1.2 Origin of Infrared Emission in Galaxies

Infrared continuum emission from gas is either free-free (bremsstrahlung) or bound-free radiation from fairly low density gas or from stellar photospheres. For more exotic objects, such as active galactic nuclei and supernovae, we may also see non-thermal (synchrotron) IR emission. The spectral energy distribution from gas at temperatures higher than 1000 K peaks at wavelengths shorter than 5 $\mu$m, so stars, especially red ones, are bright in the near-IR.

Figure 1.3: Spectral Energy Distribution (SED) of a Galaxy. The SED of a starburst galaxy undergoing an intense episode of star formation is shown. The spectrum due to cirrus emission by heated dust is also shown together with two types of energy distributions that appear in some Seyfert galaxies and other active galactic nuclei (AGN): non-thermal emission (power-law) and emission due to the dust heated by the central engine. It is possible for a galaxy to show at the same time emission due to starburst, cirrus and AGN (Telesco, 1999).

Dust and radiation are ubiquitous in the Universe, so IR emission from dust heated by radiation is also always present. The IR spectral energy distribution (SED) of dust emission is characterized by a broad bump, with the maximum flux density occurring somewhere between 10 and 250 $\mu$m, depending on the energy density of the radiation field at the location of these particles. Since dust particles evaporate at temperatures above 1500-2000 K, the IR emission from the hottest dust is often detected in $K$, but it is usually weak in $J$ and $H$. For most IR sources, the main body of the broad emission bump closely resembles a blackbody emission spectrum that is modified by a wavelength-dependent emission efficiency of the form $\lambda^n$, where $n$ is usually taken to be in the range -1 to -2. IR emission in excess of this blackbody on the short-wavelength side of the bump is usually present and indicates that there is dust at temperatures higher than that characterizing most of the bump. "Classical" dust consists of small particles about 0.01 to 0.25 $\mu$m in diameter, such as graphite and silicate grains. Some features of dust emission are usually attributed to the presence of a component of very small grains ($\sim 4-10$ Å) and/or very large molecules containing more than 50 atoms known as PAHs (Polycyclic Aromatic Hydrocarbons).

Dust emission tends to result in excess emission in the infrared over what would be expected from the apparent temperature of the astronomical object. It usually follows a blackbody spectrum modified by a wavelength-dependent emissivity term

$$B(\nu) = k_\nu \, \epsilon(\nu) \, \frac{2h\nu^3}{c^2} \frac{1}{e^{h\nu/kT}-1}$$

where $k_\nu \sim \nu^\alpha, \ \alpha=1,1.5,2,\dots$.

One of the most important results of the first ever infrared all-sky survey conducted by IRAS (see Section B.3) was the discovery that the sky at high Galactic latitudes ( $\vert b\vert\geq 10^\circ$) is covered by a vast complex of patchy and filamentary emission. This emission looks like cirrus clouds in the Earth's sky and so it is referred to as infrared cirrus. The cirrus, which is more evident at 60 and 100 $\mu$m but which is also present at 12 and 25 $\mu$m, is emission from interstellar dust particles. About 80% is emitted by dust mixed with a low-density distribution of HI and HII heated by the interstellar radiation field (ISRF). The spectral energy distribution of the cirrus emission holds some important clues to the nature of the dust grains. Cirrus has a very peculiar SED characterized by a cool far-IR slope (high 100 $\mu$m / 60 $\mu$m ratio) and a hot mid-IR slope (high 12 $\mu$m / 25 $\mu$m ratio). The dust temperature derived from the typical far-IR flux ratio is 20-30 K, assuming the dust emission efficiency varies with frequency as $\nu^{1.5}$. The 12 and 25 $\mu$m fluxes imply temperatures in the range 80-500 K. There seems to be hot dust and cold dust and not much in between. In fact, the SED implies that there are essentially two distinct populations of grains: particles large enough to be in radiative equilibrium with the radiation field and very small particles, each of which, because of its small size and therefore its small heat capacity, experiences a large increase in temperature when it absorbs a single photon. Since the radiation field heating the cirrus emitting dust has a relatively low energy density, the larger grains, which are in equilibrium with it, are very cool, thus giving rise to the extremely red far-IR colour. On the other hand, the temperature of a small grain after it absorbs a UV photon is independent of the radiative energy density. Even if a large population of very small grains is present in an HII region, their SED is not as distinct as that seen in the cirrus, because the IR emission from the larger grains in the higher energy density environment of the HII region is much more prominent at shorter wavelengths. Based on a group of emission features in the 3-12 $\mu$m region observed in Galactic reflection nebulae and other sources, PAHs are thought to constitute one component of this population of small grains.

The dominant source of diffuse background in the near and mid-IR regions is the so called "zodiacal light". This arises from interplanetary dust which scatters sunlight in the optical and near-IR, but which emits thermally in the near-IR. The spectrum of the zodiacal light in the mid-IR is featureless and corresponds to a temperature of $261.5 \pm 1.5$ K. The zodiacal light consists of slowly varying diffuse emission which is brightest in the plane of the ecliptic.