Michela Mapelli

Impact of dark matter decays and annihilations on reionization

One of the possible methods to distinguish among various dark matter (DM) candidates is to study the effects of DM decays. We consider four different DM candidates [light dark matter (LDM), gravitinos, neutralinos and sterile neutrinos], for each of them deriving the decaying/annihilation rate, the influence on reionization, matter temperature and cosmic microwave background (CMB) spectra. We find that LDM particles (1–10 MeV) and sterile neutrinos (2–8 keV) can be sources of partial early reionization (z ~< 100). However, their integrated contribution to Thomson optical depth is small (~< 0.01) with respect to the 3-yr WMAP results (τe = 0.09 ± 0.03). Finally, they can significantly affect the behaviour of matter temperature. On the contrary, effects of heavy DM candidates (gravitinos and neutralinos) on reionization and heating are minimal. All the considered DM particles have completely negligible effects on the CMB spectra.

Dynamical age differences among coeval star clusters as revealed by blue stragglers

Globular star clusters that formed at the same cosmic time may have evolved rather differently from the dynamical point of view (because that evolution depends on the internal environment) through a variety of processes that tend progressively to segregate stars more massive than the average towards the cluster centre. Therefore clusters with the same chronological age may have reached quite different stages of their dynamical history (that is, they may have different ‘dynamical ages’). Blue straggler stars have masses greater than those at the turn-off point on the main sequence and therefore must be the result of either a collision or a mass-transfer event. Because they are among the most massive and luminous objects in old clusters, they can be used as test particles with which to probe dynamical evolution. Here we report that globular clusters can be grouped into a few distinct families on the basis of the radial distribution of blue stragglers. This grouping corresponds well to an effective ranking of the dynamical stage reached by stellar systems, thereby permitting a direct measure of the cluster dynamical age purely from observed properties.

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 ≤ m BH /M ⊙ ≤ 80, via direct collapse. We derive the number of massive BHs (N BH ) 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 N BH correlates with the number of observed ULXs per galaxy (N ULX ) 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 N ULX , normalized to the SFR, and the metallicity. A larger and more homogeneous sample of metallicity measurements is required, in order to confirm our results.

The radial distribution of blue straggler stars and the nature of their progenitors

The origin of blue straggler stars (BSS) in globular clusters (GCs) is still not fully understood: they can form from stellar collisions, or through mass transfer in isolated, primordial binaries (PBs). In this paper we use the radial distribution of BSS observed in four GCs (M3, 47 Tuc, NGC 6752 and ω Cen) to investigate which formation process prevails. We find that both channels co-exist in all the considered GCs. The fraction of mass-transfer (collisional) BSS with respect to the total number of BSS is around ∼0.4‐0.5 (∼0.5‐0.6) in M3, 47 Tuc and NGC 6752. The case of ω Cen is peculiar with an underproduction of collisional BSS. The relative lack of collisional BSS in ω Cen can be understood if mass segregation has not yet driven to the core a sizeable number of PBs, which dominate stellar collisions through threeand four-body processes. The spatial distribution of BSS provides strong hints to their origin: the BSS in the cluster outskirts form almost exclusively from mass transfer in PBs, whereas the BSS found close to the cluster core most likely have a collisional origin.

The Contribution of Primordial Binaries to the Blue Straggler Population in 47 Tucanae

The recent observation (Ferraro et al. 2003b) of the blue str aggler population in 47 Tucanae gives the first detailed characterization of their spatial distribution i n the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance and a rise bey ond twenty core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multi-mass King model for 47 Tucanae. We find that the observed spatial distribution can not be expl ained within a purely collisional scenario in which blue stragglers are generated exclusively in the core throu gh direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generate d in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass-trans fer. Subject headings: stars: blue stragglers - binaries: general - globular clust ers: individual (47 Tuc)

The impact of dark matter decays and annihilations on the formation of the first structures

We derive the effects of dark matter (DM) decays and annihilations on structure formation. We consider moderately massive DM particles ( sterile neutrinos and light DM), as they are expected to give the maximum contribution to heating and reionization. The energy injection from DM decays and annihilations produces both an enhancement in the abundance of coolants (H-2 and HD) and an increase of gas temperature. We find that for all the considered DM models the critical halo mass for collapse, m(crit), is generally higher than in the unperturbed case. However, the variation of mcrit is small. In the most extreme cases, that is, considering light DM annihilations (decays) and haloes virializing at redshift z(vir) > 30 (z(vir) similar to 10), m(crit) increases by a factor of similar to 4 (similar to 2). In the case of annihilations the variations of mcrit are also sensitive to the assumed profile of the DM halo. Furthermore, we note that the fraction of gas which is retained inside the halo can be substantially reduced ( to approximate to 40 per cent of the cosmic value), especially in the smallest haloes, as a consequence of the energy injection by DM decays and annihilations.

A Panchromatic Study of the Globular Cluster NGC 1904. I. The Blue Straggler Population

By combining high-resolution (HST/WFPC2) and wide-field ground-based (2.2 m ESO/WFI) and space (GALEX) observations, we have collected a multiwavelength photometric database (ranging from the far-UV to the near infrared) of the galactic globular cluster NGC 1904 (M79). The sample covers the entire cluster extension, from the very central regions up to the tidal radius. In the present paper, such a data set is used to study the BSS population and its radial distribution. A total number of 39 bright (m218 ≤ 19.5) BSSs have been detected, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of NGC 1904, in contrast to other clusters (M3, 47 Tuc, NGC 6752, M5) studied with the same technique. Such evidence, coupled with the large radius of avoidance estimated for NGC 1904 (ravoid ~ 30 core radii), indicates that the vast majority of the cluster heavy stars (binaries) has already sunk to the core. Accordingly, extensive dynamical simulations suggest that BSSs formed by mass transfer activity in primordial binaries evolving in isolation in the cluster outskirts represent only a negligible (0%-10%) fraction of the overall population.

Dynamics of stellar black holes in young star clusters with different metallicities – II. Black hole–black hole binaries

In this paper, we study the formation and dynamical evolution of black hole-black hole (BH-BH) binaries in young star clusters (YSCs), by means of N-body simulations. The simulations include metallicity-dependent recipes for stellar evolution and stellar winds, and have been run for three different metallicities (Z = 0.01, 0.1 and 1 Zsun). Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. We find that BH-BH binaries form efficiently because of dynamical exchanges: in our simulations, we find about 10 times more BH-BH binaries than double neutron star binaries. The simulated BH-BH binaries form earlier in metal-poor YSCs, which host more massive black holes (BHs) than in metal-rich YSCs. The simulated BH-BH binaries have very large chirp masses (up to 80 Msun), because the BH mass is assumed to depend on metallicity, and because BHs can grow in mass due to the merger with stars. The simulated BH-BH binaries span a wide range of orbital periods (10^-3-10^7 yr), and only a small fraction of them (0.3 per cent) is expected to merge within a Hubble time. We discuss the estimated merger rate from our simulations and the implications for Advanced VIRGO and LIGO.

Dynamics of stellar black holes in young star clusters with different metallicities - I. Implications for X-ray binaries

We present N body simulations of intermediate-mass (3000 4000 M⊙) young star clusters (SCs) with three different metallicities (Z = 0.01, 0.1 and 1 Z⊙), including metal-dependent stellar evolution recipes and binary evolution. Following recent theoretical models of wind mass loss and core collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. In particular, massive metal-poor stars (Z � 0.3 Z⊙) are enabled to form massive stellar black holes (MSBHs, with mass � 25 M⊙) through direct collapse. We find that three-body encounters, and especially dynamical exchanges, dominate the evolution of the MSBHs formed in our simulations. In SCs with Z = 0.01 and 0.1 Z⊙, about 75 per cent of simulated MSBHs form from single stars and become members of binaries through dynamical exchanges in the first 100 Myr of the SC life. This is a factor of > 3 more efficient than in the case of low-mass ( 25 M⊙) stellar black holes. A small but non-negligible fraction of MSBHs power wind-accreting (10 20 per cent) and Roche lobe overflow (RLO, 5 10 per cent) binary systems. The vast majority of MSBH binaries that undergo wind accretion and/or RLO were born from dynamical exchange. This result indicates that MSBHs can power X-ray binaries in low-metallicity young SCs, and is very promising to explain the association of many ultraluminous X-ray sources with low-metallicity and actively star forming environments.

Constraining dark matter 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 corre- sponding energy input affects the hydrogen kinetic temperature and ionized fraction, and contributes to the Lybackground. 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 (an- nihilations) induce differential brightness temperature variations with respect to the non decaying/annihilating DM case up to ��Tb = 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.