Astronomical Techniques -- Infrared Astronomy Overview
Mike Skrutskie -- November 18, 2003


The Infrared Wavelength Domain

  • In round numbers 1.0um - 1000um (with room for argument)

    • This is largely the range from

      • silicon CCD long-wavelength cutoff (photon response) to the

      • limit of radio heterodyne sensitivity (wave response).

    • Technically 0.7-1.0um is infrared, but is better considered the "far-visible" given that it is detectable by silicon CCD's.

    • At the other extreme, radio heterodyne techniques have infringed upon the far-infrared up to 300um and to shorter wavelengths every day.


Visible/Infrared Similarites


Infrared/Optical Differences

  • Astrophysics

    • Access to low-excitation energy transitions

      • Molecular vibrational and rotational lines

        • Vibrational energies correspond to wavelenths of a few microns.

          • Rotational energies are an order of magnitude or more smaller -- wavelengths of a few hundred microns.

            • The CO rotational fundamental is at 2600um (2.6mm). H2 is at 28um. The difference arises because of the moments of inertia of the molecules.

          • Vibrational examples include

            • CO ro-vibrational bandheads at 4.6um (and overtones at 2.3 and 1.6um).

              • Seen in cool stellar atmospheres and circumstellar disks.

            • Molecular hydrogen, the most common molecule in the Universe, has a vibration fundamental at 2.1um (recall the rotational fundamental is at 28um).

              • Quadrupole emission is permitted for this symmetric molecule (dipole is forbidden) -- these lines are weak.

              • Since the excitation temperature of the 2.1um vibrational lines is of order 1000K, they are only seen in fluorescence or in shock excited regions where molecular material is present -- e.g. star formation, starburst galaxies, AGN...

        • Related to the molecular lines are broad absorption/emission features seen in the ISM due to silicate/carbon/ice grains (some are identified with polycyclic aromatic hydrocarbons (PAH's)).

          • These broad features originate from bends and stretches of atomic bonds in large molecules and solids.

          • They tend to be excited by UV photons and can be a good proxy for obscured star formation.

        • Atomic fine structure lines

          • OI [63um] and CII [158um] are primary coolants in the ISM

          • Why so? In part, because a typical temperature in any galaxy is about 30K.

      • Emission from warm dust

        • T=30K dust emission (i.e. most of the dust in the universe) peaks at 100um

        • Most of the energy emitted into the universe by galaxies is in the mid-and far infrared, particularly by starburst galaxies.

          • Much of the emitted starlight is reprocessed by dust.

          • In actively star forming galaxies or AGN-dominated galaxies the emitted infrared energy can far exceed the visible-wavelength flux.

        • Star formation occurs in dusty regions. Stellar flux is reprocessed in the circumstellar environment and in the parent molecular cloud.

        • Tenuous warm dust around normal stars, discovered in the mid-infrared, betrays the presence of mature solar systems.

      • Transparency of dust

        • Av=1 corresponds to AK=0.1

        • There are 30 magnitudes of visual extinction toward the Galactic center.

        • Star formation occurs in dusty environments.

      • Cosmological redshift

        • The peak of "visible" starlight emission in the local universe shifts to the near-infrared at z=2 and greater.

        • Conversely, the 100um peak due to warm dust in galaxies shifts into the submm regime for z=2 and greater.

        • Radio/Far-infrared observations have succeeded in revealing the values of the basic cosmological parameters (z=1000).

      • Sensitivity to cool blackbodies

  • Hardware and Techniques

    • Detectors

      • The energy of an infrared photon is small compared with atomic valence/covalent bond energies.

      • Photovoltaic detector materials must be customized to respond to the feeble energy of infrared photons. Some relevant cutoff energies:

        • Silicon -- 1.05um
        • InGaAs -- 1.7um (tunable depending on "chemistry")
        • InSb -- 5.5um
        • HgCdTe >2.5um depending on "chemistry"
        • Si:As BIB -- 28um
        • Ge:Ga -- 100um or more depending on stress!

      • Infrared arrays consist of a detector layer mechanically bonded to a silicon electronics wafer.

        • The differential expansion between these two layers has been a limiting factor in making large arrays.

        • In visible CCD's the same material, silicon, serves as detector and electronics, simplifying the requirements.

      • bolometers can take advantage of bulk thermal response to detect the longest wavelengths

    • Cryogenics

      • Near-infrared detectors, particularly HgCdTe, can operate at LN2 temperature (77K).

      • Mid/far-infrared detectors require cooling to close to liquid helium temperature (4K).

      • Bolometers work most efficiently at the lowest possible temperatures -- typically 100 mK.

    • Optical materials must be selected for appropriate infrared transmission.

    • Atmospheric transmission

      • Water vapor contributes substantial opacity across the infrared spectrum.

        • significant time variability and thus difficulty in calibration.

        • significant improvement with altitude

        • much of the infrared becomes accessible from aircraft altitude.

    • Thermal background becomes significant beyond a wavelength of 2um at 300K.

      • Temperature changes of 0.001K can overwhelm astronomical signals.

      • Rapid chopping between a target and a comparison/blank field can significantly supress the background variations.

      • Still, Poisson statistics of the vast number of background photons produce significant background noise limiting the depth of ground-based observations.

        • A one-minute integration with an 0.5-m cryogenic space-based telescope can match the performance of a one-hour measurement with a 15-m ground-based telescope.

        • Significant effort is being put into exploiting the South Pole for infrared observations.

    • Atmospheric turbulence becomes less significant at longer wavelengths.

      • The seeing disk diameter improves as (lambda)-0.2

      • 10-meter class telescopes on good sites become diffraction limited at wavelengths of a few microns.

      • Deformable mirrors can produce high-strehl ratio (nearly diffraction limited images) in near-infrared adaptive optical systems.

        • These systems tend to sense wavefront error in the far red in order to correct the images for the near-infrared.

  • Infared instrument and future project examples