Home Up Contents FAQ Search

Cosmic Background
Back Up Next

 

Community
International
Site Testing
Science Goals
Instrument Design
Budget & Schedule

Report of Cosmic Microwave Background Session

Session Leader: Jeffrey Peterson

Specifications of the Proposed Observations

At frequencies near the CMB peak, 40 - 400 GHz, the ten meter telescope will have beam sizes ranging from 3 arc-minutes to 0.3 arc minutes, assuming edge illumination typical of anisotropy telescopes. Ray tracing calculations for the proposed 10 meter optics indicate that a beam throw of at least 20 arcminutes peak to peak should be possible. Several 2-d mapping strategies may be possible with the ten meter, including conical scan, fast raster patterns, and array pixel differencing. The relative effectiveness of these alternatives was left for future discussion and for the purpose of this study it was assumed that, by some technique, 20' x 20' areas could be mapped. This gives the instrument l space coverage from 800 to 16000. At the Pole a horizontal chop sweeps across the sky at constant declination, which forces us to throw the beams vertically in order to make 2-d maps. At other sites 2-d maps can be created entirely from horizontal scans, by allowing field rotation to turn the throw angle on the sky. This is a negative aspect of polar observations, but because of the stability of the South Pole atmosphere and because of small throw of the proposed observations 2d mapping should never-the-less be possible with this telescope. Assuming 2' pixels the sensitivity for 300 hour integrations on these fields is expected to be 2 to 4 micro kelvin per pixel. This estimate was made assuming detector noise rather than sky noise dominates and assuming that a 10' x 10' array is available. A minimum of three frequencies will be required to remove sky noise and allow S-Z y parameter and delta T information to be separately determined. Additional spectral information will be valuable in order to study and remove radio point source contamination, to measure relativistic distortions of the S-Z spectrum and to detect extended galactic free-free and synchrotron emission. Interest is increasing in the CMB community in polarization measurements. By the time the ten meter is built it should become clear whether polarization sensitivity will be valuable at this angular scale.

Primary Anisotropy

The expected anisotropy spectrum at these l values depends strongly on the cosmological model. In a flat universe (lambda + omega = 1) primary anisotropy is expected to fall below 3 microkelvin at the low end of our l range. If the universe is flat and is described by an inflationary model, the ten meter telescope could measure all of the detectable acoustic peaks out to the end of the damping tail, complementing the measurements of Planck Surveyor which will likely map out the first six or so acoustic peaks but will not have the resolution to measure the entire damping tail. The shape and angular position of the damping tail is one of the most fundamental and model-independent features of the CMB fluctuation spectrum. However, the prediction of very low anisotropy at high l leads one to wonder if all possible source of fine scale anisotropy have been considered. At degree scale, the emerging anisotropy signals have pushed theorists to carry out careful anisotropy predictions. At arc minute scales the theorists have not been pushed by observation. If we provide these observations the theory community will respond with much more detailed analyses. In other words we should not be scared off by the current low anisotropy predictions. Better data will prompt more analysis. Also, even if current theories are correct an upper limit at high-l at the 10-6 level would be an important contribution. If the universe is open, we will provide important information on acoustic peaks number 3 and higher that PS may not be able to supply. Anisotropy measurements with the ten meter will be essential if the universe is open. Also, this telescope may be the only instrument with enough sensitivity at high frequencies to separate thermal S-Z sources from primary anisotropy.

Secondary Anisotropy

At the small angular scales probed by this instrument, non-linear effects become significant contributors to CMB anisotropies. Although detailed calculations have not yet been performed, it may be possible to extract information about the following effects from small-scale CMB measurements: S-Z distortions due to structures at high redshift; doppler shifts from ionized bulk flows; Rees-Sciama effect (non-linear evolution of gravitational potentials); non-linear evolution of acoustic waves; Ostriker-Vishniac contribution to polarization after reionization; and gravitational lensing from large-scale structure. Planck Surveyor will be confusion limited by S-Z distortions in the clusters that will be unresolved and numerous (18,000 to 80,000 on the sky). Finer scale observations will be needed to follow up on PS data, and it will be particularly valuable to have sufficient spectral coverage to allow seperation of S-Z thermal spectra from temperature fluctuations. The low South Pole sky noise combined with the low switched offsets afforded by an off-axis telescope make the observations possible with this instrument uniquely valuable for PS follow-up.

Resolved sources

The ten meter will resolve clusters across the CMB frequency range allowing cluster peculiar velocity measurements to be made for a large sample of clusters. The velocity sensitivity of these observations is redshift independent, so with this telescope it will be possible to map out peculiar velocities on much smaller spatial scales than will be possible with PS. If the universe is open or lambda dominated there are a large number clusters of small angular size (< 1 arc-minute) on the sky. The fine beam available at high frequencies with this large aperture will be particularly valuable in this case.

Unresolved Sources

At radio wavelength there are a large number of weak point sources, presumably quasars and active galactic nuclei. Some of these sources have flat flux spectra at centimeter wavelengths. With this telescope we will be breaking new ground, observing at higher frequency with a finer beam, and with higher sensitivity than has been available before. It is quite likely that we will detect point sources on our anisotropy fields. With frequency coverage from 40 - 400 GHz we should be able to identify these sources because they have spectra that distinguish them from CMB structure and from the S-Z effect. This will provide a list of interesting candidates for further study.

Complimentary Observations

The ten meter can compliment observations made with a broad set of other observatories. MAP will detect many clusters as unresolved decrement feature in its sky maps. The spectral coverage and fine angular scale available with the ten meter will allow identification of SZ effect in clusters as the source of MAP decrement signals. BIMA will also be useful for this purpose but only at 30 GHz. Plank Surveyor will spectrally identify S-Z effect but will not resolve the clusters. The ten meter will provide the resolution and spectral range needed to study these cluster. MMA will have a 2' field of view at 30 GHz, allowing mapping of SZ decrements. The ten meter can provide the high frequency complimentary observations the will be necessary to measure peculiar velocities of these clusters. Also MMA can be used to study, at finer angular scales, any sources the ten meter detects. CBI will have a resolution of a few arc-minutes at it's operating frequency 30 GHz. The ten meter will extend CBI measurements to higher l and provide multi-frequency follow up on any objects detected by CBI. It might appear at first glance that the ten-meter duplicates the function of JCMT and CSO, but CSO and JCMT are on-axis telescopes at a site with much higher sky noise than the pole. In addition, the fast cassegrain design of these telescopes gives them a smaller field of view than the ten meter will have. Some of the observations discussed here for the ten-meter are currently being attempted on CSO and soon will be with SCUBA on JCMT. We can expect S-Z velocity detections, from CSO in particular, long before the ten meter is finished. What can be provided with the ten meter is the low side lobe, low sky noise, large array observing program that will turn these types of observations into a productive tool for cosmology. OVRO and BIMA will be detecting clusters at 30 GHz on blank sky fields within the next few years. The ten meter can provide wide frequency range observations that these instruments cannot.

 

Send mail to help@cfa.harvard.edu with questions or comments about this web site.
Last modified: October 31, 1999