The GEM project is an on-going international collaborative effort in the fields of radio astronomy and cosmology. It aims to accurately determine the spatial distribution and absolute intensity in the radio and microwave spectrum of the radiation emitted by the Milky Way galaxy and by the unresolved blend of external galaxies.
Institutions actively participating in the project through bilateral cooperation agreements are:
The project has been supported in part by the US Department of Energy, by the US National Science Foundation, by the Progetto Italia Antartide, by INPE-Brazil, and by Colciencias of Colombia. Some of the publications to date are on the following GEM publication list.
Some preliminary GEM maps i.e. 408, 1465, and 2300 MHz.
Any instrument trying to measure the intensity or anisotropy of the Cosmic Microwave Background (CMB) sees the galactic radiation as a foreground signal, which cannot be avoided by instrumental or observational techniques, and must be accounted for and subtracted from the data during the analysis.
The uncertainties in the maps and in the frequency dependence of the galactic emission at radio and microwave frequencies have become the limiting factor in the accuracy and interpretation of the low frequency measurements of the spectrum of the CMB, and of the COBE-DMR measurements of the angular distribution of the CMB.
The overall sky brightness results from the superposition of signals generated by the acceleration of relativistic electrons (synchrotron radiation), by thermal bremsstrahlung inside hydrogen clouds (HII radiation), and by thermal radiation of dust clouds, plus smaller signals from external galaxies. The exact mixture of synchrotron, HII, and dust signals depends on the observing frequency and the region observed. The dust component is dominant at the high end of the radio spectrum and in the IR region. Because of its spectral index, ~1.5, the dust contribution is negligible for observations below 50 GHz.
At intermediate frequencies the thermal HII emission is dominant, especially in the plane of the galaxy, where giant gas clouds provide the material and conditions for large concentrations of ionized hydrogen. This radiation originates in the interaction of free electrons with other ions. The HII radiation has a spectral index of about -2.1, weakly dependent on the observing frequency and on the (poorly known) temperature of the electrons.
At low frequencies and away from the galactic plane, the synchrotron radiation is dominant. Synchrotron radiation is generated by the energy loss of electrons with relativistic velocities, when their trajectories are deflected around the field lines of the interstellar magnetic field. The intensity of this radiation depends on the number density of relativistic electrons along the line of sight, while the spectral index (typically -2.75 at low frequencies) depends, and can be determined from the energy spectrum of the electrons, and the intensity of the magnetic field
The galactic signal has been modeled by adding together the synchrotron emission measured at low frequency and away from the galactic plane, with the HII signal deduced from measurements at intermediate frequencies, and the dust emission measured in the IR. Each component is then frequency scaled according to a power law. This approach suffers from:
- uncertainty in the spectral index of synchrotron emission, because of poorly determined electron spectrum and magnetic field intensity
- uncertainty in the spectral index of HII emission because of unknown electron temperature distribution
- uncertainty in the spectral index of dust emission because of unknown size and chemical composition of the dust grains
- poorly determined zero level of the measurements of sky emission
- uncertainty in the absolute value of the instrument gain
- uncertainty in the time dependency of the instrument gain
- lack of sky coverage in the existing data-base
The final products of the GEM project will be a set of new, self consistent maps at several frequencies. Features that will link these maps together, and set them apart from the existing ones are:
The GEM project will also overcome the shortcomings of the existing data-base by providing:
- maps of constant angular resolution and beam pattern will be produced at several frequencies between 408 MHz and 10 GHz.
- absolute calibration of the zero level of the map to better than (1 K) * (f/408)-2.75, [where f is the observing frequency in MHz] for f > 1500 MHz, and to better than 0.1 K for f < 1500 MHz
- total sky coverage
- accuracy of the gain level to better than 3%
- sensitivity to the circularly polarized component (total intensity) of the galactic signal
GEM Instrument Operation
The Berkeley team has developed a compact and portable 5.5-m diameter radio antenna, which has been used for the first-stage observations. The first observations were made from near Bishop, California (fall 1993 through fall 1994 with time out for refurbishment), from de Leyva, Colombia which is close to the equator (first half of 1995), Teide, Tenerife, Spain (from fall 1995 through fall 1997), and was moved to Cachoeria Paulista Brazil in 1998 with plans to move and operate in Rio Grande de Sul, Brazil beginning early 1998. The project fell far behind schedule (funding and other issues) and the current plan is to move to Brazilia in 2001.
Receivers currently are operational at 0.408, 1.5, 2.3, and 5.0 GHz. A prototype for 10 GHz has been constructed and upgrades are planned.
The results of the observations and the equipment operating in each location are shown in these pictures.
- 1991 Construction Bldg 60 LBL
- 1991/1992 South Pole, Antarctica
- 1993-1994 Bishop, California, USA 1995 Education , Outreach from Bishop effort
- 1995 (1st half) Villa de Leyva, Colombia
- 1995-1997 Teide, Tenerife, Spain
- 1998- Cachoeria Paulista, Brazil
- 2001- Brasilia, Brazil
- 2001- Mounting the Antenna, Brazil
- 2005- 5GHz Receiver installed at Cachoeria Paulista, Brazil
GEM Instrument Description
Berkeley team has developed a compact and portable 5.5-m diameter radio antenna, which will be used for the first-stage observations. It consists of a deep parabolic reflector, which can be illuminated either via a prime-focus mounted feed antenna, or a Cassegrain optics. The reflecting surface is surrounded by a set of metallic ground screens, which reduce the beam spillover and the signal from the ground via the antenna side lobes. The ground screens extend the diameter of the parabolic antenna to 9.5 meters.
GEM description material
Design of an IF section for a Galactic Emission Mapping experiment
Authors: Miguel Bergano, Luis Cupido, Domingos Barbosa, Rui Fonseca, Dinis M. Santos, George Smoot Feb 2007 astro-ph/0702629
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