CSOLOGO1 CSOLOGO2Bolocam studies the Universe using the Sunyaev-Zel'dovich effect.


  Bolocam, a millimeter-wave bolometer camera, was built to achieve two main science goals.  One was to learn more about galaxy and large-scale structure formation via a blind survey for dusty galaxies (so called submillimeter galaxies) and the other was to learn more about the dark matter and dark energy that compose 96% of our universe via a blind survey for clusters of galaxies using the Sunyaev-Zel'dovich effect (SZE).   Since it was designed as a survey instrument, the field of view was made as large as possible while still providing acceptable optical performance over the entire field. The detectors were designed to have the best sensitivity possible, limited only by the unwanted signal caused by the atmosphere above Mauna Kea.  Jack Sayers, a graduate student at Caltech, joined the Bolocam project because the SZE survey provides a method to potentially place high-precision constraints on the properties of dark matter and dark energy. Since fall 2003, Sayers and his advisor, professor Sunil Golwala, along with several of their colleagues at Caltech, JPL, CU Boulder, and the University of Cardiff, have made a variety of observations of the SZE using the CSO Bolocam, and Sayers was recently awarded a PhD based the results of the CSO Bolocam SZE survey.

   The SZE involves scattering of the oldest photons in the universe, the cosmic microwave background radiation (CMBR), by the 10 - 100 million degree electron gas in clusters of galaxies. These electrons are composed of 60 - 70% of the baryonic mass of the cluster, compared to only 10 - 15%  from the visible stars within the cluster. Since the temperature of the CMBR is a few degrees above absolute zero, these hot electrons increase the energy of the CMBR photons. However, less than 1% of the CMBR photons are scattered by the electrons, so the overall change in the spectrum of the CMBR is fairly small. The magnitude of the change in the CMBR caused by the SZE is proportional to the pressure of the electron gas multiplied by the size of the cluster, typically a couple of Megaparsecs, which is more than 10 trillion miles or about 5 light years. Since the SZE increases the energy of some CMBR photons, it basically takes some of the low energy (low frequency) photons and moves them to higher energy (higher frequency).  The result is that the CMBR looks colder, or dimmer, at a low frequency and hotter, or brighter, at higher frequencies. Additionally, the magnitude of the SZE signal is the same no matter how far away the cluster is, so it provides a method to study the oldest, or most distant, clusters in the universe. By studying a large number of these distant clusters it's possible to determine some of the global properties of the universe, including the rate of expansion, the amount of dark energy, and the amount of dark matter.

  Bolocam was designed to observe at 143 GHz, which is near the frequency where the maximum dimming of the CMBR due to the SZE occurs (see Figure 1). This frequency is between the far infrared, which is the thermal radiation absorbed by greenhouse gasses in the atmosphere, and microwaves, which are used to cook food. At this relatively low frequency the clusters appear as cold, or dim, spots compared with the CMBR background in their maps. For the most massive clusters these cold spots are less than 0.001 degrees different from the background CMBR, which is about one part in 10000.

   The first set of observations made were maps of "blank" fields, or fields that contain no bright stars, galaxies, or other objects. They mapped two fields, each one covering an area of 0.5 square degrees, or about twice the size of the moon. These observations were surveys, to discover previously unknown clusters. To date, no one has discovered a previously unknown cluster using the SZE. However, there is still a signal in the map due to all of the unresolved clusters that are too dim for them to be seen. To look for this signal the angular power spectrum of their maps was determined, which shows how the signal is distributed as a function of angular scale. This is similar to determining the frequency spectrum of a musical instrument, so you can find which notes it plays. They did not detect this power spectrum signal, but they placed the first upper limit at 143 GHz for these angular scales.

   Their next set of observations focused on mapping the SZE signal in known clusters. To date, there have been no published maps of the SZE at 143 GHz, so these ongoing observations may produce the first ones. Since the SZE directly measures the pressure of the electron gas, it provides a fairly clean measurement of the cluster. Additionally, compared to other observations of the cluster electron gas, the SZE signal diminishes slowly when observing positions at small distances away from the cluster center. Therefore, they can study the cluster further away from its center. Their observations will help determine the astrophysics that describe cluster formation, and provide a better picture of how structure has formed and evolved in our universe.  Additionally, understanding the cluster astrophysics is critical to being able to learn more about the global properties of our universe from the distant clusters that will be discovered using the SZE in the future. A Bolocam map of the cluster MS0451.6+0305 is shown in Figure 2.


Figure 1.  The relative distortion of the CMBR spectrum for a typical massive cluster as a function of frequency.



Figure 2. Bolocam map of a massive galaxy cluster, MS0451.6+0305.  The plots shows the signal-to-noise ratio obtained by Bolocam, with contours of S/N = 2, 4, and so forth.  The central surface brightness is about 0.0002 Kelvin, or more than 10000 times dimmer than the CMBR.