Optical Spectroscopy
Introduction & Overview
Ian Browne & Chris O’Dea
Acknowledgements: Jerry Kriss & Jeff Valenti
Aims for this lecture
 What is Spectroscopy?
 Spectrographs
 Information in a Spectrum
• Emission Lines
• Absorption Lines
 Astrophysical Results from Spectroscopy
What is Spectroscopy?
A picture may be worth a thousand words,
but a spectrum is worth a thousand pictures.
—Blair Savage
 Spectroscopy is the study of radiation that has been dispersed into
its component wavelengths.
 First astronomical spectrum—the Sun (Newton 1666; Wollaston
1802; Fraunhofer 1814, 1817).
Spectroscopic Discoveries in Astronomy
 Chemical Abundances
• Discovery of Helium (in solar spectra, Lockyer & Janssen 1868)
• Stellar evolution & nucleosynthesis (Fowler, Burbidge2 1955)
• Big-bang nucleosynthesis (Peebles 1966)
• Measuring D/H is the primary mission of the Far Ultraviolet
Spectroscopic Explorer (FUSE)
 Radial velocities/redshifts
• Galactic structure and rotation (Oort 1927)
• Expansion of the universe (Hubble 1929)
• Dark matter in clusters of galaxies (Zwicky 1937)
• Discovery of quasars (Schmidt 1963)
• Planets around nearby stars (Mayor,Queloz, Marcy, Butler 1995)
• g-ray bursters are at high redshift (Metzger et al. 1997)
• High-z supernovae/accelerating universe (Riess et al. 2000)
Spectroscopic Discoveries in Astronomy
 Line Widths
• Stellar surface gravities (white dwarfs)
• Stellar rotation (Schlesinger 1909)
• Velocity dispersions in ellipticals and bulges
• Ellipticals are not rotationally supported (Illingworth 1977; Schechter
& Gunn 1979)
• Black holes in galactic nuclei (e.g., Kormendy & Richstone 1995)
What are those Squiggly Lines?
 Spectroscopic observations rarely receive press attention since the
results aren’t as photogenic or as easily understood as astronomical
images:
• 2 of 32 HST press releases during 2000 were based on
spectroscopic observations.
• Neither shows a spectrum!
 Some exceptions:
• Black hole in M87, FOS & WFPC2 (Ford, Harms, et al. 1994)
• He II in the IGM, HUT (Davidsen, Kriss, Zheng 1996)
• Black hole in M84, STIS (Bower et al. 1997)
• SN1987A, STIS (Sonneborn et al. 1997)
Kinds of Spectrographs
 1-dimensional (1D)
• Dispersed light is obtained from a single spatial point, or aperture
• Advantages:
• Only requires a 1-D detector
• Simple optical design since entering light confined to the optical axis
• Examples: FOS, GHRS, HUT, COS
 2-dimensional (2D)
• Light entering through a long slit is dispersed at each point
• Advantage:
• Spatial multiplexing increases efficiency by >10x
• Disadvantages:
• Requires a 2-D detector
• Greater optical complexity to handle off-axis rays
• Examples: STIS, nearly all ground-based telescopes
Kinds of Spectrographs
 3-dimensional (3D)—Integral Field Spectrographs
• An entire area of the sky is imaged, and light from each pixel is
separately dispersed into a spectrum. From this one can
construct “data cubes” giving intensity as a function of (x, y, l).
• For compact objects, multiplexing the additional spatial element
provides another order of magnitude increase in efficiency.
(The tradeoff is the size of the field covered.)
• Examples:
•
•
•
•
Lenslet arrays: TIGER, OASIS (CFHT)
Fiber arrays: DensePack (KPNO, retired), INTEGRAL (WHT)
Image slicers: popular for IR applications, MPE’s “3D”
Fabry-Perot interferometers: Rutgers (CTIO), TAURUS-2 (AAT)
The OASIS Integral Field Spectrograph at the CFHT
Sample Data from an Integral Field Spectrograph
Atmospheric Transmission (300-1100 nm)
Definition of Spectral Resolution
Intrinsic
Thorium
Profile
Resolution:
R  l or
cl
o
l
(Units : A or km s-1 )
Resolving Power:
l
R
l
(Units : Dimensionl ess)
Observed Profile
l
FWHM
l
Morphological Features in Spectra
l
Line Flux 
2
 Fl dl
l
Continuum Fit
1
(Units : erg s-1 cm-2 )
Continuum
Emission
Lines
Absorption
Lines
Information in a Spectrum
 A spectroscopic observation provides the following information:
• Spatial location (point, one, or two dimensions)
• Spatial resolution is instrument dependent
• Intensity (flux) as a function of wavelength
• Spectral resolution is instrument dependent
• Polarization as a function of wavelength
• The FOS could do spectropolarimetry, but STIS cannot
 Spectroscopic observations provide a direct view of atomic and
molecular processes via their radiative transitions, thus enabling us
to probe physical conditions in astronomical sources.
Quantitative Measurements of Emission Lines
 Flux, Centroid, Full-width at Half Maximum (FWHM)
• 0th, 1st, and 2nd moments of a spectral feature
• Fluxes 
physical conditions (density, temperature)
ionization state
abundances
• Centroids  Kinematics (velocities)
Outflow? Inflow? Rotation?  Black Hole Mass
• FWHM 
Dynamics, temperature
 Making physical inferences
• Use individual lines as plasma diagnostics (Osterbrock 1989)
• Compare to models
• Collisional (or, coronal) equilibrium models
• Photoionization (CLOUDY, XSTAR)
• Shock models (MAPPINGS)
Optical Temperature Diagnostic
From Osterbrock (1989)
Optical Density Diagnostics
From Osterbrock (1989)
UV Density Diagnostic
From Osterbrock (1989)
Residual Intensity
Line Depth
Residual Intensity is the Flux Spectrum Divided by Continuum Fit
Lineo Width
(Units : A or km/s)
v  l

c
l
Equivalent Width:
l
2
W   1  rl dl
eq
l
1
o
(Units : A)
Quantitative Measurements of Absorption Lines
 Equivalent Width (EW), Centroid, FWHM
• Again, these are related to the 0th, 1st, and 2nd moments
• EW = ∫ (f(l) – fc(l)) / fc(l) d l ~ Flux/ fc(lo)
• EW  Column density  physical conditions
ionization state
abundances
• Centroids  Kinematics. Stellar lines  Black Hole Mass
• FWHM  Dynamics. Thermal motion? Turbulence?
 Opacity and Line Profiles
• Absorption cross section is s  f pe2/mc),
where “f” is the oscillator strength.
• Opacity t (n) = N s f(n)  N f pe2/mc) f(n)
• Lorentzian profile: f(n) = Fo (g/4p2 / ((n  no)2 + (g/4p2)
• Doppler profile: f(n) = Fo exp((n  no)2 c2 /b2 no2)(c/(bno√p))
• Voigt profile: Convolve the Lorentzian and Doppler profiles
Absorption Line Profiles
Doppler
Lorentzian
Curves of Growth
 Curve of growth for the line equivalent width is
Wn = ∫ (1  etn) dn
Square-root portion: Wl /l ~ (Nfl
Flat portion: Wl /l ~  ln(Nfl
Linear portion: Wl /l ~ Nfl
Spectral Features due to Hydrogen
Ultra-deep Echelle Spectra of the Orion Nebula
Baldwin etal 2000, ApJS, 129, 229
Region of the Balmer limit.
Hydrogen lines up to n=28 are
detected.
Emission lines of OII multiplet
line 1 and very week NIII and
NII lines
Measuring the Mass of Black Holes in Galaxies
 Use stellar motions (rotation and velocity dispersion) to constrain
models of stellar orbit distributions in the potential of a galaxy plus a
central supermassive black hole (e.g., van der Marel et al. 1997).
 When gas disks are present, rotational velocities can be measured
using line emission from the gas. Model as Keplerian rotation in the
potential of the galaxy plus a central supermassive black hole (e.g.,
Harms et al. 1994).
Ford et al. (1994); Harms et al. (1994)
Model for Disk Velocities in M87
Courtesy L. Dressel
STIS Observations of the LINER NGC 3998
STIS Long-Slit Spectrum of NGC 3998
L. Dressel/STScI
Flux
Fitting the Ha+[N II] and [S II] Emission Lines
Wavelength (Å)
Courtesy L. Dressel
Rotation Curve of NGC 3998
Courtesy L. Dressel
The Mass of the Black Hole in NGC 3998
2.0x108 Msun
1.5x108 Msun
Courtesy L. Dressel
BH Mass vs. Galaxy Bulge Mass
There is a relationship between BH mass and bulge
luminosity. And an even tighter relationship with the
bulge velocity dispersion. M(BH) ~ 10-3 M(Bulge).
Ferrarese & Merritt 2000, ApJ, 539, L9
Consistency Between Different Methods
 BH Mass vs bulge
magnitude relation is
similar for both active
and quiescent
galaxies.
BH Mass vs bulge magnitude for quiescent galaxies, Seyferts and
nearby quasars. Size of symbol for AGN is proportional to the Hβ
FWHM. Merritt & Ferrarese 2001, astro-ph/0107134
The Structure of AGN
Seyfert 1
Narrow Line Region
Torus
Central Engine:
Accretion Disk+Black Hole
Seyfert 2
Broad Line Region
The AGN Paradigm
Annotated by M. Voit
Radio Luminosity – Optical Line Correlation.
There is a strong
correlation between radio
luminosity and optical
emission line luminosity
for both RL and RQ
objects. (see also Baum &
Heckman 1989)
Xu etal 1999, AJ, 118, 1169
Emission Lines are Powered by Accretion Disk
Luminosity.
There is a strong
correlation between X-ray
luminosity and optical
emission line luminosity
for both RL and RQ
objects.
Xu etal 1999, AJ, 118, 1169
The Alignment Effect in CSS Sources
CSS radio galaxies show extended emission line gas which is
aligned with the radio source axis (De Vries etal 1998, Axon
etal 2000)
The Alignment Effect in CSS Sources
The emission line gas
is more strongly
aligned in the CSS
radio galaxies than in
high redshift radio
galaxies.
Histogram of difference in radio and
optical position angle. De Vries etal
1999, ApJ, 526, 27
The Alignment Effect in CSS Sources
HST STIS long slit
spectroscopy of CSS
Sources
O’Dea etal in preparation.
HST STIS Long Slit Spectroscopy of CSS
Sources
Distance
along slit
Wavelength (Velocity)
O’Dea etal in preparation.
HST STIS Long Slit Spectroscopy of CSS
Sources
There are systematic offsets
in velocity on the two sides of
the radio source
There are complex line
profiles with possibly
multiple components
Velocity shifts are ~300500 km/s
Association of velocity
shifts with radio lobes
suggests that the CSS radio
lobes are accelerating gas to
these velocities.
O’Dea etal in preparation.
Relative Flux
Intrinsic absorption in the FUSE spectrum of a Seyfert 1 galaxy (Kriss et al. 2000).
Physical Properties of the Absorbers in Mrk 509
Component
#
1
v
(km s–1)
–438
2
NOVI/NHI
log Ntot
log U
0.37
18.3
–1.64
–349
1.51
18.8
–1.19
3
–280
0.48
18.3
–1.79
4
–75
0.19
19.2
–1.73
5
–5
20.7
–0.43
6
+71
2.14
18.9
–1.41
7
+166
2.76
18.8
–1.46
13.9
PG 1634 +706 (z=1.335)
Ly–a
STIS
E140M
Ly–
He II in the Intergalactic Medium
Optical depth to H I in
a Standard Cold Dark Matter
Model at z = 2.336
Optical depth to He II
From Croft et al. (1997)
Ford et al. (1994); Harms et al. (1994)
Supermassive Black Hole:
M = 3108 Msun
STIS Slit
Nucleus
– 400 km/s
16 pc
+400 km/s
References
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The End