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The mid- and far-infrared range:
radiation emission processes from
interstellar dust and gas
A.G.G.M. TielensI
Abstract
The thermal-infrared wavelength region contains the spectral signatures of solidstate compounds as well as a variety of lines emitted by atoms and molecules in
the gaseous state. Together, these signatures provide unique diagnostic probes of
the physical and chemical conditions of the cool and dusty Universe. This article
summarizes the contribution of space-based observations in the thermal infrared to
our understanding of the Universe. In particular the formation of stars and planets,
the characteristics of interstellar dust and of polycyclic aromatic hydrocarbons as
well as the interstellar media of galaxies are discussed.
Introduction
All cool and dusty objects in the Universe emit infrared (IR) radiation. Atoms
and neutral molecules and dust attain temperatures in the range of about 5 K to
1000 K and therefore emit most of their energy at mid- and far-IR wavelengths.
Generally, the near-IR range is defined as extending from 0.75 µm to 2.5 µm and
the mid-IR range runs from 2.5 µm to about 25 µm. The far-IR region covers
wavelengths from 25 µm to 100 µm, and is then going over into the sub-millimetre
domain. The spectra of cool interstellar and circumstellar regions show absorption
or emission, characteristic for dust compounds. In addition, broad emission bands
are often present due to fluorescence of large molecules, as well as narrow emission
lines that can be assigned to individual atomic and molecular transitions. These
features provide ‘fingerprints’ of the absorbing or emitting compounds. Indeed,
infrared spectroscopy is an excellent astronomical tool for studying the chemical
composition and the physical characteristics of dust and gas in space. Based upon
extensive laboratory studies, observed spectral features of dust can be identified
with definite minerals and carbonaceous compounds and the abundances of these
species can be derived. For a gas, the presence of multiple lines allows determination
I Space
Sciences Division, NASA ARC, Moffett Field, USA
current address: Leiden Observatory, The Netherlands
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7. The mid- and far-infrared wavelength range
both of the abundance of the emitting species and of the excitation conditions of
the gas (such as temperature and density).
Over the last three decades our understanding of interstellar dust and gas has increased dramatically. To a large extent this has been driven by rapid developments
in IR detector technology. Sensitive IR spectrometers operating in all the IR windows are now standard at all major ground-based infrared observatories. The opening of the full infrared window by space missions further increased our knowledge
of interstellar dust. The Low Resolution Spectrometer (LRS) on IRAS — a slitless
spectrometer sensitive from 7.5 µm to 23 µm with a resolving power of R ≈ 20 — has
provided a first systematic overview (Olnon et al 1986) of the spectral complexity
of interstellar and circumstellar dust.1 The Short Wavelength Spectrometer, SWS
(de Graauw et al 1996), and Long Wavelength Spectrometer, LWS (Clegg et al
1996), on board ISO, represented the next big step forward by providing complete
2.5 µm to 200 µm spectra of a multitude of sources — essentially all IR-luminous
galactic sources — with resolutions ranging from 100 in the long wavelength range
to up to ≈ 2000 at shorter wavelengths. The Infrared Spectrometer (IRS) on board
Spitzer — operating from 5.2 µm to 38 µm at low spectral resolution (60 to 130)
and from 10 µm to 37 µm at moderate resolution (R ≈ 600) — brought further
improvements (Houck et al 2004). With its superior sensitivity, the IRS permits
systematic spectroscopic studies of dust in typical sources in nearby galaxies and
in bright galactic nuclei out to redshifts of ≈ 2.
In the next section, we will review the observational techniques and the advantages and disadvantages of space-based versus ground-based platforms in the
thermal IR. Subsequently, we will briefly review the physical processes leading to
emission and absorption in the IR and illustrate the impact of space-based observations in the thermal IR on our view of the Universe. We shall focus on star and
planet formation, crystalline silicates, interstellar polycyclic aromatic hydrocarbons
(PAH) and on galaxies and ultraluminous IR galaxies (ULIRG).
Thermal infrared and space
Ground-based astronomy is limited by the atmosphere in the IR. Telluric absorption in the rotational and ro-vibration bands of atmospheric gases completely
blocks transmission at wavelengths between ≈ 30 µm and 300 µm and allows observations only in narrow windows shortwards and longwards of this (Figure 7.1).
In the mid-IR, windows are the M band from ≈ 4.5 µm to 5.2 µm, the N band from
≈ 8 µm to 13 µm, and the Q band from (18 to 23) µm, but these windows are
still riddled with narrow atmospheric features. In the sub-millimetre range, there
are several windows: including around (340, 410, 650, 690 and 800) GHz. Going
to space eliminates the telluric absorption and allows full spectral coverage over
a wide wavelength range. The SWS and LWS on ISO have used this to survey
the 2.4 µm to 200 µm range, the IRS on Spitzer has sampled the 5 µm to 40 µm
range, while Herschel will cover the full far-IR and sub-millimetre range (60 µm to
600 µm).
1 available
through http://irsa.ipac.caltech.edu/IRASdocs/surveys/lrs.html
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Figure 7.1: Atmospheric transmission in the thermal IR from an excellent groundbased site (Mauna Kea). Because of molecular absorptions, over most of the midand far-IR (5 µm < λ < 1 mm) the atmosphere is completely opaque, except
for a few narrow windows in the mid-IR and sub-millimetre ranges. Background
image: the Sombrero galaxy at IR wavelengths obtained by the IRAC instrument
on Spitzer, courtesy NASA/JPL-Caltech.
In addition, ground-based observations are limited by thermal emission from
telescope and atmosphere. A bright source may only have a contrast of 10−3 relative
to this background emission. This thermal background changes rapidly and on small
scales. It dominates as background noise over Poisson noise and further hampers
detection. As a result, special ‘chopping’ and ‘nodding’ techniques are required to
extract the source signal as the difference between emission from the object location
and nearby patches of sky. Chopping the secondary mirror at a rapid rate (a few
hertz) compensates for the sky noise: the signal at the source position (including
the sky) is compared with that at a nearby patch of (blank) sky (typically at
a distance of some 15′′ ). Nodding involves moving the source position into the
other chop-beam and repeating the chop-sequence at a much slower (nodding) rate
(typically once a minute) to correct for small differences in telescope optics between
the two chop positions. In space, active cooling by use of a cryostat or passive
cooling by use of sunshields cuts down the thermal background considerably and
leads to a great gain in sensitivity. The low and stable background from space
allows sensitive IR studies throughout the thermal IR. Figure 7.2 compares the
sensitivity for line observations with space-based and ground-based IR and submillimetre observatories, illustrating the gain in sensitivity. This is typically well
over two orders of magnitude. One disadvantage of space-based operations is the
severe limitation on telescope aperture: ISO and Spitzer had mirrors with diameters
less than 1 m because they had to fit within a dewar. Herschel is and JWST will
be passively cooled. Herschel has a solid 3.5 m primary mirror which could be fit
within the Ariane nose cone; JWST, on the other hand, will have to deploy its
primary mirror of 6.5 m diameter after launch.
The cold environment of space prevails over the larger ground-based apertures. Spitzer, for example, is well over an order of magnitude more sensitive
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7. The mid- and far-infrared wavelength range
Figure 7.2: A comparison of sensitivities for line detection from point sources by
space-based and ground-based observatories throughout the IR and sub-millimetre
range. The curves correspond to the line flux (i.e., the irradiance in an isolated
spectral line) required to achieve a 10 σ detection in 104 s for the different observatories. Detection of wider features depends on the resolution of the instrument.
These are for the grating instruments on JWST R ≈ 3000, on Spitzer R ≈ 600,
on Gemini R ≈ 200 in N and R ≈ 100 in Q, on ISO R ≈ 1500 for λ < 45 µm,
R ≈ 200 for λ > 45 µm, Herschel R ≈ 1500, and SOFIA R ≈ 300 and 105 . For
the sub-millimetre heterodyne systems on Herschel, SOFIA, and for ground-based
observations, a line width of 1 km/s (R = 3 × 105 ) has been assumed.
than an 8 m-class ground-based telescope in the mid-IR (Figure 7.2). In the submillimetre region, heterodyne techniques (see Chapter 31, Wild 2010) have reached
the quantum-noise limit and high spectral resolution leads to high sensitivity. In this
wavelength range, sensitivities are very similar for space-based and ground-based
observatories: the advantage of increased aperture size for ground-based telescopes
is nullified by the limited atmospheric transmission (≈ 0.2 to 0.6). ALMA, the
Atacama Large Millimeter Array, will be some three orders of magnitude more
sensitive at long wavelengths than single-dish telescopes because of sheer collecting area and moreover will provide the high spatial resolution of an interferometer.
Nevertheless, even with ALMA fully operational, space offers the distinct advantage
of contiguous coverage of the full far-IR and sub-millimetre wavelength regions and
permits, for example, unbiased spectral surveys as well as the observation of lines
from key species such as H2 O, the light hydrides, and the (modestly redshifted)
line at 157.4 µm in the C ii spectrum.
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We note that, for space-based missions, even if a telescope is passively cooled,
the instruments will need active cooling to much lower temperatures. Cryogen then
becomes a limitation, and lifetimes of IR space missions are typically two to five
years. Likewise, because of the long design and construction phases as well as the
lack of access during the mission, science instrumentation generally lags behind
current technology. In the far-IR, besides the increase in detector sensitivity still
possible, an increase in the size of the telescope area and the use of heterodyne
arrays provide other clear avenues for increasing observing “speed”, in the latter
case, particularly when mapping large areas on the sky. Thus, for extended objects,
the 5 × 5 element spectrometer array PACS on Herschel gains substantially over
the LWS on ISO. Of course, mapping on a space-based observatory always comes
at a hefty penalty in terms of pointing overheads. A nimble airborne telescope such
as SOFIA holds a substantial advantage for (spectroscopic) mapping of (bright)
very extended emission regions, actually throughout the thermal IR. Airborne observatories such as SOFIA can also react more rapidly to changing technology (e.g.,
Moore’s law, in principle, would provide a factor of 10 in observing “speed” every 5
to 10 years). SOFIA thus provides an important testbed for future instrumentation
and a driver for technology development in the thermal IR. For science objectives
which require high spectral resolution, SOFIA is already very competitive in the
sub-millimetre range. Future heterodyne instruments throughout the thermal IR as
well as heterodyne arrays will reinforce SOFIA’s distinct and unique scientific niche
in high-resolution spectroscopy. SOFIA also holds an advantage in that weight and
cryogen restrictions are largely non-existent on a Boeing 747 aircraft, which accomodates instruments of larger volume and mass than those designed for use in
space. Nevertheless, as Figure 7.2 demonstrates, the future of moderate-resolution
spectroscopy in the mid- and far-IR resides in space-based observatories such as
JWST, and missions such as SPICA and SAFIRE.
Physical processes
Interstellar gas is heated by ultraviolet (UV) photons (h ν > 6 eV) through
ionization and dissociation of atoms and molecules. Neutral and ionic atomic gases
emit primarily through emission in fine-structure lines of the dominant elements
when collisional excitation is followed by radiative decay. For molecular gas, pure
rotational transitions provide efficient cooling, and molecular gas therefore is generally relatively cold. The energy-level separation will have to match the thermal
energy of the gas for efficient collisional excitation of the upper level. In the two-level
approximation, the critical density of a transition i → j is given by ncr = Aij /γij
with Aij the Einstein coefficient and γij the collisional de-excitation coefficient of
the transition. For a multilevel system, the radiative and collisional rates have to
be summed over all possible downward transitions (cf., Tielens 2005). The critical
electron density is the density at which downward radiative and collisional transitions are equal. Below the critical density every upward collision is followed by
photon emission, this leads to cooling. At higher densities collisional de-excitation
takes over and cooling is suppressed.
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7. The mid- and far-infrared wavelength range
Figure 7.3: IR atomic fine-structure lines for different ionization stages of some
relevant elements. The horizontal axis shows the energy range over which ionization
can occur. Underneath each atomic or ionic species, the wavelength and critical
density of the transition(s) is indicated. The critical density for each transition is
expressed as ne /cm−3 = a × 10b , where a and b are given by a(b) in italics. It has
been evaluated at a temperature of 7000 K appropriate for an H ii region. (Figure
reproduced with permission from Martı́n-Hernández et al (2002).)
As a rule of thumb, a line will give information on gas with temperatures and
densities matching its energy-level separation and critical density. The abundance
of atoms in a given ionization stage, however, depends on density and temperature
as well. Figure 7.3 summarizes the characteristics of atomic and ionic fine-structure
transitions. In H ii regions, photo-ionization and electron recombination set the ionization balance and the abundance of an ionic species. This is controlled by the
energy required to ionize a given species. Because of their low excitation energy,
the IR fine-structure lines are not sensitive to the temperature of ionized gas. The
ratio of IR fine-structure lines, where two lines from the same ionization stage
of given element are available, nevertheless provides a good handle on the density of the emitting gas in the range spanning the two critical densities involved.
Thus, the [O iii] lines are used to study gas with densities in the range ≈ (102 to
104 ) cm−3 , while the [Ne iii] lines are sensitive to somewhat higher densities ≈ (104
to 105 ) cm−3 . The ratios of lines from adjacent ionized stages are very sensitive to
the ionizing radiation field. The lines from Ne, Ar and S include the dominant ionization stages of these elements and hence, when combined with H i recombination
lines, provide a direct handle on elemental abundances. ISO SWS and LWS have
provided a coherent data set for galactic H ii regions which have been analyzed
along these lines (Martı́n-Hernández et al 2002). Although the Spitzer IRS has
more limited spectral coverage, its superior sensitivity has permitted a systematic
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Jes K. Jørgensen - after Genzel (1991)
H2 density [cm-3]
102
104
H2 - low J (rot.)
emission/absorption
28 & 17 !m
CO - low J (rot.)
mm emission
OH, CH absorp.
(FIR, radio)
H2 - (ro-vib.)
emission 1-2 !m
CO - mid J (rot.)
submm emission
NH3 inversion
emission (1.2 cm)
metastable
CO - high J (rot.)
far-infrared emission
NH3 inversion
emission (1.2 cm)
non-metastable
Heavy top (rot.)
mm emission
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H2CO, HCN
HCO+, HC3N
Heavy top (rot.)
submm emission
Light hydride
rot. emission
non-metastable
H2O, OH, CH, NH3
108
H2 - mid J (rot.)
emission 3-10 !m
1010
CO - mid J (ro-vib.)
emission 4.6 !m
CO - high J
overtone bandhead
emission 2.3 !m
OH, SiO, H2O
maser emission
30
100
300
1000
3000
Kinetic temperature [K]
Figure 7.4: An overview of the molecular lines and the range of physical conditions
in molecular clouds for which they are effective probes. Figure from J.K. Jørgensen
based upon a figure from Genzel (1991).
study of elemental abundance gradients in galaxies such as M83 and M33 (Rubin
et al 2007).
Figure 7.4 illustrates the range in temperature and density where specific IR
transitions will be important. These reflect the energy-level separation and critical
density of the transitions involved. As an example, mid-J rotational transitions
(with J being the rotational quantum number of the molecule, cf., Herzberg 1959)
of CO occur at shorter wavelengths than low-J transitions and hence they probe
warmer and denser gas. Likewise, as a hydride, H2 transitions occur at much shorter
wavelengths than those of the heavier molecule CO. However, as a homonuclear
molecule, H2 has only quadrupole-allowed transitions with much lower Einstein
coefficients than dipole-allowed transitions of heteronuclear molecules. Hence, the
pure rotational H2 transitions originate from warmer gas than low-J CO transitions, but the densities are quite similar. Molecular gas can always provide information on the physical conditions in the emitting gas at any temperature or density;
if several transitions of a species can be measured, the physical conditions of the
emitting gas can be determined more reliably. In addition, atomic or molecular
abundances can be measured, and more specifically, the organic inventory of space
may be determined.
Molecular species can also radiate in molecular ro-vibrational transitions due
to fluorescence, if pumped through UV absorption via electronic transitions. Of
particular importance are the CH and CC stretching and bending modes in large
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PAH molecules. In the interstellar medium (ISM), a PAH molecule typically absorbs
a UV photon every year. Rapid internal conversion after the absorption of the UV
photon leaves the molecule in a high vibrational excitation, from where it cools
through IR emission on a timescale of ≈ 1 s.
Interstellar dust grains are in radiative equilibrium with the local UV and visible
radiation field at temperature ranges from approximately 15 K in the diffuse ISM,
to about 75 K at 0.1 pc from an O star, to up to the sublimation temperature of
the material (typically 1000 K) near, i.e., 0.1 ua to 1 ua away from a protostar (cf.,
Tielens 2005). The peak of the emission from interstellar dust therefore shifts from
the far-IR to the mid-IR, depending on location. Dielectric interstellar dust grains
such as silicates and oxides show strong resonances due to various stretching and
bending modes. These bands are characteristic for a material and thus can be used
for identification purposes. In addition, if the temperature of the emitting grains is
known, the column of dust can be calculated as well.
The Universe at mid- and far-infrared wavelengths
Star and planet formation
In a seminal paper, Adams et al (1987) explained the classes of spectral energy
distributions uncovered by IRAS and from ground-based photometric studies of
sources embedded in molecular clouds as an evolutionary progression in the birth
of low-mass protostars. In this way, five classes were recognized: starting with a
quiescent prestellar core, followed by a collapse phase with a central object and
surrounding circumstellar disk, a clearing of the collapsing envelope by accretion
and a strong stellar wind, a “naked” protostar with surrounding planetary gas and
dust disk, and finally a star surrounded by a planet/planetesimal disk. Subsequent
studies have confirmed this general scheme for low-mass star and planet formation,
have filled in many of the details, and raised it to paradigm status.
ISO has revealed the important influence of flaring and shadowing on the spectral energy distribution of intermediate-mass protostars, the Herbig AeBe stars
(Meeus et al 2001). In addition, ISO was unique in opening up moderate-resolution
spectroscopy over a wide wavelength range and this has revealed the ubiquitous
presence of a wide variety of emission features including crystalline silicates and
PAH molecules in regions of star and planet formation. Spitzer has extended these
observations to disks around low-mass protostars as far down as brown dwarfs!
Much progress has been made in our understanding of star and planet formation
since then and this field is too rich to be reviewed here. The interested reader is
referred to van Dishoeck (2004) and Werner et al (2006).
Here, we do want to stress the importance of spectroscopy over a wide wavelength range. The 2.4 µm to 45 µm spectrum of one of the strong shock peaks in the
Orion KL region (Figure 7.5; Rosenthal et al 2000) illustrates the richness and diagnostic value of spectroscopy in the mid-IR. Several different emission complexes
contribute to the observed spectrum from this region of massive star formation.
First, there is the emission from the warm molecular gas heated by a shock driven
by the powerful outflow from the massive protostar, which is deeply embedded in
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Figure 7.5: The SWS 2.4 µm to 45 µm spectrum of Peak 1 in Orion (Rosenthal
et al 2000) reveals a wealth of spectral detail including a multitude of rotational
and ro-vibrational lines from H2 and other molecules, atomic fine-structure lines,
Hi recombination lines, emission from PAHs and absorption by molecular ices and
silicates. Shown on the ordinate axis is the flux density in 10−9 Jy/sr.
the KL nebula. This gives rise to the H2 as well as the CO and H2 O emission lines.
The cold molecular cloud material in which this shock is propagating is probed
by the H2 O and CO2 molecular ice and silicate absorption features. In the foreground, the ionized gas associated with the Orion H ii region (M42) powered by
Θ1 C — a newly formed star which has already disrupted its natal cloud — gives
rise to the atomic fine-structure lines (notably [Ar ii], [Ar iii], [Ne ii], [Ne iii], [S iii],
[S iv]) and the H i recombination lines. The atomic gas in the photodissociation
regions (PDR) separating the ionized gas from the molecular cloud contributes to
the prominent PAH emission features, the [Si ii] fine-structure line, and to the rovibrational emission from the higher vibrational states of H2 . Together with their
different excitation energies and critical densities, this spectrum has been used to
determine the physical conditions in the various emission zones in this complex
region and to unravel the energetic interaction of newly formed massive stars with
their environment (Rosenthal et al 2000).
Crystalline silicates
One of the greatest surprises of the ISO mission was the incredible richness of
the circumstellar silicate spectra at long wavelengths. Spitzer has extended measurements to much fainter objects that ISO could not probe, including disks around
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7. The mid- and far-infrared wavelength range
Figure 7.6: Comparison between the ISO-SWS spectrum from comet C/1995 O1
(Hale-Bopp) (Crovisier et al 1997) with the spectrum of the young star, HD
100546 (Malfait et al 1998). The bottom trace shows the IR spectrum of forsterite
(Mg2 SiO4 ) measured in the laboratory (Koike et al 1998). As this comparison
shows, most of the observed bands are due to small forsterite grains. Prominent
crystalline bands are indicated by tick marks at the top. Note that the spectrum of
HD 100546 also shows weak spectral signatures of PAH bands in this wavelength
range. (The ordinate shows the product of wavelength and spectral irradiance.)
T Tauri stars and recently also brown dwarfs, late-type objects in external galaxies,
ULIRG nuclei, and active galactic nuclei (AGN) toroids (Bouy et al 2008; Bouwman et al 2008; Markwick-Kemper et al 2007; Sloan et al 2006; Armus et al 2007;
Spoon et al 2007). Invariably, the 10 µm to 45 µm spectra of sources with circumstellar dust show a great number of features due to crystalline olivine and pyroxene
(cf., Figure 7.6; Waters et al 1996; Waters 2000; Waters et al 2000). These features
are ubiquitous and, besides O-rich AGB stars and their descendants, they have
now been observed in such diverse objects as luminous blue variables (e.g., η Car),
Herbig AeBe stars, nominally C-rich objects such as the Red Rectangle and BD
+30 3639, and comet Hale-Bopp (Crovisier et al 1997; Malfait et al 1998; Molster
et al 2002a; Waters et al 1998a,b). While some objects are dominated by forsterite
(Mg2 SiO4 ) (e.g., Hale-Bopp and HD 100546; Figure 7.6), other objects (e.g., AGB
stars) show strong bands due to enstatite (MgSiO3 ) as well.
These narrow features clearly imply a crystalline carrier. The study of the IR
characteristics of crystalline (olivine and pyroxene) silicates by Koike et al (1993,
1998) has been instrumental in the analysis of these long-wavelength features. Ex-
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tensive laboratory studies have also been performed by Jaeger et al (1994, 1998).
Based on these studies, the prominent bands at 23.6 µm and 33.6 µm can be attributed to olivine while the bands at (26.2, 32.9, 35.8, 40.6 and 43.1) µm are
assigned to pyroxene (Waters et al 1996; Jaeger et al 1994, 1998; Molster et al
2002b).
Herschel ’s contributions to this field will be limited in view of its (long) wavelength coverage. However, the PACS instrument on Herschel is well suited to study
the 69 µm band, which is revealing in terms of the composition (particularly regarding the Mg/Fe ratio) and temperature of emitting olivine grains (Molster et al
2002b; Bowey et al 2002). Due to the interaction of the cation and anion, the exact peak position of all of these modes is sensitive to the particular metal present
with shifts from 0.3 µm in the 10 µm region to 1 µm to 2 µm at longer wavelengths (Farmer 1974). In addition, the peak position — as well as the width of the
bands — is sensitive to the temperature of the emitting grains. The 69 µm band is
spectrally isolated and provides an ideal probe of these effects. Finally, JWST will
have the sensitivity and spectral coverage to probe the characteristics of interstellar
silicate grains for galaxies out to redshifts of ≈ 2.5; Spitzer has shown that this
is a promising field of research (Armus et al 2007; Markwick-Kemper et al 2007;
Spoon et al 2007).
Interstellar PAHs
IRAS has discovered widespread mid-IR emission in the Galaxy even far from
illuminating stars where no warm dust was expected: the so-called IR cirrus (Low
et al 1984). At that time, ground-based and airborne spectroscopy had already
revealed the presence of broad emission features at (3.3, 6.2, 7.7, and 11.3) µm in
the spectra of some objects illuminated by strong UV sources such as Hii regions,
planetary nebulae and reflection nebulae. This cirrus was generally thought to trace
these same emission features. Subsequently, ISO unambigously demonstrated that
these IR emission features dominate the mid-IR spectra of C-rich post-AGB objects
and planetary nebulae, the PDRs associated with Hii regions powered by O stars
and those associated with reflection nebulae illuminated by late B stars, planetary
disks associated with young stars (such as Herbig AeBe stars and T Tauri stars)
in the somewhat later stages of evolution, and starburst regions associated with
galactic nuclei (Boulanger et al 1998, 2000; Peeters et al 2002; Verstraete et al
1996, 2001; Sloan et al 2007; Acke and van den Ancker 2004; Geers et al 2006;
Habart et al 2006). In addition, while stretched to the limit of its capabilities, ISO
showed that the mid-IR spectrum of the cirrus is dominated by these IR emission
features (Mattila et al 1996). Spitzer has shown that this is a general characteristic
of the ISM in all spiral galaxies (Flagey et al 2006; Engelbracht et al 2005; Brandl
et al 2006; Sloan et al 2007). Likewise, the spectra of ULIRGs are often bright in
these IR emission features (Genzel et al 1998; Armus et al 2007). Spitzer has now
detected these features in such sources out to redshifts of ≈ 3 (Yan et al 2005; Lutz
et al 2007; Pope et al 2007; Rigby et al 2007).
Together these observations reveal the incredible richness of the mid-IR emission
spectrum of the ISM of galaxies (Figure 7.7). Besides the well-known IR emission
features at (3.3, 6.2, 7.7, 8.6, and 11.3) µm, the observed interstellar spectra show
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7. The mid- and far-infrared wavelength range
Figure 7.7: The mid-IR spectra (spectral irradiance vs. wavelength) of the PDR in
the Orion Bar and in the Planetary Nebulae NGC 7027 are dominated by a rich
set of emission features. Assignments of these features with vibrational modes of
PAH molecules are labelled at the top. Figure adapted from Peeters et al (2002).
a wealth of weaker features, including bands at (3.4, 3.5, 5.25, 5.65, 6.0, 6.9, 10.5,
11.0, 12.7, 13.5, 14.2, and 16.4) µm. Moreover, many of the well-known features shift
in peak position, vary in width, and/or show substructure (Peeters et al 2002; van
Diedenhoven et al 2004). These IR emission features are due to IR fluorescence from
PAH molecules containing some 50 C-atoms. Assignments for the various modes
involved are shown in Figure 7.7. The observed variations of these IR emission
features imply a sensitivity to the local physical conditions, and this property is now
beginning to be employed as a diagnostic tool for astronomy (Galliano et al 2008).
Overall, driven by these observational developments as well as by laboratory and
theoretical studies, it has become clear over the past decade that PAH molecules
are an abundant and important component of the interstellar medium throughout
the Universe (Tielens 2008).
Galaxies and ULIRGs
IRAS opened the realm of external galaxies for studies in the 10 µm to 100 µm
range and discovered emission from tens of thousands of normal and active galaxies.
ISO followed up with detailed imaging, spectroscopy, and spectrophotometry of
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Figure 7.8: The ratio of the [C ii] 158 µm line to the far-IR continuum as a function
of the IR luminosity. Open (closed) circles are galaxies for which the aperture
encompasses less (more) than 10 kpc. Crosses mark ULIRGs. The filled squares
are high-redshift objects. The large filled square represents the most distant known
quasar, SDSS J114816.64+525150.3 at a redshift of z = 6.42 (Maiolino et al 2005).
Figure adapted from Malhotra et al (2001).
many galaxies detected by IRAS, as well as deep surveys in the mid- and far-IR.
Spitzer with its superior sensitivity has extended this by dedicated surveys of the
emission characteristics of nearby normal and starburst galaxies and by probing
even farther back in the history of the Universe. COBE has shown that the [C ii]
158 µm line is the dominant emission line of the Milky Way with a luminosity
of 5 × 107 L⊙ ; e.g., about 0.003 of the total IR luminosity of the Milky Way is
emitted in this single line (Bennett et al 1994). This is a very general feature
of galaxies: in a sample of 60 normal, star forming galaxies, the [C ii] line is in
general the dominant IR cooling line. The [O i] 63 µm line is a close second and
in a handful of galaxies even takes over (Malhotra et al 2001). The origin of the
[C ii] line is controversial (Hollenbach and Tielens 1999). Theoretically, because of
their density and temperature, it is expected that (Spitzer-type) H i clouds (e.g.,
the cold neutral medium, CNM) will radiate most of their energy through the
[C ii] line (cf., the section above on physical processes or Dalgarno and McCray
1972). This is supported by measurements of the [C ii] emission from high-latitude
clouds by use of sounding rockets (Bock et al 1993). However, COBE demonstrated
that the irradiance of the [N ii] line correlates with the irradiance of the [C ii] line
(to the 1.5 power), suggesting that the low-density ionized gas (the warm ionized
medium, WIM) contributes a portion of the observed [C ii] emission (Heiles 1994).
In addition, given the high observed irradiance of [O i] (where ions in the upper level
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7. The mid- and far-infrared wavelength range
have a critical density of 2×105 cm−3 ) in the sample of star-forming galaxies studied
by ISO-LWS, a substantial fraction of the observed [C ii] emission likely originates
from dense, bright PDRs associated with molecular cloud surfaces near regions of
massive star formation (Hollenbach and Tielens 1999). Theoretical models have
been developed based upon these three premises (CNM, WIM, and PDR origin)
and all are in reasonable agreement with the COBE observations of the Milky Way.
Herschel (and later SOFIA) permits the study of Doppler shifts of the [C ii]
and [O i] emission from the Milky Way, nearby normal galaxies, starburst galaxies
and ULIRGs. This will provide a better understanding of the origin of these lines.
Understanding of the origin of the [C ii] line on a galactic scale has recently received
additional impetus with the detection of this line in the spectrum of the mostdistant quasar, J1148, at a redshift of 6.42 with the 30 m IRAM telescope (Maiolino
et al 2005). The observed flux of this line — in conjunction with other PDR tracers
(e.g., CO J = 7 to J = 6; Bertoldi et al 2003) — has been interpreted as evidence
for vigorous star formation (3000 M⊙ /a) in the host galaxy. Because of the high
luminosity in this single spectral line, the [C ii] line has often been considered as a
key tracer of star formation in the early Universe, particularly for heavily obscured
galaxies. Thus, one of the three key scientific goals of the ALMA project is to
use the [C ii] line to probe star formation in the high-redshift Universe. However,
the irradiance of the [C ii] line in extreme star-formation environments such as
ULIRGs is not well understood (Malhotra et al 2001; Luhman et al 2003). Typically,
the strength of this line is a factor of ten too small relative to the far-IR dust
continuum in these environments if compared with normal and starburst galaxies
(see Figure 7.8). While that might indicate that the gas in ULIRG evironments
is denser than in normal or starburst galaxies, other tracers (e.g., CO J = 1 to
J = 0, [C i] J = 1 to J = 0, and PAH emission) do not seem to be compatible
with this solution (Luhman et al 2003). Possibly much of the (ionizing and nonionizing) ultraviolet flux is absorbed by dust in the H ii region and reradiated as
far-IR dust continuum rather than as ionic or neutral atomic fine-structure lines
or by PAHs (Luhman et al 2003). In this and many other ways, the star-formation
environment of ULIRGs may resemble that of hypercompact or ultracompact H ii
regions (Martı́n-Hernández et al 2002; Peeters et al 2004; Lahuis et al 2007).
Achievements and outlook
The past IR space missions — IRAS, ISO, Spitzer, Akari — have provided us
with a wealth of data and great new insights into the detailed physics and chemistry
of gas and dust in space and thereby on the origin and evolution of the interstellar
medium of galaxies. As detailed in this chapter, these missions have, however,
also raised numerous new questions. In the near future, SOFIA will start regular
operations and bring IR spectroscopy over a wide wavelength range and at a wide
range of spectral resolutions. Herschel, launched in May 2009, has now opened the
far-IR and sub-millimetre range for systematic studies of the Universe. The more
sensitive JWST, slated to be launched in 2014, will be able to probe even deeper.
With this prospect, and with potential new missions such as SPICA and SAFIRE
appearing on the horizon, the future for infrared astronomy looks bright, indeed.
145
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