Baryons in Clusters:

One of the most important discoveries in cosmology is that it appears that much, if not most, of the mass in the Universe is made not of stars and glowing gas that is familiar to us from astronomical images of the sky, but of what has been termed ``dark matter''. This dark matter emits little or no light or other electromagnetic radiation, and so far makes its presence known only through the gravitational force it exerts upon the luminous matter. There is some indication that the dark matter may in fact not even be baryonic. Just what fraction of the mass is in the form of non-interacting non-baryonic particles is of great interest to cosmologists and physicists.

Measuring the mass in baryons along with the total mass in a region of the Universe that could be considered a ``fair sample'' would provide a crucial direct determination of the dark matter content. In recent years, just such a test-bed has been found in the guise of massive clusters of galaxies. These clusters contain large amounts of gas (baryons) in the form of a highly ionized ``atmosphere'' of gas at temperatures of millions of degrees, which emit X-rays. Nearly all of the baryons in the clusters are believed to be in the hot phase, and so it is likely that we are truly measuring the baryonic mass in the cluster.

In addition to emitting X-rays, the hot cluster gas also Compton scatters the cosmic microwave background (CMB) radiation. This scattering, called the Sunyaev-Zeldovich effect (SZE), is measurable using radio telescopes. The CMB blackbody has a temperature of 2.7 K which peaks at a wavelength of around 1mm. Observations at centimeter wavelengths see the SZE as a reduction in the brightness toward the cluster, as the photons are scattered upward in energy. The observed decrement is proportional to the integrated electron density along the line-of-sight through the cluster.

The SZE depends linearly on the gas density, and with a different power of H₀, than the X-ray emission, so the two quantities together will yield a measure of both the mass and the angular diameter distance (and thus the Hubble constant) if the cluster depth is the same as the observed angular diameter. This has long been noted and used as a distance indicator (eg. Birkinshaw et al. 1994) that is independent of the standard distance ladder. However, no attempt had been made to study a complete and unbiased sample of clusters. Clusters tend to be ellipsoids (likely prolate) and thus there are orientation effects to consider, such as enhancement of the surface brightness when elongated along the line-of-sight, which will lead to an underestimation of the Hubble constant if the targets are surface brightness selected. Thus, it is imperative that a sample based on total X-ray flux be used.

Nearby clusters, for which there is a wealth of X-ray images and information, would be the ideal targets for investigation using this method. To this end, I have been leading a program using the small 5-meter radio telescope at OVRO to measure the SZE in an X-ray flux limited sample of 11 clusters of galaxies. Results from the first phase of this program for A2142, A2256 and A478 (Myers et al. 1997) have been published; the sample also includes the Coma cluster previously observed (Herbig et al. 1995). This work was carried out during my postdoctoral fellowship at Caltech primarily by myself and an undergraduate, Jonathan Baker (now a graduate student in astronomy at U.C. Berkeley), for his senior thesis. I am continuing work on the rest of the rest of the sample with Brian Mason (graduate student at Penn), with preliminary results from the clusters A399, A401 and A754 obtained in 1997-98 season. We plan to complete the nine clusters that can be measured with the OVRO 5-meter (ie. those not badly contaminated with radio sources) by the end of 1998. For these clusters, comparison of our SZE measurements with existing X-ray measurements gives a Hubble constant of

as reported in Myers et al. 1997. This result was updated, through the observation of further clusters, to

in Mason, Myers, and Readhead (2002). Recently, the Cosmic Background Imager was used to measure the SZE in a similar sample of z<0.1 clusters accessible to the CBI, with

resulting for the Hubble constant (Udomprasert et al. 2004).

The intra-cluster medium (ICM) itself is of interest also. If the X-ray temperature of the cluster gas is known, and thus the ionization state (baryons per electron) of the gas, then SZE measures the baryon surface mass density in the telescope primary beam. Note that this is independent of H₀ and of and elongation or orientation effects, and is only dependent on the distance through the angular size of the (approximately Gaussian) beam as projected on the cluster. If this surface density is integrated over the beam, then the baryonic mass within the Gaussian cylinder of the beam can be determined. Comparison with dynamical estimates then leads to the baryon mass fraction in the ICM.

Our SZE measurements (Myers et al. 1997) find typical surface gas mass densities of 7.5 × 10¹³ Msun/Mpc² in the centers of these clusters, and a baryonic fraction of

(for all but A478). For these clusters, we find a baryonic mass fraction

Related Publications and References:

See My Publications Page for my contributions to this field (and others).

Steven T. Myers