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

Birth of radio astronomy Karl Jansky ( ) 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 = 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 1964 Onsala 20m 1976 SEST (Chile) Odin 2001 APEX (Chile) 2005 ALMA (Chile) 2012 Student antenna 2005

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).

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.

An artist’s view of the Milky Way (Credit & Copyright: Mark Garlick, Space-Art. Astronomy Picture of the Day on 2005 Jan 4).

Hydrogen 21 cm line Hydrogen (H) – the most abundant element in the universe 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

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 Hydrogen 21 cm line

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 Observations in the Galactic disc The purple line: line-of-sight Radio lines correspond to spiral arms

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: –Replace alpha with l –Assume V=V 0 R 0 =8.5 kpc= ly V 0 =220 km/s

Rotation Curve 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 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