arXiv subjects

Xavier Hernández

Publications and source records attributed to Xavier Hernández.

5 recordsLinked to original sources

arXiv2004

Scalar field "mini--MACHOs": a new explanation for galactic dark matter

We examine the possibility that galactic halos are collisionless ensembles of scalar field ``massive compact halo objects'' (MACHOs). Using mass constraints from MACHO microlensing and from theoretical arguments on halos made up of massive black holes, as well as demanding also that scalar MACHO ensambles of all scales do not exhibit gravothermal instability (as required by consistency with observations of LSB galaxies), we obtain the range: $m\alt 10^{-7} M_\odot$ or $30 M_\odot\alt m\alt 100 M_\odot$. The rather narrow mass range of large MACHOs seems to indicate that the ensambles we are suggesting should be probably made up of scalar MACHOs in the low mass range (``mini--MACHOs''). The proposed model allows one to consider a non--baryonic and non--thermal fundamental nature of dark matter, while at the same time keeping the same phenomenology of the CDM paradigm.

astro-ph↗

arXiv2004

Using Entropy to Discriminate Annihilation Channels in Neutralinos Making up Galactic Halos

Applying the microcanonical definition of entropy to a weakly interacting and self--gravitating neutralino gas, we evaluate the change in the local entropy per particle of this gas between the freeze out era and present day virialized halo structures. An ``entropy consistency'' criterion emerges by comparing theoretical and empirical estimates of this entropy. We apply this criterion to the cases when neutralinos are mostly B-inos and mostly Higgsinos, in conjunction with the usual ``abundance'' criterion requiring that present neutralino relic density complies with $0.2<Ω_{\tildeχ^1_0} < 0.4$ for $h\simeq 0.65$. The joint application of both criteria reveals that a much better fitting occurs for the B-ino than for the Higgsino channels, so that the former seems to be a favored channel along the mass range of $155 \hbox{GeV} < m_{\tildeχ^1_0} < 230 \hbox{GeV}$. These results are consistent with neutralino annihilation patterns that emerge from recent theoretical analysis on cosmic ray positron excess data reported by the HEAT collaboration. The suggested methodology can be applied to test other annihilation channels of the neutralino, as well as other particle candidates of thermal WIMP gas relics.

hep-ph↗

arXiv2003

A New Method to Infere Which Type of Neutralinos Make Up Galactic Halos

Applying the microcanonical definition of entropy to a weakly interacting and self--gravitating neutralino gas, we evaluate the change in the local entropy per particle of this gas between the freeze out era and present day virialized halo structures. An ``entropy consistency'' criterion emerges by comparing the obtained theoretical entropy per particle of the virialized halos with an empirical entropy per particle given in terms of dynamical halo variables of actual galactic structures. We apply this criterion to the cases when neutralinos are mostly B-inos and mostly Higgsinos, in conjunction with the usual ``abundance'' criterion requiring that present neutralino relic density complies with $0.2<Ω_{\tildeχ^1_0} < 0.4$ for $h\simeq 0.65$. The joint application of both criteria reveals that a much better fitting occurs for the B-ino than for the Higgsino channels, so that the former seems to be a favored channel along the mass range of $150 \hbox{GeV} < m_{\tildeχ^1_0} < 250 \hbox{GeV}$. These results are consistent with neutralino annihilation patterns that emerge from recent theoretical analysis on cosmic ray positron excess data reported by the HEAT collaboration. The suggested methodology can be applied to test other annihilation channels of the neutralino, as well as other particle candidates of thermal WIMP gas relics.

hep-ph↗

arXiv1996

Primordial Cosmological Fluctuations on Galactic Scales

The galactic evolutionary theory is now sufficiently mature to provide information about the galaxy formation phase. From evolutionary models we establish a link between the present features of late type galaxies and the protogalactic density fluctuations. This link is used to estimate the fluctuation power spectrum amplitude at galactic scales. The agreement with the power spectrum derived from galaxy distribution and extrapolated to galactic scales, is satisfactory. Meanwhile, the comparison with respect to the standard cold dark matter power spectrum normalized to COBE satellite meassurements reveals only a marginal agreement. Among the most interesting results, the Tully-Fisher relation and the constant central brightness pointed out by Freeman (1970) appear to be the natural consequences of the initial cosmological conditions.

astro-ph↗

arXiv1996

The Building up and Evolution of Galactic Disks

The formation and evolution of disk galaxies in the cosmological context is studied. We consider the observable properties of disk galaxies and treat the disk formation and galactic evolutionary processes in a self-consistent fashion. We find the matter accretion regime to be the dominant ingredient in establishing the Hubble sequence. The accretion regime is a phenomenon directly related to the statistical properties of the primordial density fluctuations from which disk galaxies emerged. In this paper we assumed a Gaussian density field and a standard CDM model.

astro-ph↗