Mika Juvela, Helsinki University Observatory - October 2003

Planck and astronomical foregrounds

Planck is a science satellite that is being built by the European Space Agency (ESA). Planck will be launched in 2007, and during the planned operation of a year and a half it will make a detailed map of the whole sky using several radio frequencies between 30 and 860 GHz. The main objective of the mission is to study cosmic background radiation, which originates from young universe at the time when atoms were first formed. The maximum of this cosmic microwave background (CMB) is around the frequency of 100GHz. Planck will, however, cover a wide frequency range, and the sensitive observations can be used for studies of various other astronomical objects ranging from solar system objects to distant galaxies.

The Helsinki University Observatory participates within the Planck consortium in several science projects that are relevant for our studies into the physics of the interstellar matter and star formation. Planck will detect radiation produced by different processes in the interstellar space. Our main interest lies in the the study of the thermal radiation emitted by dust grains that are heated by nearby stars or the general interstellar radiation field. The dust emission can be used to study the structure and evolution of interstellar clouds and to locate sites where new stars will be born. Similar studies can be conducted in nearby galaxies, and catalogs of more distant galaxies can be used when the birth and evolution of galaxies are being studied and modelled.

Researchers at the Observatory are participating in eight separate science projects that are to be carried out during the proprietary period before all Planck data are released to public domain. Scientists from the Helsinki University Observatory are acting as coordinators for three Planck science projects: Cold cores (Mattila), Local Interstellar Medium (Juvela) and Dust in Local Universe Galaxies (Mattila). There are additional technical working groups that are doing preparatory work to ensure efficient and rapid analysis of the future Planck observations. Juvela participates in the coordination of two groups that work on the diffuse Galactic emission creating simulated maps and developing tools for the analysis of Planck data.

In the following we give some examples of the work that is being done.


Cold cores

In dense clouds the gas transforms from atomic to molecular form and the line radiation of molecules transfers energy out from the cloud. This leads to a temperature drop which enables the cores to collapse under gravitation and eventually to compress into new stars. Similarly, the dust inside the cores is shielded from external radiation and the cold dust radiates mostly at very long wavelengths (>0.1mm). For this reason many cloud cores were invisible to previous all-sky surveys that were carried out at shorter wavelengths. Because of the high sensitivity and wide wavelength coverage Planck will be able to detect a large number of previously unknown cores. It may even find a new class of very small clouds with masses comparable to the mass of Jupiter. Such cores and small clouds may form a significant fraction of the dark mass - mass that is known to exist in galaxies but which has so far not been identified.

We have started a project where we observe known cold cores in molecular lines. The all-sky survey made by the IRAS satellite was not sensitive to cold dust since it covered only wavelengths up to 100µm. The recent ESA infrared satellite ISO could make observations at longer wavelengths. ISO made pointed observations of selected targets, and as it moved between targets it observed the sky continuously at the wavelength of 170µm. These slews form the ISOPHOT Serendipity Survey which covers some 15% of the sky. A comparison with IRAS data makes it possible to identify previously unknown cold cores. We have selected 50 such cores and observed from them molecular line radiation of e.g. different CO isotopes. In this study we have used Onsala radio telescope in Sweden, Effelsberg 100 meter telescope in Germany and the SEST telescope in Chile. Once Planck data becomes available we can compare our findings with measurements of dust emission. We want to study the correlations between emission from gas phase molecules and dust grains (e.g. to what extent molecules freeze out onto the dust grains) and how these are related to different phases of the starformation process.

Cloud B68
Dark cloud Barnard 68 is one of the objects included in our study of cold cores. Left frame: the intensity of C18O line emission (green contours) in relation to the surface brightness of scattered light observed in optical light (greyscale). The blue symbols show the line over the cloud that was observed in ISOPHOT Serendipity Survey (ISOSS, 170µm). Right frame: one-dimensional scans over the cloud showing the relative intensities at 170µm (ISOSS) and 100µm (IRAS) and the number of C18O molecules along the line-of-sight. (Hotzel et al. 2002)

Working group activities

We participate in the work of a technical working group that is developing radiative transfer tools. The radiative transfer problem defines the relationship between the physical parameters of a source and the radiation observable from that source. In the case of dust emission the radiative transfer models must make assumptions of the external radiation field heating the grains and the physical properties of the grains themselves. The size distribution of dust is believed to extend from ~1µm grains down to nm scales, and features observed in mid-infrared may be caused by large molecules. Large grains remain at an equilibrium temperature determined by the balance between the total absorbed energy and the emitted infrared radiation. The temperature of small grains may, however, jump tens or even hundreds of Kelvin degrees after grain absorbs an ultraviolet photon. Therefore, small grains have a wide distribution of different temperatures. Most of the emission takes place when these grains are transiently heated to high temperatures.

Interstellar clouds are very inhomogeneous and this alters the way different parts of the cloud are heated. Realistic modelling of dust emission requires three-dimensional cloud models. Such calculations are already possible for large grains. The calculation of the temperature distribution of small grains is, however, far too time consuming so that it could be done directly for the millions of cells of a three-dimensional model cloud. We have developed an approximative method that allows the inclusion of the transiently heated small grains in three-dimensional models (Juvela & Padoan 2003). The method is at least a factor of 100 faster than direct solution and we can still calculate dust emission spectra with an accuracy of a few per cent. The Planck satellite will mostly observe emission from large dust grains but the modelling of the whole dust emission spectrum is still important when the Planck measurements are compared with infrared surveys. It is believed that e.g. in cold cores smaller grains stick onto the surfaces of bigger grains forming large aggregates. Such phenomena can be studied only by comparing and modelling emission from both small and large grains.

Cloud model
An example of the kind of calculations possible with the approximative radiative transfer method developed by us. The figures show the ratios of dust emission between 12µm, 25µm, 60µm, and 100µm wavelengths calculated towards one direction from a three-dimensional model cloud consisting of over two million cells (1283). The model represents an interstellar cloud with a diameter of some 30 light years. The darker colours correspond to lower temperatures found in denser filaments. The density distribution of the three-dimensional model cloud shown in the figures is the result of a magneto-hydrodynamic simulation. (Juvela & Padoan 2003)

We participate in the work of a working group that is producing template maps of various Galactic emission components e.g. dust emission, synchrotron emission and free-free emission (Juvela is one of the coordinators). The Planck observations will be a mixture of these components, CMB and radiation from extragalactic sources. The separation of these components will be one of the first tasks in the analysis of Planck data. The simulated template maps are used in the development of component separation algorithms and later as a priori information in the component separation process.

Surveys carried out at frequencies close to 10GHz have shown that there may be yet another component in the diffuse Galactic emission. This so called anomalous microwave emission peaks somewhere below 100GHz i.e. close to the maximum of the CMB spectrum. The component is spatially correlated with dust emission, and Draine & Lazarian (1998) suggested that this emission could be caused by very small rapidly spinning dust grains. The actual cause remains still uncertain. We have, however, created a preliminary template based on the spinning dust hypothesis and estimates of the amount of ionized, neutral and molecular material towards different directions on the sky. The predicted intensity of the signal in the Planck 30GHz channel is shown below.

Spinning dust
Simulated map of the so called anomalous microwave emission at the frequency of 30GHz. The figure shows the intensity (logarithm of antenna temperature) over the whole sky. In the projection used in this figure the Galaxy shows as a bright arc.

Summary

Planck will be launched in three years time but preparations for the analysis of the observations have already begun. Currently most of the work takes place in technical working groups but in the following years the groups formed around science proposals will become more active. The culmination of the long project is only a year or two after the launch as the results of the scientific work will be published. Active participation is, however, essential already now since only that guarantees access to Planck data immediately when measurements start and not only after a proprietary period of two years.

The Planck related work at the Helsinki University Observatory was supported in part by the ANTARES space science program (2001-2004) funded by Tekes and the Academy of Finland.