What is Submillimeter Astrophysics?

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A Definition of Submillimeter

The 'submillimeter' region of the electromagnetic spectrum is at the border between the far infrared and the short-wavelength radio regions. As such, it borrows technologies from both regimes: bolometers from the infrared, heterodyne receivers from the radio.
Wavelength: 300 microns to 1 millimeter
Frequency: 300 Gigahertz to 1 Terahertz
In Practice: the frequencies transmitted by the atmosphere over Mauna Kea. The atmosphere is the greatest problem faced by submillimeter astronomers, which, combined with the lack of high-performance instrumentation, explains why the submillimeter region of the spectrum is currently the least well studied. Astronomy can only be done in the submillimeter at sites with extremely dry atmospheres, such as the tops of mountains and the antarctic.

Where does submillimeter emission come from in the universe?

Molecular Clouds (like the Horsehead Nebula)

The Horsehead Nebula in the CO 3-2 molecular line

Evolved stars & planetary nebulae (like The Ring Nebula, M-57)

M-57 in the CO 3-2 molecular line

Galactic star-forming regions (such as Ultracompact HII Regions)

Galactic UCHII G10.62

External galaxies (as in the Whirlpool Galaxy, M51)

CO 3-2 and CI 1-0 in M51:

The Cosmic Microwave Background Radiation

COBE observations of the CMBR

What are the most common emission mechanisms?

Continuum emission: modified blackbody from cold dust

Dust spectrum of G45

Spectral line emission: rotational transitions from molecular gas (CO, CS, HCO+, H2O etc.) and fine-structure transitions from atomic species (CI)

Spectra of the G45.12+13 outflow