Spectroscopy. B. Nikolic bn204/ February 2012

Transcription

1 bn204/ Astrophysics Group, Cavendish Laboratory, University of Cambridge February 2012

2 Outline

3 is a key tool in many areas of astronomy from astrophysics It is used from the shortest to longest wavelengths observed in astronomy A wide range of instruments is available to match possible science goals

4 areas Measurement of redshift (of distant galaxies, quasars, etc) Quantitative measurement of recent star formation in near-by galaxies Measurement of metallicity of stars Composition, chemistry, physical conditions of the interstellar medium Dynamics of galaxies, clusters of galaxies, the interstellar medium Radial velocity planet searches Studies of interstellar dust through extinction measurements Many more! Most imaging studies lead to spectroscopic followup

5 Damped Lyα systems Sloan Digital Sky Survey spectrum of a Q Flux (1e-17 cgs units) DLA Observed Wavelength From: [Kulkarni et al.(2012)kulkarni, Meiring, Som, Peroux, York, Khare, and Lauroesc

6 Damped Lyα systems VLT/UVES zoom in Observed Flux (1e-16 cgs units) log NHI = log NHI = 21.95, Normalized Flux log NHI = 21.85, Observed Wavelength From: [Kulkarni et al.(2012)kulkarni, Meiring, Som, Peroux, York, Khare, and Lauroesc

7 Gas outflows from proto-stars JCMT/HARP-B CO 3 2 (Focal Plane heterodyne array) 10 Blueshifted Lobe Redshifted Lobe 8 12 CO(3-2) Tb [K] v LSR [km s -1 ] From: [Gottschalk et al.(2012)gottschalk, Kothes, Matthews, Landecker, and Dent]

8 Stellar dynamics of nearby galaxies LBT long-slit near-ir spectroscopy From: [Greco et al.(2012)greco, Martini, and Thompson]

9 Stellar dynamics of nearby galaxies LBT long-slit near-ir spectroscopy From: [Greco et al.(2012)greco, Martini, and Thompson]

10 Tracing star formation and dynamics using Hα VLT/VIMOS IFU spectroscopy From [Bremer et al.(2012)bremer, Scharwächter, Eckart, Zuther, Fischer, and Valenc

11 Tracing star formation and dynamics using Hα Fabry-Perot imaging From [Dicaire et al.(2008)dicaire, Carignan, Amram, Marcelin, Hlavacek-Larrondo, de

12 Outline

13 Things to think about 1. Spectral resolution 2. Range of wavelengths that needs to be covered 3. Size/location/distribution of objects 4. Spatial resolution 5. Dynamic range 6. Sensitivity

14 Basic approaches 1. Interfere incoming radiation with itself with different/variable path length (Optical/infrared/sometimes in sub-mm). Dispersion technique: Refractive index (Prisms/Grisms) Diffraction (Gratings) Variable path length (Fourier transform spectroscopy/fabry-perot) 2. Voltage measurement (possibly after down-mixing) and do spectral analysis in electronics (mm-wave/radio)

15 Spectral resolution Resolving power R = λ δλ = ν δν (1) R L λ where L maximum path length inside the instrument Type R Interference filter Diffraction grating Fabry-Perot Fourier Transform (2)

16 Interference filter Essentially a fixed version of Fabry Perot Layers created by thin film deposition onto glass Astronomical workhorse

17 Diffraction Grating d Constructive interference when: d sin(θ) + d sin(θ ) = mλ (3) d Spacing between rulings θ Incident angle m Diffraction order λ Wavelength

18 Important mathematical relations where N: Number of rulings R = λ = Nm (4) δλ m: Diffraction order used If not diffraction limited then: R = λ δλ = Nm χλ/d (5) where χ: Seeing/angular size of slit used D: Telescope diameter

19 Some practicalities Artifacts from bright sources are common (reflections/ghosting/etc) Accurate positioning of slit/fibre/mask is crucial and can be challenging Wavelength calibration is usually required in addition to usual detector calibrations Dispersion of incoming radiation over many pixels means that measurements take much longer to be limited Outcome from newer instruments are large data cubes can be difficult to interpret and analyse when there are many lines/features

20 Outline

21 References M. Bremer, J. Scharwächter, A. Eckart, J. Zuther, S. Fischer, and M. Valencia-S. Optical Integral Field of NGC ArXiv e-prints, Feb I. Dicaire, C. Carignan, P. Amram, M. Marcelin, J. Hlavacek-Larrondo, M.-M. de Denus-Baillargeon, O. Daigle, and O. Hernandez. Deep Fabry-Perot Hα Observations of NGC 7793: A Very Extended Hα Disk and A Truly Declining Rotation Curve. AJ, 135: , June doi: / /135/6/2038. M. Gottschalk, R. Kothes, H. E. Matthews, T. L. Landecker, and W. R. F. Dent. The JCMT 12CO(3-2) Survey of the Cygnus X Region: I. A Pathfinder.