Infrared Spectroscopy Stuart Ryder Anglo-Australian Observatory

OverviewOverview What makes infrared (IR) spectroscopy so special?What makes infrared (IR) spectroscopy so special? Observing MethodObserving Method Data reductionData reduction What can we learn?What can we learn? ExamplesExamples SummarySummary

The Infrared Sky. I The sky (atmosphere) is ~500  brighter in the near-IR than in the optical.The sky (atmosphere) is ~500  brighter in the near-IR than in the optical.

The Infrared Sky. II The sky (atmosphere) is ~500  brighter in the near-IR than in the optical.The sky (atmosphere) is ~500  brighter in the near-IR than in the optical.

The Infrared Sky. III Atmospheric absorption (CO 2, H 2 O) varies with water column/airmass.Atmospheric absorption (CO 2, H 2 O) varies with water column/airmass.

The Infrared Sky. IV Airglow from OH ¯ dominates J, H, and (most of ) K.Airglow from OH ¯ dominates J, H, and (most of ) K.

OH Suppression e.g., Cambridge OH Suppression Instrument (COHSI): pre-disperse IR light (R~5000), mask out the OH lines, undisperse, then re-disperse at lower resolution (R~500). Gold mirror in suppressor unit, with masking applied at the location of known OH lines. Suppressor unit is the large black box shown at left, coupled to UKIRT via an optical fibre.

The Art of Nodding: Star Place star at position 1, integrate for ~30 seconds.... Star on slit Resulting K-band spectrum (vertical lines are OH features; bright band on right edge is thermal emission from sky)

The Art of Nodding Place star at position 2, integrate for ~30 seconds....

The Art of Nodding ….and take the difference. With such a strong source and short integrations, OH line residuals are negligible.

The Art of Nodding: Galaxy Place galaxy at position 1, integrate for a few minutes....

The Art of Nodding Repeat at position 2…. (for large objects, may need to nod to completely blank sky)

The Art of Nodding ….and take the difference. Residuals may be worse than for the star, but weak features in the source now become apparent.

Data Reduction. I Wavelength calibrate using a Kr or Ar arc (at some wavelengths, OH lines can be put to good use!)

Data Reduction. II Extract and co-add the nodded spectra of the object. Most of the absorption features are due to the atmosphere….

Data Reduction. III ….but the standard star spectrum is affected in the same way. So divide this spectrum into the previous one….

Data Reduction. IV ….after first “patching” absorption intrinsic to the star, e.g. Brackett  at  m….

Data Reduction. V ….which removes almost all atmospheric effects. But we still need to correct for the spectral shape of the star….

Data Reduction. VI ….which is basically the spectrum of a blackbody having the same effective temperature as the star….

Data Reduction. VII ….and multiplying by this function gives us the final, clean spectrum, ready for scientific analysis!

What IR spectroscopy can do Reduce dust extinction: A K ~ 0.1 A VReduce dust extinction: A K ~ 0.1 A V Redshift and SFR surveys:Redshift and SFR surveys: –H  6563 (0.7 < z < 2.8) –[O II ] 3727 (2.0 < z < 5.7) Diagnostic lines, e.g. H S(1) 2.12  m / H S(1) 2.25  m ~2 for UV-pumped fluorescence cf. ~10 for thermal excitation by shocksDiagnostic lines, e.g. H S(1) 2.12  m / H S(1) 2.25  m ~2 for UV-pumped fluorescence cf. ~10 for thermal excitation by shocks Spectral types of red/obscured starsSpectral types of red/obscured stars MetallicityMetallicity Dust and Ice absorption features Dust and Ice absorption features 

Example: CSF in M100 UKIRT + IRCAM3 (2x mag) AAT (D. Malin) JHK “true colour” image of circumnuclear star forming (CSF) regions in M100.

Example: CSF in M100 Targeted spectroscopy of these CSF regions with CGS4 on UKIRT allows us to measure the equivalent widths of Br  (from H II regions), and CO (from young red supergiant stars)….

Observations Confront Models ….which can be compared with models in the literature for starbursts, allowing their ages (and thus the triggering mechanism) to be determined.

UNSWIRFUNSWIRF University of New South Wales InfraRed Fabry-Perot a near-IR version of the Taurus Tunable Filter (see S. Cianci’s talk), used with IRIS on the AAT.

UNSWIRFUNSWIRF Good for isolating the H  m emission from the cometary nebula Parsamyan 18…. ….and comparing it with the H  m emission allows the excitation source (Star “A”) to be determined.

UNSWIRFUNSWIRF Contours of the H  m (left) and Br  (right) emission from the dust columns of the Eagle Nebula. Note how the molecules survive inside the dust, but are split up and ionised at the surface by stellar radiation.

SummarySummary Infrared spectroscopy is only slightly more complicated than optical spectroscopy (mainly due to the bright, and rapidly-varying atmosphere).Infrared spectroscopy is only slightly more complicated than optical spectroscopy (mainly due to the bright, and rapidly-varying atmosphere). There are well-established techniques and data reduction software available to overcome these problems.There are well-established techniques and data reduction software available to overcome these problems. There are a swag of atomic, molecular, and solid-state features available, which are not present (or heavily obscured) at other wavelengths.There are a swag of atomic, molecular, and solid-state features available, which are not present (or heavily obscured) at other wavelengths. IRIS2 is coming, and with it, the ability to once again carry out near-infrared spectroscopy at the AAT.IRIS2 is coming, and with it, the ability to once again carry out near-infrared spectroscopy at the AAT.

ReferencesReferences Handbook of Infrared Astronomy, by Ian Glass (Cambridge University Press 1999)Handbook of Infrared Astronomy, by Ian Glass (Cambridge University Press 1999) COHSI: UNSWIRF: United Kingdom Infrared Telescope (UKIRT) utilities: Kingdom Infrared Telescope (UKIRT) utilities: