The Dark Galaxy Hypothesis Forthcoming , Philosophy of Science

Arxiv preprint arXiv:1102.1184, 2011

This Review Talk concerns, in a detailed way, the mass discrepancy phenomenon detected in galaxies that usually we account by postulating the presence of a non luminous non baryonic component. In the theoretical framework of Newtonian Gravity and Dark Matter Halos, we start by recalling the properties of the latter, as emerging from the state-of-the-art of the numerical simulations performed in the current ΛCDM scenario of cosmological structure formation and evolution. We then report the simple, but much-telling, phenomenology of the distribution of dark and luminous matter in Spirals, Ellipticals, and dwarf Spheroidals. It will be shown that a coherent observational framework emerges from reliable data of different large samples of objects. The findings come after applying different methods of investigation to different tracers of the gravitational field. They include RCs and dispersion velocities profiles fitting, X-ray emitting gas properties analysis, weak and strong lens signal mass decompositions, analysis of halo and baryonic mass functions. We will then highlight the impressive evidence that the distribution of dark and luminous matter are closely correlated and that have universal characteristics. Hints on how this phenomenological scenario of the mass distribution in galaxies, including the Milky Way and the nearby ones, has a cosmological role, are given. Finally, we discuss the constraints on the elusive nature of the dark matter particle that observations pose to its direct and indirect searches.

The distribution of the non-luminous matter in galaxies of different luminosity and Hubble type is much more than a proof of the existence of dark particles governing the structures of the Universe. Here, we will review the complex but well-ordered scenario of the properties of the dark halos also in relation with those of the baryonic components they host. Moreover, we will present a number of tight and unexpected correlations between selected properties of the dark and the luminous matter. Such entanglement evolves across the varying properties of the luminous component and it seems to unequivocally lead to a dark particle able to interact with the Standard Model particles over cosmological times. This review will also focus on whether we need a paradigm shift, from pure collisionless dark particles emerging from "first principles", to particles that we can discover only by looking to how they have designed the structure of the galaxies.

2016

This Review Talk concerns, in a detailed way, the mass discrepancy phenomenon detected in galaxies that usually we account by postulating the presence of a non luminous non baryonic component. In the theoretical framework of Newtonian Gravity and Dark Matter Halos, we start by recalling the properties of the latter, as emerging from the state-of-the-art of the numerical simulations performed in the current ΛCDM scenario of cosmological structure formation and evolution. We then report the simple, but much-telling, phenomenology of the distribution of dark and luminous matter in Spirals, Ellipticals, and dwarf Spheroidals. It will be shown that a coherent observational framework emerges from reliable data of different large samples of objects. The findings come after applying different methods of investigation to different tracers of the gravitational field. They include RCs and dispersion velocities profiles fitting, X-ray emitting gas properties analysis, weak and strong lens signal mass decompositions, analysis of halo and baryonic mass functions. We will then highlight the impressive evidence that the distribution of dark and luminous matter are closely correlated and that have universal characteristics. Hints on how this phenomenological scenario of the mass distribution in galaxies, including the Milky Way and the nearby ones, has a cosmological role, are given. Finally, we discuss the constraints on the elusive nature of the dark matter particle that observations pose to its direct and indirect searches.

In this universe, not all of the matter around us can be readily seen. The further an object is away from us and the less luminous it is, the less visible it becomes. Just by looking at an object is usually difficult, if not impossible, to tell the amount of mass it contains. But astronomers have been using the measured luminosity to estimate the luminous mass of stars, based on empirically established mass-to-light ratio which seems to be only applicable to a special class of stars---the main-sequence stars---with still considerable uncertainties. Another basic tool for astronomers to determine the mass of a system of stars or galaxies comes from the study of their motion, as Newton demonstrated with his law of gravitation, which yields the gravitational mass. Because the luminous mass can at best only represent a portion of the gravitational mass, finding the luminous mass to be different or less than the gravitational mass should not be surprising. Using such an apparent discrepancy as compelling evidence for the so called dark matter, which has been believed to possess mysterious nonbaryonic properties having a dominant amount in galaxies and the universe, seems to be too far a stretch when seriously examining the facts and uncertainties in the measurement techniques. In our opinion, a galaxy with star type distribution varying from its center to edge may have a mass-to-light ratio varying accordingly. With the thin-disk model computations based on measured rotation curves, we found that most galaxies have a typical mass density profile that peaks at the galactic center and decreases rapidly within ~ 5% of the cut-off radius and then declines nearly exponentially toward the edge. The predicted mass density in the Galactic disk is reasonably within the reported range of that observed in interstellar medium. This leads us to believe that ordinary baryonic matter can be sufficient for supporting the observed galactic rotation curves; speculation of large amount of non-baryonic matter may be based on an ill-conceived discrepancy between gravitational mass and luminous mass which appears to be unjustified.

Foundations of Physics, 2018

The evidence of the phenomenon for which, in galaxies, the gravitating mass is distributed differently than the luminous mass, increases as new data become available. Furthermore, this discrepancy is well structured and it depends on the magnitude and the compactness of the galaxy and on the radius, in units of its luminous size R opt , where the measure is performed. For the disk systems with −13 ≥ M I ≥ −24 all this leads to an amazing scenario, revealed by the investigation of individual and coadded rotation curves, according to which, the circular velocity follows, from their centers out to their virial radii, an universal profile V U RC (r/R opt , M I) function only of the properties of the luminous mass component. Moreover, from the Universal Rotation Curve, so as from many individual high quality RCs, we discover that, in the innermost regions of galaxies, the DM halo density profiles are very shallow. Finally, the disk mass, the central halo density and its core radius, come out all related to each other and to two properties of the distribution of light in galaxies: the luminosity and the compactness. This phenomenology, being absent in the simplest ΛCDM Cosmology scenario, poses serious challenges to the latter or, alternatively, it requires a substantial and tuned involvement of baryons in the formation of the galactic halos. On the other side, the URC helps to explain the two-accelerations relationship found by McGaugh et al 2016, in terms of only well known astrophysical processes, acting in a standard DM halos + luminous disks scenario.

Recent observations have revealed the structural properties of the dark and luminous mass distribution in spirals. These results led to the vision of a new and amazing scenario. The investigation of single and coadded objects has shown that the rotation curves of spirals follow, from their centers out to their virial radii, an universal profile that implies a tuned combination of their stellar disk and dark halo mass distributions. This, alongside with accurate mass modeling of individual galaxies, poses important challenges to the presently theoretically favored ΛCDM Cosmology.

arXiv (Cornell University), 1997

We present evidence that all galaxies, of any Hubble type and luminosity, bear the kinematical signature of a mass component distributed differently from the luminous matter. We review and/or derive the DM halo properties of galaxies of different morphologies: spirals, LSBs, ellipticals, dwarf irregulars and dwarf spheroidals. We show that the halo density profile LSBs

Journal of Cosmology and Astroparticle Physics, 2016

The failure to find evidence for elementary particles that could serve as the constituents of dark matter brings to mind suggestions that dark matter might consist of massive compact objects (MACHOs). In particular, it has recently been argued that MACHOs with masses > 15M may have been prolifically produced at the onset of the big bang. Although a variety of astrophysical signatures for primordial MACHOs with masses in this range have been discussed in the literature, we favor a strategy that uses the potential for magnification of stars outside our galaxy due to gravitational micro-lensing of these stars by MACHOs in the halo of our galaxy. We point out that the effect of the motion of the Earth on the shape of the micro-lensing brightening curves provides a promising approach to testing over the course of next several years the hypothesis that dark matter consists of massive compact objects.

Science, 2001

The Milky Way galaxy contains a large, spherical component which is believed to harbor a substantial amount of unseen matter. Recent observations indirectly suggest that as much as half of this ``dark matter'' may be in the form of old, very cool white dwarfs, the remnants of an ancient population of stars as old as the galaxy itself. We conducted a survey to find faint, cool white dwarfs with large space velocities, indicative of their membership in the galaxy's spherical halo component. The survey reveals a substantial, directly observed population of old white dwarfs, too faint to be seen in previous surveys. This newly discovered population accounts for at least 2 percent of the halo dark matter. It provides a natural explanation for the indirect observations, and represents a direct detection of galactic halo dark matter.

Physical Review D, 2008

We study the effects of substructure in the Galactic halo on direct detection of dark matter, on searches for energetic neutrinos from WIMP annihilation in the Sun and Earth, and on the enhancement in the WIMP annihilation rate in the halo. Our central result is a probability distribution function (PDF) P (ρ) for the local dark-matter density. This distribution must be taken into account when using null dark-matter searches to constrain the properties of dark-matter candidates. We take two approaches to calculating the PDF. The first is an analytic model that capitalizes on the scale-invariant nature of the structure-formation hierarchy in order to address early stages in the hierarchy (very small scales; high densities). Our second approach uses simulation-inspired results to describe the PDF that arises from lower-density larger-scale substructures which formed in more recent stages in the merger hierarchy. The distributions are skew positive, and they peak at densities lower than the mean density. The local dark-matter density may be as small as 1/10th the canonical value of ≃ 0.4 GeV cm −3 , but it is probably no less than 0.2 GeV cm −3 .