensity, speed and momentum (lecture, 9 February):Number of particles that hitwallTypical momentum per particleTime interval in which they hitConsider a certain class of stars to be gas particles, andconsider the component of their motion perpendicular to theGalactic plane.Suppose the distribution of these stars extends above andbelow the plane by some scale height H/2. Consider starslying on the ends of a cylinder of Galactic matter that extendsabove and below the plane.Astronomy 142 12

Mass per unit area of the Galaxy’s disk, in theSun’s neighborhood (continued)Weight of the cylinder, approximately:dAIf stellar “gas pressure” balances gravity,thendWH/2Compare to force on test mass m frominfinite plane with mass per unit area µ(presumably done in PHY 121 or 122):Force from gravity:dW(All on RHS observable.)Astronomy 142 13

Proof of the previous assertionmdFhrdrµ mass per unit area Astronomy 142 14

An easier way to estimate the force from gravityfrom a thin sheet using Gaus’s lawAAstronomy 142 15

Mass per unit area of the Galaxy’s disk, in theSun’s neighborhood (concluded)Putting the numbers in, for the solar neighborhood:Star counts in the solar neighborhood enable us to estimatethe luminosity per unit area L (also called the surfacebrightness) of the disk locally. This leads to the mass to lightratio:Thus the solar neighborhood on average emits light lessefficiently than the Sun does – consistent with there beingmore lower-mass stars than higher-mass stars.Astronomy 142 16

Dark matter I: the Galactic disk in the solarneighborhoodWhen this procedure was first applied to observations by Oortin the late 1940s, the resulting value of µ was greater than thatof visible stars and interstellar gas in the Solar neighborhood bya factor of about 2.! Missing mass, or dark matter: mass that emits no light butcan be detected by its gravity?Since then,! Better (less biased) samples of stars have led to smallerestimates of the total µ.! The discovery of neutral atomic and molecular gas in theISM 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.Astronomy 142 17

Relation between dynamics and stellar populationAstronomy 142 18

Dark matter IIAstronomy 142 19

Adding up the componentsAstronomy 142 20

Spiral structureAstronomy 142 21

The winding problem for transient structureAstronomy 142 22

Density wave theory for spiral structureAstronomy 142 23

Spiral densitywaves,HI, CO clustersand higherstellar densityall lie along thespiral armsAstronomy 142 24

Spiral structureAstronomy 142 25

Views of Spiral structureAstronomy 142 26

X-shaped bulgeAstronomy 142 27

SummaryThe Milky Way as an edge on GalaxyGalactic fountainSpiral density wavesComponents of the Milky WayThe Milky Way’s rotation curveHow to compute the vertical force from gravity from a thinsheetAstronomy 142 28