The Swedish contribution to EU-HOU: A Hands-On Radio Astronomy exercise Mapping the Galaxy using hydrogen Daniel Johansson Christer Andersson
Outline Introduction to radio astronomy Onsala Space Observatory SALSA Onsala Our Galaxy The Milky Way Exercise Mapping the Galaxy using hydrogen Observations and analysis
Atmospheric transparency http://en.wikipedia.org/wiki/radio_frequency
Birth of radio astronomy Karl Jansky (1905-1950) Discovered a radio source using this antenna (1932) The antenna operated at a wavelength of 14.5 m He had detected radio emission from the Galactic center 1 Jansky = 10-26 W/m 2 /Hz
One of the greatest discoveries of radio astronomy Cosmologists had predicted a background radiation The Cosmic Microwave Background (CMB) allsky blackbody radiation at 3 K Discovered in 1964 by A. A. Penzias & R. W. Wilson Nobel Prize in Physics 1978 Big Bang theory
Radio telescopes Resolution of telescope ~ wavelength/diameter Radio telescopes are large compared to optical telescopes Interferometry Two or more telescopes are connected Higher resolution VLBI (Very Long Baseline Interferometry) Using telescopes all over the earth as a giant interferometer
Other great discoveries of radio astronomy Pulsars Neutron stars with high rotation period Discovered in 1967 using radio telescopes Also emit at other frequencies Quasars Astronomical objects at huge distances Discovered in the 1950 s Matter falls into a supermassive black hole, causing an enormous outburst of energy Appears in the telescope as a faint star
Onsala 25 m 1964 Odin 2001 Onsala 20m 1976 SEST (Chile) 1987-2004 Student antenna 2005 APEX (Chile) 2005 ALMA (Chile) 2012
The back structure Wheels to track any source on the sky The horn and the cable to the receiver
The receiver is in this box
Specifications of SALSA Onsala Diameter 2.3m Angular resolution 7 degrees at 1420 MHz Radio receiver Bandwidth 2.4 Mhz 256 frequency channels
The Northern Milky Way (Credit & Copyright: Jerry Lodriguss, astropix.com. Astronomy Picture of the Day on 2003 Aug. 25).
An artist s view of the Milky Way (Credit & Copyright: Mark Garlick, Space-Art. Astronomy Picture of the Day on 2005 Jan 4).
The Milky Way Our Galaxy A spiral galaxy consisting of 100 billions of stars, most of them in a rotating disk lots of interstellar gas. We look at it from inside, and see it as a luminous band, stretching across the sky. Some regions are darker than others: the light from stars is absorbed by interstellar dust. Radio observations don t suffer from extinction => One can probe the Galaxy at much larger distances.
Hydrogen 21 cm line + Proton Electron Hydrogen (H) the most abundant element in the universe The electron s spin reverses direction Emission at 21 cm Abundant in our Galaxy Atomic hydrogen in the ground state hyperfine transition The electron s spin becomes anti-parallel to the proton s Radiation at 1420 MHz 21 cm is emitted Radio frequency the atmospheric window is open + Proton Electron
Hydrogen 21 cm line Spin flips probability: Once every ten million years should be hard to detect But: Huge amounts of atomic hydrogen in the Galaxy Makes the 21 cm line easy to detect Theoretical prediction: H.C. van de Hulst (1944) Observational discovery Ewen & Purcell USA 1951 Muller & Oort Holland 1951
The Galactic plane Geometrical situation when observing cloud M at galactic longitude l. The cloud and the sun S move on circular orbits and with the same velocity View of the Galactic plane. Galactic coordinates (l,b) are shown
Radio spectrum l=180 Quadrant III Perseus arm Quadrant II Cygnus arm Galactic rotation l=270 Orion arm Sun l=90 Sagittarius arm Centaurus arm C Quadrant IV Quadrant I Observations in the Galactic disc The purple line: line-of-sight Radio lines correspond to spiral arms l=0 10 kpc = 32 600 light-years
Mapping the Galaxy using hydrogen We can use observations of hydrogen to detect the spiral arms of the Milky Way 1. Observe at different galactic longitudes 2. Calculate the distance to clouds of hydrogen 3. Make a map of the observations
Geometry Use trigonometry Observed velocity: V obs = V cos( α) V0 sin( l) Replace alpha with l R cos( α) = R0 sin( l) Assume V=V 0 R = R0V0 V sin 0 sin( l) l + V obs R 0 =8.5 kpc=28 000 ly V 0 =220 km/s
Rotation Curve V d Keplarian rotation (Solar system) V~1/R Solid body rotation (cdrom ) V~R Differential rotation (The Milky Way) V=Constant Dark matter
Geometry Distance to the cloud 2 SM = r 2 2 ± = ± R R0 sin( l) + R0 sin( l) Two solutions to second degree equation Discard negative solutions Two positive solutions require further observations Observe at higher galactic latitude We now have a map of the Milky Way
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