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Astronomical Spectroscopy
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.
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
Example: the undispersed image
Example: the dispersed image
Extracting information from the dispersed image:
the digital read-out and the extracted spectrum
The extracted spectrum is
what is used to do science
Wavelength (Angstrom)
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)
Spectra: continuum emission, line emission, and
line absorption
These are emission lines,
some are isolated, some are
in densely-packed bands of
lines
Argon
Helium
This is a continuum
spectrum (the Solar Black
Body), with absorption lines
by intervening gas
Mercury
T = 5,800 K
Sodium
Shall we try to see real spectra using a simple, but absolutely real spectrometer?
Neon
Identifying the nature of sources
Star-forming
galaxy
Star
Quasar
Passive galaxy
The chemistry of sources: spectral chemical patterns
Argon
These are spectra
obtained in the Lab.
We use them as
template to identify
the chemicals in the
observed spectra of
sources
Helium
Mercury
Sodium
Spectral lines (both emission and absorption ones) are like a cosmic barcode
Neon
system for chemical elements.
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
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
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
Measuring Rotation
We determine the rotation velocity by measuring the Doppler shift
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 bandpass filters of longer
and longer wavelength
Expansion stretches photon wavelengths, causing a
cosmological redshift directly related to lookback time
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
Continuum emission, line emission, and line
absorption: let’s observe them!
These are emission lines,
some are isolated, some are
in densely-packed bands of
lines
Argon
Helium
This is a continuum
spectrum (the Solar Black
Body), with absorption lines
by intervening gas
Mercury
T = 5,800 K
Sodium
Shall we try to see real spectra using a simple, but absolutely real spectrometer?
Neon