The Swedish contribution to EU-HOU: Mapping the A Hands-On Radio Astronomy Galaxy using exercise 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) – all- sky 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 Odin 2001 Onsala 20m 1964 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. − + Electron Hydrogen 21 cm line Proton

The electron’s spin reverses direction • Hydrogen (H) – the most abundant element in the Emission at 21 cm universe

• Abundant in our Galaxy + − Electron Proton • 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 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

Galactic Quadrant III Quadrant II rotation

Perseus arm Cygnus arm

Orion arm l=270 Sun

l=90

Sagittarius arm Centaurus arm

C

Quadrant IV Quadrant I

• Observations in the Galactic disc • The purple line: line-of-sight l=0 • Radio lines correspond to spiral arms 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 = V α −V l obs cos( ) 0 sin( )

– Replace alpha with l R α = R l cos( ) 0 sin( )

– Assume V=V 0 R V sin( l) R = 0 0 V l + V 0 sin obs

R0=8.5 kpc=28 000 ly

V0=220 km/s Rotation Curve

• Keplarian rotation (Solar system) V – V~1/ R • Solid body rotation (cdrom…) – V~R • Differential rotation (The Milky Way) – V=Constant d – Dark matter Geometry

• Distance to the cloud

= r = ± R2 − R 2 l 2 + R l SM ± 0 sin( ) 0 sin( )

• 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

Thank you for listening