Our Place Our Place in in the the Cosmos Cosmos Way from within - PDF document

The Milky Way • The night sky is filled with a single galaxy - our home, the Milky Way • Unlike all other galaxies, we view the Milky Our Place Our Place in in the the Cosmos Cosmos Way from within • We see individual stars and nebulae and also a band of milky light from billions of unresolved Lecture 16 stars [gala = Greek for milk] The Milky Way Galaxy • Obscuring clouds of dust appear as dark bands against the diffuse light • The Milky Way looks similar to barred spiral galaxies seen edge-on The Milky Way is a Observing the Milky Way Barred Spiral Galaxy • Although we have a close-up view of the Milky Way Milky Way, our view is obscured by dust that as seen lies in the plane of the Galaxy from • This was not known to William Herschel Earth around the turn of the 19th century, whose model for the Milky Way was based simply on counting the number and brightness of stars The in different directions barred spiral galaxy NGC 891 Herschel’s Model of the Measuring the Milky Way Milky Way (c1800) • Our primary distance measure - trigonometric parallax - works only to distances of a few hundred light-years • A key distance indicator within the Milky Way are globular clusters • These are large spheroidal groups of stars held together by gravity, much like very small elliptical galaxies • The 150 or so catalogued globular clusters have luminosities from 400 L � to 1 million L � • They typically contain 500,000 stars within a radius of 15 light-years

Globular Cluster M80 Globular Cluster Distribution About 3/4 of globular clusters occupy a • large volume surrounding the disk and bulge known as the halo of the Milky Way Three advantages of studying globular • clusters 1. They are very luminous and so can be seen to large distances 2. Those outside the disk are relatively unobscured by dust 3. All stars within them are at approximately the same distance from us and are of approximately the same age and chemical composition Distances and Ages of Globular Clusters The globular cluster M92 If the globular clusters contain stars of • known luminosity L and apparent brightness b , then neglecting dust obscuration, the distance to the cluster is given by The colour of the main sequence turnoff on Stars of known luminosity are referred to as • standard candles the H-R diagram corresponds to a mass Examples include the luminosities of the • lowest-mass stars to have left the main of 0.8 M � and an age sequence - which also provides an age of about 13 billion estimate years Stereoscopic Size of the Galaxy views of the distribution of • Harlow Shapley used RR Lyrae variable stars globular clusters (whose luminosity is proportional to period of in the Milky variability) to estimate distances to globular Way clusters Colours denote • Knowing distances to the clusters and their average colour position on the sky, Shapley could make a 3d of stars in each map of the distribution of globular clusters cluster • He found that they occupied a roughly spherical region of space with a radius of Sun’s location about 300,000 light-years shown by a green cross

Distribution of Globular Clusters • Globular clusters move under the gravitational influence of the Galaxy • Symmetry requires that the centre of the distribution of globular clusters coincides with the centre of mass of the Galaxy • By determining the distance to the centre of the distribution, Shapley found our distance from the Galactic centre • Modern determinations show that the Sun is about 27,000 light-years from the centre - about halfway toward the edge of the disk Rotation of the Galaxy • Rotation of the Galaxy is best determined by In this observing radio waves to which dust clouds direction gas are transparent clouds move • The 21-cm line of hydrogen is particularly away from us useful � red-shift • Doppler shift of measured radiation gives the radial velocity of the emitting gas cloud In this direction gas • Observations show that the galaxy is rotating clouds move towards us � blue-shift • Sun is moving on a near-circular orbit at about 220 km/s Milky Way probably looks like this… Map of the Milky Way NGC6744 (SBbc) Milky Way appears to be a typical SBbc giant barred spiral

Milky Way Rotation Curve Dark Matter in the Milky Way • Applying Newton’s laws to the observed rotation curve of the Galaxy yields a total mass of about 6 x 10 11 M � • Amount of luminous material would suggest a much lower mass, hence Milky way is mostly dark matter • Inner parts are dominated by visible matter, outer parts, to at least 150,000 light years, Like nearly all spiral galaxies, the rotation speed of (the “halo”) are dominated by dark matter the Milky Way is almost independent of radius to large distances � Milky Way is mostly dark matter What is the Halo Dark Matter? The MACHO Experiment • One possibility is dark astronomical-sized objects such as “Jupiters”, cooled white dwarfs or neutron stars, and black holes • These are collectively known as Massive Astrophysical Compact Halo Objects (MACHOs) • Although dark, their presence may be detected by their deflection of light, as predicted by Einstein’s General Theory of Relativity • If they pass directly in front of a distant star, the star’s light will be deflected and amplified - an effect known as gravitational lensing A Supermassive Black Hole at The MACHO Experiment the Galaxy’s Centre • The MACHO experiment monitored tens of • We saw in the last lecture that most galaxies millions of stars over several years in the have a supermassive black hole (SMBH) at Large and Small Magellanic Clouds, two small their centres satellite galaxies of the Milky Way • The Milky Way is no exception • Around twelve gravitational lensing events • Observations of the orbits of stars very close were found, but not nearly enough to explain to the centre of the Galaxy indicate the all of the dark matter in our halo presence of a SMBH of mass around four • Thus while some MACHOs exist, it is unlikely million M � that our dark matter halo is primarily • Our Galaxy was probably once “active” - all composed of MACHOs that now remains of the AGN is a quiescent SMBH

Near-infrared image of the centre of the Galaxy (blue=hot, red=cool) Centred on Sgr A* 1 ly = 8” A Closer View in K-band 2.1 µ Results Mass within orbit • Position of the star “S2” measured over a 10- • Speed at closest approach ~5000 km/s (200 year period 1992-2002 gives almost a times faster than Sun’s orbital speed around complete orbit: an ellipse with Sgr A* at one Galaxy) focus • For star in circular orbit of radius r about • Orbital period = 15.2 years (cf 200 Myr for point mass M , Kepler’s laws give velocity v as Sun) v 2 = G M / r • Pericentric distance (closest approach) 17 light • Allowing for eccentric orbit (e=0.87) gives a hours - only 3 times Sun-Pluto distance (cf mass of 3.7 x 10 6 Solar masses within a ~25,000 light years for Sun) radius of 17 light hours of Sgr A*

Summary • We believe the Milky Way is a fairly typical barred spiral galaxy • We live about half-way out in the disk, about 27,000 light-years from the centre • The mass of the Galaxy is dominated by dark matter • A supermassive black hole of around four million M � lies at the Galactic centre