Astronomy across the spectrum: telescopes and where we put...
Transcript of Astronomy across the spectrum: telescopes and where we put...
Astronomy across the spectrum: telescopes and where we put them
Martha Haynes Exploring Early Galaxies
with the CCAT June 28, 2012
CCAT: 25 meter submm telescope
Me, at 18,400 feet in the high Atacama desert in Chile, at the site of the future CCAT (submillimeter wavelength telescope)
ALMA 12m antenna Oct ‘11
CCAT Site on C. Chajnantor
Thermal radiation • A blackbody is an object whose radiation depends only on its
temperature.
• If an object (star, planet, galaxy) behaves like a blackbody, then its radiation is said to be thermal, and its spectrum is given by “Planck’s function”).
• Spectrum: the variation in the intensity of light with wavelength.
B is the spectral radiance, the energy per unit time per unit surface area per unit solid angle per unit frequency (or wavelength)
h is Planck’s constant = 6.625x10-27 erg s
k is Boltzmann’s constant = 1.38x10-
16 erg K-1
B(,T) = 2h3
c2
1
exp(h/kT) -1
Wikipedia.org
Blackbody radiation • A blackbody is an object whose radiation depends only on its
temperature.
• If an object (star, planet, galaxy) behaves like a blackbody, then its radiation is said to be thermal, and its spectrum is given by “Planck’s function”).
• Spectrum: the variation in the intensity of light with wavelength.
h is Planck’s constant = 6.625x10-27 erg s
k is Boltzmann’s constant = 1.38x10-
16 erg K-1
B(,T) = 2h3
c2
1
exp(h/kT) -1
Wikipedia.org
B(λ,T) = 2hc2/λ5
exp(hc/λkT) -1
Non-thermal radiation • Not all sources that exhibit continuous
spectra are thermal, meaning that their temperature does not determine how their apparent brightness changes with wavelength. => non-thermal sources
• The most important source of non-thermal radiation is synchrotron emission, which is emitted when very fast moving electrons are accelerated as they spiral around lines of magnetic field.
For example, the radio source SgrA*: a supermassive black hole at the center of the Milky Way.
Here: 3C31
Blue: optical starlight Red: radio synchrotron
Observing the universe
Optical light: • Light from stars • Bright lines from
ionized (hot) gas near very hot stars and supermassive black holes in galactic nuclei
We need other telescopes to reveal: cold gas, cool gas, superhot gas, dust, and non-thermal sources!
Spectral energy distribution (SED) of galaxies
2h3 1 c2 exp(h/kT) - 1
I()=
Typical spectrum of active galaxy, i.e. one
with accreting supermassive black hole
in its nucleus
In the optical regime, we detect the integrated starlight.
Thermal emission = black body radiation I
But at other wavelengths, we detect other important constituents like gas, dust, and synchrotron radiation
Darkness: Absence of (visible) light
Extinction due to foreground dust: makes a star appear redder and fainter
Interstellar Dust
• Probably formed in the atmospheres of cool stars
• Mostly observable through infrared emission - very cold < 100 K.
• Radiates lots of energy - surface area of many small dust particles adds of to very large radiating area
• Infrared and radio emissions from molecules and dust are efficiently cooling gas in molecular clouds.
• Whispy nature indicates turbulence in ISM
IRAS (infrared) image of
infrared cirrus of interstellar
dust.
“Dark cloud” Barnard 68 B Z V
K H J
Electromagnetic spectrum
Astronomical Images
• Position on the sky • Morphological appearance • Apparent brightness (flux) at some l
• Images at different times:
• Does source move? => parallax?
• Does it change size/shape? • Does it change brightness?
• Images in different wavelength bands • Flux => temperature, if thermal source
• What is the image’s field-of-view? • What is the image’s angular resolution? • What is the image’s spectral sensitivity? • When was the image taken?
Different telescopes provide different clues
Images
Wide field High resolution Morphology: appearance, structural details Astrometry: position, relative to other objects Photometry: apparent brightness, color
Spectra:
temperature, density, chemical composition, motions
Elliptical galaxy spectra
Elliptical galaxy spectra
Color: difference in the flux at two wavelengths
Spectral energy distribution
More measures of flux => more accurate representation of the true spectral energy distribution (SED)
Activity at 11am: The CMD of galaxies
Red: ellipticals Blue: spirals
Galaxy spectra
• Redshift
• Velocity dispersion/rotational velocity
• Star formation rate
• AGN activity
• Abundances
Trivial understanding of the Hubble sequence
Elliptical galaxies • Formed all stars long ago (red) • Little gas (fuel for new stars) • Random stellar motions • Found in clusters
Spiral galaxies • Still forming stars today (blue) • Lots of gas and dust • Rotation in disk plane • Avoid clusters
Spectral evolution
What is the purpose of a telescope?
1. A telescope acts like a light bucket, to gather photons.
• “bigger is better” => collecting area
2. In addition to gathering light, a telescope allows a more detailed view of the structure of a celestial object and/or to discern the presence of multiple objects. This is called the telescope’s ANGULAR RESOLUTION Example: Palomar 5m telescope The diameter of the telescope is 5 m = 500 cm Let’s find the diffraction limit at 500 nm.
1.22 X 500 nm X 10-7 cm/nm
500 cm Θ = = 0.025 arc seconds
But image quality at Palomar isn’t that good! At optical wavelengths, the images are not diffraction limited => atmospheric turbulence
The “seeing” of an image
The “seeing” of an image is a measure of its quality or sharpness. The seeing is always bigger than either (1) the diffraction limit or (2) the atmospheric seeing, whichever is greater.
High-Resolution Astronomy Solutions:
• Put telescopes on mountaintops, especially in deserts
• Put telescopes in space
• Active optics – control mirrors based on temperature and orientation
Source “Confusion” • Especially at longer wavelengths, telescopes with angular resolution
detect the collective radiation from lots of sources within the beam but which are unresolved by it.
• Because of confusion, even if you keep on integrating longer and longer, the noise level will not decrease.
Herschel and CCAT
Large Optical/IR telescopes
Telescope Location Diameter Access
Hubble space 2.4 m National/international
VLT Chile 4 x 8 m Europe
Keck Mauna Kea 2 x 10 m Caltech/U California/Hawaii
Gemini Mauna Kea and Chile
2 x 8 m National/international
Subaru Mauna Kea 7 m Japan, U Hawaii
Magellan Chile 2 x 6.5 m Carnegie, Harvard, MIT, Michigan, Chile
Palomar Calif. 5 m Caltech, JPL, National
Access to some telescopes is restricted to astronomers from certain countries/institutions
Radio Astronomy R-M-S: Radio – millimeter – submillimeter wavelengths Radio: Meter to centimeter wavelength
• Long wavelengths (relative to IR/opt/UV/X-rays)
• By Wien’s law, we expect cold temperatures (partly true)
• But also, not all radiation is thermal (i.e. follows Wien’s law and reflects the object’s temperature)
•Synchrotron radiation •Bremsstrahlung radiation
1 meter 1 cm
1 mm
Telescopes across the E-M spectrum
Name Wavelength range
Diameter Location Main science
Fermi Gamma ray (complex) Low earth Time domain
Chandra X-ray (complex) Elliptical orbit Imaging/spect
GALEX 125-280 nm 0.5m Low earth Imaging/spect
HST UV/opt/NIR 2.4m Low earth Imaging/spect
Spitzer NIR/MIR 0.9m Earth trailing Imaging/spect
Herschel 60-670 μm 3.5m L2 (Lagrange point) Imaging/spect
WISE 3.4-22 μm 0.4m Low earth Imaging
ALMA 350μm–10mm 54 x 12m 5000 m in Chile Continuum/spect
EVLA 7mm to 1m 27 X 25m 2124 m in NM Continuum/spect
Arecibo 2 cm to 1 m 305 m Puerto Rico Pulsars; HI; Solar system radar
Telescope design considerations
• Aperture size (collecting area, diffraction limit) • Wavelength/frequency coverage • Elevation/transparency of atmosphere • Angular resolution/point spread function • Field of view • Spectral bandwidth • Spectral resolution • Sampling rate (time domain)
• How much human intervention can there be? • Construction practicalities • Data rates/transfer/reduction • Politics/opportunities • Who pays the bill for (1) construction and (2) operations?
1
3
10
12 10 8 6 4 2 Billions of years after the BB
Today
Star Formation Rate in the Universe The Universe is far less active now than 10 billion years ago
Galaxy-galaxy interactions stimulate star formation, as well as the production of elements heavier than Hydrogen through nuclear reactions (*)
(*) We care because we are, after all, made of nuclear waste
?
Optically obscured galaxies in the early universe
Submillimeter galaxies
HST HST Spitzer
Spitzer
Wang, Barger &
Cowie 2009 ApJ 690,
319
GOODS field object
at z>4