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ApJ 717, L66 : Dhanraj
ApJ 691, 1042, YoungHyun Lee
ApJ 739, 12, Shu Woo Young
ApJ 712, 139, Park Keun Hong
Mid-infrared observations of the
transitional disks around DH Tau, DM
Tau, and GM Aur (Grafe et al., 2011)
Song Donguk
IR Universe
1. Our Solar System
 All of the planets and moons in our solar system emit strongly
in the infrared.
 This infrared emission is the heat from atmospheres and
surfaces which peaks in the mid to far infrared (15 to 100
microns).
 Solar system objects also reflect infrared radiation from the Sun.
 This reflected radiation peaks in the near infrared at about 0.5
microns.
 For planets and moons which have atmospheres, infrared
studies can show us how the abundance and composition of
atmospheric gases as well as the temperature of the
atmosphere vary with depth.

In addition, infrared astronomy has led to the discovery of new
comets and asteroids and bands of dust which lie in our solar
system.
Planets
 Planets
Infrared
Image of
the Sun
Venus
Both images were taken by the
Galileo satellite : the infrared
image of the clouds of Venus on
the left with the visible light
image of Venus on the right.
 Sunlight passing through the atmosphere of Venus is
absorbed by its surface which causes it to heat up,
giving off radiation which is primarily infrared.
 The carbon dioxide in Venus's thick atmosphere
traps much of the infrared heat, giving Venus a
surface temperature of 750K (890 degrees
Fahrenheit) which is hot enough to melt lead.
 By studying the infrared spectra of Venus,
astronomers found that sulfuric acid droplets exist in
its atmosphere.
Mars
 Some observations of Mars may be
made with PACS and SPIRE, but these
instruments are so sensitive that Mars
will cause the detectors to be overexposed. But the HIFI instrument is
ideally suited to investigating the thin
martian atmosphere, and it can detect
the presence of molecules such as
methane and water in the atmosphere.
JUPITER
an infrared image of Jupiter and
one of its moons, Io ( has several
active volcanoes and is heated by
tidal forces caused by the
gravitational pull of Jupiter and the
other Galilean moons.)
 Jupiter radiates ~1.6 times as much heat, in the form of
infrared energy, as it receives from the Sun. This indicates that
Jupiter has an internal source of energy - probably heat
created by Jupiter's collapse when it was formed.
 By studying the infrared emission from Jupiter we have learned
much about its cloud structure.
 Jupiter's belts (its dark horizontal bands) are brighter in the
infrared than its zones (its bright horizontal bands) This
indicates that the belts are regions of hotter gas. The
temperature of Jupiter also increases towards its center, so the
zones are at higher levels in Jupiter's atmosphere than the belts.
SATURN:
The HST/NICMOS
infrared image of
Saturn shows the
details of its
cloud structure.
 Saturn also radiates about twice as much radiation in
the infrared as it received from the Sun.
 ISO was recently used to study the concentrations of
heavy and ordinary hydrogen in the atmosphere of
Saturn.
 These measurements give information about the
composition of the original cloud of gas and dust
from which the sun and planets formed.
TITAN
 a set of
four near
infrared
pictures of
Titan
 visible light
image of Titan
on the right
taken by
Voyager 2.
 In 1944, Gerard Kuiper discovered that Titan, the
largest moon of Saturn, had an atmosphere. He
detected the methane in Titan's atmosphere by
studying its infrared emission.
 In 1994, astronomers using the Wide Field Planetary
Camera on the Hubble Space Telescope made the
first images of the surface of Titan.
 These images were made in the near-infrared since
infrared radiation is able to penetrate the hazy
atmosphere of Titan. The infrared images of Titan's
surface show a bright area which is a surface feature
that is about 2,500 miles across (about the size of
Australia).
Uranus and Neptune
 Emission from Uranus and Neptune at different wavelengths in
the Herschel range comes from different levels in their
atmospheres, so will be sensitive to properties such as
temperature, pressure, and composition and how they vary with
altitude.
 Tracing molecules at various altitudes provides information on
how convection and winds move gases in the atmospheres – in
other words, we can examine the weather deep inside the
atmospheres of the giant planets.
 Gases like water, methane, carbon monoxide and ammonia are
present and will show up through characteristic spectral
features at particular wavelengths.
Neptune
 The diagram shows a theoretical model of what the spectrum
of Neptune may look like as observed by SPIRE.
 By comparing the actual observations with such models, we
will learn a lot about the structure and composition of
Neptune’s atmosphere.
Uranus
 Uranus will be used as a standard “calibration” source
by the SPIRE instrument – in other words, the
brightness of other sources observed by SPIRE
(galaxies, stars, comets, etc.) will be measured by
comparing them to Uranus. The reason for the choice
of Uranus as the standard calibration source is that we
already know quite accurately how warm it is.
 It turns out to be the best choice, since Uranus was
first discovered by William Herschel himself.
ASTEROIDS:
 Asteroids are rocky-metallic objects which range in size
from about 1 to 1000 km. They orbit the Sun and are
thought to be leftover material from the formation of the
planets in our solar system.
 Most of the asteroids are found in the Asteroid Belt which
lies between the orbits of Mars and Jupiter.
 Astronomers have also identified a group of asteroids
whose orbits cross Earth's orbit.
 The infrared radiation from an asteroid can be used to get
information about its location, composition, rotation and its
shape and size.
 The IRAS mission discovered over 400 new asteroids and
Kuiper Belt objects, dwarf
planets
 Although most of the well-known asteroids are to be
found between the orbits of Mars and Jupiter, it has
recently been discovered that there is a large population
of asteroids much further out from the Sun, past the orbit
of Neptune. These are so-called , Kuiper Belt objects,
trans-Neptunian objects, about which we do not know
very much, but we believe them to be remnants of the
primitive planetesimal disk from which the solar system
formed. Some of them are large enough to have been
given the name “dwarf planets”. In fact, Pluto, was
recently relegated to the status of a dwarf planet. The
pictures below show the location of the trans-Neptunian
belt and the sizes of some known dwarf planets in
Kuiper Belt objects, dwarf
planets
 Fig
the location of the trans-Neptunian belt and the sizes of some
known dwarf planets in comparison to that of the Earth.

The trans-Neptunian asteroids and dwarf
planets are very cold (about 40 degrees above
absolute zero) – but that’s actually warm
enough for them to emit light at Herschel
wavelengths. The diagram to the left shows the
expected thermal spectrum that Herschel could
measure for dwarf planets of different sizes
and distances from Earth.
COMETS
 Comets are basically dusty snowballs which orbit the Sun. They
consist of an icy nucleus surrounded by a large cloud of gas
and dust (called the coma). The coma is created as the ice in
the nucleus is warmed and vaporizes.
 Comets have 2 tails, a straight gas tail and a curved dust tail.
 The gas tail is created by the solar wind whose magnetic fields
pull the gas away from the comet's coma.
 The dust in the coma is not affected by magnetic fields but is
pushed out by the Sun's radiation.
 The dust in the tail reflects sunlight and radiates in the infrared.
 The infrared emission from comets can be used to get
information on the nature of the dust they contain as well as on
the rate at which material is being lost from the nucleus.
comets
IRAS found that dust from comets fills the Solar System and that comets
are dustier than they were thought to be. Many of the meteors may be
the larger pieces of this comet dust.
 IRAS discovered 6 new comets including comet IRAS-Araki-Alcock and
collected infrared data on 25 previously known comets.
 Comets are balls of dust, rock and ice left over from the formation of the
solar system.  We can learn about the raw material from which the Sun
and the planets formed.
 Herschel will look at comets, detecting the thermal radiation from the
nucleus and the spectral features of the gases that evaporate from the
nucleus when the comet gets close to the Sun. Water is especially
important to reconstruct the early development of the solar system, and
determine whether comets were the source of water and pre-life
chemicals on primitive Earth. It is another pleasing connection with the
Herschel family that Caroline Herschel, sister of William, became famous
herself as the discoverer of eight.
COMET IMPACT WITH JUPITER
 In 1994 several fragments of comet Shoemaker-Levy 9, which
had broken apart during its previous orbit in 1992, collided with
the planet Jupiter. These impacts released a tremendous
amount of energy into Jupiter's atmosphere. Spectacular
images of comet Shoemaker-Levy's collision with Jupiter were
taken in the infrared.
The image shows the impact of
fragment A (Courtesy of NASA/NSSDC,)
The image is an infrared color
composite showing fragments
A,E F,G and H impacting Jupiter
(Courtesy of NASA/NSSDC).
ZODIACAL DUST BANDS
 IRAS also discovered bands of infrared emission that girdle our solar
system.  Called the zodiacal dust bands, these are likely to be
debris from colliding asteroids.
 Two bands appear 9 degrees above and 9 degrees below the ecliptic,
which result from debris in an orbit about the sun that is inclined by 9
degrees to the ecliptic. The bands result from the particles spending
more time at the extremes of their orbit, causing an apparent increase
of density at plus and minus 9 degrees.
 Another one is found in the ecliptic plane.
 The infrared emission of these bands show a temperature of 165-200K
and a distance of 2.2 - 3.5 AU from the Sun This places these dust
bands between Mars and Jupiter in the region of the asteroid belt.
2. Star Formation
 Dense molecular clouds are always observed around starforming regions. New stars condense out of these clouds
through gravitational collapse triggered by an instability
(e.g. Jeans instability for self-gravitation) or shock wave
(e.g. from an exploding supernovae).
 The formation of protostars and their evolution to the
main-sequence of the Hertzsprung-Russell diagram are
main topics of stellar astrophysics.
 But we will be focuss in here on the observable effects of
star formation on the ISM: photoionisation of surrounding
gas, absorption and re-radiation of starlight by dust.
protostar
 Many of the most interesting infrared objects are associated
with star formation. Stars form from collapsing clouds of gas
and dust. As the cloud collapses, its density and temperature
increase. The temperature and density are highest at the center
of the cloud, where a new star will eventually form.
 The object that is formed at the center of the collapsing cloud
and which will become a star is called a protostar
 Since a protostar is embedded in a cloud of gas and dust, it is
difficult to detect in visible light. Any visible light that it does
emit is absorbed by the material surrounding it.
 Only during the later stages, when a protostar is hot enough for
its radiation to blow away most of the material surrounding it,
can it be seen in visible light.
 Until then, a can be detected only in the infrared.
star forming regions
 The light from the protostar is absorbed by the dust surrounding it,
causing the dust to warm up and radiate in the infrared.
 Infrared studies of star forming regions will give us important information
about how stars are born and thus on how our own Sun and Solar
System were formed.
 Infrared telescopes have cataloged thousands of hot, dense cores within
clouds of gas and dust which could be newly forming stars.
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Two infrared images of star forming regions:
an IRAS view of the constellation Orion in
which there are several regions active star
formation, and an image from the new
Spitzer Space Telescope of newborn stars
in the nebula NGC 7129.
T-Tauri stars and Herbig-Haro
objects
 Protostars which are starting to blow away the gas and dust
surrounding them are called T-Tauri stars.
 The warm dust remaining around T-Tauri stars still radiates in the
infrared.
 There is evidence that the remaining dust and gas surrounding T-Tauri
stars form rotating disks which may mark the beginnings of planetary
systems.
 Herbig-Haro objects, which are also associated with newly forming
stars, can be studied in the infrared. These are small nebulae which
vary in shape and brightness over a period of only a few years.
 Both Herbig-Haro objects and T-Tauri stars are found in regions of
active star formation.
 It is thought that these nebulae represent high speed gas flowing from
young stars colliding with interstellar clouds.
 The study of T-Tauri stars will help us understand the details on how
T-Tauri stars and KleinmannLow Nebula
an infrared adaptive optics image
of a T-Tauri stars (UY Aur).
an infrared image of the Kleinmann-Low
Nebula, a region of intense star
formation in the constellation Orion.
CISCO, Subaru 8.3-m Telescope, NAOJ
newborn stars
 In visible light much of this region is hidden by dust however in
the infrared you can see the effects of the hot winds produced
by newly formed massive stars.
 These hot winds heat up the surrounding gas and cause them
to radiate strongly in the infrared.
 The winds will eventually clear much of the gas and dust
surrounding the stars.
The infrared image shows stars and
glowing interstellar dust heated by the
intense starlight of the newborn stars.
Here is a comparison of a visible light
(left) and an infrared (right) view of
OMC-1 (OMC stands for Orion
Molecular Cloud) taken by the Hubble
Space Telescope
Globules
 Globules are also found in star forming regions. These are small
clouds (~ 1 light year in diameter) which contain 10 to 1000 solar
masses of gas and dust. In visible light, globules are seen in dark
silhouette against bright nebulae. They produce no visible light of their
own and are thought to be collapsing clouds which will produce stars.
 Infrared observations from IRAS showed that some globules contain
protostars.
an infrared image of the globule in the nebula
IC 1396 taken by the new Spitzer Space
Telescope. Spitzer's sensitive infrared detectors
unveiled the brilliant hidden interior of this
opaque cloud of gas and dust and revealed, for
the first time, a glowing stellar nursery with
never-before-seen young stars and stars still in
the process of formation. The inset shows a
visible light view of the same region.
3. Stars
Infrared observations
have led to the
discovery of a large
number of stars which
are too cool to be
detected by their
visible light or are
hidden behind
obscuring dust.
 Infrared observations
have also led to the
discovery of several
stars which have
orbiting material.
Above is an image of infrared point sources in the
entire sky as seen by the Infrared Astronomical
Satellite (IRAS). The plane of our Galaxy runs
horizontally across the image. Blue sources are cool
stars within our Galaxy, which show an obvious
concentration to the galactic plane and center.
Yellow-green sources are galaxies which are basically
uniformly distributed across the sky, but show an
enhancement along a great circle above the galactic
plane. This enhancement is caused by galaxies in the
Local Supercluster. Reddish sources, the infrared
cirrus, are extremely cold material close to us in our
own Galaxy. Black areas were not surveyed by IRAS.
the brightest stars in our galaxy
 In 1997, the infrared camera on the Hubble Space
Telescope (NICMOS) revealed one of the brightest
stars in our galaxy .This star, which is 10 million
times more radiant than our Sun, was discovered in
the center of our galaxy where it was hidden from
visible light telescopes by thick dust.
In the image you can see two expanding
shells of gas being ejected by the star in
one of the largest stellar eruptions ever
seen. Don F. Figer (UCLA) and NASA,
AURA/STScI
Wolf-Rayet 104, a "spiral" star
 A "spiral" star which was discovered in the infrared, Wolf-Rayet 104 is
3 times the size of our sun and 100,000 times brighter. Because it is
so large and radiates so intensely, part of its outer atmosphere is
being blown off. Wolf-Rayet 104 is a binary star system - its
companion is a smaller OB star.
 The material being ejected from Wolf-Rayet 104 is swept into a spiral
pattern by the stellar winds of both stars.
UC Berkeley Space Sciences
Laboratory/WM Keck Observatory
New Star Cluster
 Stars are often form in groups called star clusters. The stars in a
cluster will usually move slowly away from each other as they age.
Many new star clusters are still partially hidden by the dust and gas
leftover from star formation, making them more difficult to view in
visible light. Infrared light, however, can penetrate this dust and
provide us with a deeper view of the cluster.
an infrared view of the Quintuplet Star
Cluster which lies near the center of our
Milky Way Galaxy. Since the center of
our galaxy is a very dusty place, infrared
observations are often the best way to
view objects embedded in and hidden
by this dust. This image taken with the
NICMOS instrument on the Hubble
Space Telescope is the clearest view yet
of this cluster. Don Figer (STScI) et al.,
Brown Dwarfs
 brown dwarfs - objects whose mass
is between twice that of Jupiter and
the lower mass limit for nuclear
reactions (0.08 times the mass of our
sun).
 They can be discovered by infrared
telescopes, even deep within thick
clouds.
 An infrared image of the Trapezium
star cluster in the Orion Nebula, was
part of a survey done at the United
Kingdom Infrared Telescope ( UKIRT )
in which over 100 brown dwarf
candidates were identified in the
infrared.
an infrared image of the
Trapezium star cluster in the
Orion Nebula.
4. Extrasolar Planets
 In the 1980's, astronomers using IRAS data discovered about two
dozen stars which had infrared-emitting dust surrounding them,
extending hundreds of astronomical units from the stars.
 This discovery inspired astronomers to make more detailed
observations of these stars.
 What they found around these stars were flat, disk-shaped, areas of
dust in which planets have formed or could be forming.
 These findings have led the way to one of the most exciting new areas
of research in astronomy - the search for planets around other stars.
 Among the stars studied were Beta Pictoris, HL Tauri, Vega, Epsilon
Eridani and Alpha Piscis Austrinus. The discovery of these disks
provided the first significant evidence that other solar systems might
exist.
Beta Pictoris
HR 4796A
 An infrared image of Beta Pictoris : The
an infrared view of a disk
around the star HR 4796A
presence of a warp in this disk indicates the
existence of a Jupiter-sized planet around this taken with the Hubble's
NICMOS camera. the light from
star. There is also evidence for the existence of the star (which is about 1000
comets around Beta Pictoris..
times brighter than the disk) is
 The visible light from a planet or disk of material blocked so we can better see
the ring of material around the
is hidden by the brightness of the star that it
star.
orbits.
 In the infrared, where planets have their peak
brightness, the brightness of the star is reduced,
making it possible to detect a planet in the
infrared.
 To aid in the detection of planets, use occulting
disks to mask out the light from a star allowing Beta Pictoris : J.-L. Beuzit
for a better view of possible planets.
et al. . (Grenoble Obs.), ESO
5. Our Galaxy
 Our galaxy, the Milky Way, radiates about half of its luminosity in the
infrared. The Milky Way is a spiral galaxy containing over 100 billion
stars. It is over 100,000 light years in diameter and has a disk
containing spiral arms and a dense central sphere or bulge. The center
of our galaxy is not visible at optical wavelengths because it is hidden
behind numerous clouds of gas and dust.
 However we can view the center of our galaxy in the infrared, since
infrared rays can penetrate gas and dust.
The image is a combination of infrared
data from the 2MASS. The Galactic plane
runs horizontally along the image, and the
Galactic center is the bright (yellow)
object near the middle. In the blue regions
(2MASS) many stars invisible to optical
telescopes can be seen in the infrared.
The red areas (MSX) show the distribution
of dust near the center of our galaxy.
2MASS Project, Umass, IPAC/Caltech,
our galaxy
from 2MASS
from the COBE satellite
 The center of our galaxy is one of the brightest infrared sources in the
sky. It is about one thousand times brighter in the infrared than at radio
wavelengths. Infrared observations show that the center of our galaxy
consists of a very dense crowding of stars and that stars and gases
near the center are orbiting very rapidly (probably due to the existence
of a black hole).
 The image, a near-infrared view of the center of our Milky Way Galaxy
from 2MASS, shows multitudes of otherwise hidden stars. The central
core of our galaxy is the brighter region at the upper, left portion of the
image.
An infrared image of the entire sky from the COBE satellite shows the
bright band in the middle of the image, our Milky Way galaxy. This is
how our galaxy appears from our vantage point in the sun's orbit about
the center of our galaxy. Our solar system is located far out in the disk
of our galaxy at a distance of about 30,000 light years from the center
Galaxies
 The infrared emission from galaxies comes
primarily from three sources: stars, interstellar gas,
and dust.
 The emission from stars peaks in the near-infrared
(1-3 microns).
 Emission from atoms and molecules in interstellar
gas makes up only a few percent of the infrared
output of galaxies.
 The primary source of infrared radiation beyond 3
microns is thermal emission from dust grains
heated by starlight.
An infrared image of the spiral galaxy M83
(European Southern Observatory) showing the
infrared glow of stars in the spiral arms.
Starburst Galaxies
 The brightest infrared galaxies are usually the
ones which have a lot of dust (in star-forming
regions for example).
 Astronomers using the IRAS satellite observed
20,000 galaxies in the infrared.
 Many of these were starburst galaxies galaxies which are forming enormous numbers
of new stars, and are thus extremely bright in
the infrared.
 Further infrared studies of these galaxies may
find the cause of this star-forming frenzy.
 About half of the luminosity of an average
spiral galaxy is radiated at far-infrared
wavelengths. Elliptical galaxies are faint in the
infrared because they no longer have much
gas and dust.
An infrared image of the
starburst galaxy M83.
Spiral galaxies, which are
rich in gas and dust, are
strong infrared sources
and are still forming new
stars.
Galaxies
 infrared images of three : Andromeda galaxy taken by the IRAS
satellite (Notice the regions where young stars are forming
shown in yellow and red).
 The barred spiral galaxy NGC 1364 (see the bright areas of star
formation, as well as young star clusters) taken in 1999 (shows
the improvement in resolution since the days of IRAS (sixteen
years earlier).
 The edge on spiral galaxy NGC891(clearly see the lanes of
dust along the edge of the galaxy).
Infrared images of the Andromeda galaxy (NASA/IPAC/IRAS), the barred spiral galaxy NGC 1364 (C.
Marcella Carollo (JHU, Columbia U.), NASA, ESA), and the edge-on spiral galaxy NGC 891 (JC
Barentine (PSI) et al., KPNO, NOAO). (JC Barentine (PSI에) 외., KPNO, NOAO).
Star formation in spiral galaxies
 Star formation in spiral galaxies is concentrated in the
spiral arms. This is evident from the observed Balmer
recombination emission (red) from photoionised regions
around hot young stars, interspersed with dark regions
obscured by dense dust and molecular clouds.
Messier 81
 The magnificent, dusty, star-studded arms of a
nearby spiral galaxy, Messier 81 : infrared
image from NASA's Spitzer Space Tele. shows
us old stars, interstellar dust heated by star
formation activity, and embedded sites of
massive star formation within this galaxy.
 The bluish-white central bulge of the galaxy
contains older stars and only a little dust.
Winding outward from the bulge are the grand
spiral arms which are very rich in infrared
emitting dust.
 The infrared-bright clumpy knots within the
spiral arms show the places where massive
Spitzer view of Messier 81
stars are being born in giant molecular clouds.
The greenish areas are regions dominated by
the infrared light radiated by hot dust that has
Antennae galaxies
 Sometimes galaxies, each containing billions of stars, collide with
each other.
  These collisions trigger star formation in these galaxies
 They do this by compressing gas and dust to the point where this
material starts to collapse under its own gravity.
 Due to a high rate of star formation, colliding gas-rich galaxies radiate
very strongly in the infrared.
an infrared image of two galaxies (called the
antennae galaxies) in collision. the bright
areas of intense star formation and the glow
from the centers of the two galaxies.
(Bernhard Brandl and the WIRC team
(Cornell), Palomar Observatory)
Hubble Deep Field : HST
 During December 1995, the Hubble
Space Telescope scanned a small area
of the sky to make the deepest image of
the sky ever - this area is called the
Hubble Deep Field.
 In 1996, Astronomers using ISO found
that many of the faint galaxies detected
by the Hubble Space Telescope in the
Hubble Deep Field radiate most of their
energy in the infrared and are going
through a period of very active star
formation.
 Recently, several new galaxies were
discovered behind the Milky Way in
Infrared View of the 30 Largest
near-infrared 2MASS images.
Galaxies (2MASS)
5. ISM
 Much of the space between the stars is filled with
gas (primarily hydrogen and helium) and tiny pieces
of solid particles or dust (composed mainly of
carbon, silicon and oxygen).
 In some places this interstellar material is very
dense, forming nebulas.
 In other regions the gas and dust density is very
low. The image shows an infrared view of the gas
and dust in our galaxy along the plane of our Milky
Way galaxy. Here you can see areas of dense gas
and dust as well as areas which are nearly empty.
 Most of this gas and dust originates from the death
of stars which either exploded (supernova) or blew
off their outer layers, returning their material to
interstellar space.
W. Waller and F.
Varosi (GSFC),
IRAS, SkyView,
NASA
 Often, the gas and dust between the stars can be
detected only in the infrared.
 By using infrared detectors, astronomers can
penetrate the often invisible interstellar gas and dust
clouds and gain much information about their
composition and structure
 By using infrared detectors, astronomers can
penetrate the often invisible interstellar gas and dust
clouds and gain much information about their
composition and structure
dying stars
 Below are two infrared images taken by
NICMOS of material being ejected into
space by dying stars.
Credits (Left Image): Rodger Thompson, Marcia
Rieke, Glenn Schneider, Dean Hines (University of
Arizona); Raghvendra Sahai (Jet ProInstrument
Definition Team, and NASA/AURA/STScI
Credits (Right Image): Credits: William B. Latter
(SIRTF Science Center/Caltech) and
NASA/AURA/STScI
pulsion Laboratory); NICMOS
"infrared cirrus
 A surprise discovery from the IRAS mission was that space is
filled with faint wisps of dust which cannot be seen in visible
light.This has been given the name "infrared cirrus" because it
resembles the cirrus clouds in the Earth's atmosphere.
 Infrared cirrus is very cold (15-30 K) and can only be detected
in the infrared.
IRAS image of cirrus at the south
celestial pole and ISO-LWS
spectra of infrared cirrus
 Astronomers using ISO discovered
emission lines from interstellar water
vapor in a variety of sources including
star form ing regions, planetary nebulae
and near formed stars.
 ISO also discovered for the first time
hydrogen cyanide ice molecules in a
dusty cloud surrounding a newly
forming star.
ISM
 The ISM is a multi-component, multi-phase medium. Its main constituent
is hydrogen gas, but all components produce distinctive emission and
absorption spectral signatures. Not all phases are in pressure equilibrium.
Interstellar dust, whilst comprising < 0.1% of the ISM, plays an
exceptionally important role in the physics and chemistry of the ISM, star
formation, and our interpretation of spectra from astrophysical sources.
 gas component
phase
 neutral
molecular

cold

warm
 ionised
diffuse

HII

coronal
T (K)
N (m−3)
50 − 100
109
100
25 × 106
8 × 103
0.25 × 106
8 × 103
0.03 × 106
104
106−10
5 × 105
6 × 103
M(Mo)
109?
1.5 × 109
1.5 × 109
109
5 × 107
108?
Dust
 mostly grains or aggregates of molecules containing carbon, graphite,
silicates
 • grain sizes a range from nm to μm
 • intimately mixed in with gas
 • formed in atmospheres of old stars, novae, supernovae
 • destroyed by UV irradiation (evaporation)
 • re-radiated IR dust emission traces star formation
 • grains act as catalysts for chemical reactions in the ISM, providing
surfaces on which molecules (e.g. H2) can form
 • charged grains align themselves along the ISM magnetic field B and
contribute to polarisation
 • grains have a broad distribution of sizes, shapes,T and N l
 • reduction in overall optical brightness of objects seen through dust is
called interstellarextinction; strong wavelength dependence on
absorption and scattering – generally, l < a, implying UV/optical light
suffers most from dust extinction:
 • absorbed UV/optical light is re-radiated in the IR
Nebulae
 These are clouds of gas and dust in
the ISM. They fall broadly into 3
categories:
 1. Emission nebulae, or HII regions
 Appear red, violet, blue Balmer
reddish in colour due to emission of
recombination lines (m -> n = 2
transitions in HI, where m = 3, 4, 5
are the most important transitions).
These transitions are: Ha lambda
6563 °A , Hb 4861 °A and Hg
4340 °A .
Nebulae
 2. Reflection nebulae: clouds
illuminated by
 starlight; the flux is too low to
photoionise hydrogen;
 appear blue in colour since we see
scattered light
 which is preferentially scattered at
short wavelengths
 by dust in the clouds
 3. Dark nebulae: dense clouds which completely extinguish
starlight from our line of sight; the dust in these clouds absorbs
and scatters the optical/UV starlight; we could only see the
stars in the IR, where there is less extinction; this is called
interstellar reddening
IS Gas
• atomic hydrogen (HI) produces the 21 cm (1420MHz) radio signal, which
can be either in emission or absorption
 • neutral ISM gas produces interstellar absorption lines, which are much
narrower than stellar absorption lines because the ISM gas is much
cooler gas is shock-ionised by local novae/super-novae and emits
rcan use absoprtion lines to determine abundances of elements in ISM
gas
 • coronal radio and X-ray synchrotron radiation due to relativistic,
nonthermal electrons and magnetic fields
 • ionised ISM gas is coupled to a highly turbulent B-field
Multi-wavelength images
Molecular clouds
 Although atomic gas contains most of the mass of the ISM, molecular
gas is the densest component and is responsible for star formation. Most
of what we know about ISM molecules comes from spectroscopy.
 Molecules can have 3 types of transitions: electronic, vibrational and
rotational. The transitions are almost always seen in absorption because
of the extremely low temperatures of ISM molecular gas. Electronic
transitions usually occur in the far-UV, whilst the weaker vibrational and
rotational transitions occur in the mm/submm wavebands (i.e. between
radio and FIR). The most abundant ISM molecule is H2, although the
most observed molecule is CO.
Orion molecular cloud,
 A portion of the sub-mm spectrum of the Orion molecular
cloud, showing strongest lines and atmospheric transmission.
The lines are seen in emission due to the relatively high
temperature of the cloud (T ~60 K), such that some highly
excited vibrational levels and rotational sublevels are
populated.
6. The Early Universe
 The billions of galaxies outside our own galaxy range in distance
from hundreds of thousands to billions of light years away. For the
most distant galaxies, we see them as they were billions of years
ago.
 As a result of the Big Bang (the tremendous explosion which marked
the beginning of our Universe), the universe is expanding and most
of the galaxies within it are moving away from each other. All distant
galaxies are moving away from us and the farther away they are, the
faster they are moving.
 This recession of galaxies away from us has an interesting effect on
the light emitted from these galaxies, the light that galaxy emits is
"redshifted" , the wavelengths of light get longer and are shifted
towards the red part of the spectrum.
 This means that infrared studies can give us much information about
the visible spectra of very young, distant galaxies.
The Early Universe
 An infrared view of some of the farthest galaxies ever seen,
taken by the Hubble Space Telescope's NICMOS camera.
 Infrared studies have also found a potential protogalaxy (a
galaxy in the process of formation) more than 15 billion light
years from Earth.
 This object, named IRAS 10214+4724, may be a huge,
contracting hydrogen cloud just beginning to shine with
newborn stars. This is close to the edge of the observable
universe and its light has taken since nearly the beginning of
the universe to reach us.
 Protogalaxies provide us with a look at the era when galaxies
were first coming to life.