Infrared Spectroscopic Observations in Astronomy

Adwin Boogert NASA Herschel Science Center IPAC, Caltech Pasadena, CA, USA

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 1 Scope

●Lecture 1 (Monday): What you need to know when planning, reducing, or analyzing infrared spectroscopic observations of dust and ices.

●Lecture 2 (Tuesday): Basic physical and chemical information derived from interstellar ice observations. Not discussed: laboratory techniques (see Palumbo lectures) and surface chemistry (see Cuppen lectures).

●Lecture 3 (Tuesday): Infrared spectroscopic databases. What's in them and how (not) to use them.

●Drylabs (Tuesday): Using databases of interstellar infrared spectra and of laboratory ices. Deriving ice abundances and analyzing ice band profiles.

NOTE: Please download all presentations and drylab tar file:

spider.ipac.caltech.edu/~aboogert/Cuijk/

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 2 Topics

●Infrared wavelength definitions ●Infrared facilities ●Vibration modes ●Observational challenges: ● Atmospheric absorption ● Atmospheric and telescope background emission ● Chopping and nodding ● Celestial background emission ●Infrared detectors ●Spectral resolution ●Spectrometer types ●Spatial resolution ●Sensitivity ●Data reduction ●Summary: Ground vs Space Based IR Astronomy

05 June 2012 Interstellar Dust School (Cuijk): Interstellar Ices (Boogert) 3 Reading Materials Lecture 1

Basic reading material on observing techniques:

● Chapters 2 (Infrared Sky) and 6 (Infrared Techniques) of Handbook of Infrared Astronomy by I. S. Glass

● Gemini Mid-IR pages: www.gemini.edu/sciops/instruments/michelle/mid-ir-resources

More advanced reading materials on spectrometers and telescopes:

● Astrophysical Techniques by C.R. Kitchin

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 4 Infrared Astronomy

Somewhat subjective definitions of infrared wavelength regions in astronomy (I.S. Glass, p. 27):

● Near-infrared: 0.75-5 um ● Mid-infrared: 5-25 um ● Far-infrared: 25-350 um ● Sub-millimeter: 350-1000 um

Roughly based on key wavelengths:

● Human eye cutoff: 0.75 um ● Optical CCDs cutoff: 1.1 um ● Background emission dominates: >2.3 um ● Background emission peaks (T~300 K): ~10 um ● Longest wavelength mi-ir window: 25 um ● Heterodyne techniques feasible (<2008): >350 um

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 5 Facilities Selection of past, current, and future 3-200 um spectrometers suitable for ice and dust feature observations (not complete!). We will get back to aspects of this table during the lecture.

Instrument Detector  R Remarks (>3 um) m *1000 ISAAC/VLT InSb 1-5 1.4-10 - NIRSPEC/Keck InSb 1-5 2.0-25 optional ad. opt. SpecX/IRTF InSb 1-5 1.0-2.0 x-dispersed NIRSpec/JWST HgCdTe 1-5 0.1-3 IFU SWS/ISO InSb,SiGa, 2-45 0.1-1.5  scanning SiAs,GeBe IRC/AKARI Insb,SiAs 2-26 0.1 prism+grating TreCS/GeminiS SiAs 8-26 0.1-1.0 - MIRI/JWST SiAs 5-28 0.1-3 IFU IRS/Spitzer SiAs,SiSb 5-35 0.06-0.6 - FORCAST/SOFIA SiAs,SiSb 5-50 0.1-1.0 5-15 um x-disp LWS/ISO GeBe,GeGa 45-200 0.2 - PACS/Herschel GeGa 57-210 1.5 IFU 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 6 Dust and Ice Transitions

●Typical wavelength ranges in which dust and ice vibrational modes occur. ●Whether a particular mode can be observed and with which instrument and technique depends on absorption and emission spectrum of the earth's Allamandola (1984) atmosphere (see next slides).

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 7 Challenge: Atmospheric Absorption ●Windows of Mauna Kea: 1 mm (black) and 3 mm H2O (red) good/fair transmission between 1-26 μm. ●Strong wavelength dependence, even within windows. ●Transmission depends strongly on water vapor column above site (use H2O water vapor monitor to assess!), and elevation on sky. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 8 Challenge: Atmospheric Absorption

Mauna Kea: 1 mm (black) and 3 mm H2O (red) ●Same plot as previous, but on log wavelength scale, highlighting NIR transmission.

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 9 More Challenges: Infrared Background Emission

While atmospheric absorption reduces the stellar signal strength, strong infrared background emission increases the noise. Atmospheric and telescope background emission often much stronger than stellar emission:

●Fluctuations in background emission strength produce systematic noise effects. Can be minimized using sophisticated subtraction methods.

●Background emission important component statistical noise, because photon noise follows Poisson statistics. Noise in observed stellar signal (after full reduction):

 star∝N phot = N phot bgN phot  star...

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 10 Infrared Background Emission

Main sources of background emission:

(1) Earth's atmosphere

● Solutions: go cold (I∝T4), go high. Space, balloon (airship?), airplane, Antarctica, high mountains.

● Monitor weather conditions: good conditions at Mauna Kea have ~1 mm Precipitable Water Vapor (CSO 225 GHz ~0.05), but can be much higher.

● Causes 'sky noise': unstable weather, thin cirrus and other structured cloud, wind-borne dust (e.g., Saharan dust storms affecting Canary Islands).

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 11 Infrared Background Emission

Mauna Kea: 1 mm (black) and 3 mm H2O (red) ●Atmospheric emission spectrum (model)

●Rise by 3-5 orders of magnitude above 3 m!

●Sky temperature similar at most wavelengths, but O3 emission from higher and colder layers.

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 12 Infrared Background Emission

Mauna Kea: 1 mm (black) and 3 mm H2O (red) ●Same plot as previous, but on log wavelength scale, highlighting NIR background emission (OH lines).

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 13 Infrared Background Emission

Main sources of background emission (continued):

(2) Telescope mirrors + support structures. Solutions:

● Low emissivity coatings (~5% for aluminum [most telscopes], ~2% for silver [Gemini]), needs re-coating every ~5 years.

● Thermally stable telescope

● Keep mirrors uniformly clean. M1 segment gaps M2 support structure 10 um entrance pupil image of Canaricam at Grantecan

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 14 Infrared Background Emission

● Telescope emission peaks at ~15μm, corresponding to temperatures of ~270 - 290 K ● Mauna Kea sky emission compared to emission from a telescope with 2% emissivity (Gemini)

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 15 Infrared Background Emission

● Keep them cold and avoid temperature fluctuations: ● Go to space ● Go far from earth's radiation, avoid going in and out of Earth's shadow (Herschel, JWST: L2, Spitzer: earth trailing) ● Sun still heats telescope. Use Helium to make telescope mirror+structures very cold (Spitzer ~5.5 K, ISO ~4 K, AKARI ~6 K) or cold (Herschel ~80 K).

L2 orbit for Herschel and JWST

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 16 Infrared Background Emission

Main sources of background emission (continued):

(3) Instrument window. ● Window separates cooled spectrometer from outside world. Prone to condensation and ice formation. Solved by using a fan, esp. if humid conditions. If it happens anyway, do careful flatfielding.

Background cancellation via nodding or nodding+chopping secondary, want small stable residual offset signals

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 17 Nodding and Chopping Background emission subtracted by

●Telescope nodding only, if the background is stable on time scales of 10s of seconds (e.g., NIRSPEC/Keck at Mauna Kea).

●Beamswitching: move (chop) secondary mirror between 2 sky positions at fast rate. Background emission not perfectly canceled as beams have slightly different optical paths, which have different defects, dust, etc., leading to radiative offset between the two chop positions. Compensate by nodding the telescope so that the object and reference positions are switched. Most used beam switch variants:

● Nod the telescope by a distance equal to the chop throw along the chop axis ● On-array and off-array nodding

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 18 Nodding and Chopping

●A-B gives net signal corrected for radiative offset. ●BUT flexure and temperature changes mean that offset changes with time, so take data in sequence A,B,B,A to remove linear gradient in offset (instead of A,B,A,B). 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 19 Nodding and Chopping

●Actual 10 m beam- switch observation performed with T-ReCS at Gemini-South. ●This is the variant with off-array nodding.

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 20 Sky Background

●Galactic Cirrus and Solar System Zodiacal light largest contributors to extraterrestrial sky background in 3-200 um wavelength region.

●Intensity strongly direction dependent.

●Affects sensitivity of infrared satellite observations

Diffuse background emission, away from ecliptic. Leinert et al. 1988

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 21 Infrared Detectors

●Infrared detection done using N2 or He cooled semiconductors: bound electrons liberated by sky photons. Detectors can be photovoltaic (measure current of electrons) or photoconductive (measure change in resistance).

●Nomenclature: Infrared detector arrays are not CCDs! They are arrays of individual detectors, each of which are read out individually. Advantage: one bad or saturated pixel does not affect an entire column!

●Infrared arrays hard to make and expensive. Still improving, especially at longer wavelengths (motivated/funded by space missions—HST, JWST).

●Each detector material and array type has its own properties (quantum efficiency, dark current,readout noise) and problems (memory effects, standing waves, effect of cosmic ray hits). 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 22 Spectral Resolution

Relatively broad dust and ice features (>0.1 m) need spectral resolutions of ~100 to a few thousand. But higher spectral resolution may matter (e.g., for narrow CO, 13CO ice features):

● improved sensitivity by looking 'between' sky lines (instead of them being smeared out)

● separation circumstellar gas phase emission and/or absorption lines from ice band

Proper grating AND narrower slits needed for higher spectral resolution. Narrower slits transmit less light. Trade-off with seeing important.

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 23 High Spectral Resolution Needed? Ro-vibrational transition rules lead to characteristic P and R branch spectrum superimposed on ice absorption band. May be problem at low spectral resolution. Example: R=25,000 spectrum shows CO fundamental (J=1, v=1) absorption lines. Other sources show broad emission severely compromising the analysis.

Boogert et al. (2002)

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 24 Spectrometer Types

●Standard (but cooled!) grating spectrometers often used for ice and dust observations (R~

●Higher R sometimes desired: use echelle grating (optimized for high diffraction orders), which in combination with cross disperser (prism or grating) 'packs' orders efficiently on detector array provided slit is short.

●Longer slits (e.g. for larger field of view for extended emission) may only give one order on array.

●If desired wavelength range does not fit on array, move grating. But ISO/SWS moves mirror to scans spectrum over (small) array.

●Grism: prism+grating: all objects in field of view display spectra at position of direct imaging (e.g., AKARI satellite). Spectra of different objects may overlap. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 25 Spectrometer Types

Use of Echelle grating and normal grating modes for NIRSPEC/Keck

Low resolution mode: move High resolution mode: Echelle Echelle grating out of the way grating+cross disperse and only use cross disperse grating grating 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 26 Spectrometer Types ●1D spectroscopy for spatial distribution of solid state features possible using long slits.

●2D spectroscopy becoming more popular as more instruments (Herschel/PACS, JWST/MIRI+NIRSPec) have Integral Field Units.

●IFU image slicer images sky pixels onto entrance slit of grating spectrometer

Example: Herschel/PACS IFU 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 27 Spatial Resolution

●Telescopes seeing limited in near-infrared (although adaptive optics using (laser) guide stars can remove much of the seeing effects), but often diffraction limited in mid-infrared: m arcsec =0.252∗ diameter m

●Some infrared spectrometers are coupled with adaptive optics for high spatial resolution (CRIRES, NIRSPEC). E.g., One could study ice/dust features in circumstellar disks and envelopes as function of distance to star (see example in lecture 2).

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 28 Spatial Resolution

source: www.gtc.iac.es/en/media/canaricam/

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 29 Sensitivity ●Some ground-based and all space-based telescopes use online integration time calculators to determine time needed for certain S/N, source brightness, instrument settings (, and atmospheric conditions).

●For other facilities one needs to use sensitivity tables and tables with other noise contributions and then take into account instrument overheads.

●Noise sources: ● Readout noise: property of readout electronics. Expressed as standard deviation in unit of electrons per read-out. ● Photon or shot noise: intrinsic property of particle nature of light. Scales with square root of number of photons per readout, following Poisson statistics. Note: includes ALL photons, incl. science object, thermal background, dark current! Main reason why ground-based thermal infrared observations are tough. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 30 Sensitivity ● Sky background variations, even if the weather is good the sky background varies. It follows behavior of “1/F noise” and decreases if the chopper frequency F increases. Can be corrected a bit by subtracting neighboring sky on 2D arrays.

●In summary, signal-to-noise ratio achieved is

2 N el obj N integr÷N el obj2 N el atmtel2 N eldarkread

● Nel(obj) is the number of electrons from the science target after photon absorption in atmosphere, filters, mirrors, and taking into account detector quantum efficiency. ● At >3 m, Nel(atm+tel)>>Nel(obj), so main source of noise, e.g., L=13.0 mag object yields 25 electrons/sec with ISAAC/VLT, while background is 25,000 electrons/sec! ● For satellites Nel(atm+tel) much smaller, but extraterrestrial background becomes significant (cirrus, zodiacal) 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 31 Data Reduction Reduction of data taken with modern infrared detector arrays fairly similar to optical CCD data reduction:

(1) [Bias level subtraction]. Optional, because taken care of automatically in step (2) or (4).

(2) [Dark current subtraction]. Needs same integration time as on science, calibration, flatfield targets. Optional, because taken care of automatically in step (4).

(3) Division by flatfield: ● spectral response curve for 1D spectra ● Taken using lamp or on thermal background

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 32 Data Reduction (Continued) (4) Subtract Chop/Nod pairs to get rid of ● Telescope+earth background emission ● Sky background emission (Cirrus, interstellar) ● Hot pixels (Spitzer)

(5) Extract 1D spectrum from 2D image ● Second order background subtraction using neighboring columns: ● Thermal background variations between nod/chop ● Local, spatial variable extended source emission

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 33 Data Reduction (Continued) (6) Divide by telluric line standard ● Can be used for photometric calibration as well ● Preferred spectral type, airmass difference, brightness ● Need stellar model spectra (e.g., from Kurucz) ● Correct for grating shifts before division. Telescope pipelines generally don't do this. ● Gets rid of other multiplicative effects in case no flatfielding done or that flatfield did not remove such as standing waves (though not perfectly). ● Optionally use atm models to remove sky lines or residuals thereof

04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 34 Data Reduction (Continued) (7) Wavelength calibration: ● Use calibration lamp lines ● Lamp lines generally not available >4 um ● Use atmospheric emission lines for wavelength calibration or fine-tune the lamp calibration (grating can shift between science and lamp observations). Atmospheric lines come for free with infrared observations and should give very accurate calibration.

Software packages for infrared data reduction:

●Some telescopes, especially satellites, provide packages optimized for data reduction of specific instruments (VLT: MIDAS, IRTF: an IDL-based package, ISO/SWS: IDL+OSIA, Herschel: HIPE). ●Spitzer/IRS provides tools for certain aspects of data reduction (e.g., SPICE for source extraction). ●General purpose tools such as IRAF and IDL can be used, but may require extensive script writing. 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 35 Summary :Ground vs Space Based IR Astronomy Given huge advantage of low background emission of satellites, why bother doing ground based infrared spectroscopy?

●Large ground based telescope apertures give high spatial resolution (there will be JWST, but then there will be ELT, TMT..).

●Ground based telescopes can be equiped with new, more specialized instruments (e.g., high spectral resolution).

●Infrared satellites have limited lifetime, making follow up spectroscopy on new discoveries hard.

●Windows of good/fair transmission between 3-4, 4.6-5, 8-13 and 16-25 μm. Benefit from cold high dry sites (Mauna Kea/Chile) and low emissivity, high cleanliness.

●Last 3 points are in favor of SOFIA airplane too 04 June 2012 Interstellar Dust School (Cuijk): Infrared Spectroscopy (Boogert) 36