by Boris SHUSTOV, RAS Corresponding Member, Director of the Institute of Astronomy, Moscow

The hidden mass idea implies that we live in the Universe where unobservable matter dominates. The nature of this matter is obscure and may be quite unusual. But the hidden mass is perceived by most astronomers as a quaint but a plainly established thing. Since different and not always consistent definitions of this unobservable component of the Universe occur even in the literature, I shall use what appear to me as the most cogent definitions.

The hidden mass (HM) is matter which exists in the Universe but is unobservable. It consists of two different components: (1) Dark matter (DM) of the unknown nature whose existence is manifest only implicitly through gravitational action on different objects of the Universe and (2) baryonic dark matter (BDM) which is just common matter, unobservable though due to our limited possibilities.

Physicists and astronomers discuss a wide range of possibilities in explaining the physical nature of hidden mass carriers--from hypothetic elementary particles down to dwarf stars and black holes. The masses of candidate carriers differ by more than 70 orders of magnitude.

The main question about dark matter is quite straightforward: What is it? So far there is no generally accepted and cogent answer despite the eighty-year history of research. Only one thing is obvious: DM exceeds BDM greatly.

Baryonic matter is just common substance which forms us and the world we live in. The nature of baryonic matter and many of its characteristics are studied mostly (with respect to astronomical objects anyway), by observation methods. We observe such matter in the

Fig. 1. Microphotometer curve, i.e. radial brightness distribution (upper panel) and the galaxy rotation curve NGC~3198 obtained by observations of neutral hydrogen at 21 cm (lower panel). The rotation curve is resolved into three components: disk-the anticipated rotation curve of a galaxy model whose radial distribution of stars proportionally corresponds to brightness distribution (upper panel); gas--contribution of the gas component; halo--contribution of the nonobservable component, the dark halo.

Universe by using various instruments, first of all, ground- and space-based telescopes; and yet a considerable part of it (probably more than a half!) is hidden from us. This is what we call baryonic dark matter (BDM). The main question here: Where is it, BDM, and in what form?

In this article we shall deal with the hidden mass problem as it stands today, and with recent changes in the approach to it, at least, within the limits of our Galaxy and its surroundings. We shall accentuate ultraviolet and the role of extraatmosphere observatories in detecting some of the signatures of baryonic matter still hidden from us.

OBSERVATIONAL EVIDENCE ON HIDDEN MASS

The fact that a part of the surrounding world is hidden from us despite the great strides of science (some things will always be hidden anyway) is an obvious truth. The following bits of evidence are considered to be the strongest arguments in favor of the hidden mass existence: the kinematics within galactic clusters; the rotation curves of (disk) galaxies; the observations of X-ray radiation of hot gas in galactic clusters; the gravitational microlensing.

The first group includes studies carried out by Swiss astronomer Fritz Zwicky and published as early as the 1930s. He measured the dispersion of galaxies speed in the Coma cluster and evaluated the (dynamic) cluster mass proceeding from the celestial mechanics laws. He evaluated also the total luminance of galaxies. It appeared that the ratio of mass to radiated energy was 400 times higher than that of the Sun! By that time stellar physics fundamentals were formed already, and in accordance with them the above fact just could not be in a normal stellar world. Therefore Zwicky concluded there had to be some very massive component in galactic clusters or in a surrounding space between them which was not luminant but dark. This component gets galaxies moving with great speed (dispersion velocity is around 1,000 km/s) in a cluster.

Fig. 3. Mass spectrum evolution of the DM halo beginning from the age of the Universe (corresponds to the cosmological factor value z=30) to our epoch z=0. The picture is adapted from H. - W. Rix 3/2009 IMPRS Heidelberg Galaxies Block Course.

Measurement of rotation curves of galaxies, i.e. relationship between the rotation speed and the distance to the galactic center, is considered the strongest evidence on the existence of a hidden mass in galaxies. It appeared that the linear rotation speed is constant over a long stretch. Fig. 1 shows the rotation speed of the galaxy NGC 3198. The rotation curve obtained from observations of HI on the 21 cm (lower panel) is resolved into three components. The curve marked as "disk" is the anticipated rotation curve of a galactic model in which the distribution of radial surface density corresponds (proportionally) to the distribution of surface brightness. Galactic gas also contributes to the rotation curve. Obviously, the joint contribution of gas and stars is not sufficient to explain the rotation curve observations. It is necessary to add a component (halo) composed of unobservable dark matter. At long distances from the galactic center the contribution of this halo (sometimes the term "dark halo" is used) is predominant.

This situation, i.e. the existence of a massive halo, is typical of actually all spiral galaxies where one could observe peripheral areas composed of neutral hydrogen. A similar situation has been observed in irregular dwarf galaxies and in galaxies with a low surface brightness, though the latter may have a lower concentration of dark matter towards the center.

Hidden mass is certainly present in giant elliptic galaxies and also in large clusters of galaxies. Observations of hot gas in these objects irradiating in the X-ray band are viewed as an important tool for hidden mass studies there. Gas particles heated to a temperature of dozens of millions of degrees centigrade move at very high speeds, and so a powerful gravitation pull is needed to keep this gas from escaping. And again, like it was found by Zwicky, the observable mass is far too small to retain the gas. Since these hot gas halos should be close to a hydrostatic equilibrium, measurements of temperature distribution over X-ray images and spectra make it possible to assess the total mass distribution. As shown by many researchers, ordinary matter dominates within giant elliptic galaxies at distances R<Re, where Re, is the effective radius, with dark matter making up just 20 percent at most; if R>>Re, dark matter predominates.

Microlensing observations are yet another method of hidden mass detection. The gist of this method consists in that the gravitation field of an invisible solid body moving close to the line of sight between a distant emitting source (a star from another galaxy, quasar, etc.) and an observer, acts on a source radiation as a lens and produces a marked brightness gain of this source, i.e. a flash, closer to the line of sight. Objects that trigger microlensing are not far from us as compared with extragalactic bodies. Accordingly, the angular rates of their movement perpendicular to an observer's line of sight are relatively high. Therefore the effect of every microlens can be observed only for dozens of days. Experiments on detecting such flashes are carried out in other countries as well. Thousands of such events have been registered.

The observation results of microlensing events in the OGLE experiments and other similar projects have made it possible to assume that small-mass (not above several tenths of the solar mass) stars, possibly the brown dwarfs, are one of the hidden mass components. By some estimates the number of such small-mass stars in our Galaxy exceeds that predicted by the modern theory of star origin and evolution. But there are also other conclusions, too. The hypothesis postulating a great number (as many as 1012) of small-mass stars (white dwarfs) in the halo of our Galaxy was tested experimentally. Actually all nonexotic scenarios of the Galaxy origin exclude such a possibility. A microlens can be in the form not only of a star or a planet but also

be a bunch of dark matter (if bunches of such mass can exist). In the OGLE experiment about two million stars of the Magellanic Clouds were observed. Its aim was to observe flashes caused by the microlensing effect on objects in the halo of our Galaxy. The overall conclusion is as follows: it is unlikely that dark matter bunches capable of causing the microlensing effect can be in the halo of our Galaxy. This is the problem our scientists have to grapple with.

DARK MATTER AND COSMOLOGY

All modern generally accepted cosmological models imply that dark matter constitutes a major part of the Universe. It is believed that the total density of matter as the energy in the Universe is close to a critical density (simply speaking, critical density is a function of the following condition: if density is below the critical value, the Universe will expand eternally, and if it is above this value, its expansion will change to compression sometime). According to modern concepts, the main total density constituents are as follows: dark matter (about 23 percent of the total density), baryonic matter (about 4 percent) and dark energy (about 73 percent). The latter component is introduced in order to explain the nature of the accelerated expansion of the Universe as it follows, for example, from observations of supernovae. It is fair to say that many scientists do not accept implicitly this theory as conclusive. Thus they query the assumption that supernovae of the 1st type, which are not very numerous and which have not been observed accurately enough, could become a basis for the dark energy hypothesis and regarded as reference sources ("standard candles").

The assessment accuracy of the Universe parameters is considered rather high--not below several percent. However, the main question for an unbiased critic is this: To what extent the values of these parameters (their estimation errors) can be considered to be model independent. Without going into discussions we may note that, according to the American astrophysicist Joseph Silk, the available estimation accuracies were made under rather harsh a priori assumptions. In particular, the thin structure invariance is an important a priori assumption. If this assumption is ignored, there will be more degrees of freedom, particularly, in determining baryonic density.

The present cosmological models do not furnish information on concrete dark matter carriers but place restrictions on the characteristics of these carriers. In particular, dark matter should be cold, that is its particles should not move fast. For example, neutrinos moving as fast as velocity of light are not fit for the role of dark matter carriers exactly because of that. Only cold matter provides conditions for small inhomogeneities, the nuclei of future galaxies and clusters of galaxies. As it will be seen below, the mass of even the smallest stable structures in dark matter exceeds the solar mass by dozens and hundreds of thousands of times!

Astrophysicists suggested various and often very exotic candidates for dark matter carriers. Their hypotheses are based on most sophisticated theories from elementary particle physics. But no particles fit for this purpose have been found among the known elementary particles (whereas their number is above 300 counting in those obtained at accelerators). About a dozen different theories on the nature of dark matter have been put forward, from the possibility of hypothetic particles arising from the symmetry of a different kind to a mirror world where all particles were similar to our particles but interacted with them through gravitation. Naturally each particle like that needs experimental verification that calls for significant

Fig. 5. Gravitational potential distribution in the Galaxy. The constituent conditioned by the baryonic matter of the Galaxy is denoted by MW (Milky Way), and the constituent conditioned by dark matter is denoted by "DM halo".

efforts and facilities including extra-atmospheric equipment. The Russian-Italian project PAMELA is a telling example. However, so far no hypothesis has been confirmed in such experiments.

We do not know yet what dark matter (DM) means. Yet proceeding from the idea of its general properties (it should be cold, its carriers should not be very massive, etc.) we can construct a consistent pattern of the evolution of the Universe, more precisely, relevant to the formation of such giant structures as galaxies and their clusters.

According to generally accepted simulation data on the formation and evolution of galaxies and their clusters, initial fluctuations (ripples) of the density spread of cold dark matter are a most important process preceding the formation of the first galaxies (protogalaxies). The process of growing density fluctuations of dark matter is described as the crowding (clusterization) of dark matter. (Cold dark matter is abbreviated as CDM in the literature.) It is easy to observe this process on computer models. By assigning a vast number of point masses in some space and distributing them uniformly, we can trace a further evolution of this ensemble under the action of gravity. Even minor initial deviations from homogeneity (which are inevitable) lead to an increase of inhomogeneities in the distribution of gravitating point objects. The rising density fluctuations give birth to large and solid clusters of DM. These clusters are the first gravitationally coupled (and relatively stable because DM particles can no longer escape) objects in the Universe. They are called "dark matter halos". From the very beginning these halos were relatively massive, at least dozens of thousands times as massive as the Sun. There were a great number of such DM halos in the nascent Universe. Later on a whole spectrum of halo masses up to giant halos with a mass of more than a hundred trillion suns came into being in the course of changes brought about by mutual collisions and fusions of the initial halos. Fig. 3 shows the temporal spectrum of DM halo masses.

Since there is much less of baryonic matter than of DM, and its gravitational action is much weaker than that of DM, it follows DM "obediently", i.e. dark matter exerts gravitational control over further structurization a baryonic matter (gas) which accumulates in inside DM halo. Thereupon this gas forms the first stars (in primary halos), galaxies and larger halos and galactic clusters in the largest halos. The role of DM in the formation of the first stars and galaxies is discussed in greater detail in my lectures at the annual All-Russia Astronomical Conference of Students at Ural Federal University (Yekaterinburg).

In general, clusterization leads to the formation of a cellular structure (like a sponge) with condensation in nodes. The DM spreading pattern is often called "cosmic spider web". Nodes of a different scale are just DM halos with baryonic matter concentrated inside.

Fig. 4 shows the growth of structures in the Universe according to the most up-to-date computer model Bolshoi. Today it is the most accurate and comprehensive model of the evolution of the Universe as developed by astronomers of the USA, Germany and Russia under the guidance of Professor Anatoly Klypin of the University of New Mexico. The model is designed with the aid of the supercomputer Pleiades installed in the NASA Ames Research Center.

HIDDEN MASS IN THE GALAXY AND ITS SURROUNDINGS: DARK MATTER?

It is quite natural to conclude: if there is too much of dark matter, then it should be quite nearby. It was the subject of many studies of the Galaxy and its surround-

Fig. 6. Relative contribution of DM phases to cosmological evolution.

ings. Thousands of works were published accordingly. Overall, we can say that DM gravitational manifestations are not noticeable either in thin or thick disks, or in the bulge (central condensation of the Galaxy). Dark matter is in a large halo with a typical linear scale ~200 kps. The halo mass makes up, as it follows from data on the galactic rotation curve and globular cluster movement, ~2x10¹²M_ө and, in any case, does not exceed 6x10_ө¹²M_өn. The lower limit of the halo mass is estimated at 1.4x10¹²M_ө. A similar assessment is provided by a method suggested as early as 1959. This method is based on an analysis of kinematics of movements and of the gravitational effect of our Galaxy and the galaxy M31 (both are the largest and most massive members of the local galactic cluster) on each other.

Distribution of the gravitational potential in the Galaxy is shown in outline above. The baryonic component of the Galaxy is designated by MW (Milky Way), and the DM component, by "DM halo". This distribution pattern agrees with the density distribution in the sense that the higher the density of mater, the deeper the gravitational hole is. The scale of the galactic dimensions is 30 kiloparsec, and that of the dark halo dimensions, about tenfold as large. That is why, despite the lower density in the center, the total mass of DM halo exceeds the galactic mass manifold. The DM mass within the Galaxy is relatively small.

Scientists wonder how much of dark matter is within the Sun. The answer is definite: the relative mass of the solar hidden matter cannot be above several percent (<2 percent at an uniform mixing of ordinary and dark matter). Hence, there can be no DM in stars in practical terms.

Distribution of the potential and thus of matter density is shown in rough outline. Identification of DM distribution parameters is an important question. By simulation data, DM halos are not necessarily spherical. Their characteristic oblateness (ratio between the minor and major axes) is ~0.5. Such assessment is based on an analysis of hot X-ray gas distribution in elliptical galaxies, of orbits of low-mass satellites captured by galaxies and on gas disk thickness calculations.

The simulation of the formation and evolution of galaxies in CDM models brought out a number of problems. First of all, these models produce a distribution of DM overmuch concentrated towards the center ("DM cusp problem". In line with the CDM cosmological models the density distribution in DM central areas halo should have an expressed density peak (cusp) but observations do not confirm that. The fact that, according to numerical evolution scenarios, thousands of small DM halos should remain after fusions and dwarf galaxies could be formed there, is even more critical. Only several dozen galaxies like that are observed in our Galaxy. Astrophysicists are tackling these problems. For instance, an inventory of astration in the galactic center will defuse the controversy over the cusp problem.

The astration phenomenon gives rise to supernovae whose explosions extrude matter from the galactic center, and since baryonic matter density is substantially higher here than that of dark matter, DM follows in the "coat-tails" of ordinary matter, and the peak is off.

BARYONIC MATTER IN THE UNIVERSE

Thus, baryonic matter density in the Universe is assessed at about 4 percent of the total. Such assessment for a selected model can be made by analyzing the primary nuclear fusion and also checked by measurements of the relative amount of primary deuterium and hydrogen (D/H ratio). Since a part of baryons is concentrated within galaxies, the 4 percent value is an upper parameter for determination of intergalactic matter. But of these 4 percent only a small portion is detectable. It was possible to assess the distribution of luminous (observable) mass in the Universe and determine that observable matter density there makes up only 10-30 percent of the total amount of baryons.

Therefore hidden baryonic matter search in the Universe is an outstanding problem of basic science.

Here different forms of unobservable objects are suggested, such as low-mass stars, black holes, planetary or comet bodies, small gas clouds... Some authors contend that baryonic matter of the Universe can be divided into four phases, depending on its density and temperature. The condensed phase, i.e. gas in galactic clusters, is observed by X-ray radiation with a temperature above 10 mln degrees C. The diffusion phase represents a majority of structures observed in the hydrogen absorption line 121.6 Nm (Lyman alpha line) in the spectra of distant quasars. The heat-hot phase is a gas heated by shock processes to temperatures ranging from hundreds of thousands to dozens of millions degrees centigrade. Such gas is hard to detect by absorption lines due to high ionization and low radiation intensity. A relative portion of the above components changed in the course of the evolution of the Universe. In compliance with these data a substantial part of dark baryonic matter can be detected in phases 3 and 4 (Fig. 6).

Observations of cold gas intergalactic clouds which absorb energized quanta in systems radiation (diffused phase) can be made with ground-based telescopes in the Layman alpha line, but only for very distant clouds. The point is that the Layman alpha line wavelength is in the far ultraviolet spectral range in which the Earth's atmosphere is absolutely opaque. Due to the so-called cosmological red shift for distant objects, the Layman alpha line wavelength in the spectrum registered by an observer shifts to the red, that is to a more long-wave region. For very distant objects it shifts to the visible spectrum and can be registered by ground instruments. But only large clouds can be observed at such long distances. Since the observed spectrum of intergalactic clouds mass, smallest clouds including, is a very important scientific mission, it would be natural to concentrate on closer objects. But in the near Universe, for which the cosmological factor z is not above value 2, and it, this near Universe, constitutes about 80 percent of its total volume, it is possible to observe intergalactic clouds in the Layman alpha line only by space telescopes of the ultraviolet range. Thus, a space ultraviolet telescope provides for a significant increase in the effectiveness of a search for dark baryonic matter in the diffused phase.

A substantial part of baryonic matter at small z values, i.e. in the near Universe, is believed to be highly ionized. It is heated to hundreds of thousands, even to dozens of millions of degrees centigrade. At such high temperatures atoms lose a major part of their electrons because in this state matter can irradiate or absorb only very energized quanta. The wavelength of such quanta is in the ultraviolet spectrum inaccessible to ground observations. For this purpose an instrument should be placed in outer space to avoid the effect of atmospheric interferences, for the atmosphere absorbs radiation completely.

There is also evidence that many baryons can be in emptiness, or out of galaxies. Therefore, of much importance is high-resolution spectroscopy in the ultraviolet range to determine more exactly the mass of baryonic component of the Universe and the chemistry of this component.

HIDDEN MASS IN THE GALAXY AND ITS ENVIRONMENT: DARK BARYONIC MATTER?

Detection of a great number of absorbing clouds of highly-ionized gas in the Galaxy and its environments is another proof of great prospects in search of baryons in the intergalactic medium. It was effected by observations of the OVI absorption lines in the far ultraviolet spectrum. The space telescope FUSE furnished spectra of 100 quasars in which OVI lines associated with objects (clouds) were found in phase 4. Such clouds fill the galactic corona and the space occupied by the local cluster of galaxies. The total mass of baryons is assessed at the content of oxygen 0.1 relative to the content of O₂ in one trillion solar masses. It is enough to explain a considerable part of the hidden mass in the halo of our Galaxy!

Thus, observations by the relatively small space telescope FUSE led to a remarkable result running counter to the existing concepts. It turns out that a substantial part of the hidden mass of the Galaxy and its environment can be explained by the presence of the warm-hot component of the baryonic constituent of the Universe, a widespread component hard to detect. The international extra-atmospheric observatory (World Space Observatory) Spektr-UF now being created under Russia's guidance and equipped with a 170 cm telescope mirror and a set of modern spectrographs will be best for this problem solving.