Astronomy 218 Finding Interstellar Gas Empty Space? The black space between the stars in the night sky leads to a perception that interstellar space is largely empty.

Viewed through different “eyes”, infra-red & radio, this empty space is revealed to be full of gas that is just too cool for your eyes to perceive it. Interstellar Matter A wider-angle view, in this case of the Milky Way, reveals a myriad of stars, but also glowing regions of gas and dark regions that blocking light from the stars beyond.

The collection of gas and dust between the stars is called the interstellar medium. The gas is largely atoms and small molecules, dominated by hydrogen and helium, with composition similar to the Sun and other stars. Finding Cold Gas How can we detect the material between the stars? 1) Gas seen against a hotter background leaves absorption lines. 2) Warmer gas seen against a dark background exhibits emission lines. Radio waves and IR open new avenues to detect cold gas. 3) Acceleration of electrons produces radio waves. 4) Aligning the spins in hydrogen produces radio waves. 5) Even in cold gas, collisions can excite molecules. Interstellar absorption It is common when looking at stellar spectra to see the broad stellar lines superimposed with narrower lines. Starlight has been absorbed as it passed through an intervening cloud of gas. The narrow lines indicate the gas is colder, lower pressure gas than the star. The narrowness, doppler shift and low ionization state of these lines generally allow distinction between the spectra of the star and the gas cloud or clouds. Learning from Absorption

We can learn a lot about the nature of these clouds by their absorption lines. The Doppler shift reveals the cloud’s movement (radial velocity). The linewidth reveals the pressure & temperature. The pattern of lines is determined by the composition and the ionization state (temperature). The depth of the lines reveals the column density, the total number of absorbers (per m2) along the line of sight through the cloud. N (x) = n x if n is constant. τ = σ N (x) if σ is constant. Nebulae Nebula is an observational term used since antiquity (e.g. Abd al-Rahman al-Sufi in 964) for fuzzy objects in the sky. Emission nebulae are glowing interstellar gas. Dark nebulae are obscuring clouds of gas and dust. Planetary nebulae are a form of emission nebula whose typically circular shape was mistaken for planets through early telescopes. Spiral galaxies were called spiral nebulae before they were determined to be extra-galactic. Emission Nebulae Emission nebulae generally glow red, their light is dominated by the hydrogen Hα line, the transition from n = 3 to n = 2. The visible dust lanes are part of the nebula. The reflection nebula is separate, caused by intervening clouds. IR reveals active star formation in the dust lanes. Interstellar Fluorescence

Hα emission indicates hydrogen gas excited above the ground state. Collisional excitation is inefficient in the low density gas cloud. Ultraviolet photons from hot stars photo-excite the nebula, which emits visible photons as the ionized gas de-excites and recombines. This conversion of UV light to visible is an example of fluorescence. Reflection Nebula Reflection nebulae are illuminated clouds of gas and dust which scatter light toward the observer. The characteristic bluish color is the result of the increase in scattering probability with decreasing wavelength, as in the daylight sky. “Forbidden” Transitions Atomic states are defined by their respective quantum numbers; n, l, m. The probability of a specific atomic transition occurring is dependent on the change in quantum numbers between the initial and final states. The fastest transitions, termed dipole transitions, were historically labeled “permitted” as they were seen in laboratory plasmas. Slower transitions, e.g. quadrupole, octupole, etc., were labeled “forbidden” as they were not seen in laboratory plasmas because these states collisionally de-excited before the transitions occurred. Each successive -pole is roughly 1000 times slower. Forbidden Lines The low densities of interstellar gas inhibit collisional de- excitation, allowing forbidden transitions to occur. A forbidden transition in doubly ionized oxygen [OIII] produced the greenish (500 nm) color in the Orion nebula and other forbidden transitions also contribute. Planetary Nebula A particular class of emission nebulae are planetary nebulae, so named by William Herschel, the discoverer of Uranus, because they appeared similar to that planet in early telescopes. Planetary nebulae exhibit Abell 39 a variety of shapes but Cat's Eye most are circular and all Nebula are centered around a white dwarf, which excites the surrounding gas. Aside from emission lines of hydrogen, they exhibit strong forbidden lines of OIII, OII and NII. Bremsstrahlung Thermal electrons in an ionized gas interact frequently with the ions in the gas. The attraction of these positively charged ions accelerates the electron, causing it to radiate energy. This process is known as free-free emission or thermal bremsstrahlung (German for braking radiation). The energy of the photon is limited by the thermal energy of the electron, thus synchrotron & cyclotron Electrons also experience acceleration in the presence of magnetic fields. A non-relativistic electron spiraling in a helix with the Larmor radius,

spirals at the cyclotron frequency, It emits radio waves with A relativistic electron, with Lorentz Factor spirals at the synchrotron frequency ωs = eB/γc. Synchrotron radiation exhibits a broader peak than cyclotron radiation with Spin Flip Perhaps the most astrophysical important forbidden transition is the electron spin-flip transition. A hydrogen atom whose proton and electrons have their spins parallel has a slightly higher energy, 5.9 μeV, than one with antiparallel spins. This highly forbidden transition has a lifetime of 107 years and is strongly subject to collisional de-excitation, making it is difficult to observe in terrestrial laboratories. 21-Centimeter Line The large volume of low density neutral interstellar hydrogen (HI) allows the low-energy, 21 cm radiation, from this spin-flip to be observed. The very long lifetime of the spin-flip transition makes the 21 cm line intrinsically very narrow. Real 21-cm spectra are generally very complex, as the lines are Doppler shifted and broadened, revealing the kinematics of the cold neutral hydrogen gas. Mapping the Galaxy The relative immunity of 21 cm radiation to absorption allows these radio waves to be observed from across the galaxy.

Mapping the 21 cm line in the 1950s gave the first indication that the Milky Way is a spiral galaxy. Interstellar Molecules Forming a dense gas clouds in the interstellar medium requires the gas to be very cold, ~20 K. This fosters the formation of molecules, rather than atoms. These clouds are mostly molecular hydrogen, which does not emit the 21cm line or strongly in any other portion of the spectrum. Fortunately, other molecules are present include carbon monoxide (CO), hydrogen cyanide (HCN), ammonia (NH3), water (H2O), methanol (CH3OH) and formaldehyde (H2CO). In all, more than a hundred molecules have been identified in the interstellar medium. Many emit copiously at radio wavelengths. Molecular Emission Beyond their emissions due to atomic transitions of the atoms in these molecule, molecules like H2 and CO have additional degrees of freedom which allow additional, lower energy, modes of excitation and emission or absorption of photons. Primary among these are vibrational and rotational modes. Locating Cold Gas Fortunately, radio waves are not absorbed much by warmer parts of the interstellar medium, so molecular gas clouds can be detected even though there may be other gas and dust clouds in the way. For example, spectra of formaldehyde (H2CO) from different parts of M20 reveal the molecular gas is prevalent in the dark dust lanes and other dark regions around the nebula. Molecular Thermometer Molecules are also detected in emission, with different energy photons corresponding to different rotational or vibrational transitions. This makes it is possible to map the temperature distribution of the molecular gas. This is a contour map of H2CO near the M20 nebula, with the differently colored lines correspond to different rotational transitions. Five Phases Observations like these reveal 5 different phases for gas in the interstellar medium. 1) Cold Molecular Clouds (n > 109 m−3, T ~ 10 K) 2) Cold Neutral Medium (n ~ 108 m−3, T ~ 100 K) also called HI regions. 3) Warm Neutral Medium (n ~ 4 ×105 m−3, T ~ 7000 K) also called the Intercloud Medium. 4) Warm Ionized Medium (n ~ 106 m−3, T ~ 104 K) also called HII regions. 5) Hot Ionized Medium (n < 104 m−3, T ~ 106 K) also called coronal gas. Local Bubble Coronal gas is quite common, taking up more than half of the volume of the interstellar medium. For example, our Sun lies near the center of a 100 pc diameter bubble of coronal gas. This local bubble encompasses most of the nearby stars. Despite the large volume of coronal gas, it contains much less mass than the cooler denser phases. Next Time The Interactions of Gas, Dust and Starlight.