ASIAA Interferometry Summer School – 2006

Introduction – Radio Astronomy

Tatsuhiko Hasegawa (ASIAA)

1. Atmospheric window to the electromagnetic waves2. Radio wavelengths band – advantages and disadvantages 3. What do we probe with the radio ?

Arizona Radio Observatory

What is sub-millimeter astronomy ? (http://aro.as.arizona.edu/documentation.htm)

Advantages and Disadvantages of Radio Astronomy● No interstellar extinction – You can see through a dark cloud. ● Difficulty in separating radio sources along the same line of sight – It is difficult to

separate multiple emission sources if they have the same radial velocity. ● High spectral-resolution – A velocity resolution of 0.1 km/s is readily attained. ● Low angular-resolution – Largest single-dish millimeter telescopes have angular

resolutions similar to a human eye (about 20 arcseconds). => This is overcome by

combining many radio telescopes – interferometer. ● Detections of low-temperature (low energy) sources – 10 K gas does not emit in IR.

Angular resolution ~ beam size (HPBW) ~ / D (radian) .

Freq= 5 10 5 GHz 1000 GHz 100 GHz 10 GHz = 0.6 m 0.3 mm 3 mm 3 cm

D = 6 mm 3 m 30 m 300 m / D = 10 –4 10 –4 10 –4 10 –4 (radian) = 21''

Max Planck D = 100 m

Nobeyama D = 45 m

IRAM D = 30 m

JCMT D = 15 m

A = A V ( 0.55 m / ) + c

(0.3 m < < 3 m)

Whittet 1981 QJRAS, 22, 3.

What do we probe with the radio ? What does the electromagnetic wave in the (sub-)mm

band reflect ? What kind of phenomena or material do we see in the radio band ?

1. Continuum emission in the (sub-)mm band.

(i) Thermal dust emission.

(ii) Free-free emission from ionized gas.

2. Line emission from molecules and atoms.

(i) From the line frequency, we identify the molecular species.

(ii) From line ratios, we can estimate the density and temperature.

(iii) Line emission from cold (10K) gas is most easily observed in the mm band.

(iv) Sub-mm lines selectively probe warm molecular gas (protostars, evolved stars,

photodissociation regions, active star-forming regions).

(v) Kinematics of molecular gas can be studied.

log

log

Free-free continuum emission from an HII region at

> 1mm ( < 300 GHz). Thermal continuum dust

emission from HII and neutral regions (Tdust = 200

– 40 K) at < 1mm ( > 300 GHz).

Hoare et al. 1992 MNRAS, 258, 257 (right).

Knapp et al. 1993 ApJS, 88, 173 (left).

<= (m)

Keen et al. 1980 ApJL, 240, L43 Crapsi et al. 2004 A&Ap, 420, 957.

Phillips and Vastel 2002 ASPH 0211610

(m) =>