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Astronomy across the spectrum:
telescopes and where we put them
Martha Haynes
Discovering Dusty
Galaxies
July 7, 2016
CCAT-prime: next generation telescope
CCAT Site on
C. Chajnantor
Me, at 18,400 feet in the
high Atacama desert in Chile,
at the site of the future
CCAT-prime (submillimeter
wavelength telescope)
ALMA 12m
antenna at
5000 meters
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(ν,T) =
2hν3
1
c2
exp(hν/kT) -1
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 = 1.38x1016 erg K-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.
B(ν,T) =
B(λ,T) =
2hν3
1
c2
exp(hν/kT) -1
2hc2/λ5
exp(hc/λkT) -1
h is Planck’s constant = 6.625x10-27
erg s
k is Boltzmann’s constant = 1.38x1016 erg K-1
Wikipedia.org
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!
Astronomical Images
• Position on the sky
• Morphological appearance
• Apparent brightness (flux) at some λ
• 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?
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.
Θ=
1.22 X 500 nm X 10-7 cm/nm
500 cm
= 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.
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
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
Activity this pm: The CMD of galaxies
Red: ellipticals
Blue: spirals
Galaxy spectra
• Redshift
• Velocity
dispersion/rotational
velocity
• Star formation rate
• AGN activity
• Abundances
Spectral evolution: as f(z) (or lookback time)
Infrared and radio waves penetrate the dust
Optical light
is absorbed
by the dust
so the cloud
is “dark”
Infrared light
penetrates
the dust so
we can see
the stars
hidden by the
dust above
blue
green
red
Dusty NGC 3628: a galaxy viewed edge-on
• Stars in our own Milky Way (white, isolated dots)
• Starlight in NGC3628 (white, in flattened disk)
• Dust in NGC3628 (darker regions where the dust blocks the starlight
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.
Astrochemistry: dust and molecules
NASA/ESA/J.Lake
H. Busemann
• Interstellar dust is made of carbon, oxygen, silicon, magnesium, iron
• Stars are born when cold, dusty molecular clouds collapse and heat up.
• Dust and cold gas are required for stellar birth!
Spectral energy distribution (SED) of galaxies
In the optical regime, we detect the
integrated starlight.
I
Thermal emission = black body radiation
3
2hν
1
I(ν)=
c2 exp(hν/kT) - 1
But at other wavelengths,
we detect other important
constituents like gas, dust,
and synchrotron radiation
Typical spectrum of
active galaxy, i.e. one
with accreting
supermassive black hole
in its nucleus
Submillimeter galaxies
Optically obscured galaxies in the early universe
HST
Wang,
Barger &
Cowie 2009
ApJ 690,
319
GOODS
field object
at z>4
Spitzer
HST
Spitzer
CCAT site: Cerro Chajnantor
Cerro Chajnantor (5600 m) has
better observing than South Pole,
ALMA plateau, & Mauna Kea
(Radford & Peterson,
arXiv:1602.08795)
Conversion of 350 µm opacity to
PWV robust:
PWV[mm] = 0.84 τ(350µm) – 0.31
– 350µm: routine
– 200µm: best 10%
– Longer λ: increased
sensitivity & efficiency
Telescope design considerations
What telescope attributes does the science require?
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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)
•
•
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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?
Telescopes across the E-M spectrum
Name
Wavelength
range
Diameter
Location
Main science
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
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
Fermi
Spitzer
Let’s build CCAT-prime up there!