by Christophe Balland
The first joint Berkeley-Strasbourg Workshop in Astrophysics was held early this spring on the campus of the University of California at Berkeley. Co-organized by the Center for Particle Astrophysics (CfPA) and the Observatoire Astronomique de Strasbourg, this meeting offered the opportunity for two communities to meet and interact : more than 80 people, mostly from American and French institutions, attended the five day workshop to debate on our current understanding of the role baryons play in cosmology.
BARYONIC DARK MATTER IN THE HALO AND THE DISK OF THE GALAXY
The workshop opened with a discussion of recent results from various microlensing experiments. These experiments suggest that a substantial fraction of the matter in the disk and the halo of the Milky-Way galaxy may be present as baryonic substellar objects known as MACHOs (acronym for MAssive Compact Halo Objects). The MACHO collaboration, involving researchers from the CfPA, reported 8 halo candidates with mass ranges between 0.1 and 1 solar mass from a 2-year microlensing search towards the LMC (Bennett). This result implies that as much as 50 % of the halo mass may be accounted for by MACHOs assuming a standard spherical halo model, in agreement with the limits set on MACHO masses by the French experiment EROS (Aubourg).
Both the MACHO and EROS teams discussed the search for small timescale events (t<20 days) in order to probe low-mass objects in the galactic halo (Griest & Renault). No such event has been detected in either experiment so far putting an upper limit of 20 % for the contribution of 10^{-6}-10^{-2} solar mass objects to the halo mass, a result fairly independent of the assumed galactic model and initial mass function.
The nature of MACHOs, and more generally of the galactic dark matter, was discussed by various speakers. While faint hydrogen-burning stars and neutron stars have been ruled out, M-dwarfs, White dwarfs and Brown dwarfs are still viable candidates. The mass function of low-mass stars in the galactic disk has been determined from different observed luminosity functions (Chabrier) and suggests that, in order to explain both star counts and microlensing experiments, a substantial amount of these objects is present in the galactic disk. In addition to MACHOs, the possibility that hidden gas, either diffuse or in cold condensed clouds, may pervade the galactic disk and/or halo was dicussed on theoretical grounds (Gerhard) and possible observational signatures such as enhanced galactic gamma-ray emission were emphasized (Salati).
THERMAL HISTORY OF THE GAS AND GALAXY FORMATION
Various aspects of the gas and dark matter physics during galaxy formation were the main topic of the workshop's second day. First insights may be obtained by modeling nearby galaxy properties to infer star formation and gas processes histories (Wyse). Population synthesis models provide a powerful tool for this purpose (Charlot). However, this approach is limited by the large number of poorly constrained parameters involved in star formation modeling as well as by substantial uncertainties in population synthesis models.
Highlights of this session included a discussion of the Lyman-alpha forest as a new tool to understand galaxy formation and chemical evolution. The Ly-alpha forest probes the small-scale structure of the universe (Weinberg) and contains 80 % of all baryons at redshift 2, far more than what is seen in the form of stars today. Where did all these baryons go ? Cooling is inevitable so heating mecanisms must be invoked to heat the gas up to 10^6 K to prevent star formation. Photoionization has been shown to be insufficient to solve the overcooling problem (Blanchard) but the possible existence of a hot intergalactic medium (IGM) offers an alternative solution. However, no Gunn-Peterson signature has been observed so far, raising a potential problem for hierarchical scenarios of structure formation.
Hierarchical galaxy formation is studied assuming initial conditions for dark matter and a prescription for structure formation. Reconstruction of the merger history of halos analytically or by use of N-body simulations (Evrard, Steinmetz) is then possible, allowing predictions for the distribution of large-scale structure. The incorporation of the physics of gas was stressed as an essential step (Chieze, Teyssier). A crucial point is to explain the angular momentum of galaxies (Steinmetz): tidal torques account for angular momentum of dark halos but not disks which are too small if the baryon fraction of the universe is of the order of 5 %. This problem can be solved with a higher baryon fraction (Steinmetz).
BARYONS IN THE EARLY UNIVERSE
The theory of nucleosynthesis in the first minutes of the universe is today viewed as a major success of Big Bang Cosmology. Determination of primordial abundances of light elements provides an estimate of the ratio of photons to baryons densities. Abundance ratios give direct access to the baryon density since the photon number density is known with accuracy from COBE measurements. Until recently, only local measurements of light elements ratios were possible, implying an accurate understanding of how Deuterium, 3He, 4He and 7Li abundances (and others) may be affected by stellar processes.
The most conservative determination of the baryon density from local measurements (including uncertainty on the value of the Hubble parameter) leads to an upper limit of OmegaB < 0.1 (Audouze, Schramm). D/H ratio has been stressed as the most accurate baryometer (Schramm). However, two observations of extragalactic deuterium abundance, going in opposite direction, have recently challenged the Big-Bang nucleosynthesis picture. The large D/H ratio measurement from Songaila et al which gives a low value of OmegaB is consistent with current observations of 4He, 7Li and the existence of three neutrino species but not with 3He abundance (Audouze). On the other hand, the low value of D/H ratio (Tytler) favoring a high value of OmegaB is in contradiction with 4He and 7Li abundances. The question of whether the current dispersion in deuterium values is real or not has been discussed. High D/H observations may be, for example, contaminated by high column density interlopers in the line-of-sight of quasar absorbers. Low values of the D/H ratio (Tytler) are very close to solar system values implying little or no astration of Deuterium.
Recent results on other elements abundances and the importance of stellar physics for cosmology were discussed. 7Li has been shown to be largely depleted by rotation (Deliyannis, Vauclair) whereas destruction of 3He in low-mass stars implies the (D+3He) constraint moves towards lower OmegaB values (Deliyannis).
Constraining the baryon density from Cosmic Microwave Background (CMB) anisotropies was another topic of this session (Scott). OmegaB is related to the height of the first accoustic peak so that firm constraints on the CMB angular power spectrum at intermediate scales (l ~ 100) may put a constraint on OmegaB, provided other cosmological parameters are known.
EXISTENCE AND PHYSICAL STATE OF THE INTERGALACTIC MEDIUM
A full day was devoted to the intergalactic medium. Discussion focused on high redshift damped Ly-alpha clouds today interpreted as galaxies in the first stages of formation. These high column density clouds thus offer a way of directly probing the chemical evolution history of galaxies over a large fraction of the age of the universe (Lu, Rauch, Cowie). The density of Ly-alpha systems at redshift 3 is comparable to the density seen in stars but they are metal-poor, consistent with type-II supernovae enrichment. Little or no star formation is seen in these systems implying that they may be protoellipticals or metal-poor dwarf galaxies.
The state of the IGM was also discussed (Bartlett): the abscence of any Gunn-Peterson signature implies a highly ionized, possibly reheated IGM. In this case, inverse Compton scattering of CMB photons on IGM electrons should induce spectral distorsion in the CMB, an effect however not detected by COBE.
BARYONIC FRACTION IN GROUPS AND CLUSTERS OF GALAXIES
The last day of the workshop focused on the problem of the baryonic content of groups and clusters of galaxies. The baryonic fraction in groups and poor clusters is about 15 %, with most baryons in stars (Mulchaey, Mamon), whereas baryons in rich clusters may make up 20 % of the total mass, mostly in the form of X-ray emitting intra-cluster medium (Mushotzky) with large spatial variation seen. This result is inconsitent with predictions of Big-Bang nucleosynthesis if Omega0=1. Possible segregation of baryons in clusters has been shown to be inefficient in solving this problem, and unless BBN is wrong or Omega0 is of the order of 0.3, we face what is now known as the baryon catastrophy in clusters. Accurate cluster mass determinations are crucial with this respect. Cluster masses may be derived either using weak shear gravitational lensing distortions to recontruct surface mass densities (Squires, Broadhurst) or using the X-ray gas as a tracer of cluster gravitational potential (Evrard, Mushotzky). Though this latter method might suffer from delicate assumptions on the state of the gas and underlying mass models, these two approaches seem to lead to roughly consistent results.
The various aspects of the physics of baryons discussed during this workshop contribute to the emergence of a consistent picture of the universe, graphically summarized below (Silk):
The workshop was ponctuated by a number of social events among which was a visit to the San-Francisco Exploratorium.
This workshop was made possible thanks to financial support from the Center for Particle Astrophysics, the Observatoire Astronomique de Strasbourg and the Mission scientifique et technologique de l' Ambassade de France aux Etats-Unis. The second Berkeley-Strasbourg workshop should be held in the near future in Strasbourg Observatory.
balland@physics.berkeley.edu
25Jul96
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