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/m2/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) 1987-2004

Odin 2001

APEX (Chile) 2005

ALMA (Chile) 2012

Student antenna 2005

The back structureWheels to track any sourceon 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 planeGeometrical 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

rotationGalactic

Perseus armCygnus arm

Orion armSunl=270

l=0

Quadrant I Quadrant IV

Sagittarius arm

Centaurus arm

C

Quadrant IIQuadrant III

l=90

l=180

10 kpc = 32 600 light-years

Radio spectrum

• Observations in the Galactic disc

• The purple line: line-of-sight

• Radio lines correspond to spiral arms

Mapping the Galaxy using hydrogenWe 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=V0

)sin()cos( 0 lVVVobs

)sin()cos( 0 lRR

obsVlV

lVRR

sin

)sin(

0

00

R0=8.5 kpc=28 000 ly

V0=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

)sin()sin(SM 02

02 2

lRlRRr

Thank you for listening