Primordial gas heating by dark matter and structure formation

Dark matter (DM) decays and annihilations might heat and partially reionize the Universe at high redshift. Although this effect is not important for the cosmic reionization, the gas heating due to DM particles might affect the structure formation. In particular, the critical halo mass for collapse is increased up to a factor of ~2. Also the fraction of gas which collapses inside the smallest halos is substantially reduced with respect to the cosmological value. These effects imply that DM decays and annihilations might delay the formation of the first structures and reduce the total star mass in the smallest halos.Comment: 4 pages, 4 figures, to appear in the "Memorie della SAIt", proceedings of the "LI congresso della Societa' Astronomica Italiana", Firenze, April 17-20 200

Broad [OIII] in the globular cluster RZ 2109: X-ray ionized nova ejecta?

We study the possibility that the very broad (~1500 km/s) and luminous (L_5007 ~ 1.4e37 erg/s) [OIII] line emission observed in the globular cluster RZ 2109 might be explained with the photoionization of nova ejecta by the bright (L_X ~ 4e39 erg/s) X-ray source hosted in the same globular cluster. We find that such scenario is plausible and explains most of the features of the RZ 2109 spectrum (line luminosity, absence of H emission lines, peculiar asymmetry of the line profile); on the other hand, it requires the nova ejecta to be relatively massive (>~ 0.5e-3 Msun}), and the nova to be located at a distance <~ 0.1 pc from the X-ray source. We also predict the time evolution of the RZ 2109 line emission, so that future observations can be used to test this scenario.Comment: 11 pages, 2 figures, 6 tables; accepted for publication on MNRA

Constraining DM through 21 cm observations

Beyond reionization epoch cosmic hydrogen is neutral and can be directly observed through its 21 cm line signal. If dark matter (DM) decays or annihilates the corresponding energy input affects the hydrogen kinetic temperature and ionized fraction, and contributes to the Ly_alpha background. The changes induced by these processes on the 21 cm signal can then be used to constrain the proposed DM candidates, among which we select the three most popular ones: (i) 25-keV decaying sterile neutrinos, (ii) 10-MeV decaying light dark matter (LDM) and (iii) 10-MeV annihilating LDM. Although we find that the DM effects are considerably smaller than found by previous studies (due to a more physical description of the energy transfer from DM to the gas), we conclude that combined observations of the 21 cm background and of its gradient should be able to put constrains at least on LDM candidates. In fact, LDM decays (annihilations) induce differential brightness temperature variations with respect to the non decaying/annihilating DM case up to Delta_delta T_b=8 (22) mK at about 50 (15) MHz. In principle this signal could be detected both by current single dish radio telescopes and future facilities as LOFAR; however, this assumes that ionospheric, interference and foreground issues can be properly taken care of.Comment: 9 pages, submitted to MNRA

Ultra-luminous X-ray sources and remnants of massive metal-poor stars

Massive metal-poor stars might form massive stellar black holes (BHs), with mass 25<=mBH/Msun<=80, via direct collapse. We derive the number of massive BHs (NBH) that are expected to form per galaxy through this mechanism. Such massive BHs might power most of the observed ultra-luminous X-ray sources (ULXs). We select a sample of 64 galaxies with X-ray coverage, measurements of the star formation rate (SFR) and of the metallicity. We find that NBH correlates with the number of observed ULXs per galaxy (NULX) in this sample. We discuss the dependence of our model on the SFR and on the metallicity. The SFR is found to be crucial, consistently with previous studies. The metallicity plays a role in our model, since a lower metallicity enhances the formation of massive BHs. Consistently with our model, the data indicate that there might be an anticorrelation between NULX, normalized to the SFR, and the metallicity. A larger and more homogeneous sample of metallicity measurements is required, in order to confirm our results.Comment: 21 pages, 8 figures, accepted for publication in MNRA

Impact of dark matter on reionization and heating

We derived the evolution of the energy deposition in the intergalactic medium (IGM) by different decaying (or annihilating) dark matter (DM) candidates. Heavy annihilating DM particles (with mass larger than a few GeV) have no influence on reionization and heating, even if we assume that all the energy emitted by annihilations is absorbed by the IGM. In the case of lighter particles, the impact on reionization and heating depends on the efficiency of energy absorption by the IGM. We calculated the fraction of energy produced by decays and annihilations which is effectively absorbed by the IGM. We found that this fraction is generally high at very high redshift (>>100), but drops at more recent epochs.Comment: 3 pages, 2 figures, to appear in the proceedings of the 11th Marcel Grossmann Meeting held in Berlin, Germany, July 23-29 2006 (parallel session on Dark Matter

The effect of early dark matter halos on reionization

The annihilation of dark matter particles releases energy, ionizing some of the gas in the Universe. We investigate the effect of dark matter halos on reionization. We show that the effect depends on the assumed density profile, the particle mass, and the assumed minimum halo mass. For NFW halos and typical WIMPs, we find the effect to be quite small. However, light dark matter candidates in the MeV range can contribute significantly to reionization and can make an important contribution to the measured optical depth. This effect may be used to constrain light dark matter models. We also study the effect of varying the halo density profile on reionization.Comment: Minor changes from v2. Accepted for publication in Phys. Rev.

Discovery of a 6.4 h black hole binary in NGC 4490

We report on the discovery with Chandra of a strong modulation (~90% pulsed fraction) at ~6.4 h from the source CXOU J123030.3+413853 in the star-forming, low-metallicity spiral galaxy NGC 4490, which is interacting with the irregular companion NGC 4485. This modulation, confirmed also by XMM-Newton observations, is interpreted as the orbital period of a binary system. The spectra from the Chandra and XMM-Newton observations can be described by a power-law model with photon index ~1.5. During these observations, which span from 2000 November to 2008 May, the source showed a long-term luminosity variability by a factor of ~5, between ~2E+38 and 1.1E+39 erg/s (for a distance of 8 Mpc). The maximum X-ray luminosity, exceeding by far the Eddington limit of a neutron star, indicates that the accretor is a black hole. Given the high X-ray luminosity, the short orbital period and the morphology of the orbital light curve, we favour an interpretation of CXOU J123030.3+413853 as a rare high-mass X-ray binary system with a Wolf-Rayet star as a donor, similar to Cyg X-3. This would be the fourth system of this kind known in the local Universe. CXOU J123030.3+413853 can also be considered as a transitional object between high mass X-ray binaries and ultraluminous X-ray sources (ULXs), the study of which may reveal how the properties of persistent black-hole binaries evolve entering the ULX regime.Comment: Fig. 1 in reduced quality; minor changes to match the MNRAS versio

Radiation from early black holes - I. Effects on the neutral intergalactic medium

In the pre-reionization Universe, the regions of the intergalactic medium (IGM) which are far from luminous sources are the last to undergo reionization. Until then, they should be scarcely affected by stellar radiation; instead, the X-ray emission from an early black hole (BH) population can have much larger influence. We investigate the effects of such emission, looking at a number of BH model populations (differing for the cosmological density evolution of BHs, the BH properties, and the spectral energy distribution of the BH emission). We find that BH radiation can easily heat the IGM to 103-104K, while achieving partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal (δTb∼ 20-30 mK at z≲ 12) which should be observable with forthcoming experiments (e.g. LOFAR). We also find that at z≲ 10 BH emission strongly increases the critical mass separating star-forming and non-star-forming haloe

Dispersal and population structure at different spatial scales in the subterranean rodent Ctenomys australis

This study was funded by grants from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, PIP5838), Agencia de Promoción Científica y Tecnológica de la Argentina (PICTO1-423, BID-1728/OC-AR), and the programme ECOS-Sud France/Argentina (A05B01).Background: The population genetic structure of subterranean rodent species is strongly affected by demographic (e.g. rates of dispersal and social structure) and stochastic factors (e.g. random genetic drift among subpopulations and habitat fragmentation). In particular, gene flow estimates at different spatial scales are essential to understand genetic differentiation among populations of a species living in a highly fragmented landscape. Ctenomys australis (the sand dune tuco-tuco) is a territorial subterranean rodent that inhabits a relatively secure, permanently sealed burrow system, occurring in sand dune habitats on the coastal landscape in the south-east of Buenos Aires province, Argentina. Currently, this habitat is threatened by urban development and forestry and, therefore, the survival of this endemic species is at risk. Here, we assess population genetic structure and patterns of dispersal among individuals of this species at different spatial scales using 8 polymorphic microsatellite loci. Furthermore, we evaluate the relative importance of sex and habitat configuration in modulating the dispersal patterns at these geographical scales. Results: Our results show that dispersal in C. australis is not restricted at regional spatial scales (similar to 4 km). Assignment tests revealed significant population substructure within the study area, providing support for the presence of two subpopulations from three original sampling sites. Finally, male-biased dispersal was found in the Western side of our study area, but in the Eastern side no apparent philopatric pattern was found, suggesting that in a more continuous habitat males might move longer distances than females. Conclusions: Overall, the assignment-based approaches were able to detect population substructure at fine geographical scales. Additionally, the maintenance of a significant genetic structure at regional (similar to 4 km) and small (less than 1 km) spatial scales despite apparently moderate to high levels of gene flow between local sampling sites could not be explained simply by the linear distance among them. On the whole, our results support the hypothesis that males disperse more frequently than females; however they do not provide support for strict philopatry within females.Publisher PDFPeer reviewe