Measuring the Sunyaev-Zel'dovich Effect at 850 Microns Using SCUBA

This article is based on papers submitted to MNRAS listed on www.arXiv.org as astro-ph/0306300 and astro-ph/0302471. Please see those works for details not provided here, including a complete list of references.

We report on our measurement of the Sunyaev-Zel'dovich (SZ) effect increment in two galaxy clusters using SCUBA. One of the most versatile probes of large scale structure of the universe, the Sunyaev-Zel'dovich SZ effect is an important tool for cosmologists which is difficult to measure at frequencies above 200 GHz. As the increment has been measured in only 8 clusters, our measurement is a significant addition to this field.

The SZ effect is caused by the interaction of low energy cosmic microwave background (CMB) photons and the high energy (approximately 10^7 K) electrons generally found in galaxy clusters. On average, photons gain energy from the electrons in this process. Because photon number is conserved, a characteristic change in the spectrum of the CMB as seen through the cluster results. This change manifests itself as a substantial decrease in the CMB temperature between approximately 10 GHZ and 200 GHz (called the SZ decrement) and a substantial increase in the CMB temperature between about 200 and 1000 GHz. This increase is called the SZ increment and SCUBA is well positioned to measure it.

Observations of galaxy clusters at SZ increment frequencies are important for a variety of reasons. Constraining the full spectral shape of a cluster's SZ distortion allows separation of the thermal SZ effect, which is caused by the random motions of the cluster's electrons, and the kinetic effect, which is caused by the cluster's motion relative to the CMB rest frame. This effect can be used as a probe of large scale motions and structure formation in the universe. Because the effects of CMB cooling and cosmological dimming cancel in the SZ effect, a cluster's SZ intensity is independent of redshift. This means that, unlike with other probes, structure in both location and velocity distributions at virtually any distance can be probed with multiple frequency SZ measurements.

In principle, combination of SZ data with X-ray brightness allows measurement of the cluster's temperature, electron density, pressure, and proper distance. This works well for nearby clusters. Unfortunately, this strategy becomes difficult to implement at high redshifts because X-ray fluxes become very small at redshifts greater than 1.

While measurements of the SZ decrement are becoming routine, detection of the SZ increment is still a challenging task. This is largely because detection of SZ emission requires sensitive instruments which can sample a wide range of spatial scales. Sub-mm instruments are beginning to make useful progress at increment wavelengths. Our group has used SCUBA to measure the SZ effect in 2 galaxy clusters. Non-standard observation and data analysis techniques are required to make this experiment a success. A large (180") chop throw is used to sample the full cluster image. We use the 650 GHz (450 micron) data to remove the effects of atmospheric emission from the 350 GHz (850 micron) data since standard, in-band atmospheric corrections would cancel the SZ intensity. JCMT's high resolution allows us to reject possible point source contaminants which plague SZ measurement with smaller instruments. We fit the SZ amplitude directly to our measured data rather than fitting to a map made from the data. As a check for point source contamination, we also fit the data to an annular model. Measurements are made of control fields which should give null results to check these methods.

The result for ClG 0016+16 is shown in Figure 1 (above left), where the grey band shows the fit of the time stream to the extended emission profile, and the points with error bars are determined by fitting to an annular model. We have also performed Monte Carlo simulations in an attempt to understand the effects of confused sources on our measurement. It is found that the likelihood of obtaining our result due to confused point sources if no SZ increment were present is less than 1%. After correcting the model fit value for the effects of atmospheric subtraction, the result shown in red in Figure 2 (right) is obtained. This can be combined with decrement measurements (inset) to limit the peculiar velocity to v_pec = 400⁺¹⁹⁰⁰_-1400 km/s via the kinetic effect (dotted line). We are currently analyzing the data from a number of cluster fields in the hopes of placing useful limits on the 350 GHz intensity for many of them.

This work shows that with a carefully designed experiment, the JCMT/SCUBA combination can be used to provide robust measurements of the SZ effect. It is hoped that it will be possible to use SCUBA 2 for similar measurements which will compliment the many upcoming SZ experiments working at other wavelengths.

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