Ay 20 - Fall 2004 - Lecture 16Our Galaxy, The Milky WayOur Galaxy - The Milky Way• Overall structure and major components• The concept of stellar populations• Stellar kinematics• Galactic rotation and the evidence for a dark halo• Galactic centerCOBE/DIRBEIR image of our GalaxyOur Sun lies within the galactic disk, ~ 8 kpc from thecenter of the GalaxyShapley used RR Lyrae to determine distances to globularclusters, and from there the approximate position of theSun within the Galaxy.Shapley’s Globular Cluster Distribution3020101020 4010201020kpcA Modern View of the GalaxyActually, thereis the thindisk, and thethick diskActually, there isthe stellar halo,the gaseouscorona,and thedark haloAnother Schematic View• There are ~ 2 1011 starsin the Galaxy• An exponential disk ~ 50kpc in diameter and ~ 0.3- 1 kpc thick; containsyoung to intermediate agestars and ISM• Nested “spheroids” ofbulge and halo, containingold stars, hot gas, and darkmatter• The Sun orbits around thecenter with V ~ 220 km/s,and a period of ~ 2 108yrOther Spiral Galaxies Indicate How TheMilky Way Might LookNGC 628Face-On ScNGC 891Edge-On SbThe disk: thin, roughly circular disk of stars with coherent rotation about the Galactic center.€ Ldisk≈ 15 − 20 ×109LsunMdisk≈ 6 ×1010MsunDisk extends to at least 15 kpc from the Galactic center.Density of stars in the disk falls off exponentially, bothradially and vertically:€ n(R) ∝ e−R hRdisk scale length hR ~ 3 kpcMost of the stars (95%) lie in a thin disk, with a vertical scaleheight ~ 300 pc. Rest form a thick disk with a vertical scaleheight ~ 1 kpc. Thin disk stars are younger.Also a gas disk, thinner than either of the stellar disks.Major Components of the GalaxyMajor Components of the Galaxy• The bulge: central, mostly old spheroidal stellar component:• The halo, contains:€ Lbu lg e≈ 5 ×109LsunMbu lg e≈ 2 ×1010MsunGalactic center is about 8 kpc fromthe Sun, the bulge is a few kpc inradius(i) Field stars - total mass in visible stars ~109 Msun. All are old, metal-poor, have random motions. Very low density.(ii) Globular clusters. A few % of the total halo stellar content.(iii) Gas with T ~ 105 - 106 K. Total mass unknown.(iii) Dark matter. Physical nature unknown. About 90% of thetotal mass.Principal Components of the GalaxyThe Concept of Stellar Populations• Originally discovered by Baade, who came up with 2populations:Pop. I: young stars in the (thin) disk, open clustersPop. II: old stars in the bulge, halo, and globular clusters– Today, we distinguish between the old, metal-rich stars inthe bulge, and old, metal-poor stars in the halo– Not clear whether the Pop. I is homogeneous: young thindisk, vs. intermediate-age thick disk• A good modern definition of stellar populations:Stellar sub-systems within the Galaxy, distinguished bydensity distributions, kinematics, chemicalabundances, and presumably formation histories.Could be co-spatial.Due to the dust obscuration, the best ways to probe theGalactic structure are in infrared, and H I 21 cm line,which also provides the kinematics.An IR View of the Galaxy:(2MASS JHK composite, clipped a bit in longitude)Note the boxy bulge; probablya signature of the central barLMCSMCOB associations, H II regions, and molecularclouds in the galactic disk outline the spiral armsKinematics of the Galaxy:The Rotating, CylindricalCoordinate SystemΠ ≡ dR/dtΘ ≡ R dθ/dtZ ≡ dz/dtThe Local Standard of Rest• Defined as the point which co-rotates with the Galaxy atthe solar Galactocentric radius• Orbital speed of the LSR: ΘLSR = Θ0 = 220 km/s• Define the peculiar velocity relative to the LSR as:u = Π - ΠLSR = Πv = Θ - ΘLSR = Θ - Θ0w = Z - ZLSR = Z• The Sun’s peculiar motion relative to the LSR:u = - 9 km/sv = +12 km/sw = +7 km/sStellar Kinematics Near the SunStellar Kinematics Near the Sun• Velocity dispersion of stars increases with their meanage: the evidence for a stochastic acceleration due toGMC and spiral arm encounters in a differentiallyrotating Galaxy• The shape of the velocity ellipsoid also changes: olderstars rotate more slowly; the thick disk rotates with aspeed of about a half of that of the thin disk; and the halodoes not seem to have a detectable rotationDifferential RotationInner orbit pulls aheadOuter orbitlags behindThe Appearance ofDifferential RotationQuantifying theDifferentialRotationThus, by measuringradial velocities, if weknew the distances,we could map out thedifferential rotationpatternThe trick, of course, isknowing the distances…Photometric distances toOB stars and youngclusters are used.Combining Distances and Velocities• Since the spiral densitywaves concentrate the HI, and also may triggerstar formation, we canassociate young stars, OBassociations and clusterswith ISM peaks• Since these stars must be young, they could not have movedvery far relative to the gas• Fortunately, they are also very bright and can be seen far away• Of course, the extinction must be also understood very wellGas Responds tothe Spiral DensityWave Pattern, andthe Rotating Bar… And the Result Is:A Flat Rotation Curve!Schematic Spiral Galaxy Rotation Curve:Very common, our Galaxy is not special in this regard00252001005 10 15 20Radius from the Center (kpc)Rotation Speed (km/sec)Solid-BodyRotationDifferential RotationInterpreting the Rotation CurveMotions of the stars and gas in the disk of a spiral galaxyare approximately circular (vR and vz << vφ). Define the circular velocity at radius r in the galaxy as V(r). Acceleration of the star moving in a circular orbit must be provided by a net inward gravitational force: € V2(r)r= −Fr(r)To calculate Fr(r), must in principle sum up gravitationalforce from bulge, disk and halo.If the mass enclosed within radius r is M(r), gravitationalforce is:€ Fr= −GM(r)r2(From P. Armitage)Simple model predicts the rotation curve of the Milky Wayought to look like:€ v ≈GMgalaxyR= 210 Mgalaxy8 ×1010Msun      1 2R8 kpc      −1 2 km s-1This number is about right - Sun’s rotation velocity is around 200 km s-1.Scaling of velocity with R-1/2 is not right - actual rotation velocity is roughlyconstant with radius.Implies:• gravity of visible stars and gas largely explains therotation velocity of the Sun about the Galactic center.• Flat rotation curve requires extra matter at largerradii, over and above visible components.Dark