Introduction to Astronomy Galaxies

Peder Norberg (OCW129) [email protected] Contributors to the material of these lectures: Ian Smail, Adrian Jenkins, Michele Fumagalli, Peder Norberg et al. Course Content The aim of this part of the Introduction to Astronomy course is to provide an introduction to:

The properties of our own galaxy (the “Milky Way”) Evidence for Dark Matter from galaxies The different types of external galaxies & galaxy mergers Measuring distances and the expansion of the Universe The properties of clusters and superclusters Black holes, Active galactic nuclei, quasars & gamma-ray bursts Lecture Summary 1. The Milky Way: mapping the Galaxy.

2. Galaxies - Morphology: the Hubble Sequence, spiral and elliptical galaxies

3. Galaxies - Distances: measuring distances & the most distant object 4. Clusters and superclusters: the most massive structures in the Universe 5. Active galaxies, Quasars & GRBs: the most powerful explosions in the Universe Useful Numbers Gravitational constant: G=6.67x10-11 m3 kg-1 s-2

30 Mass of the Sun: M0=1.99x10 kg

26 Luminosity of the Sun: L0=3.84x10 W Sun’s absolute magnitude (V-band)=4.8 1 light year = 9.5x1015 m 1 parsec = 3.26 light years = 3.09x1016 m

Age of the Universe: THubble=13.7Gyrs

-1 -1 Hubble Constant: H0=70 km s Mpc Matter density in the Universe as a fraction of the critical density: ΩBaryons=0.05 ΩDark matter=0.26 ΩΛ=0.69 Introduction to Astronomy Galaxies

Lecture 1 The Milky Way

Peder Norberg [email protected] Introduction to Astronomy PHYS1081 e.g. Zeilik Ch17 Lecture 1

By the end of this lecture, you should know:

The structure of our Galaxy The position of the Sun within the Galaxy Galactic coordinates Mapping the Galaxy: defining spiral arms Where do the arms come from? Orbits in the Galaxy, the rotation curve and evidence for dark matter The Shape of our Galaxy The Milky Way Local Hero

Thomas Wright (1711–1786) Born in Byers Green, built an Observatory at Westerton (near Bishop Auckland) In “A New Theory of the Universe” (1750) he explained the Milky Way as "an optical effect due to our immersion in what locally approximates to a flat layer of stars.” Another of Thomas Wright's ideas was that the faint nebulae visible on the sky are actually incredibly distant galaxies. Herschel’s view of our galaxy

William Herschel (1738-1832) counted stars in different directions in sky, assumed the are uniformly distributed, and derived this map of the Galaxy with the Sun at centre Kapteyn’s view of our galaxy

Jacobus Kapteyn (1851-1922) did a similar analysis of star counts from photographic plates, with some parallax measurements. He placed the Sun close to the centre of the galaxy, which is ~15kpc in size. The thickness of the disk is about right, but the size is wrong due to dust absorbing the light of stars in the plane of the disk. Optical map of the Milky Way

http://galaxy.phy.cmich.edu/~axel/mwpan2/ Far-infrared map: Dust in emission The “Real” Milky Way

The Milky Way is a typical spiral galaxy…

Milky Way: Past

10 -1 Luminosity is 2x10 L0 and the current star formation rate (SFR) is 3M0 yr 10 -1 Tform~L/SFR= 2x10 L0/3M0 yr ~ 10Gyrs The MW could form all its stars at the present SFR during the age of Universe

(THubble=13.7Gyrs). In fact the star formation of galaxies may change with time. Galactic Coordinates Galactic Latitude/Longitude

Equatorial coordinate system (Right Ascension, Declination) also Ecliptic coordinates…

Galactic coordinates: galactic latitude (b) galactic longitude (l)

l=0 direction Sun to galactic centre (increases eastwards) b=0 is plane of the galaxy (b=90 is North) The Size of our Galaxy & Mapping our Galaxy Globular Clusters

How can we map the size of our galaxy? Using tracers… Use globular clusters (GC) - group of ~106 stars. GC contain some of the oldest stars in the galaxy (population II stars). Bright (MV= -4 to -10) and so can be seen to large distances and used to probe the distant parts of the MW. Globular Clusters

Harlow Shapley (1885-1972) noted that GCs are not distributed like the disk. They are most easily observable objects that lie outside the disk and so can be used to map the size and structure of the galaxy. Globular Clusters

Sun Sun

Equal numbers in all directions More in one direction than other

Observed Equally distributed above distribution and below the plane, but on the sky: concentrated in a direction. Globular Clusters

Sun Sun

Equal numbers in all directions More in one direction than other

Observed Equally distributed above distribution and below the plane, but on the sky: concentrated in a direction. The Sun’s place in the Galaxy

Side-on: Face-on distribution:

Sun kpc

Galactic centre kpc

Shapley assumed GCs were clustered around galaxy and used Cepheid variable stars in them to measure their distances. He concluded the Galactic centre was nowhere near the Sun, but lay about 20kpc away in the direction of Sagittarius (because of dust in the way, he got the value wrong). Current value is 7.8+/-0.6 kpc and we are 26pc above the plane of the disk. Our Galaxy

sun

Shapley's model of our galaxy is still roughly right (other than scale): The Disk is ~30 kpc across and only 0.6 kpc thick (Pop I+II stars) The Bulge is about 2 kpc across (Pop II stars) The Halo is a diffuse structure reaching to 100 kpc radius (Pop II) There is a “thick” disk of Pop II stars (thickness ~2-3kpc) A very crude model. Can we learn more: make a more detailed map? Mapping the Galaxy A better map of the Galaxy

Globular Clusters

Stars HI = gas

Few globular clusters, so a crude tracer of structure of the galaxy. Stars would be better, but we need distances which are hard to measure. Work in progress: https://www.sdss3.org/surveys/apogee.php (Apogee) http://sci.esa.int/gaia/ (GAIA) Or use Hydrogen atoms: 21cm emission from spin flip of e- in neutral Hydrogen (HI). A better map of the Galaxy

Globular Clusters

Stars HI = gas

We can use neutral Hydrogen (HI) to map the structure of the Galaxy. The wavelength of the HI emission is Doppler shifted by the motion of the gas in the spiral arms. We can convert this velocity shift into an estimate of distance using geometry.

Diagram from A. Rudolph Georgelin & Georgelin (1976) Our Galaxy 3 kpc Spiral Arms

Early theory had fixed spiral pattern. OK if disk rotates as solid-body (but we’ll see this is not true). Differential rotation would make the arms wind-up after ~100 Myrs. So how are the arms formed? Spiral Arms: Density Waves?

Young stars

Old stars Spiral Arms What causes the initial waves? In some cases the causes are external.

Generally however instabilities in the disk naturally produce spiral waves which travel around the disk. https://www.youtube.com/watch?v=v39UtTfOHAU Rotation of the Galaxy & Dark Matter Galactic Rotation Curve

Sun -1 VC=220km s

Clemens (1985)

Use HI or molecular/hydrogen emission lines to map variation in rotation velocity as a function of radius in the galactic disk. Rotation velocity at Sun’s galactic radius is ~220km/s in almost circular orbit. Sun also oscillates vertically around the plane of the galaxy. The orbits of stars in the halo and the bulge are much less ordered. Differential Galactic Rotation For stars travelling around the galaxy at radius R with speed V on a circular orbit the centripetal acceleration must balance the acceleration due to the mass interior to the orbit:

2 V GM (< R) GM (< R) acentripetal = = 2 V = R R R If all the mass in the GM V = ∝ R−1/2 galaxy was concentrated Kepler R in the centre we have: 4π R3ρ Alternatively, if the mass M (< R) = is in a sphere with 3 constant density (ρ): G 4π R3ρ 4πGρR2 V = × = ∝ R Solid R 3 3 Differential Galactic Rotation For stars travelling around the galaxy at radius R with speed V on a circular orbit the centripetal acceleration must balance the acceleration due to the mass interior to the orbit:

GM V = ∝ R−1/2 Kepler R

Keplerian rotation

4πGρR2 V = ∝ R Solid 3

Solid body rotation Galactic Rotation Curve

Solid Body orbits

0 10 20 Distance from Galactic Centre (kpc) Mass Profile of the Galaxy What density profile is needed to obtain a constant rotation velocity as a function of radius, assuming a spherical mass distribution? V 2 GM (< R) G = V = M (< R) R R2 R If V=const then M (< R) ∝ R i.e. isothermal halo: ρ(R) ∝ R−2 R R ρ M < R = 4πr2ρ r dr = 4πr2 0 dr = ( ) ∫ ( ) ∫ 2 0 0 r R 4πρ dr = 4πρ R ∫ 0 0 0 This decline is shallower than the light in the disk (exponential) or halo (falls as R-3) Mass Profile of the Galaxy What density profile is needed to obtain a constant rotation velocity as a function of radius, assuming a spherical mass distribution? V 2 GM (< R) G = V = M (< R) R R2 R If V=const then M (< R) ∝ R i.e. isothermal halo: ρ(R) ∝ R−2 R R ρ M < R = 4πr2ρ r dr = 4πr2 0 dr = ( ) ∫ ( ) ∫ 2 0 0 r R 4πρ dr = 4πρ R ∫ 0 0 0 This decline is shallower than the light in the disk (exponential) or halo (falls as R-3) Galactic Rotation Curve

Total 250

200 Dark halo

150

(km/s) Disk+bulge+halo c

V 100 (visible matter)

0 10. 20 30 Radius (kpc) -1 Isothermal halo can produce flat rotation curve with vc~220km s (at 8kpc) -1 Gravity of visible matter within solar orbit only gives vc~150km s (at 8kpc) …so this massive halo has to be mostly dark 11 Within 20kpc the total enclosed mass in MW is ~2x10 Mo. Total mass to edge the halo 12 at ~100kpc is ~10 Mo (90% of mass of galaxy is dark). Dark Matter: MACHOs or WIMPS? Is the “dark” halo baryonic - i.e. made of Black Holes, Brown dwarfs or Jupiters (MACHOs= MAssive Compact Halo Object)? We can test this using gravitational lensing (which produces brightening) of stars in the bulge by foreground MACHOs in MW’s halo

Time scale of event is set by MACHO’s mass. Black hole mass lens event take years. Jupiter mass lens events ~few days. Brown dwarf a ~month.

Current limits on numbers of events and the masses of these lenses suggest in fact that most of the halo is not in the form of MACHOS (~1%). Dark Matter: WIMPS

Weakly Interacting Massive Particles (non-baryonic) Search for recoil of atoms due to massive “invisible” particles

Many experiments ongoing… no clear detection yet, but limits are improving quickly. Dark Matter Halo

Now believe the MW is inside a massive non-baryonic dark matter halo that extends out to ~100-200kpc … Summary: Lecture 1

You have now seen: The structure of our Galaxy (disk/bulge/halo) The use of globular clusters to determine the position of the Sun within the Galaxy Definition of Galactic coordinates Mapping our Galaxy and the nature of spiral arms The form of the galactic rotation curve Evidence for non-baryonic dark matter