Interstellar Medium - Spring 2018


These lecture notes provide an overview of the material covered. These few pages provide more descriptive material and some links to relevant figures. Note that most of the notes will be handed out in class and not posted.

I. Introduction


Why should we care about the Interstellar Medium (ISM) in galaxies? Several reasons come to mind:

a. the ISM is the building block for new star formation, and also the repository of gas expelled from stellar atmospheres and outer layers in the end phases of the life of stars. It also may include in-falling ("accreted") gas from the intergalactic medium. We often refer to the latter as the IGM. It appears unlikely we will find any pristine gas (i.e. still as metal poor as the gas formed in the early universe). Studies of the IGM show that even in the early Universe some metal enrichment has already taken place. The physical processes going on in the ISM and IGM will be quite similar, though not identical. This is mostly related to the density of the gas (which tends to be lower in the IGM), and the physical processes that occur. E.g. photo ionization by star light is likely more prevalent in the ISM, although in the early universe, an as yet TBD source was responsible for the re-ionization of the IGM.

b. The hot diffuse gas phase in the IGM may be the dominant repository of baryonic material in the Universe, containing more mass than stars and ISM inside galaxies. This medium is though to be a part of the "cosmic web" between galaxies.

c. While the astrophysics of gaseous nebulae is not simple, in many cases we can find beautiful demonstrations of basic concepts in e.g. radiative processes in the ISM which are easier to understand than the more complex processes in stellar or planetary atmospheres. There are several good examples where the physical processes separate out, one being the dominant over the other.

d. The ISM provides one of the best probes for the study of galactic dynamics, especially in the outer parts of galaxies. Radial velocity measurements  using radio and sub-mm lines found in the ISM are far more accurate than stellar velocities from optical spectra. Gas often extends much farther out in the halo than that stars can be traced, especially in external galaxies, and gas will more closely follow circular orbits due to its tendency to settle in disks as it collides with other gaseous material. That means that at least in principle an analysis of the gas kinematics is inherently simpler than that of stellar kinematics, where triaxial distributions may be common.

e. The ISM provides a richness of probes, through continuum emission, absorption lines, and emission lines that allows chemical abundances to be determined in many different places. It is the best current probe of chemical composition as a function of redshift. In spite of its overall small density, the probes are very sensitive.

The ISM is almost a perfect vacuum if we compare it with the best vacuum achievable in laboratories on earth. The typical average density is 1 hydrogen atom per cm3. This corresponds to a mass of about 2400 kg for an object with the volume of the Earth. The medium is highly structured, however, especially in the colder phases. Structure is found down to the smallest scales that have been investigated.

In any direction we look from our location in the Milky Way, we will find ISM along the line of sight. Lockman described this as "Looking for nothing in the ISM and not finding it".  The traditional approach towards ISM research, both observationally and theoretically has been almost exclusively based on understanding of the Galactic ISM, in particular that in the solar neighborhood. However, observational techniques have reached the point where we can begin to address the physical parameters of the ISM in various external systems, greatly enhancing the richness in environments and conditions present. In addition, external galaxies offer a much better vantage point for determining the global, galaxy-wide properties of ISM parameters.

Subjects we will discuss in this course include:

                             validity of laws of statistical mechanics
                             atomic and molecular physics
                             radiative transport and processes
                             collisional & radiative ionization and excitation


Even if we manage to cover all this, it is good to keep in mind what we left out: astro-chemistry (the formation and destruction of (complex) molecules, much of the detail on molecular spectroscopy and molecular cloud physics, most of magnetic field physics, the entire field of star formation, etc.

II. Brief Historical Overview

It took quite some time for people to realize there was an ISM.

1904
Discovery by Hartmann of narrow, stationary, Ca+ absorption lines in spectrum of a spectroscopic double star. These lines did not take part in the motion of the other lines, which suggested their origin was outside the stars.

Questions:

How would one discover these lines are interstellar and not circumstellar?
What is the significance of the fact that the lines are narrow?

1913
Discovery of increasing ionization rate above 1 km in Earth's atmosphere. This was attributed to some unknown form of radiation ("Hoehenstrahlung"), probably not solar in origin.

1927
Clay found that Hoehenstrahlung is less important at the equator than at the poles of Earth; attributed it to charged particles approaching Earth from interstellar space, hence "cosmic rays" from then on (Millikan, 1927).

1930's

Discovery of interstellar extinction.  This was not found by Kapteyn (think of Kapteyn model for our Galaxy!) but by Trumpler (1930) & van de Kamp (1932). Trumpler estimated the distance of open clusters from their angular size. He then noticed that the cluster light from the star seemed to dim faster than distance-2 with those estimated distances.  Van de Kamp discovered that galaxy number counts decreased towards the Galactic plane.

1930's and 40's
Karl Jansky and Grote Reber discovered radio continuum emission from the Milky Way. The emission mechanism was only fully understood later in the 50's and 60's. (What radiation is it?)

1937,40
Swings and Rosenfeld and McKellar find diatomic molecules in interstellar absorption lines (CH, CH+, CN)

1939
Stromgren develops concept of "Stromgren sphere", which is ionized gas region around massive stars. I suspect he did not give it that name.

1944
Prediction by Van de Hulst of the existence of a 21-cm line transition in HI. Observationally verified in 1952 by Ewen and Purcell, first conclusive proof of the existence of neutral hydrogen in the ISM. Possibly the most important prediction and discovery made in ISM research.

1949
Serendipitous discovery by Hall and Hiltner that the polarization of star light is correlated with reddening, hence with extinction. The interpretation of this is that the dust grains may be non-spherical but elongated, and systematically aligned in the interstellar magnetic field to cause polarized scattered light.

1952
Shlovsky predicts synchrotron radiation and its polarization. This provided further evidence for the existence of interstellar magnetic fields and cosmic rays.

1963
Discovery of OH molecule 18-cm emission lines and masers. This was also about the time of the discovery of the first pulsar, and pulsars have turned out to be important background sources for us to learn more about the ionized ISM.

1968 and beyond
Discovery of NH3, H2O, H2CO and subsequently many more complex molecules in ISM.

1960's
Discovery of soft X-ray background, providing direct evidence for hot ISM (million degree) in solar neighborhood.

1972
Copernicus satellite for first time detects UV absorption lines from ISM. Confirmation of existence of molecular hydrogen, and underabundance of many elements (C,N, O,...) in ISM compared to solar abundance. Discovery of OVI absorption lines confirming existence of hot medium in ISM.

1970's
Development of IR astronomy, culminating with flight of IRAS satellite in 1983, which produced Far-infrared maps of Milky Way and other galaxies, of HII regions, and many other objects. Also discovered extensive mid-IR emission from PAHs (polycyclic aromatic hydrocarbons), which are large molecules in the dust, much smaller than the typical dust grains causing interstellar extinction. There were also significant developments in X-ray and even gamma-ray observatories providing new data on the high energy ISM.

We stop our little overview here and will cover more recent discoveries as we go along in this course. Significant space missions since the 70's have included the International Ultraviolet Explorer, the Hubble Space Telescope (ongoing), FUSE satellite, Einstein, ROSAT, Chandra, and XMM in X-ray (the latter two are still operational), COS-B and Gamma Ray Observatory in gammma-ray, and ISO, SPITZER, Herschel, and WISE in infrared. Other missions relevant to ISM include various probes of cosmic microwave background (which detect the Galactic ISM whether they want to or not).  Important discoveries of the cool and warm HI, the molecular gas, and the ionized warm gas have come from ground-based telescopes.

I. Overview of the interstellar medium. See also Chapter 1, textbook.


Discuss list of ISM components from text book.


The ISM in a galactic disk on large scales is likely in some sort of equilibrium situation, although on smaller scales it may be subject to various instabilities (e.g. those causing a cloud to collapse and start forming stars). As an example, its vertical scale height and distribution will be determined by the disk potential (defining the vertical force of gravity as a function of height above the plane) and the velocity distribution of the gas in the z-direction. Since the disk potential is due to stars, gas, and dark matter, all of which will have some extended distribution with height above the plane, the situation is not as simple as an atmosphere on a planet, where we can assume that the gas all experiences the same gravitational force. Still, we do often describe the gas distribution in terms of exponential scale heights, even if an exponential density distribution is not necessarily the correct solution to the hydrostatic equilibrium problem.

The second parameter that determines the scale height is of course the velocity dispersion of the gas. This has several contributions, first it can never be less than the thermal velocity width if the gas is at constant temperature. Most of the ISM actually experiences significant bulk motions on top of this thermal motion, the origin of which is still subject to debate. It is clear that mechanical energy input from stars in the form of stellar winds and supernovae stir up the medium. This may produce turbulence down to small scales. In addition, the gas could be subject to its own gravitiational forces (self-gravity) and establish a velocity dispersion between gas clumps in a self-gravitating larger cloud (e.g. for molecular gas). In this case, the disk potential itself matters less, although that was responsible in the first place for establishing the thickness of the gas layer from which the molecular cloud clumps formed. In addition, once the molecular cloud is dispersed due to star formation, the remaining clumps will move in the disk potential and obtain a scale height commensurate with their velocity dispersion.

Some images of ISM in Milky Way and other galaxies

Here is an overview picture of the Milky Way in different components From Radio to gamma-ray.

Textbook Plate 1, 2, 3, etc.
Maps of the entire Milky Way in various bands. Notice there is HI in all directions. Only at high galactic latitudes can you see much structure, since in the plane the gas is present over a wide velocity range and that has been integrated over to give a total HI map.

The Milky Way in the light of H-alpha recombination line, observed with the Wisconsin H-alpha Mapper telescope and other surveys is also shown. This is a low resolution imaging telescope which worked from the northern sky and provided a very sensitive image of the ionized gas distribution. Note that in optical (Halpha!) we cannot see all the way through the Milky Way so much of what you see in this picture if relatively nearby gas.


Here is what our Milky Way might look like from above. An HI image of the spiral M31, in a project I am working on with colleagues in Europe. This is among the highest resolution HI images we have of any external spiral. M31 in HI with the Westerbork telescope



Ionized or neutral?

The ISM is considered neutral if the dominant form of hydrogen gas (H) is neutral. The hydrogen gas can be in atomic form (HI) or molecular form (H2). Whenever hydrogen is neutral in the ISM, the dominant number of atoms will be in the electronic ground-state (n=1) because at the prevailing temperatures, collisions cannot excite the gas to higher levels at significant rates. There is enough hydrogen around that the ISM is essentially opaque to radiation with wavelengths below 912 Å (photon energy equal to 13.6 eV). Hydrogen can be ionized of course, but there are not enough such photons around to keep all hydrogen ionized in the ISM. Photons with such energy are called Lyman continuum photons. A similar ionization limit exists for He. At what wavelength?

Many elements have ionization potentials less than H, so even if H is neutral there will be trace ions present, because there are plenty of photons at wavelengths that can ionize those elements (why, where from?). E.g. Sulphur has 10.36 eV ionization energy, Mg 7.65 eV, and Na 5.14 eV.