Observing the 21-cm Line: Insights into Neutral Hydrogen and Molecular Clouds in Space
This overview of neutral hydrogen observation highlights the significance of the 21-cm radio line, revealing the forbidden hyperfine transition of hydrogen atoms. We explore the delicate balance of molecular clouds, where UV radiation from stars threatens molecular integrity unless shielded within dense structures. Giant Molecular Clouds (diameter 15-60 pc, temperature ~10 K) are crucial for star formation, with hydrogen and carbon monoxide (CO) serving as key tracers. Detailed observations, such as CO emission from NGC 5194, illuminate the complex interactions in the interstellar medium.
Observing the 21-cm Line: Insights into Neutral Hydrogen and Molecular Clouds in Space
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Presentation Transcript
0 Observing Neutral Hydrogen:The 21-cm (radio) line Forbidden hyperfine-structure transition: A = 2.95x10-15 s-1 Equal magnetic fields repel => Higher energy Opposite magnetic fields attract => Lower energy Magnetic field due to proton spin 21 cm line Magnetic field due to electron spin
0 Observations of the 21-cm Line G a l a c t i c p l a n e All-sky map of emission in the 21-cm line
0 Molecular Clouds • Molecules are easily destroyed (“dissociated”) by ultraviolet photons from hot stars. They can only survive within dense, dusty clouds, where UV radiation is completely absorbed. “Molecular Clouds”: UV emission from nearby stars destroys molecules in the outer parts of the cloud; is absorbed there. Largest molecular clouds are called “Giant Molecular Clouds”: Molecules survive Cold, dense molecular cloud core Diameter ≈ 15 – 60 pc Temperature ≈ 10 K HI Cloud Total mass ≈ 100 – 1 million solar masses
0 Molecules in Space • CO = Carbon Monoxide • OH = Hydroxyl • Radio emission from rotational (and vibrational) transitions • H2 = Molecular Hydrogen Ultraviolet absorption and emission: Difficult to observe! But: Where there’s H2, there’s also CO. Use CO as a tracer for H2 in the ISM!
