AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

Astrophysical Techniques IV –Spectroscopy

B. Nikolichttp://www.mrao.cam.ac.uk/˜bn204/mailto:b.nikolic@mrao.cam.ac.uk

Astrophysics Group, Cavendish Laboratory, University of Cambridge

February 2012

AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

Outline

Science background

Spectroscopy techniques

Back matter

AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

Science

I Spectroscopy is a key tool in many areas ofastronomy from astrophysics

I It is used from the shortest to longest wavelengthsobserved in astronomy

I A wide range of instruments is available to matchpossible science goals

AstrophysicalTechniques IV

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Science areas

I Measurement of redshift (of distant galaxies, quasars,etc)

I Quantitative measurement of recent star formation innear-by galaxies

I Measurement of metallicity of starsI Composition, chemistry, physical conditions of the

interstellar mediumI Dynamics of galaxies, clusters of galaxies, the

interstellar mediumI Radial velocity planet searchesI Studies of interstellar dust through extinction

measurementsI Many more! Most imaging studies lead to

spectroscopic followup

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‘Damped’ Lyα systems

Sloan Digital Sky Survey spectrum of a Q1135-0010

4000 5000 6000 7000 8000 9000Observed Wavelength

-5

0

5

10

15

20

25

Flu

x

(1e-

17

cg

s u

nit

s)

DLA

From:[Kulkarni et al.(2012)Kulkarni, Meiring, Som, Peroux, York, Khare, and Lauroesch]

AstrophysicalTechniques IV

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‘Damped’ Lyα systems

VLT/UVES zoom in

3700 3800 3900 4000 4100

0

0.5

1

1.5

Ob

serv

ed F

lux

(1

e-1

6 c

gs

un

its)

3700 3800 3900 4000 4100Observed Wavelength

0

0.5

1

1.5

No

rmal

ized

Flu

x

log NHI = 22.05

log NHI = 21.95, 22.15

log NHI = 21.85, 22.25

From:[Kulkarni et al.(2012)Kulkarni, Meiring, Som, Peroux, York, Khare, and Lauroesch]

AstrophysicalTechniques IV

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Gas outflows from proto-stars

JCMT/HARP-B CO 3→2(Focal Plane heterodyne array)

0

2

4

6

8

10

-50 -40 -30 -20 -10 0 10 20 30 40

12C

O(3

-2)

Tb [K

]

vLSR [km s-1

]

Blueshifted LobeRedshifted Lobe

From:[Gottschalk et al.(2012)Gottschalk, Kothes, Matthews, Landecker, and Dent]

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Stellar dynamics of nearby galaxies

LBT ‘long-slit’ near-IR spectroscopy

From: [Greco et al.(2012)Greco, Martini, and Thompson]

AstrophysicalTechniques IV

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Stellar dynamics of nearby galaxies

LBT ‘long-slit’ near-IR spectroscopy

From: [Greco et al.(2012)Greco, Martini, and Thompson]

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Tracing star formation and dynamics usingHα

VLT/VIMOS IFU spectroscopy

From[Bremer et al.(2012)Bremer, Scharwachter, Eckart, Zuther, Fischer, and Valencia-S]

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Tracing star formation and dynamics usingHα

Fabry-Perot imaging

From[Dicaire et al.(2008)Dicaire, Carignan, Amram, Marcelin, Hlavacek-Larrondo, de Denus-Baillargeon, Daigle, and Hernandez]

AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

Outline

Science background

Spectroscopy techniques

Back matter

AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

Things to think about

1. Spectral resolution2. Range of wavelengths that needs to be covered3. Size/location/distribution of objects4. Spatial resolution5. Dynamic range6. Sensitivity

AstrophysicalTechniques IV

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Basic approaches

1. Interfere incoming radiation with itself withdifferent/variable path length(Optical/infrared/sometimes in sub-mm). Dispersiontechnique:

I Refractive index (Prisms/Grisms)I Diffraction (Gratings)I Variable path length (Fourier transform

spectroscopy/Fabry-Perot)

2. Voltage measurement (possibly after down-mixing)and do spectral analysis in electronics(mm-wave/radio)

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Spectral resolution

Resolving power ≡ R =λ

δλ=

ν

δν(1)

R ∼ ∆Lλ

(2)

where ∆L maximum path length inside the instrument

Type R

Interference filter 102 – 103

Diffraction grating 103 – 106

Fabry-Perot 104 – 106

Fourier Transform 104 – 106

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Interference filter

I Essentially a fixed version of Fabry PerotI Layers created by thin film deposition onto glassI Astronomical workhorse

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Diffraction Grating

d

Constructive interference when:

d sin(θ) + d sin(θ′) = mλ (3)

d Spacing between rulingsθ Incident angle

m Diffraction “order”λ Wavelength

AstrophysicalTechniques IV

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Important mathematical relations

R =λ

δλ= Nm (4)

whereN: Number of rulingsm: Diffraction order used

If not diffraction limited then:

R =λ

δλ=

Nmχλ/D

(5)

whereχ: Seeing/angular size of slit usedD: Telescope diameter

AstrophysicalTechniques IV

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Some practicalities

I Artifacts from bright sources are common(reflections/ghosting/etc)

I Accurate positioning of slit/fibre/mask is crucial andcan be challenging

I Wavelength calibration is usually required in additionto usual detector calibrations

I Dispersion of incoming radiation over many pixelsmeans that measurements take much longer to be‘background limited’

I Outcome from newer instruments are large data‘cubes’ – can be difficult to interpret and analyse whenthere are many lines/features

AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

Outline

Science background

Spectroscopy techniques

Back matter

AstrophysicalTechniques IV

B. Nikolic

Sciencebackground

Spectroscopytechniques

Back matter

References

M. Bremer, J. Scharwachter, A. Eckart, J. Zuther,S. Fischer, and M. Valencia-S.Optical Integral Field Spectroscopy of NGC 5850.ArXiv e-prints, Feb. 2012.

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: AVery Extended Hα Disk and A Truly Declining RotationCurve.AJ, 135:2038–2047, June 2008.doi: 10.1088/0004-6256/135/6/2038.

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.ArXiv e-prints, Feb. 2012.

J. P. Greco, P. Martini, and T. A. Thompson.Measurement of the Mass and Stellar PopulationDistribution in M82 with the LBT.ArXiv e-prints, Feb. 2012.

V. P. Kulkarni, J. Meiring, D. Som, C. Peroux, D. G.York, P. Khare, and J. T. Lauroesch.A Super-Damped Lyman-alpha QSO Absorber atz=2.2.ArXiv e-prints, Feb. 2012.