arXiv subjects

E. Ripamonti

Publications and source records attributed to E. Ripamonti.

27 records · Page 2Linked to original sources

arXiv2007

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.

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arXiv2007

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.

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arXiv2007

Impact of dark matter decays and annihilations on structure formation

We derived the influence of dark matter (DM) decays and annihilations on structure formation. The energy deposited by DM decays and annihilations into metal free halos both increases the gas temperature and enhances the formation of molecules. Within the primordial halos the temperature increase generally dominates over the molecular cooling, slightly delaying the collapse. In fact, the critical mass for collapse is generally higher than in the unperturbed case, when we consider the energy input from DM. In presence of DM decays and/or annihilations the fraction of baryons inside collapsed metal free halos should be slightly less (~0.4) than the expected cosmological value.

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arXiv2007

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.

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arXiv2007

The role of HD cooling in primordial star formation

The role of HD cooling in the formation of primordial objects is examined by means of a great number of 1-D models of the collapse of halos, exploring a wide range of masses and virialization redshifts. We find that HD has very little effect upon the critical mass separating the objects which are likely to form stars from those which are not. We also find that, once the proto-stellar collapse has started, HD effects are quite negligible. Instead, HD effects can be important during the intermediate stage of gas fragmentation: objects below a certain mass scale (~3x10^5 Msun at z_vir=20 in our ``fiducial'' case) can be cooled by HD down to T~50-100 K, whereas H2 cooling never takes the gas below T~200 K. The lower temperature implies a reduction of a factor ~10 in the Jeans mass of the fragmenting gas, and stars forming in such low-mass halos are probably less massive than their counterparts in larger halos. We estimate the importance of this mode of star formation through a variation of the Press-Schechter formalism, and find that it never exceeds the contribution of halos which are cooled by H2 only. Halos where HD is important account at best for a fraction ~0.25 of the total primordial star formation. However, HD cooling might provide a channel through which long-lived low mass stars could be formed in primordial conditions.

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arXiv2006

Intergalactic medium heating by dark matter

We derive the evolution of the energy deposition in the intergalactic medium (IGM) by dark matter (DM) decays/annihilations for both sterile neutrinos and light dark matter (LDM) particles. At z > 200 sterile neutrinos transfer a fraction f_abs~0.5 of their rest mass energy into the IGM; at lower redshifts this fraction becomes <~ 0.3 depending on the particle mass. The LDM particles can decay or annihilate. In both cases f_abs~0.4-0.9 at high (> 300) redshift, dropping to ~0.1 below z=100. These results indicate that the impact of DM decays/annihilations on the IGM thermal and ionization history is less important than previously thought. We find that sterile neutrinos (LDM) decays are able to increase the IGM temperature by z=5 at most up to 4K (100K), about 50-200 times less than predicted by estimates based on the assumption of complete energy transfer to the gas.

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arXiv2006

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

We derive the effects of 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 (H2 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 m_crit is small. In the most extreme case, i.e. considering light DM annihilations (decays) and halos virializing at redshift z_vir>10 (z_vir~10), m_crit increases by a factor ~4 (~2). In the case of annihilations the variations of m_crit 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 is substantially reduced (to ~40 per cent of the cosmic value), especially in the smallest halos, as a consequence of the energy injection by DM decays and annihilations.

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arXiv2006

Numerical simulations of the metallicity distribution in dwarf spheroidal galaxies

Recent observations show that the number of stars with very low metallicities in the dwarf spheroidal satellites of the Milky Way is low, despite the low average metallicities of stars in these systems. We undertake numerical simulations of star formation and metal enrichment of dwarf galaxies in order to verify whether this result can be reproduced with "standard" assumptions. The answer is likely to be negative, unless some selection bias against very low metallicity stars is present in the observations.

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arXiv2006

A metal rich molecular cloud surrounds GRB 050904 at redshift 6.3

GRB050904 is the gamma-ray burst with the highest measured redshift. We performed time resolved X-ray spectroscopy of the late GRB and early afterglow emission. We find robust evidence for a decrease with time of the soft X-ray absorbing column. We model the evolution of the column density due to the flash ionization of the GRB and early afterglow photons. This allows us to constrain the metallicity and geometry of the absorbing cloud. We conclude that the progenitor of GRB050904 was a massive star embedded in a dense metal enriched molecular cloud with Z~0.03 Z_solar. This is the first local measurement of metallicity in the close environment of a GRB and one of the highest redshift metallicity measurements. We also find that the dust associated with the cloud cannot be similar to that of our Galaxy but must be either sizably depleted or dominated by silicate grains. We discuss the implications of these results for GRB progenitors and high redshift star formation.

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