Problemas de agencia entre propiedad y control

The power spectrum of the transmission in the Lyman-αforest (the flux power spectrum, FPS), exhibits a suppression on scales smaller thanλmin = 1/kmax ∼30 km s−1. Several physical effects may contribute to this observed cutoff: (i) Doppler broadening resulting from the finite temperatureT0of the IGM, (ii) Jeans smoothing due to the finite pressure of the gas, and (iii) Dark Matter free streaming; these suppress power below scalesλb,λpand λFS, respectively. We have shown in Section5.7that, whenλis expressed in velocity units, λb andλp are independent of redshiftzfor a given value ofT0, whereasλDM ∝(1 +z)1/2. This means that any smoothing of the density field due to WDM free-streaming will be most easily observable at high-redshift, and the observed FPS may provide constraints on the nature of the Dark Matter [150,235–237], and possible be a ‘WDM smoking gun’.

In our work we tried to answer two questions:

• Does the observed cutoff in the FPS favour Cold or Warm Dark Matter, or can both models provide acceptable fits to the existing data?

• Are the WDM models with largeλFS that were previously excluded allowed if one considers a less restrictive thermal history?

To answer these questions we run a set of cosmological hydrodynamical simulations at very high resolution, varyingλb,λpandλFS independently. We then compute mock spectra that mimic observational limitations (noise, finite spectral resolution and finite sample size), and compare the mock FPS to the observed FPS.

We demonstrate that all three effects (i.e. Doppler broadening, Jeans smoothing and DM free-streaming) yield a cutoff in the FPS that resembles the observed cutoff. Of course in reality all three effects will contribute at some level. In particular, Doppler broadening and Jeans smoothing both depend on the temperatureT0of the IGM, and so always work together.

To answer the two questions posed above, we have tried to fit the observed FPS at redshifts z = 5 and 5.4 with (i) a CDM model (which has λDM = 0), varying T0 and the thermal history, and (ii) the particular case of a resonantly produced sterile neutrino WDM model (characterised by the mass of the particle,mDMc2 = 7keV, and the Lepton asymmetry parameterL6, [153]), varyingL6,T0 and the thermal history.

In addition to motivations based on particle physics (seee.g.[54]) our particular choice of WDM particle is motivated by the fact that: (i) its decay may have been observed as a 3.5 keV X-ray line in galaxies and clusters of galaxies [55–57], (ii) it produces galactic (sub)structures compatible with observations [136,137], and (iii) it is apparently ruled out by the observed FPS [225].

Fig.5.22shows how theHIRESdata is compatible with CDM and SN cosmologies if we choose relatively late reionisation model (LateRof [253]) so that λp is small and T0 ≈(7−8)·103K as predicted by this model. Both the assumed late reionisation redshift,

and the relatively low value of T0, are reasonable and consistent with expectations and previous work. Crucially, a WDM model with L6 = 12 and the same late redshift of reionisation also provides an acceptable fit to the data, providedT0 67000K. With such a low value ofλb andλp, the FPS cutoff is mostly due to WDM free streaming.

From this comparison we conclude that the observed suppression in the FPS can be explained by thermal effects in CDM model but also by the free-streaming in a WDM model: current data do not strongly favour either possibility. We also find a reasonable fit for a WDM model that was previously ruled out by [150] and [235–237]. Our present analysis differs in a number of ways:

1. We vary the thermal history of the IGM within the allowed observational limits as discussed by [210,253]. The previous works modeled the UVB according to [211]. The latter scenario is known to reionise the Universe too early with respect to current observations [210], plausibly overestimatingλp.

2. We did not use any assumptions about the evolutionT0(z)but inferred ranges ofT0 atz = 5.0andz = 5.4based on theoretical considerations and limits inferred from the Lyman-αdata (see also [234]).

We also reconsidered the impact of peculiar velocities (‘redshift space distortions’), which were claimed to affect the appearance of a cutoff at the smallest scales [246], but found these not to be important at the much higher resolution of our simulations.

We also demonstrated that spatial fluctuations in temperature, which are expected to be present close to reionisation, may dramatically affect the FPS. Spatial variations inT0 can dramatically increase the amplitude of the FPS at the scale of the imposed fluctuations, effectively decoupling the large-scale and small-scale FPS. Unfortunately, this means that a model without fluctuations inT0 will yield incorrect constraints on parameters if such fluctuationsarepresent in the data. Interestingly, the nuisance caused by fluctuations inT0 may actually be rather helpful if the cutoff in the FPS is in fact due to WDM, since in that case there would be no spatial fluctuations in the location of the cutoff – and the evolution with redshift of the cutoff would followλDM ∝(1 +z)1/2.

Moving away from Lyman-α and studying the small-scale Universe in the H I 21- cm line during the ‘Dark Ages’ [265] instead is currently almost science fiction, but ultimately may be the most convincing way of determining once and for all whether most of the Dark Matter in the Universe is warm or cold.

In the meantime, we have produced a new robust constraint on WDM from the most recent QSO spectra available from HIRES at high redshift. We have assumed a conservative thermal history for the IGM that is still consistent with reionization, and that gives the minimal size of the intergalactic structures. We have assumed that the intergalactic medium is optically thin, and that reionization happens uniformly in all the space. We aim to discuss

a more realistic reionization scenario in an upcoming publication. We have neglected the effect of undetected metals that may increase the power on the smallest scales, implying a weaker cutoff.

Appendix