Integral Field Spectroscopy. David Burnham & Trystyn Berg

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1 Integral Field Spectroscopy David Burnham & Trystyn Berg

2 Introduc;on Integral Field Units (IFUs) are rela;vely new instruments in astronomy for obtaining spa;ally resolved spectra. The largest and most advanced telescopes today are being equipped with IFUs, including plans for future space telescopes. Taking mul;ple spectra over large resolvable objects is important for many modern astronomical projects. IFUs provide a datacube of two spa;al dimensions and one wavelength dimension. Thus, various features of a single object (or mul;ple objects) in the field of view can be studied effec;vely. Both spa;al and spectral resolu;on have to be sacrificed for the combina;on of the two types of data. Image slice at a single infrared wavelength Galac;c nucleus seen in combined infrared light Spectral slice showing the spectra across the en;re galac;c nucleus

3 Theory IFUs use a similar concept to long slit spectrographs: Light from an extended object is passed through a single long slit The light is then diffracted with a gra;ng in the direc;on perpendicular to the slit The output is a two dimensional image of distance (along the slit) versus wavelength Spa;al informa;on is constrained along the slit itself, but Integral Field Units aim to add another spa;al dimension to provide spectral informa;on over the en;re field of view. Wavelength Using a single slit spectrograph is an unrealis;c method of obtaining a spa;ally resolved spectrum of extended objects. Long integra;on ;mes and telescope instabili;es inhibits piecing together mul;ple images efficiently.

4 Designs IFUs have 3 main designs: 1) Lenslets An array of lenslets focuses the incident light onto a single diffrac;on gra;ng. The gra;ng is arranged such that the spectra are dispersed at an angle on the CCD to prevent overlap. 2) Lenslets + Fibres The light from the lenslets is sent through individual fibre op;c cables and put through a single slit. The light is then dispersed perpendicular to that slit, providing the spectrum for each spaxel. The spa;al resolu;on in the datacube is limited not by the CCD, but by spaxels, the number of spectral inputs on the area of the detector.

5 Image Slicer 3) Image Slicer This design is the most similar in concept to the single long slit spectrograph: The light from the telescope is sent onto a slicing mirror (a mirror with rows to reflect light from the object into different direc;ons) These image slices are directed with mirrors onto the same spectrograph slit The light is diffracted by a gra;ng, providing a spectrum for each row

6 Designs Advantages Table 1 Lenslets LiYle light loss Simplicity in design Lenslets + Fibres Uses full detector Easier to convert single slit spectrographs Larger wavelength range Image Slicer High efficiency High spa;al resolu;on along slices Low number of spaxels Disadvantages More lenslets means lower spectral resolu;on Poor op;miza;on of detector space Op;cal fibres introduce light loss Currently limited to infrared wavelengths Tend to be bulkier than the other IFU designs

7 Project: Stellar Ou`lows During star forma;on gas is ejected into large ou`lows. Observa;ons with IFUs provide the intensity for molecular transi;on lines at different points over the en;re ou`low. By comparing the ra;o of various transi;ons of H 2 molecules, the temperature and excita;on mechanisms of the gas can be obtained using sta;s;cal mechanics. We can tell by the line ra;o of the intensi;es of two molecular transi;ons, whether the molecules are being excited through shocks (line ra;o ~10) or florescence (~2). The temperature can be obtained by comparing the number density (which is propor;onal to the line emission intensity) of different energy states.

8 Projects: Luminous Infrared Galaxies (LIRGs) IFUs are very well suited to studying various aspects of LIRGs. These galaxies are bright and full of star forming regions. IFUs can help determine: The precise loca;ons of star forma;on in the galaxies Posi;on and abundance of large star clusters Occurrence of supernovae within LIRGs Lee: Interac;ng galaxies from the Hubble Space Telescope beside a single wavelength of the datacube from an IFU (SINFONI). These images illustrate the sacrifice of spa;al resolu;on for spectral informa;on in IFUs.

9 Instruments SINFONI (Spectrograph for Integral Field Observa8ons in the Near Infrared) is a near infrared ( microns) image slicer IFU on the Very Large Telescope at Cerro Paranal in Chile. Depending on the wavelength band of observa;on, the spectral resolu;on ranges from The spa;al resolu;on is between , which is a tradeoff for the The Gemini North and South telescopes field of view size (8 x8 0.8 x0.8 ). have an IFU called The Gemini Mul8 Object Spectrograph (GMOS). It is a 1500 element lenslet+fibre array opera;ng in the op;cal and near infrared ( nm). 500 of the fibres are dedicated for observing the sky for removal of background signal. The instrument has a spectral resolu;on of , spa;al resolu;on of 0.2, and field of view of 5 x7.

10 SAURON (Spectrographic Areal Unit for Research on Op8cal Nebulae) at the William Herschel Telescope at La Palma, Chile is an op;cal (450700nm) IFU composed of a 1577 lenslet array. It has a spa;al resolu;on of 0.27, spectral resolu;on, and field of view of 41 x33. It's main goal is to study the forma;on of ellip;cal galaxies and the bulges of spirals. The James Webb Spaces Telescope (JWST) also has plans to be equipped with two instruments that have IFU components. MIRI (MidInfrared Instrument) will have roughly 1221 slice image slicer, covering the mid infrared of 528 μm. JWST will also have a nearinfrared instrument with an IFS mode called NIRSpec (Near Infrared Spectrograph).

11 References General Informa;on Durham University: Integral Field Spectroscopy Wiki: ifs.wikidot.com Stellar Ou`lows: Forde K. P., Gledhill T. M., 2012, MNRAS, 421, L49 LIRGs: Vaisanen P., Escala A., Kankare E., Ko;lainen J., Masla S., Rajpaul V., Randriamanakoto Z., Reunanen J., Ryder S., Zijlstra A., 2012, ArXiv eprints Image Credits: Introduc;on: Stephen Todd (ROE) and Douglas PiercePrice (JAC) Theory: en.wikipedia.org/wiki/file:longslit_spectroscopy_example.png Designs: First image from above Durham University link above & examples in table from M. WestmoqueYe, adapted from AllingtonSmith et al., 1998 Image Slicer: BlandHawthorn J., AllingtonSmith, J. Astronomical Imaging: 3D spectrophotometric imaging opens a new window into the cosmos (2010), Laser Focus World, Volume 46, Issue 12. Stellar Ou`lows: NASA, ESA, and P. Har;gan (Rice University) LIRGs: From above LIRGS source Instruments: VLT image from European Southern Observatory, GMOSSouth from Gemini Observatory, SAURON image from project website at JWST image from NASA