0320-Component separation methods for the PLANCK mission

PAPER

Component separation methods for the PLANCK mission

A comprehensive paper on foreground removal

Exploring Cosmic Origins with CORE: B-mode Component Separation

Simulation: synchrotron, dust, anomalous microwave emission (AME) and radio and infrared point sources.

Analysis of the WMAP and Planck data has demonstrated that the polarized Galactic emission is welldescribed by a simple two component model of the interstellar medium comprising synchrotron radiation and thermal dust emission. However, this picture is likely to become more complex as the sky is measured with increasing accuracy.

Diffuse source separation in CMB observations [Delabrouille 2007]

Diffuse galactic emissions originate from the local interstellar medium (ISM) in our own galaxy. The ISM is constituted of cold clouds of molecular or atomic gas, of an intercloud medium which can be partly ionised, and of hot ionized regions presumably formed by supernovae. These different media are strongly concentrated in the galactic plane. The intensity of corresponding emissions decreases with galactic latitude with a cosecant law behaviour (the optical depth of the emitting material scales proportionnally to 1/ sin b). Energetic free electrons spiralling in the galactic magnetic field generate synchrotron emission, which is the major foreground at low frequencies (below a few tens of GHz). Warm ionised material emits free-free (Bremstrahlung) emission, due to the interaction of free electrons with positively charged nuclei. Small particles of matter (dust grains and macromolecules) emit radiation as well, through thermal greybody emission, and possibly through other mechanisms.

Extragalactic emissions arise from a large background of resolved and unresolved radio and infrared galaxies, as well as clusters of galaxies. The thermal and kinetic Sunyaev-Zel’dovich effects, due to the inverse Compton scattering of CMB photons off hot electron gas in ionized media, are of special interest for cosmology. These effects occur, in particular, towards clusters of galaxies, which are known to comprise a hot (few keV) electron gas. Infrared and radiogalaxies emit also significant radiation in the frequency domain of interest for CMB observations, and contribute both point source emission from nearby bright objects, and a diffuse background due to the integrated emission of a large number of unresolved sources, too faint to be detected individually, but which contribute sky background inhomogeneities which may pollute CMB observations.

Solar system emission comprises emissions from the planets, their satellites, and a large number of small objects (asteroids). In addition to those, there is diffuse emission due to dust particles and grains in the ecliptic plane (zodiacal light). The latter is significant essentially at the highest frequencies of an instrument like the Planck HFI

Synchrotron

Synchrotron emission arises from energetic charged particles spiralling in a magnetic field. In our galaxy, such magnetic fields extend outside the galactic plane. Energetic electrons originating from supernovae shocks, spiraling in this field, can depart the galactic plane and generate emission even at high galactic latitudes. For this reason, synchrotron emission is less concentrated in the galactic plane than free-free and dust.

In principle, synchrotron emission can be highly polarised, up to 50-70%

Free-Free

Free-free emission is the least well known observationally of the three major emissions originating from the galactic interstellar medium in the millimetre and centimetre wavelength range. This emission arises from the interaction of free electrons with ions in ionised media, and is called “free-free” because of the unbound state of the incoming and outgoing electron. Alternatively, freefree is called “Bremsstrahlung” emission (“braking radiation” in German), because photons are emitted while electrons loose energy by interaction with the heavy ions.

While free-free emission is not observed directly, as it never dominates over other astrophysical emissions, the source of its emission (mainly ionised hydrogen clouds) can be traced with hydrogen recombination emission lines, and particularly Hα emission. The connection between Hα and free-free has been discussed extensively by a number of authors

Free-free emission, being due to incoherent emissions from individual electrons scattered by nuclei in a partially ionised medium, is not polarised (to first order at least).

Thermal emission of galactic dust

Dust consists in small particles of various materials, essentially silicate and carbonaceous grains of various sizes and shapes, in amorphous or crystalline form, sometimes in aggregates or composites. Dust is thought to comprise also large molecules of polycyclic aromatic hydrocarbon (PAH). The sizes of the grains range from few nanometers for the smallest, to micrometers for the largest. They can emit through a variety of mechanisms. The most important for CMB observations is greybody emission in the far infrared, at wavelengths ranging from few hundreds of microns to few millimeters.

In principle, thermal emission from galactic dust should not be strongly polarised, unless dust particles are significantly asymmetric (oblate or prolate), and there exists an efficient process for aligning the dust grains in order to create a significant statistical asymmetry.

Spinning dust or anomalous dust emission

In the last years, increasing evidence for dust-correlated emissions at frequencies below 30 GHz, in excess to expectations from synchrotron and free-free, has aroused interest in possible non-thermal emissions from galactic dust

SZ effect

The Sunyaev Zel’dovich (SZ) effect [Sunyaev & Zeldovich (1972)] is the inverse Compton scattering of CMB photons on free electrons in ionised media. In this process, the electron gives a fraction of its energy to the scattered CMB photon. There are, in fact, several SZ effects: The thermal SZ effect is due to the scattering of photons on a high temperature electron gas, such as can be found in clusters of galaxies. The kinetic SZ effect is due to the scattering on a number of electrons with a global radial bulk motion with respect to the cosmic background. Finally, the polarised SZ effect is a second order effect due to the kinematic quadrupole of the CMB in the frame of an ensemble of electrons with a global transverse bulk motion with respect to the CMB.

SZ effects are not necessarily linked to clusters of galaxies. Any large body with hot ionised gas can generate significant effects. It has been proposed that signatures of inhomogeneous reionisation can be found via the kinetic and thermal SZ effect

Each cluster of galaxies has its own thermal, kinetic and polarised SZ emission.

The SZ effect is particular in several respects. As most of the emission comes from compact regions towards clusters of galaxies (at arcminute scales), most of the present-day CMB experiments do not resolve clusters individually (apart for a few known extended clusters). For this reason, it seems natural to use point source detection methods for cluster detection (see review by Barreiro [Barreiro (2005)]). However, the very specific spectral signature, the presence of a possibly large background of clusters with emission too weak for individual cluster detection, and the interesting possibility to detect larger scale diffuse SZ emission, makes looking for SZ effect with diffuse component separation methods an interesting option.

The infrared background of unresolved sources

The added emissions from numerous unresolved infrared sources at high redshift make a diffuse infrared background, detected originally in the FIRAS and DIRBE data [Puget et al. (1996)]. Because each source has its specific emission law, and because this emission law is redshifted by the cosmological expansion, the background does not have a very well defined frequency scaling. It appears thus, in the observations at various frequencies, as an excess emission correlated between channels. The fluctuations of this background are expected to be significant at high galactic latitudes (where not masked by much stronger emissions from our own galaxy), and essentially at high frequencies (in the highest frequency channels of the Planck HFI.)

Point sources

The “point sources” component comprises all emissions from astrophysical objects such as radio galaxies, infrared galaxies, quasars, which are not resolved by the instruments used in CMB observations. For such sources, the issues are both their detection and the estimation of parameters describing them (flux at various frequencies, location, polarisation...), and specific methods are devised for this purpose. For diffuse component separation, they constitute a source of trouble. Usually, pixels contaminated by significant point source emission are blanked for diffuse component separation.

Foreground cleaning

• select region of observation

• select frequency

• mask

• foreground cleaning