• ALMA
• high redshift galaxies
• molecular hydrogen

The study of high redshift (z) galaxies is now entering a golden era, owing both to highly sensitive instruments (i.e. the Hubble Space Telescope, Subaru and Keck telescopes) and refined selection techniques (e.g. the narrow band and the lyman break dropout selection techniques, to name a few).
In this work, we focus our attention on a specific class of high-z galaxies, the Lyman Alpha Emitters (LAEs). LAEs are detected by means of their Ly-alpha emission line (1216 angstrom in the galaxy rest frame) which can be produced by stellar sources, by the cooling of collisionally excited neutral hydrogen in the interstellar medium (ISM), by the accretion of material by
active galactic nuclei (AGN), or by the accretion of cold gas onto dark matter potential wells. LAEs have rapidly been gaining popularity as a probe of early galaxy evolution since the strength, width and continuum break at wavelengths shorter than the Ly-alpha make their detection largely unambiguous.
The aim of this work is to obtain a theoretical estimate of the molecular content of simulated LAEs at z = 5.7, 6.6. Given the paucity of information about the ISM of these sources, we use a simple analytical model to describe the structure of the molecular clouds (MCs) and calculate the molecular hydrogen (H2) content taking into account its formation on dust grains, and its destruction by ultraviolet (UV) photons. Such a study is of utmost importance to understand star formation in these early galaxies, since the MCs are the birthplaces of stars.
The physical model built to calculate the H2 content of these galaxies is now briefly summarized: we assume a disk-like distribution of the gas which extends upto the gas radius, while the MCs are assumed to be distributed within a radius equal to that of the stellar distribution. The main features of the dust, namely the physical size and the temperature are treated properly, since these affect both the absorption properties of the dust and the rate of H2 formation on the grains. We conclude that the LAE molecular fraction (fH2) anticorrelates with UV field intensity and correlates with gas density;
in addition, sources with larger fH2 have larger dust mass and star formation rates (SFR).
Once the molecular fraction is calculated, it can be translated into the mass of H2 contained in each LAE; this puts constraints on the H2 detectability of LAEs with the Atacama Large Millimiter/submillimiter Array (ALMA) that will begin the Early Science observations in 2011.
Molecular hydrogen is in fact traced by the emission lines produced by rotational transitions of the carbon monoxide (CO), whose detection is one of the main goals of ALMA.
Using the conversion factor between the mass of molecular hydrogen and the CO luminosity, we obtain the rotational lines luminosity for each of the simulated LAEs; we also show a comparison between these results and the few CO observations of high-z LAEs available as of now getting a good agreement between theoretical outcomes and experimental data.
We finally calculate the intensity of the CO high-J (6-5) rotational transitions that get redshifted within the millimeter band of the ALMA receiver; the calculation of the observing time required to detect such lines was carried out using the Sensitivity Calculator provided by ALMA staff. We achieve a minimum value of integration time of order of few hours.