• dust
• high redshift galaxies
• reonization

We have presented a dust extinction and FIR emission model that is aimed at explaining the IRX- relation for high redshift (z > 5) galaxies. Star-forming galaxies in the early universe are particularly interesting since they are the most likely sources of cosmic reionization. As a result of the many challenges in high redshift galaxies observations, well-known z = 0 correlations have been used to estimate dust extinction in typical z > 5 galaxies based on the available UV data. We focused in particular on the relationship between the infrared excess IRX, IRX = L(8-1000 micron)/L(1600 angstrom), and UV slopes beta, F(UV) = lambda^beta.

We focused on the phenomenon observed for the first time by Capak et al. and Willot et al. They studied the IRX- diagram of a diverse sample of 5.1 < z < 5.7 galaxies. More than half of the galaxies show a substantial deficit in IRX at a range of UV colors compared to the samples at z < 3. Interestingly, the wider and more recent ALMA spectroscopic survey in the Hubble ultra deep eld (ASPECS), has confirmed this evidence. Such galaxies are interesting because their location on the IRX- diagram is diffcult to be explained with current models for dust attenuation.

After reviewing the main currently available theories to explain the observed FIR faint galaxies at z > 5, we proceed to propose our own novel model for dust extinction and FIR emission. For dust infrared emission and chemical composition we referred to the WD01 model by Draine & Weingartner. We computed the temperature of grains exposed to the typical range of internal UV eld intensities of z = 6 LBG galaxies. We did so by matching the absorbed and the emitted energy rate. Such calculation allows for attenuation of the UV flux inside starless molecular clouds and account for the CMB effects both for what concerns dust temperature and dust continuum suppression.

We then placed central clusters inside the molecular clouds, using the code Starbust99 to infer the stellar luminosity at different ages. The very first step of our original work was to consider a static model, in which we kept the density distribution of the cloud unchanged with respect to the stellar-free MC model. We evaluated the resulting dust temperatures for various masses and ages of the stellar clouds (in the range 10^3 -10^7 solar masses and 1 - 20 Myr for the ages). We then produced the emission spectrum of dust located either in the diffuse ISM or in the MCs.

We then proceeded to investigate the effect of taking into account the dynamics of the HII regions. In our dynamical model we considered the Stromgren sphere to form instantaneously, and after that the ionization front expands as a dense front until the central homogeneous core of the clouds is exited. Afterwards we considered the ionization front to revert to an rareed front. We evaluated the effect on dust temperature as a function of the distance from the centre of the cloud and derived the cumulative mass fraction above a certain temperature.

In conclusion, we took into account the presence of turbulence inside the molecular clouds. We considered a log-normal distribution for the local density enhancements in the clouds. We varied the Mach number in the range typical range M = 1 - 50. As in the dynamical model, we evaluated the effect on dust temperature as a function of the distance from the centre of the cloud and derived the cumulative mass fraction above a certain temperature. We summarize out main ndings below.

- Dust in the diffuse ISM attains large temperatures while in molecular clouds it becomes very cold (but not colder than the CMB). In the diffuse ISM Td = 45 K for typical size a = 0.1 micron and Td > 60 K for smaller grains. Instead in dense MCs for a = 0.1 micron the temperature is Td < 50 K and reaches Td = 20 K in the inner layers.

- Adding star formation inside the MCs effectively heats the dust grains. Although diffuse ISM dust remains hotter with Tpeak_ISM = 104 K, the star forming MCs have Tpeak_MC = 74 K which is higher than the starless peak temperature Tpeak_MCe = 40K.

- The dynamical implementation is crucial for the larger masses of our MCs sample: Mcl > 10^4 solar masses, since for smaller masses the D-type expansion is short (or absent for Mcl < 10^3 solar masses). The temperature variations depending on the cluster masses and ages are much wider with respect to the static model (for a fixed cluster and cloud mass above Mcl > 10^3 solar masses, the dust temperature decreases 30 K below its initial value).

- For Mach number below M < 50, the effect of turbulence on dust temperature is most important for cloud masses Mcl < 10^4 solar masses. Differently, for M = 50 the dust temperature difference with respect to the static model predictions is always non negligible (e.g. for Mcl = 7 x 10^6 solar masses, the temperature difference reaches 34 K for a portion of the cloud mass around f_M = 10^-1).

In the end, our results point in the direction of dust with a higher temperature with respect to the previous model, as also suggested in other recent works. The FIR-faintness of high redshift galaxies might then be solved by assuming warmer Spectral Energy Distributions (SEDs) for high-z galaxies. There are many possible implementations of our work and future prospectives. A whole galaxy model is beyond the purpose of this Thesis, nevertheless our work represents a rst step on this direction.