Infrared SpectroscopyInfrared SpectroscopyInfrared SpectroscopyInfrared Spectroscopy

Stuart RyderStuart RyderAnglo-Australian ObservatoryAnglo-Australian Observatory

sdr@aaoepp.aao.gov.ausdr@aaoepp.aao.gov.au

http://www.aao.gov.au/local/www/sdr/http://www.aao.gov.au/local/www/sdr/

OverviewOverviewOverviewOverview• 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. IThe Infrared Sky. IThe Infrared Sky. IThe Infrared Sky. I

• The sky (atmosphere) is ~500The sky (atmosphere) is ~500 brighter in the near-IR brighter in the near-IR than in the optical.than in the optical.

Band U B V R I J H K K´

Sky (mag/arcsec-2)

21.2 22.5 21.5 20.8 19.3 15.0 13.7 12.5 13.7

The Infrared Sky. IIThe Infrared Sky. IIThe Infrared Sky. IIThe Infrared Sky. II

• The sky (atmosphere) is ~500The sky (atmosphere) is ~500 brighter in the near-IR brighter in the near-IR than in the optical.than in the optical.

The Infrared Sky. IIIThe Infrared Sky. IIIThe Infrared Sky. IIIThe Infrared Sky. III

• Atmospheric absorption (COAtmospheric absorption (CO22, H, H22O) varies with water O) varies with water

column/airmass.column/airmass.

The Infrared Sky. IVThe Infrared Sky. IVThe Infrared Sky. IVThe Infrared Sky. IV

• Airglow from OHAirglow from OH¯̄ dominates dominates JJ, , HH, and (most of ) , and (most of ) KK..

OH SuppressionOH SuppressionOH SuppressionOH Suppressione.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, withmasking applied at the location of knownOH lines.

Suppressor unit is the large black boxshown at left, coupled to UKIRT viaan optical fibre.

The Art of Nodding: StarThe Art of Nodding: StarThe Art of Nodding: StarThe Art of Nodding: StarPlace 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 NoddingThe Art of NoddingThe Art of NoddingThe Art of NoddingPlace star at position 2, integrate for ~30 seconds....

The Art of NoddingThe Art of NoddingThe Art of NoddingThe Art of Nodding….and take the difference. With such a strong source

and short integrations, OH line residuals are negligible.

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

The Art of NoddingThe Art of NoddingThe Art of NoddingThe Art of NoddingRepeat at position 2…. (for large objects, may need to nod tocompletely blank sky)

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

Data Reduction. IData Reduction. IData Reduction. IData Reduction. I

Wavelength calibrate using a Kr or Ar arc (at somewavelengths, OH lines can be put to good use!)

Data Reduction. IIData Reduction. IIData Reduction. IIData Reduction. II

Extract and co-add the nodded spectra of the object. Mostof the absorption features are due to the atmosphere….

Data Reduction. IIIData Reduction. IIIData Reduction. IIIData Reduction. III

….but the standard star spectrum is affected in the sameway. So divide this spectrum into the previous one….

Data Reduction. IVData Reduction. IVData Reduction. IVData Reduction. IV

….after first “patching” absorption intrinsic to the star,e.g. Brackett at 2.166 m….

Data Reduction. VData Reduction. VData Reduction. VData Reduction. V

….which removes almost all atmospheric effects. But westill need to correct for the spectral shape of the star….

Data Reduction. VIData Reduction. VIData Reduction. VIData Reduction. VI

….which is basically the spectrum of a blackbody havingthe same effective temperature as the star….

Data Reduction. VIIData Reduction. VIIData Reduction. VIIData Reduction. VII

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

What IR spectroscopy can doWhat IR spectroscopy can doWhat IR spectroscopy can doWhat IR spectroscopy can do

• Reduce dust extinction: Reduce dust extinction: AAKK ~ 0.1 ~ 0.1 AAVV

• 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. HDiagnostic lines, e.g. H22 1-0 S(1) 2.12 1-0 S(1) 2.12m / Hm / H22 2-1 S(1) 2-1 S(1)

2.25 2.25 m ~2 for UV-pumped fluorescence m ~2 for UV-pumped fluorescence cfcf. ~10 for . ~10 for thermal excitation by shocksthermal 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 M100Example: CSF in M100Example: CSF in M100Example: CSF in M100

UKIRT + IRCAM3 (2x mag)AAT (D. Malin)

JHK “true colour” image of circumnuclearstar forming (CSF) regions in M100.

Example: CSF in M100Example: CSF in M100Example: CSF in M100Example: 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 ModelsObservations Confront ModelsObservations Confront ModelsObservations Confront Models

….which can be compared with modelsin the literature forstarbursts, allowing their ages (and thus the triggering mechanism) to be determined.

UNSWIRFUNSWIRFUNSWIRFUNSWIRFUniversity of New South Wales InfraRed Fabry-Perot

a near-IR version of the Taurus Tunable Filter (seeS. Cianci’s talk), used with IRIS on the AAT.

UNSWIRFUNSWIRFUNSWIRFUNSWIRF

Good for isolating the H2 2.12 m emission from the cometary nebula Parsamyan 18….

….and comparing it with the H2

2.25 m emission allows theexcitation source (Star “A”)to be determined.

UNSWIRFUNSWIRFUNSWIRFUNSWIRF

Contours of the H2 2.12 m (left) and Br(right)emission from the dustcolumns of the Eagle Nebula. Note how themolecules survive insidethe dust, but are splitup and ionised at thesurface by stellar radiation.

SummarySummarySummarySummary• Infrared spectroscopy is only slightly more complicated Infrared spectroscopy is only slightly more complicated

than optical spectroscopy (mainly due to the bright, and than optical spectroscopy (mainly due to the bright, and rapidly-varying atmosphere).rapidly-varying atmosphere).

• There are well-established techniques and data reduction There are well-established techniques and data reduction software available to overcome these problems.software available to overcome these problems.

• There are a swag of atomic, molecular, and solid-state There are a swag of atomic, molecular, and solid-state features available, which are not present (or heavily features available, which are not present (or heavily obscured) at other wavelengths.obscured) at other wavelengths.

• IRIS2 is coming, and with it, the ability to once again IRIS2 is coming, and with it, the ability to once again carry out near-infrared spectroscopy at the AAT.carry out near-infrared spectroscopy at the AAT.

ReferencesReferencesReferencesReferences• Handbook of Infrared AstronomyHandbook of Infrared Astronomy, by Ian Glass (Cambridge , by Ian Glass (Cambridge

University Press 1999)University Press 1999)

• COHSI: COHSI: http://http://wwwwww//astast.cam..cam.acac..ukuk/~optics//~optics/cohsicohsi//cohsicohsi..htmhtm

• UNSWIRF: UNSWIRF: http://http://wwwwww.phys..phys.unswunsw..eduedu.au/~.au/~sdrsdr//unswirfunswirf/UNSWIRF./UNSWIRF.htmlhtml

• United Kingdom Infrared Telescope (UKIRT) utilities: United Kingdom Infrared Telescope (UKIRT) utilities: http://http://wwwwww..jachjach..hawaiihawaii..eduedu//JACpublicJACpublic/UKIRT/astronomy//UKIRT/astronomy/