Astronomical Spectroscopy

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Astronomical Spectroscopy
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Astronomical Spectroscopy

• Astronomical spectroscopy is done by attaching a
  spectrometer to a telescope
• A spectrometer is a device separates the
  individual wavelengths of the incoming light and
  measures the intensity of each
• Every spectrometer consists of two distinct
  components
• A disperser, which sorts the incoming light by
  wavelength, and then sends light of different
  wavelength along different directions, so that
  each wavelength will be observed at its own
  location.
• The disperser is what makes a spectrometer a
  spectrometer. It is the components that separates
  light of different wavelength
• There are many types of dispersers, prims,
  grisms, gratings, etc.
• Technology differs, but they all do the same
  thing sort light of different wavelength
• A camera, which images the stripe of dispersed
  light (spectral image) and focuses it onto the
  detector, which in turn records the spectral
  image.

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Example of a disperser the prism
Here the image of this beam of light is a point
Here the image of this beam of light is a line
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Example the undispersed image
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Example the dispersed image
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Extracting information from the dispersed
imagethe digital read-out and the extracted
spectrum
The extracted spectrum is what is used to do
science
Wavelength (Angstrom)
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Why Spectroscopy

• We take spectra of astronomical sources to
• Identify their nature
• The spectrum of a Star differs from that of a
  Galaxy, which differs from that of a Quasar
• Measure their chemical composition and the
  abundances of chemicals
• The universe becomes richer and richer of
  chemical elements as time passes.
• Study their motions, measure their speeds
• Fundamental to understand the evolution of the
  sources and how their interact with each other
  (e.g. merging, collisions)
• Measure their redshift, determine their distance
  from us
• Fundamental to chart the large-scale structure of
  the Universe and to study the formation of cosmic
  structures (groups, clusters, superclusters)

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Spectra continuum emission, line emission, and
line absorption
 Argon                                                                                  
 Helium                                                                                  
 Mercury                                                                                  
 Sodium                                                                                  
Neon                                                                                  
These are emission lines, some are isolated, some
are in densely-packed bands of lines This is a
continuum spectrum (the Solar Black Body), with
absorption lines by intervening gas
T 5,800 K
Shall we try to see real spectra using a simple,
but absolutely real spectrometer?
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Identifying the nature of sources
Star-forming galaxy
Star
Quasar
Passive galaxy
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The chemistry of sources spectral chemical
patterns
 Argon                                                                                  
 Helium                                                                                  
 Mercury                                                                                  
 Sodium                                                                                  
Neon                                                                                  
These are spectra obtained in the Lab. We use
them as template to identify the chemicals in the
observed spectra of sources
Spectral lines (both emission and absorption
ones) are like a cosmic barcode system for
chemical elements.
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Studying the chemistry of galaxies
This galaxy with weaker emission lines has 3x the
abundance of chemicals of our own Milky Way
Galaxy.
This galaxy with much stronger emission lines has
1/5x the abundance of chemicals of our own Milky
Way Galaxy
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Studying the chemical composition of gas
The picture shows the spectrum of Distant cold
inter-galactic gas Distant galaxies Local
galaxies The galaxies clearly show the presence
of Magnesium (Mg)and Iron (Fe) in their
spectra The Inter-galactic gas only shows
Magnesium, but not iron If confirmed, this would
be the first detection of cold, dense gas with
primordial chemical composition ever
observed. This would be the primeval gas out of
which early galaxies form
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Doppler shift studying motions (e.g. of gas)
Notice that these lines are observed at bluer
wavelength than in the Lab the gas is moving
toward us at V-350 km/s
Here the same line are observed at the same
wavelength as in the Lab no motions

• These spectra show the absorption by interstellar
  gas (Magnesium) in star-forming galaxies
• In the local galaxies, the gas absorption has the
  same wavelength as in the Lab no motions
• In the distant galaxies, the gas is observed at
  bluer (shorter) wavelengths it is moving away
  from the galaxies (toward us) at V-350 km/s

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Measuring Rotation
We determine the rotation velocity by measuring
the Doppler shift
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The effect of the cosmic expansion of space
redshift
Shown here is the spectrum of the same galaxy
placed at higher and higher redshift. The higher
the redshift (z), the more the spectrum is
observed shifted to redder wavelengths (l), the
more the galaxy appears fainter. The redshift is
induced by the stretching of space by the cosmic
expansion! Notice that to observe the same
portion of the spectrum at higher and higher
redshift, one needs to use band-pass filters of
longer and longer wavelength
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Expansion stretches photon wavelengths, causing a
cosmological redshift directly related to
lookback time
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The effect of redshift
A gallery of spectral images of galaxies at
increasingly higher redshift (labeled on the
left) These are all star-forming galaxies
observed very early in the cosmic evolution
(primeval galaxies) The emission line shown in
the circles is observed at longer wavelengths in
those galaxies located at higher redshift The
line is called Lya
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Continuum emission, line emission, and line
absorption lets observe them!
 Argon                                                                                  
 Helium                                                                                  
 Mercury                                                                                  
 Sodium                                                                                  
Neon                                                                                  
These are emission lines, some are isolated, some
are in densely-packed bands of lines This is a
continuum spectrum (the Solar Black Body), with
absorption lines by intervening gas
T 5,800 K
Shall we try to see real spectra using a simple,
but absolutely real spectrometer?