Galaxy dark halo

How will we identify the extra astrophysics required to reconcile the properties of CDM dark haloes with those of luminous galaxies We can start by developing knowledge of the evolutionary history of at least one place in at least one galaxy. We would be unlucky if that place were far from the norm alternatively, any theory that predicts such a history to be very unusual might be suspect -the galaxian Copernican principle. Kinematics and current spatial location are of course critical parameters, so that traditional stellar populations analyses are... 

However, according to the latest estimates, the fraction of our Galaxy s dark halo that could be explained by baryonic matter (low-luminosity stars and non-luminous compact, massive objects) cannot exceed 20%. These estimates are based on the effect such objects would have on the hght from stars in the Magellanic Clouds. It is concluded that the halo of our Galaxy, and probably that of other spirals of this type, is not principally made up of ordinary, atomic matter. 

Consequently, we are forced to admit that compact massive objects are not the main component of the dark halo of our Galaxy, or any of its kind. This in turn implies that the Sun and stars are lost amongst a halo of darkness, in the middle of a haze of neutralinos, hypothetical particles predicted by so-called supersymmetric particle theories. 

The dark halo hypothesis is based on the assumption that companions are physical if they are not then they do not measure the mass of the main galaxy, but characterise mean random velocities of galaxies ... 

In mid 1970s the main arguments for the presence of dark halos (coronas) of galaxies and clusters of galaxies were statistical. In particular, the masses of... 

For comparison, the primordial baryon fraction from observations at high redshift is 15%. Presumably these baryons were present initially, when the galaxy formed. Indeed modelling of disk formation requires an initial baryon fraction of 10 — 15% in order for sufficient cooling to have occurred to form the disk. The missing" galactic baryons amount to a baryon fraction comparable to what is observed, namely around 5 — 10% of the dark halo. 

Neutrinos can be cold dark matter if their masses are around few GeV or a TeV. However, fourth-generation heavy neutrinos lighter than 45 GeV are excluded by the measurement of the Z-boson decay width at the Large Electron-Positron collider at CERN. Moreover, direct searches for WIMP dark matter in our galaxy exclude Dirac neutrinos heavier than 0.5 GeV as the dominant component of the galactic dark halo (see Figure 3). Thus although heavy Dirac neutrinos could still be a tiny part of the halo dark matter, they cannot solve the cold dark matter problem. 

In the framework of semi-analytical models of galaxy formation (Mo, Mao White, 1998), the evolution of galaxy disks can be described by means of scaling laws calibrated on the Galaxy with Vc and A as parameters (Jimenez et al. 1998 Prantzos Boissier, 2000), where Vc is a measure of the mass of the dark halo and A is a measure of the specific angular momentum of the halo. 

Detailed studies have provided solid evidence for the existence of dark halos around disk galaxies of any morphological type, luminosity, and environmoit (e.g., Rubin et cU. 1985 Kent 1988). Nevertheless, quantitative assessmmt of the size and importance of the various dynamical components (e.g., disk, bulge, halo, and their M/L ratios) suffers from the limited validity of certain premises. One questionable assumption is that of constant M/L ratios for the bulge and disk components. Another question is about the mass distribution inferred from optical emission-line rotation curves, which are related to the kinematics of the ionized gas, so that rotation curves in bulge-dominated regions may not measure true rotational velocities (Kent 1988 Kormendy Westpfahl 1989). 

These considerations suggest that dynamical studies of bulge-dominated spirals can be wrong not only in their assessment of the visible M/L ratios but also in their inferences about dark halos. NIR imaging effectively probes the mass distribution due to lower extinction and to the more direct link to the stellar mass. When coupled with accurate rotation curves, NIR brightness profiles should provide the best means to determine the mass distribution in disk galaxies. 

The existence of dark matter (either baryonic or non-baryonic) is inferred from its gravitational effects on galactic rotation curves, the velocity dispersions and hydrostatic equilibrium of hot (X-ray) gas in clusters and groups of galaxies, gravitational lensing and departures from the smooth Hubble flow described by Eq. (4.1). This dark matter resides at least partly in the halos of galaxies such as our... 

Fig. A 1.1. Orbital speed as a function of distance from the centre of the galaxy NGC 3198. The flat part of the curve can only be explained under the assumption that there is a massive halo of dark matter. Points correspond to observations. Curves show contributions from the disk and the halo, calculated using a suitable model.

It is thus assumed that the (rotational) speeding offences committed in the galactic periphery are due to the existence of a massive halo of invisible matter. In our own Galaxy, there must be ten times as much dark matter as visible matter, amounting to some 1000 biUion solar masses. We may deduce that the same is true of aU... 

Gravitahonal (micro) lensing is now recognised as a way of revealing otherwise hidden matter. It is universally used to estimate the distribution and quantity of dark matter on a variety of distance scales. The study of dark matter in the halo of our own Galaxy is not the least significant amongst these. 

First, it is useful to know that a model spiral galaxy can be considered to consist of a central spherical bulge component, embedded in a rotating disk of stellar and gaseous material with the whole embedded in a spherical halo of very diffuse gas and halo stars. It is conventionally believed that this latter component is much more massive than it appears, with the deficit made up of dark matter. ... 

I conclude that approximately 25 15% of the baryons could be dark. If so, and this is far from a robust conclusion, one can pose the following question where are they Intriguingly, the possible shortfall is comparable to the mass observed in stars ( flf). Could there be a mass in dark stars comparable to that in visible stars Or could early star formation and death have resulted in the ejection of a comparable mass of baryons from the galaxy and its halo Were the situation to rest here, there would be little reason to take the issue of... 

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