Galaxies: The Nature of Galaxies

Galaxies: The Nature of Galaxies

The Milky Way The Milky Way is visible to the unaided eye at most place on Earth Galileo in 1610 used his telescope to resolve the faint band into numerous stars In the 18 th century, Immanuel Kant and Thomas Wright speculated that the Sun is embedded in the Milky Way as an ensemble of stars and that similar Island Universes exist At the end of the 18 th century, William Herschel counted stars in 683 region of the sky and produced a map Herschel also cataloged a list of nebulae, which he believed to be outside of the Milky Way He considered the possibility of them being other galaxies

The Milky Way Jacobus Kapteyn confirmed in the early 20 th century Herschel s Universe by star counting He obtained a flattened spheroid with radially decreasing stellar density Ho estimated the diameter to be about 12 kpc and the Sun about 650 pc from the center Jacobus Kapteyn 1851-1922

The Milky Way During the years of 1915 to 1919 Harlow Shapley estimated the distance to 93 globular clusters He noticed a clustering in the constellation Sagittarius, about 15 kpc from the Sun He proposed a diameter of the galaxy of about 100 kpc Harlow Shapley 1885-1972

The Milky Way In the mid-19 th century, the Earl of Rosse studied the nebulae with the largest telescope available The Leviathan of Parsonstown (Birr Castle, Ireland) was a 72 metal reflector He discovered that many of the nebulae have a spiral structure

The Milky Way In 1923, Hubble used the 100 Hooker reflector telescope on Mount Wilson to resolve the nearby Andromeda spiral galaxy M31 and also M33 into stars Hubble also found Cepheid stars in both, resulting in distances of about 300 kpc for both

The Milky Way Structure The Milky Way consists of Galactic disk about 50 kpc in diameter Galactic bulge near the center Stellar Halo with globular clusters The Sun is 8 kpc away from the center Difference to earlier calculations basically due to interstellar extinction

The Milky Way Halo The Milky Way Halo is filled with old stars and about 150 globular clusters The clusters were formed during the early days of the galaxy formation, about 10 15 billion years ago Hot ionized gas emitting gamma rays is also in the Halo

The Milky Way Disk The Milky Way disk contains young to medium aged stars, a total of about 100 to 400 billion The disk also contains most of the gas and dust of the galaxy The disk is about 30 kpc in diameter and 0.3 kpc thick The Sun is about 8 kpc away from the center of the galaxy Measured rotational speed of about 220 km/s = 225 pc / Myr For the rotational speed we get ω = 0.028 rad / Myr One revolution takes about 224 million years

Rotation Curve of the Milky Way For comparison, the Triassic period extended from 200 to 250 million years ago First mammals developing on Pangaea Measuring the rotational speed as a function of the radius shows an almost flat distribution For very small distances from the galactic center, the rotational speed increases rapidly, typical for a rigid body For larger radii, the rotational speed is almost flat around 220 km/s

The Milky Way Disk Using ω 2 = G M r r 3 we obtain for the mass M r 10 11 M Since velocity is roughly constant throughout the galaxy M r = V 2 r G r This is in contrast to the visible luminous mass d M r dr = = 4 π r 2 ρ V 2 G

The Milky Way Disk This results in a density in the outer regions of the Milky Way ρ = V 2 4 π r 2 G The radius dependence for the density from star counts in the Halo obtained a different behavior of r -3.5 Invisible dark matter may be responsible for the discrepancy

Speculated View of the Galaxy Observed and extrapolated spiral arms of the Milky Way The Sun is located in the Orion-Cygnus arm

Speculated View of the Galaxy

The Milky Way Center The center of the Milky Way is obscured by gas and dust No optical measurements possible Measurements are made in the infrared and radio Infrared light shows a dense star cluster 2 10 6 solar masses within 1 pc Stars are about 1000 AU apart (regular star collisions) Radio observations reveal a ring of dust and gas rotating around the star cluster Extend 5 to 25 ly from center Shock wave X-ray and gamma rays are also detected, signs of annihilation radiation

The Milky Way Center Radio image at 1 m wavelength of the region around Sagittarius

The Milky Way Center Stars very close to the center Sgr A* have been observed in the 21 st century The stars S2 orbits Sgr A* in 15.2 years in an elliptical orbit The semi-major axis is 1.4 10 14 m or 930 AU Using Kepler s third law we obtain for the mass of the interiors of M = 4 π 2 a 3 G P 2 = 3.5 10 6 M

Twisted Band of Gas The satellite Herschel discovered a twisted band of dense gas in the sub-millimeter region in 2011 The band is about 100 pc in diameter It is slightly off-center to Sgr A*

Twisted Band of Gas Simple sketch of the band structure S. Molinari, et al., Ap. J. 735, L33 (2011)

The Milky Way Bubbles Using the Fermi gamma ray satellite, two very large spherical gamma ray bubbles were detected in 2010 The diameter of each is about 7.7 kpc The origin is unclear