Simulation of galaxy evolution for ALMA

There are billions of galaxies in the Universe, and they are not uniformly distributed. Rather they are "clustered"; that is, they tend to form a large-scale structure of the Universe. The optical (and near infrared in the observers' frame) astronomers have found large-scale structures of galaxies nearly up to a redshift of 7. This means that the galaxies have already formed in the first several times 108 years of the history of the Universe. Then, are the optical observations enough? The answer is no, because dust in galaxies absorbs (blocks) the optical stellar light and reemit it into far infrared and submillimeter. More importantly, dust traces the metal enrichment in galaxies and becomes ingredients for planets.

Therefore, in order to reveal the blocked (i.e., "hidden") part of the stellar light and to trace the metal enrichment in galaxies, far-infrared and submillimeter observations are crucial. In particular, ALMA has just started observation in submillimeter (and millimeter) wavelengths, and we expect to be able to detect distant galaxies and large-scale structures of galaxies up to a redshift of 6 (or beyond). Below we show the example of our similation of large-scale structure of galaxies. See Suwa, Hirashita, & Tamura (2010) for details.

Here are some details for the calculation:

(a) We use the standard Lambda cold dark matter model of structure formation in the Universe to calculate the dark matter distribution, which ditermines the gravitational potential. The calculation method is summarized in Suwa et al. (2006).

(b) We identify stong concentrations of dark matter as galaxies. In each galaxy, we estimate gas mass according to the cosmic barionic fraction and form stars on a dynamical timescale determined by the dark matter potential. The dust enrichment is treated by assuming dust production in supernovae. A similar formulation can be found in Hirashita & Ferrara (2002).

(c) By an analytical treatment of radiative transfer, we calculate the far infrared luminosity emitted by dust. Here the dust optical depth is detemined consisently with the above dust enrichment.

In the figure, each point corresponds to an individual galaxy, whose luminosity of dust emission at a submillimeter wavelength of 0.85 mm is calculated individually. Therefore, the figure predicts the early Universe (z = 6) which ALMA would see. The larger a point is, the more luminous the galaxy is. The redder a point is, the larger fraction of energy is radiated in submillimeter. We only plot galaxies which are detectable by ALMA. The gray-scale shows the distribution of the dark matter, whose gravity attracts galaxies. Seeing the figure, there are actually numerous galaxies detectable by ALMA. We also observe that "red" galaxies are dominated. Here, "red" means that a galaxy radiates more in the submillimeter than in the optical (so it is different from normal red). The existence of dust makes galaxies "redder", because dust absorbs optical light and emits submillimeter radiation. We emphasize that if the galaxies have "redder" colors, the importance of ALMA is more pronounced. Therefore, a lot of red points in the figure indicate that ALMA is really essential in observing the early Universe.


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