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