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Observational Constraints on Baryonic Dark Matter

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Observational Constraints on Baryonic Dark Matter 141 OBSERVATIONAL CONSTRAINTS ON BARYONIC DARK MATTER Bernard Carr School of Mathematical Sciences, Queen Mary Westfield College, Mile End& Road, London El 4NS. ABSTRACT A large fraction of the baryons in the Universe must be dark. Some of the dark baryons may be in the form of a hot intergalactic medium, while others may have been processed into the remnants of a first generation of Population III stars. Such remnants might be numerous enough to provide the dark matter in galactic halos, although halos could also contain elementary particles relicts of the Big Bang. The remnants would need to be either brown dwarfs or massive black holes. Evidence for the first possibility may come from cluster cooling flows and gravitational microlensing events. Evidence for the second could come from detecting the background light generated by the stellar precursor of the black holes. However, the COBE results imply that the light would need to be in the near-IR rather than being reprocessed by dust into the far-IR. 142 1. INTRODUCTION It is well known that, while ordinary visible material has a density O 0.01 in units of the critical density, there is evidence for a much v = larger density of invisible material1 l, In fact, there are four contexts in which the existence of dark matter has been proposed: (i) There mav be .local dark matter2 l, associated with our isalactic disk, with a mass comparable to that of the visible disk, though the need for this may go away if there is a population of ''thick disk" stars3 l , (ii) There may be dark matter in galactic halos4 l ; the associated density parameter is rather uncertain since it depends on the unknown radius to which the typical halo extends (averaged over all galaxies) but it is probably of order 'l 0.1. (iii) There may be dark matter in with a density h = clusters, parameter in the range n 0.2-0.3, deoendinis on the scale of clustering. c = (iv) Finally, if one accepts the inflationary scenario, there may have to be background dark matter (unclustered on the scale of galaxies though probably clumped on smaller scales) in order to make the total cosmological density parameter unity. Some of the dark matter components may be the same. For example, if one believes that individual galaxies are stripped of their halos when they aggregate to form clusters (thereby forming a collective cluster halo ), it would be fairly natural to identify the halo and cluster dark matter providing the original halos were large enough. Likewise the cluster and background dark matter could be identified if one invoked some form of biased galaxy formation in which galaxies form in only a small fraction of the volume of the Universe5l, On the other hand, it is also possible that all the dark matter components are different or that they require different mixtures of dark matter candidates. Candidates for the dark matter may be grouped into non-bar,vonic types (in which the dark object is some sort of elementary particle ) and baryonic types (in which it is something astrophysical). In the first case, the existence of the dark object - which may be generically termed an "ino" - goes back to the very early Universe. In the second case, the dark object forms out of the background gas at a relatively late stage (viz. 107-109y after the Big Bang); this may be termed the "Population III" scenario. The candidates are listed explicitly in Table ( 1) in order of increasing mass. The table illustrates that there are many forms of non-luminous matter; even though some of the candidates can be rejected, 143 Table (1): baryonic and non-baryonic dark matter candidates INOS POPULATION III Axions eV) (lo-S Snowballs ? Neutrinos (10 eV) Brown dwarf� 0.08 M ) (< e Pho ti nos (1 GeV) M-dwarf s (0. 1 M ) 0 6 Monopo les oo1 Gev) Whi te dwarfs M ) (1 e 1 Planck re licts ( 1 0 GeV) Neutron stars ( M ) 9 2 e Primordial holes (> 101 5g) Stellar holes (� lo M ) e 0 Quark nuggets (<102 g) VMO holes (lo2-105M ) 0 Shadow ma tter SMO holes (> 10 M ) ? 5 e Cosmic strings ? several viable ones in both categories remain. It is therefore naive to assume that all the dark matter problems will have a single explanation and one probably needs both baryonic and non-baryonic components. The main argument for both baryonic and non-baryonic dark matter comes from cosmological nucleosynthesis considerations. In order to explain the primordial light element abundances, the latest calculations6 l - allowing for a recent reduction in estimates of the neutron lifetime - 1·equire the - baryon density parameter to lie in the range O.Olh 2�nb.;0.016h-2 (where h is the Hubble parameter H0 in units of 100 km/s/Mpc). Since H0 must be at at least 50, the upper limit implies that Qb must be well below 1, so the inflationary model requires that the "smooth" background density be dominated by some non-baryonic component. Whether Ob exceeds the density of visible baryons (Qv-0.01 ) depends crucially on the value of H0• Most of the baryons could be visible if H0=100 but, in this case, the ai;(e of the Universe would be less than the ai;;es of globular clusters unless there were a cosmological constant. On the other hand, if H0=50 (as seems more likely), Qb must exceed Qv b;c a factor of at least so there must be 4, some dark baryons and perhaps enough to provide galactic halos. Much recent work has focussed on the question of whether one can circumvent the cosmological nucleosynthesis upper limit on Qb by invoking a 1st order phase transition during the quark-hadron era7 l. The idea is that the transition would generate fluctuations in the baryon density. Neutrons would then diffuse from the overdense regions (since their 144 cross-section is less than that of the protons), leading to variations in the neutron-proton ratio8 l, It is conceivable that Ob=l, in which case 99% of baryons have turned dark9 l, However, this is not a view favoured by most cosmologists and ! •:"11 not discuss it further here. The discrepancy between Ob and Ov could be resolved if there were an appreciable density of intergalactic gas. We know that there must be some neutral gas in the form of Lyman--o: cl0ucls but the density parameter associated with these clouds is probably no more than 0.01. If one wants put all the dark baryons into an intergalactic medium, then its to temperature must lie in the range 104K to 108K, the lower limit coming from the Gunn-Peterson test and the upper limit from the new COBE limits the Compton distortion of the microwave background radiation10l, on The other possibility is that the dark baryons are in Population III ,_·emnants. Note that this does not exclude the first proposal: in a biased scenario, for example, dark baryons r,ould be in both galactic halos and an intergalactic medium, in which case inos could still dominate the halo density. However, most of the emphasis in this talk will be on the possibility that halos are dominated by Population III objects. The suggestion that the halo dark matter could be baryonic goes against the current trend to assume that all forms of dark matter except that in the disk are non-baryonic. However, the arguments advanced in support of 1 this trend 1 l are not very convincing but just reflect a prejudice that the number of forms of dark matter should be as small as possible. Table (1 l indicates that dark matter could take as many different forms as visible matter, so it is quite plausible that the efficiency with which baryons turn dark exceeds the efficiency with which they turn visible. Thus the fact that the background dark matter (if such exists ) has to be non-baryonic does not exclude the halo dark matter being baryonic. Admittedly, it might seem strange that baryonic and non-bar:vonic material should have comparable densities, although there are some scenarios where this arises naturally12l, but this is a coincidence which pertains independent of whether the baryons remain in mainly visible or invisible form. In the next two sections will discuss why one might expect Population III objects to form (§2 ) and what constraints observations already place on their nature (§3 ). The remaining sections will discuss recent developments: in particular, the implications of the results COBE (§4 ), the evidence for dark matter production in cooling flows (§5) and the cletection of dark matter through gravitational lensing effects (§6). 145 2. THE FORMATION OF POPULATION III OBJECTS Although the halo and possibly cluster dark matter may be baryonic, it cannot be in the form of ordinary gas else it would generate too many X-rays. The gas must therefore have been processed into the dark remnants of a generation of pregalactic or protogalactic "Population III" stars13 l. The reason one might expect Population III stars to form is that, in most cosmological scenarios, one would expect the first bound objects to be much smaller than galaxies. For example, in the hierarchical clusterin g scenario, the first objects have a mass around 106M0 and bind at a redshift in the range 20-100; these clouds then cluster gravitationally to make galaxies and clusters of galaxies14 ). A currently popular version of this model is the "Cold Dark Matter" scenario15• In the pancake scenario16) the first objects to form are pancakes of cluster or supercluster scale and they do so at a redshift in the range 3-10.
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