Extragalactic Foreground Sources…and You! Benjamin Recchie
The Cosmic Microwave Background • Cosmic Microwave Background (CMB) is an “echo” of the Big Bang. • Small anisotropies (less than 1 part in 100000) in CMB can be analyzed to learn about very early universe. • In theory, we can just point our radio telescopes at the sky and measure the CMB. Angular power spectrum of the CMB The COBE map of the universe.
But Reality (and the Foreground) Intervenes... • The sky is cluttered in all directions with galaxies and other extragalactic objects which contribute microwave “noise” that obscures the CMB.
What we’re trying to do • We can improve our knowledge of the CMB if we can only get rid of (or find some way to ignore) these pesky foregrounds sources.
Summary of Foreground Sources • Sunyaev-Zeldovich Effect • Thermal SZE -creates slightly shifted blackbody spectrum. • Kinetic SZE - creates blackbody spectrum at slightly different temperature. • Extragalactic Radio Sources • Active Galactic Nuclei, QSOs, etc. • Far-Infrared Sources • IR sources contribute considerable microwave radiation • Various and Sundry Galactic Sources • Free-free radiation, galactic dust, synchrotron radiation
Galactic Foreground Sources(and why I’m ignoring them) • Galactic foregrounds contribute minimally for frequencies from 10 to 100+ GHz.
The Sunyaev-Zeldovich Effect • The Sunyaev-Zeldovich Effect (SZE) arises from gas heated (mainly) by falling into a gravitational well. • Leads to distortion of the blackbody spectrum for CMB photons. • Can be used to independently verify cosmological parameters.
Thermal Sunyaev-Zeldovich Effect • CMB photons undergo Compton scattering from electrons in heated gas, and gain some energy. • Shifts CMB spectrum up in energy. • Larger than kinetic effect by at least a factor of ten at most frequencies. • Relatively easy to account for and correct.
Kinetic Sunyaev-Zeldovich Effect • Moving gas creates false appearance of blackbody at different temperature from microwave background. • Has same spectral behavior as CMB anisotropy. • Difficult to correct for, but relatively minor.
SZE Contribution to CMB Measurements Not much, up to high angular scales (l > 1000).
Extragalactic Radio Sources • These consist primarily of Active Galactic Nuclei (AGN). This includes: • radiogalaxies • QSOs • blazars • BL Lac objects • GHz Peaked Sources (GPS)
Extragalactic Radio Sources (cont’d) • Up to 200 GHz, radio sources dominate the errors; higher than that, dusty galaxies dominate. • However, not terribly important while l < 700 or so. • Even non-radio galaxies still emit microwave radiation. • MAP and Planck probably won’t have interference from extragalactic radio sources.
Far-Infrared Sources • Far-IR sources emit considerable radiation in the microwave. • Many galaxies emit most of their radiation in IR and sub-millimeter wavelengths. • Dust absorbs UV and optical wavelength light and reradiates it in the far-infrared. • Side note: there also exists a Cosmic Infrared Background (CIRB).
LIRGs and ULIRGs • Nearby LIRGs and ULIRGs are merging or otherwise interacting; presumably, most such galaxies are the same. • Hubble Deep Field image shows many objects undergoing gravitational interactions.
But do they matter for CMB studies? • Models predict Planck will detect thousand of IR sources at after filtering. • For example - at 857 GHz, 40000 sources predicted; at 545 GHz, 5000 sources. • However, models also predict virtually no far-IR sources around prime frequencies for CMB measurements.
The Future:MAP • MAP will observe background from 22 to 90 GHz, with angular resolution of 0.3 degrees. • Will measure galactic foreground more precisely, so that it can be subtracted from other studies. MAP will turn this: …to this:
The Future:Planck • Planck will observe background from 50 to 850 GHz, with resolutions of 5 to 10 arcminutes, depending on the instrument. • Will be able to observe certain high-frequency extragalactic microwave sources.
Conclusions • The thermal Sunyaev-Zeldovich effect has major effect on the CMB spectrum at large angular scales, but is easy to correct for. The kinetic Sunyaev-Zeldovich effect is much harder to correct for, but is very small. • Extragalactic radio sources make small but ultimately unimportant contributions to CMB up through large angular scales (but are interesting in and of themselves). • Far-infrared sources deserve more study, but will also probably require only minor corrections to newer measurements of the CMB. • Planck and MAP will answer many of our questions about the CMB, and should be able to bypass most foreground sources.