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? • 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). A picture may be worth a thousand words, but a spectrum is worth a thousand pictures. —Blair Savage
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)
Definition of Spectral Resolution Intrinsic Thorium Profile Resolution: Resolving Power: Observed Profile FWHM
Morphological Features in Spectra Continuum Fit 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 Residual Intensity is the Flux Spectrum Divided by Continuum Fit Line Depth Line Width Equivalent Width:
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) = Ns 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) = Foexp(-(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
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).
Model for Disk Velocities in M87 Courtesy L. Dressel
STIS Long-Slit Spectrum of NGC 3998 L. Dressel/STScI
Fitting the Ha+[N II] and [S II] Emission Lines Flux 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 ~300-500 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.
Intrinsic absorption in the FUSE spectrum of a Seyfert 1 galaxy (Kriss et al. 2000). Relative Flux
PG 1634 +706 (z=1.335) Ly– STIS E140M Ly–
He II in the Intergalactic Medium Optical depth to H I in a Standard Cold Dark Matter Model atz = 2.336 Optical depth to He II From Croft et al. (1997)