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Lecture I
History. Components of the ISM and pressure equilibrium.
• What is the interstellar medium (ISM)
• Why is the ISM important and interesting to study?
• The discovery history of the interstellar medium (ISM)
• An overview of the different components and gas phases that
make up the ISM
What is the ISM?
• The ISM is the (baryonic) ’stuff’
between the stars, i.e. gas and
dust.
• It is NOT a single entity: its
properties (i.e. density,
temperature, kinematics and
chemical abundances) vary
greatly from place to place
• http://www.eso.org/public/images/
eso0650a/zoomable/
Useful Units
• pc = parsec = 206265AU = 3.086×1018cm
• M⊙ = solar mass = 1.989×1030kg
• L⊙ = solar luminosity =1.989×1033erg s-1
• eV = 1.602×10-12erg
• Å = 10-8cm
• Jansky = 10-23erg s-1 cm-2 Hz-1
What is the ISM?
12CO
J=1-0
10 pc
Extended ~ 10 pc scales
low density ~ 102-103cm-3
1 kpc
What is the ISM?
20 μm
10 pc
1 kpc
What is the ISM?
CS J=2-1
10 pc
Compact ~ pc scales
4
6
-3
High density ~10 -10 cm
1 kpc
What is the ISM?
Critical density:
The CO ladder spans
the full density
regime encountered
in the molecular ISM
CS(2-1)
HCO+(1-0)
Why is it important?
• The Cosmic Blueprint
Why is it important?
• Galaxy formation and evolution
Why is it important?
• Galaxy formation and evolution
Why is it important?
IC 342
HI (atomic gas)
THINGS
1 kpc
Why is it important?
IC 342
HI (atomic gas)
THINGS
12CO
J=1-0
(molecular gas)
NRAO 12m
1 kpc
Why is it important?
IC 342
HI (atomic gas)
THINGS
12CO
J=1-0
(molecular gas)
NRAO 12m
Spitzer 70um
IR emission
(star formation)
1 kpc
Historical Highlights
ca. 1800: German/British astronomer
William Herschel makes catalogue of
bright patches in the sky, calls them
’nebulae’. Also discovered IR light.
ca. 1860-1900: Margaret Lindsay (‘Herschel of
the spectroscope’) and William Huggins, Irish/
English astronomer couple. Pioneers in the field
of spectroscopy. William the first to take a
spectrum of a planetary nebula (NGC6543).
Noted that some bright patches (like the Orion
Nebula) had pure emission spectra
characteristic of gas while others (like the
Andromeda Galaxy) had spectra reminiscent to
that of stars.
1904: Discovery of the ISM by
German astronomer Johannes
F. Har tmann who noticed
stationary CaII lines in the
spectrum of the spectroscopic
binary Ự Ori. Unclear whether
the material is truly
interstellar or circumstellar.
1919: Edward E Barnard (‘The man who never
slept’), American astronomer. Made first
catalogue of ‘dark nebulae’. Holes in the stellar
distribution or obscuring dark matter?
Historical Highlights
1913: Discovery of ‘Höhenstrahlung’ by
Victor Hess, German Austrian physicist,
using balloon born experiements. Shows
that they cannot come from the Sun.
1927: ‘Höhenstrahlung’ is shown to be high
energy charged particles by Dutch physicist
J. Clay. Denoted Cosmic Rays.
1950s: Cosmic rays are shown to be heavy
particles, i.e., protons and ử-particles.
Post 1960:
There are two types of cosmic rays: 1)
Galactic Cosmic Rays (GCRs) and 2) Solar
energetic particles (mainly protons).
Normally ‘Cosmic Rays’ refers only to GCRs.
The likely sources of GCRs are supernovae,
active galactic nuclei, quasars and gamma
ray bursts. So can originate in the Milky
Way or even in other galaxies.
We talk about primary (protons, ử-particles)
and secondary (neutrons, pions, positrons,
and muons) cosmic rays.
Historical Highlights
1922: First observations of diffuse
interstellar bands (DIBs) by
American astronomer Mary Lea
Heger (while graduate student at
Lick Observatory!).
1934: Paul Merrill discovers many
more DIBs and show
unequivocally that their origin is
interstellar. More than 250 DIBs
known today.
1930: Rober Trumpler finds that the linear sizes
of open clusters are systematically overestimated
with distance, thus proving the existence of
interstellar extinction by dust.
1933: Plaskett & Pearce finds that the CaII
absorption line is stronger towards more distant
stars, and concludes the absorption must be
interstellar
1937-40: Discovery of the
first interstellar molecule
(CH) by Swings & Rosenfeld.
This was based on optical
spectra obtained by Dunham
and Adams towards Oph.
Historical Highlights
1944: “Henk” van der Hulst (while a student in
Utrecht) predicts the existence of the 21cm hyperfine
structure line of neutral interstellar hydrogen.
1949: Hall & Hiltner find a correlation between the polarisation of starlight
and reddening, which is evidence of aligned dust grains and thus magnetic
fields.
1953-57: Shklovsky suggests that the Crab nebula is powered by
synchrotron radiation. Confirmed the following year by Russian
astronomers. Searches for Zeeman splitting of the 21cm line suggested by
Bolton & Wild.
1951: Detection of 21cm line by Ewen & Purcell using home-build radio
detector. Beat Oort & Muller to it. HI is seen everywhere in the sky.
1954: First maps of the HI
distribution in the Milky
Way by Hulst, Muller & Oort.
The Galactic disk is
estimated to contain
5ₒ109M⊙ of atomic gas (ca.
10% of the entire mass of the
disk). Average density <n> ≈
1cm-3.
Historical Highlights
1960-70s: First radio detections of interstellar molecules, pioneered by C. H.
Townes.
Interstellar OH masers discovered by Weinreb
et al. (1963).
First detection of polyatomic molecules (NH3)
in space (Cheung 1968).
Millimetre observations of the Orion Nebula
leads to the first detection of CO J=1-0 in
space (Wilson, Jefferts & Penzias 1970).
CO is found to be an abundant constituent of
the interstellar medium in the Galaxy
(Zuckerman & Palmer 1974).
1973: George Carruthers, inventor of
UV the camera and spectrograph,
detects UV lines of H2 using sounding
rockets to fly detectors above the
Earths atmosphere. Incontrovertible
proof that H2 exists in the interstellar
medium.
1975: First extragalactic detection of CO at millimetre wavelengths using the
36feet NRAO telescope on Kitt Peak (Rickard et al. 1975).
1970-80s: First maps of the Galactic distribution of CO. A picture of the
molecular versus atomic gas distribution in the Galaxy emerges.
Historical Highlights
1983: The Infrared Astronomical Satellite (IRAS). First ever space-born
observatory to carry out an all-sky survey at IR wavelengths. UK, US, NL
mission.
IRAS all-sky map
Full sky survey at 12, 60, 100μm
Groundbreaking observations of cirrus
clouds, dust properties and the Milky Way’s
core
Discovered a dust disk around Vega, possibly
proto planetary system.
Discovered a new population of dustenshrouded IR-luminous galaxies
1990-91: Cosmic Background Explorer (COBE). NASA mission which
mapped the temperature and temperature anisotropies of the cosmic
microwave background. Also produced maps the CO, [NII], and [CII] emission
distribution in our Galaxy.
1995-98: Infrared Space Observatory (ISO). An ESA/JAXA/NASA mission
designed to study the Universe at 2.5-40μm. With its spectroscopic
capableties in the mid- and far-IR, ISO sheds new light on:
The nature and composition of grains (silicates and
ices) and PAHs
H2 lines as probes of shocks and photodissociation
regions
IR dark clouds
The excitation of gas in starburst galaxies and
active galactic nuclei in the local universe
ISO
Antennae Galaxies
ISO: 15μm (red)+
7μm (blue) image
Optical
COBE All-sky Maps in [CII]158 and [NII]205
Historical Highlights
SWAS 13CO J=1-0 and 5-4, and [CI]609μm maps of Orion A
1998-2006: The Submillimetre Wave Astronomy Satellite (SWAS) and
ODIN. Pointed observations of H2O, H218O, O2, CI and 13CO lead to new
insight into chemistry, composition and collapse of star forming clouds.
2007: Launch of Far-Ultraviolet Space Explore (FUSE).
2003-now: Spitzer Space Telescope
Car ries three instr uments:
IRAC, IRS and MIPS.
M u c h h i g h e r s e n s i t i v i t y,
mapping speed than ISO and
much higher angular resolution
than IRAS. First large scale FIR
maps of clouds at high angular
resolution
Leads to a revolution in our
ability to study gas and dust in
n e a r b y a s we l l a s d i s t a n t
galaxies
Uncovers tens of thousands of
distant dust obscured galaxies
IRS spectra of nearby IR-luminous galaxies
IRAC image (red) of nearby galaxy
Historical Highlights
2009-2013: Herschel Space Observatory. Highly successful ESA mission.
Was co-launched with Planck.
A quantum leap forward in far-IR and sub
millimetre studies of gas and dust in our own
galaxy as well as in other galaxies - from our
cosmic backyard to the edge of the observable
universe
Left behind an astounding data archive much of which is still not fully explored
2011-now: The Atacama Large Millimeter/sub-millmeter Array.
A groundbreaking facility: 56 submillimeter
antennase working in unison as an interferometer
Continuous coverage from 83GHz to 1000GHz
Very high spectral resolution: 0.1km/s
Very high angular resolution: down to 0.01”
Small instantaneous field of view: 10-20”