Integral Field Spectroscopy. David Lee, Anglo-Australian Observatory.

Why integral field spectroscopy? Traditional spectroscopic techniques include:Traditional spectroscopic techniques include: –Longslit spectroscopy. This provides a spectrum with one dimensional spatial information along the slit. –Multiple-object spectroscopy either with multiple slits or multiple fibres. These techniques simultaneously provide spectral information from many objects (~100) but with limited spatial information along the slit and no spatial information from fibres. For observations of many types of object it would be useful to obtain information about the two-dimensional spatial structure as well as spectral information.For observations of many types of object it would be useful to obtain information about the two-dimensional spatial structure as well as spectral information. Integral field spectroscopy is a relatively new technique developed to achieve this.Integral field spectroscopy is a relatively new technique developed to achieve this.

Integral field spectroscopy. Integral field spectroscopy is a technique to produce a spectrum for each spatial element in an extended two- dimensional field. The observation produces a data- cube containing both spatial and spectral information.

Various methods of IFS:

Advantages of IFS Obtain both imaging and spectroscopic information simultaneously - maximises the information available on the detector.Obtain both imaging and spectroscopic information simultaneously - maximises the information available on the detector. In bad seeing the large field of view of an IFS will help to prevent slit losses.In bad seeing the large field of view of an IFS will help to prevent slit losses. Resolution is fixed by the fibre / mirror size not by the slit width.Resolution is fixed by the fibre / mirror size not by the slit width. Use of optical fibres allows the instrument to be removed from the telescope and located in a more stable environment.Use of optical fibres allows the instrument to be removed from the telescope and located in a more stable environment. Target acquisition is straightforward.Target acquisition is straightforward.

Disadvantages of IFS The integral field unit optics can decrease the transmission of the instrument.The integral field unit optics can decrease the transmission of the instrument. Accurate sky subtraction becomes more difficult than with a longslit or multiple-slit spectrograph.Accurate sky subtraction becomes more difficult than with a longslit or multiple-slit spectrograph. Data analysis can be difficult.Data analysis can be difficult. The “slit length” is much less than with a longslit spectrograph.The “slit length” is much less than with a longslit spectrograph.

Science with IFS Spectroscopy of extended objectsSpectroscopy of extended objects Spatially resolved spectroscopy (aperture effect)Spatially resolved spectroscopy (aperture effect) Dynamics, kinematics, velocity maps - rotation curvesDynamics, kinematics, velocity maps - rotation curves Velocity dispersion informationVelocity dispersion information Variation of spectrum within object (starburst / AGN etc)Variation of spectrum within object (starburst / AGN etc) Line strength distributionsLine strength distributions Maps of emission / absorption linesMaps of emission / absorption lines “Large aperture spectroscopy” without loss of resolution (Low Surface Brightness galaxies)“Large aperture spectroscopy” without loss of resolution (Low Surface Brightness galaxies)

Schematic of SPIRAL on the AAT. Figure courtesy of Matthew Kenworthy

How the IFU works: Fore-optics re-image the telescope focal plane.Fore-optics re-image the telescope focal plane. A micro-lens array is used to sample the magnified imageA micro-lens array is used to sample the magnified image Optical fibres are used to re-format the two- dimensional image into a one-dimensional slit.Optical fibres are used to re-format the two- dimensional image into a one-dimensional slit. The fibres feed a dedicated bench mounted optical spectrograph.The fibres feed a dedicated bench mounted optical spectrograph. The light is dispersed to form a spectrum on the detectorThe light is dispersed to form a spectrum on the detector

Photograph of micro-lens array. The LIMO micro-lens array contains mm Square lenses, all Silica construction, with anti-reflection coatings.

SPIRAL’s two-dimensional fibre array The two-dimensional fibre array containing 512 optical fibres. The fibres are positioned within a machined brass plate to an accuracy of ~5 microns RMS.

SPIRAL’s output slit. At the output slit the fibres are reformatted into a linear array which forms the entrance slit to the spectrograph. SPIRAL’s output slit is 60 mm in length.

The SPIRAL spectrograph Littrow designLittrow design Mounted on a stable optical benchMounted on a stable optical bench Operates at F/4.8Operates at F/4.8 Spectral resolution from Spectral resolution from Wavelength range nmWavelength range nm

IFS: observing sequence Due to the large amount of data obtained with a single IFS observation some care has to be taken to ensure that appropriate calibration exposures and sky observations are taken. Arc lamp exposures for wavelength calibration - each fibre is individually calibrated.Arc lamp exposures for wavelength calibration - each fibre is individually calibrated. Dome - flat exposures (white light source) to allow identification of spectra and to remove pixel to pixel variations on the detector.Dome - flat exposures (white light source) to allow identification of spectra and to remove pixel to pixel variations on the detector. Twilight sky exposures to accurately determine the transmission of each fibre / microlens - this allows flat-fielding of reconstructed images.Twilight sky exposures to accurately determine the transmission of each fibre / microlens - this allows flat-fielding of reconstructed images. Object / Sky exposures - separate object and sky observations may be required. Care has to be taken to obtain accurate sky subtraction.Object / Sky exposures - separate object and sky observations may be required. Care has to be taken to obtain accurate sky subtraction. Spectrophotometric standard stars for flux calibration or velocity templates.Spectrophotometric standard stars for flux calibration or velocity templates.

Example CCD data - MITLL2

E-IFU standard star observation.

Example data: PKS V=17 Compact radio galaxy Continuum Emission IFU image at wavelength 5550 Å

PKS

Emission line map in O[III] 5007Å (~5500Å at z=0.0985)

Sky subtraction: mean sky method Allocate sky fibres within field of view Simultaneous observation of both object and skySimultaneous observation of both object and sky object must not fill field of viewobject must not fill field of view sky spectrum obtained from mean of all sky fibressky spectrum obtained from mean of all sky fibres Problems can arise due to systematic errors such as: Wavelength calibration errorsWavelength calibration errors Errors in determination of fibre throughputErrors in determination of fibre throughput contamination of sky spectrumcontamination of sky spectrum

Sky subtraction: mean sky method Before After sky subtraction (note sky residuals)

Low Surface Brightness galaxy F Integrated photographic B magnitude = 18.6 magIntegrated photographic B magnitude = 18.6 mag Surface brightness 23.9 mag/square arc-secondSurface brightness 23.9 mag/square arc-second Object size ~10 arc-secondsObject size ~10 arc-seconds DSS image Spectrum from 45 minute exposure (3 x 900 s object, 3 x 900 s sky)

Spectrum of LSB galaxy Reconstructed IFU image Note the excellent subtraction of the night sky emission linesNote the excellent subtraction of the night sky emission lines Measured redshift z = 0.029Measured redshift z = 0.029

Planetary nebula NGC6302 Reconstructed IFU image in N[II]. This image consists of a mosaic of 8 IFU images. DSS image

The jet of R-mon (NGC 2261) SPIRAL observations of the reflection nebula around star R-mon. Green is H- alpha, blue is [OI] 6300 Å, red is [SII] 6716 Å. Image size is 35” x 20”.

Example spectra from R-Mon Spectra, S[II] 6716 and 6731 Å, from two of the SPIRAL slit blocks with the continuum subtracted. Weak emission from the nebula can be seen, with stronger blue shifted emission from the jet.

Supernova 1987A Composite colour IFU image with [OI] 6300 Å (red) and continuum (blue and green). HST image

SAURON data: NGC Reconstructed image of NGC 2549 Velocity map Velocity dispersion map

IFS: further information Lenslet - fibre type systems:Lenslet - fibre type systems: –SPIRAL - A: Kenworthy et al., 2001, PASP, Vol. 113, p 215 –INTEGRAL (WHT): Lenslet only type systems:Lenslet only type systems: –TIGER: Bacon et al., 1995, Astronomy and Astrophysics Supplement, v.113, p.347 –SAURON: Bacon et al., 2001, MNRAS, in press (astro-ph/ ) Image slicer spectrographs:Image slicer spectrographs: –NIFS (Gemini): –3D: Weitzel et al., 1996, Astronomy and Astrophysics Supplement, v.119, p.531

Summary of IFU characteristics. Field of view 22” x 11” (32” x 10”)Field of view 22” x 11” (32” x 10”) Spatial sampling 0.7” (1.0”)Spatial sampling 0.7” (1.0”) Wavelength range 480 nm nmWavelength range 480 nm nm Resolution (R=4000 with 600 l/mm)Resolution (R=4000 with 600 l/mm) IFU data reduction software available “on-line” during observations at AATIFU data reduction software available “on-line” during observations at AAT SPIRAL - Nod & Shuffle observing mode for improved sky subtraction accuracySPIRAL - Nod & Shuffle observing mode for improved sky subtraction accuracy

Diagram of fore-optics Corrector lens 2 - Magnification lens 3 - Field lens 4 - Microlens array

Fibre Transmission. SPIRAL uses “blue” fibres for better UV performance but with absorption in the red.