Astronomy across the spectrum: telescopes and where we put them. Martha Haynes Discovering Dusty Galaxies July 7, 2016

Transcription

1 Astronomy across the spectrum: telescopes and where we put them Martha Haynes Discovering Dusty Galaxies July 7, 2016

2 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

3 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) = 2hν 3 c 2 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 1 exp(hν/kt) -1 Wikipedia.org

4 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) = 2hν 3 c 2 1 exp(hν/kt) -1 B(λ,T) = 2hc 2 /λ 5 exp(hc/λkt) -1 h is Planck s constant = 6.625x10-27 erg s k is Boltzmann s constant = 1.38x10-16 erg K -1 Wikipedia.org

5 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

6 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!

7 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?

8 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 But image quality at Palomar isn t that good! At optical wavelengths, the images are not diffraction limited => atmospheric turbulence = arc seconds

9 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.

10 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

11 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

12 Activity this pm: The CMD of galaxies Red: ellipticals Blue: spirals

13 Galaxy spectra Redshift Velocity dispersion/rotational velocity Star formation rate AGN activity Abundances

14 Spectral evolution: as f(z) (or lookback time)

15 Infrared and radio waves penetrate the dust Optical light is absorbed by the dust so the cloud is dark blue green red Infrared light penetrates the dust so we can see the stars hidden by the dust above

16 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

17 Darkness: Absence of (visible) light Extinction due to foreground dust: makes a star appear redder and fainter

18 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.

19 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!

20 Spectral energy distribution (SED) of galaxies In the optical regime, we detect the integrated starlight. I Thermal emission = black body radiation I(ν)= 2hν 3 1 c 2 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

21 Submillimeter galaxies Optically obscured galaxies in the early universe HST HST Spitzer Wang, Barger & Cowie 2009 ApJ 690, 319 GOODS field object at z>4 Spitzer

22 CCAT site: Cerro Chajnantor Cerro Chajnantor (5600 m) has better observing than South Pole, ALMA plateau, & Mauna Kea (Radford & Peterson, arxiv: ) Conversion of 350 µm opacity to PWV robust: PWV[mm] = 0.84 τ(350µm) µm: routine 200µm: best 10% Longer λ: increased sensitivity & efficiency

23 Telescope design considerations What telescope attributes does the science require? 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?

24 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 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 μm 3.5m L2 (Lagrange point) Imaging/spect WISE 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

25 Let s build CCAT-prime up there!