Ay 102 Physics of the Interstellar Medium supplemental material - PowerPoint PPT Presentation

Ay 102 Physics of the Interstellar Medium supplemental material Hillenbrand – Winter Term 2019-2020

solar core CNM and WNM this room Average T and n Atomic Gas Studied via: - HI (neutral H) - metal lines (neutral and lightly ionized)

Probing CNM Gas: OI and CII (and SI) have low lying energy levels that produce far-infrared emission lines.

Probing CNM/WNM Gas: Dopita & Sutherland

Probing CNM/WNM Gas: Can infer column density N from absorption due to metals. Dopita & Sutherland

Probing CNM/WNM Gas: HI • https://www.youtube.com/watch?v=Q2mgpsTFuV8

HI Map of our Galaxy from Within

HI Map of our Galaxy from Within

Most of the HI rotates around the Galactic Center, but there are also “high velocity clouds” that differ from the expected rotation curve by >50 km/s, and trace material that is infallingon to the Milky Way.

Inferred Map of our Galaxy from Above

Within the Galactic Plane, we observe discrete clouds along any given line-of-sight, which trace spiral arms. https://www.youtube.com/watch?v=HGwkZY4E64k Data are atomic gas (HI) or molecular gas (H 2 )

Systematic changes in v r radial velocity as a function of galactic longitude let us v t construct the galactic rotation curve.

Rotation in Nearby Galaxy M 33

Atomic Levels – “Hyperfine” Structure now including electron spin coupling to nucleus spin aligned spin drawn to scale (and thus hard to see!)

Atomic Levels – not to scale now beyond “fine” to “hyperfine” lines: electron spin coupling to nucleus “spin”.

aligned spin anti-aligned spin 21 cm (radio) A. Glassgold

ê C 10 é C 01 A. Glassgold

ê C 10 é C 01 A. Glassgold

α α α J. Williams

J. Williams

A. Glassgold

J. Williams

Atomic Gas: The CNM and WNM • HI is a major diagnostic though not the major coolant. • Emission is ubiquitous; absorption is rare. • In the Milky Way, typically n(HI) = 0.1 to a few cm -3 • Density variations and temperature variations found to be anti-correlated, in a manner consistent with pressure equilibrium, so • Two-phase model consisting of: cool, dense clouds that can absorb in HI, embedded in warm, lower density “inter- cloud” medium that emits in HI.