The resulting densities are not enough to produce stim- ulated decays of the axion to photons since they occur when Γ$_{π}$ m 2 $_{p}$V$_{e}$ f$_{π}$/ ( Rm 4 $_{π}$f$_{a}$ ) > 1 which implies densities ρ > 10 15 Kg/ m 3 for m = 10 − 5 eV [21, 22]. Typical densities for axion stars are shown in table I. 4 The galactic halo will be an ensemble of axion stars and this picture is not in contradiction with observations since the size of axions stars fit into the limits coming from microlensing or gravothermal instability [11]. Previous studies that construct models for galactic dark matter halos out of scalar fields assume that each galactic halo is a spherical Bose-Einstein condensate made of an ultra- light scalar field [23–28]. In the present work, the axion stars will play the role of the scalar field mini-MACHOs, that is, a scenario where the scalar field (the axion) form a large number of stable asteroid-sized scalar conden- sations which end up clustering into structures similar to CDM halos with all their advantages. The stability of structures made of scalar fields have been extensively studied [29–31], and those analysis could be extended to our case with potential energy density given by (1). 0 1 10 -4 2 10 -4 3 10 -4 4 10 -4 5 10 -4 (x) =5x10 -4 (0)=3x10 -4 (0)=1x10 -4 0 500 1000 1500 2000 x -7e-14 -6e-14 -5e-14 -4e-14 -3e-14 -2e-14 -1e-14 0 a(x) FIG. 3. Scalar field and potential for a typical axion star with different values of σ (0) and an axion mass of m a = 1 . 0 × 10 − $^{5}$eV. TABLE I. Masses and R 99 for the configurations shown in Fig. 3 σ (0) Mass (Kg) R 99 (meters) density ρ (Kg/m $^{3}$) 5 × 10 − 4 3 . 90 × 10 13 1 . 83 6 . 3 × 10 12 3 × 10 − 4 6 . 48 × 10 13 2 . 86 2 . 7 × 10 12 1 × 10 − 4 1 . 94 × 10 14 8 . 54 3 . 1 × 10 11 Furthermore, axion stars present similar characteris- tics to the recently proposed neutralino stars [32], with the advantage that they could survive longer periods of time [33]. If DM is distributed as axion stars, their detection will be very difficult. The proposed femptolensing to detect axion compact objects [34] is close to its lower detectable limit. Another related issue is the low number of axion stars around the earth. Assuming for instance a Navarro-Frenk-White profile for the galactic halo, and a local halo density of 0 . 3 GeV / cm 3 around the Sun, there will be ∼ 1 axion star in the volume cover between Jupiter and the Sun. Nevertheless, another axion properties can shed light on the axion, such as the conversion of axions into photons in the presence of strong magnetic fields [35]. Collisions of axion stars with neutron stars [36] will produce flashes of light that could be detected by Gamma ray Obser- vatories [37]. A more detailed analysis of these ideas together with a more detailed study of the stability of the axion stars could help us to determine if DM is mainly composed by scalar field particles as the axions. mass. So, besides the change in variables (11), it is con- venient to solve for a ( x ) = 1 − A ( x ). Solving the system (9) for the new set of variables { x, σ ( x ) , a ( x ) , ˜ B ( x ) } we obtained typical nodeless configurations for these axion stars. Some of them are shown in Fig. 3, where we have taken an axion mass m = 10 − 5 eV. The total gravita- tional mass and the radius R$_{99}$ , both in physical units, for those configurations are shown in Table I. A possible scenario emerges with the hypothesis that DM is mainly composed by axions. As was already pointed out by Kolb and Tkachev [12], nonlinear effects in the evolution of the axion field in the early universe may lead to the formation of “axion miniclusters”. These miniclusters may relax, due to the collisional 2 a → 2 a process or by gravitational cooling [21], and they will evolve to a BS. In the present work we have constructed those BSs for axion particles by solving the EKG system for a real quantized scalar field which is regulated by an axion-potential (1). These self-gravitating systems, the axion stars, have very small masses and radii of me- ters (Table I) and consequently very low compactnesses. Acknowledgments: We would like to thank To- natiuh Matos, Luis Urena, Shin Yoshida and Carlos Palenzuela for very enlightening discussion and Aaryn Tonita for careful reading of the manuscript. We thank L. Rezzolla for his support and hospitality at the AEI. This work was supported in part by the CONACYT and CONACYT-SNI. 213 (2003) A. Refregier, Ann. Rev. Astron. Astrophys. 41 , 645 (2003) M. Tegmark et al. [SDSS Collaboration], Phys. Rev. D 69 , 103501 (2004) [1] W. J. G. de Blok, S. S. McGaugh, A. Bosma and V. C. Rubin, Astrophys. J. 552 , L23 (2001) H. V. Peiris et al. [WMAP Collaboration], Astrophys. J. Suppl. 148 ,