The Interstellar MediumAstronomy 216

Spring 2005

Al Glassgold & James Graham

University of California, Berkeley

The Interstellar Medium/Media (ISM)

What is the ISM?Just what it says: The stuff between the stars in andaround galaxies, especially our own Milky Way:Gas, dust, radiation, cosmic rays, magnetic fields.

Why do we study the ISM?Gas is where it all started back then at “Recombination”.Part of the effort to understand the evolution of baryonic matter through all time.

Why is the ISM important?Stars form from the ISM, and then they activate it dynamically and chemically on dying: Gas is the active chemical ingredient of galaxies.

Condensed Syllabus Spring 2005

Seeing the ISMCan we see the ISM the same way we see the Sun and the stars shine?In principle, yes, but it’s hard because gas at temperatureslike the surfaces of stars cools rapidly unless its heated somehow, e.g., by being close to a star. This kind of gas is called nebular and has characteristic emission lines.

Most of the gas in galaxies is relatively cold and, since a typical wavelength is inversely proportional to temperature, is only observable in emission in bands outside of the Visible, unavailable until the second half of the 20th century.

Large volumes of gas are hot, and require short wavelength observations (UV & X-rays) with satellite observatories.

A Little Bit of HistoryDiscovery of the Orion Nebula (1610) fueled speculation about gas clouds.

Nebulae figured prominently in the Messier Catalog.

Nebular emission lines discoveredvisually in NGC 6543 by Wm. Huggins (1864); detection in Orion Nebula by M. Huggins (1889).

First photograph of the Orion Nebulaby Henry Draper (1880) HST (O’Dell)

Discovery of Interstellar Clouds (away from stars)First interstellar absorption line: Hartmann’s Potsdam observations reprinted in ApJ 19, 268, 1904:Stationary narrow dip in broad oscillating Ca II stellar absorption line of binary Delta Orionis (O9.5)

For 75 years, the measurement at high spectral resolution (up to R = 300,000 or 1 km/s) of absorption lines in theoptical spectra of nearby bright stars was almost the only way of quantitatively studying cool interstellar clouds (especially low IP systems like Na I, K I, Ca II) .

Space Astronomy strongly impacted this field in 1973 with Lyman Spitzer’s UV satellite, Copernicus, allowing the transitions of abundant species to be used, e.g., H2, HI Ly-α, & the lines of C IV, N V, & O VI (diagnostic of warm gas). Copernicus was followed by IUE , HST and FUSE.

Interstellar Clouds Appear Dark

Lund Observatory Photos of the Milky Way

These long-known dark regions were studied by Barnard in the 1st quarter and by Bok in the 3rd quarter of the 20th century.

They appear dark here because (1) they do not emit at optical wavelengths & (2) their dust blocks the light of the stars.

Example of a Well-Studied Dark Could B68

Stars gradually appear as the observing band changes from B to K due to the decreasing absorption with wavelength of small dust particles.

This extinction property was confirmed by Trumpler (1930)by comparing luminosity and angular-diameter distances ofopen clusters.

Interstellar Dust and Space AstronomyMuch of the electromagnetic spectrum is obscured by the Earth’s atmosphere:

This is relevant for small but macroscopic dust particles that absorb continuously across the entire spectrum.

Dust particles efficiently absorb short-wavelength radiation emitted by stars and re-radiate at longer wavelengthscharacteristic of their relatively low temperatures. This includes much of the atmospherically obscured infrared: observing dust emission requires observations from space.

Atmospheric transparency from Mauna Kea(A. Tokunaga, in Allen’s “Astrophysical Quantities”

NIR

FIR

Interstellar Dust and Space AstronomyIR space observations in the last 20 years:

IRAS 1983COBE 1991ISOSIRTF/SPITZER 2003

Originally considered an embarrassing obstacle to astronomical observing, the scientific study of cosmic dust is a challenging and important part of astronomy, e.g., the terrestrial planets and the rocky cores of giant planets were formed from circumstellar dust.

IRAS All Sky Image of the Milky Way(0.5° resolution)

Zodiacal light LMC Orion

Ophiuchus

Blue: 12 µmGreen: 60 µmRed: 100 µm

Notice the wide distribution of 100 µm“cirrus” emission out of the Milky Way plane.

Far-IR Spectrum of the ISMCOBE spectrum of the Galactic plane, 0.1-4 mm

Two main components: CMBR & Dust

There also lines! The most prominent is the ground state fine-structure transition of C+ at 157.74 µm, with an energy level spacing of 93K.

Dust peak ≈ 140 µm, corresponds to Tdust ≈ 20 K

This line is characteristic of warm partially ionized gas from much of the Milky Way, where C is ionized by far UV photons (hv > 11.25 eV)

Radio Astronomy(The other long wavelength observational revolution)

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are needed to see this picture.

CO(1-0) Map of the Milky Way(Columbia/CfA Mini-telescope Project)

Rapid development from WWII & radar. ISM milestones: The detection of the H 21-cm line (1951)The radio detection of interstellar molecules (1963)CO as the tracer or molecular material (> 1970)

Radio Astronomy: Early ISM MilestonesDetection of the H 21-cm line: Predicted by van de Hulst (1945).

Post-WWII detection race won in by Ewen & Purcell over Muller & Oort (1951). Seen everywhere in the sky.

Early optical detections of interstellar molecules: CH Dunham et al. (1937)CN Swings & Rosenfeld (1937)CH+ McKellar (1940)

Radio detection of molecules pioneered by C. H. Townes:OH (Weinreb et al. (1963) NH3 & H2O (Cheung et al. 1968, 1969

(Townes & Welch Berkeley collaboration)H2CO Snyder at al. (1969)CO Wilson, Jefferts, & Penzias (1970)

CO and the > 125 interstellar molecules are crucial forstudying the cold and warm dense gas of galaxies.

General Properties of the ISM

• Mostly confined to the disksome dilute gas in the halo, as in E galaxies

• Large ranges in temperature & densityT ≈ 10 - 106 K & n ≈ 10-3 - 106 cm-3

• Even dense regions are “ultra-high vacuum”lab UHV:10-10 Torr (n ≈ 4 × 106 cm-3)

(STP conditions: n ≈ 3 × 1019 cm-3)

• Far from thermodynamic equilibriumcomplex processes & interesting physics

Cosmic (Solar) AbundancesElement Abundance Element Abundance

H 1.00 Mg 3.4×10-5

He 0.10 Al 3.0×10-6

C 2.5 × 10-4 Si 3.4×10-5

N 1.0 × 10-4 S 1.6×10-5

O 4.9 × 10-4 Ca 2.3×10-6

Na 2.1 × 10-6 Fe 2.8×10-5

Mass fractions of H, He and “heavier elements “metals”X = 0.71, Y = 0.27, Z = 0.02

Standard reference abundancesChondritic meteorites and the Sun - very similar,

presumably like the ISM 4.5 x 109 yr ago HII regions- gas phase onlyB-stars - recent ISM (some discrepancies – e.g., C)

Classification by Ionization State of Hydrogen

ISM composition similar to solar system -H is most abundant element (≥ 90% of atoms)

The regions of interest are characterized by the state of hydrogen:

Ionized atomic hydrogen (H+ or H II) “H-two”Neutral atomic hydrogen (H0 or H I) “H-one”Molecular hydrogen (H2) “H-two”

Labeled as H II, H I, or H2 regions, with thin transition zones H II → H I → H2

Ionized GasH II regions - bright nebulae associated with regions of star

formation & molecular clouds; ionized by stellar UV photons from early-type (OB) stars

T ≈ 104 K & ne ≈ 0.1 - 104 cm-3

Warm Ionized Medium (WIM)T ≈ 8000 K & ‹ne› ≈ 0.025 cm-3

Hot Ionized Medium (HIM)tenuous gas pervading the ISM, ionized by electron impact

T ≈ 4.5 x 105K & n ≈ 0.035 cm-3

Our goal: Understand how these phases are observed and how they are heated and ionized.

H I Regions

Cool clouds (CNM)T ≈ 100 K & n ≈ 40 cm-3

Warm neutral gas (WNM)T ≈ 7500 K & n ≈ 0.5 cm-3

Lyman-α forestintergalactic clouds with NHI > 1013 cm-2

Our goal: Understand how these properties are determined observationally and what are the relevant nthermal and ionizations processes.

Molecular CloudsCold dark clouds (M ≈ 10 - 1000 M )

T ≥ 10 K & n ≈ 102 - 104 cm-3

Giant molecular clouds (M ≈ 103 - 105 M )T ≥ 20 K & n ≈ 102 - 104 cm-3

with complex structure that includes cores and clumps with densities n ≈ 105 - 109 cm-3

Stars form in molecular clouds.

Our goal: In addition to understanding the observations and underlying physical processes, can we find the connection between the molecular and atomic phases & how stars form.

Other Components of the ISMDust Grains ≈ 0.1 µm in size, composed of silicates or

carbonaceous material, constituting ≈ 1% by mass of the ISM

Photons: CMB, star light (average interstellar radiation field or ISRF), X-rays (from hot gas & the extragalactic background)

Magnetic fieldsCosmic rays

Energies in the ISM

1. Kinetic energy of random (thermal) motion2. Kinetic energy of organized fluid motion3. Starlight4. Magnetic fields5. Cosmic rays6. Cosmic Background Radiation

Estimates for the LISM give typical numbers that are all about 0.5 eV cm-3, thereby offering the potentialof significant interactions between these components

James Graham’s Conception ofThe ISM as a Complex System

Understanding the ISM means understanding the physical processes which drive mass, momentum and energy exchange between the stars and its components.

Motivation for Studying the ISMStar formation is the fundamental process that determines the observational properties of galaxies.The history of star formation should yield the Hubble sequence: smooth elliptical (E), spiral disk (S) & irregular (I) galaxies .

The basic question is: How are baryons transformed into starsunder the influence of:

gravity (well understood)radiation pressure (small) ionizing fluxes (important)magnetic fields (poorly known)large scale motions (measurable) turbulent stresses (poorly understood)

Sampling of Broader Questions

Why does star formation occur mostly in spiral arms– What triggers star formation– What determines the star formation rate in different

Hubble types?– What determines the initial mass function of stars?– Why do stars form in multiples?– How do planets form?