Why ALMA

Why ALMA

Why do we require ALMA observations of galaxies in the first billion years of the universe? A significant challenge in studying galaxies discovered in the first billion years of the universe is the very limited physical information we have available on these sources. In most cases, we can only measure the brightness and color of these galaxies in the rest-frame UV. While we have lower S/N flux information from Spitzer/IRAC observations for many galaxies reaching into the rest optical, it is notoriously difficult to determine what fraction of the measured fluxes is from older, lower-mass stars and what fraction is from bright emission lines.

With ALMA observations, we can greatly improve our physical characterization of individual galaxies in the first billion years. The following properties, in particular, can be quantified much more accurately:

1. Redshifts (< 0.1% from spectroscopy vs. 3-5% from UV-optical photometry)

2. Dust-obscured SFRs (direct constraints not possible from UV photometry)

3. Dynamical Properties/Masses (constraints not possible from UV photometry)

By measuring each of the above quantities, we can greatly improve our constraints on the masses of galaxies. First of all, redshift measurements are essential for determining which IRAC fluxes specific strong nebular emission lines contribute. This dramatically improves our ability to infer stellar masses from the available UV-optical photometry. Measurements of the dust-obscured SFRs allow for a determination of the total SFRs and provide a useful link to source stellar masses (via the star-forming main sequence and the IRX-stellar mass relation). Finally, as the dynamical masses correlate with the total mass in stars, gas, and dark matter, measurements of the dynamical masses provide a convenient anchor point for deriving masses at z>6.

It is clear that by obtaining a measurement of all three of the above quantities, we should greatly improve our ability to quantify the speed at which mass build-up occurs in galaxies at early times.