External galaxies

Date
Category sciences

Nearby galaxies:

The sub-millimeter/millimeter part of the spectrum is one of the least explored parts of a galaxy‘s spectral energy distribution (SED), yet it contains primary informations on dust properties. Although interstellar dust represents only 1% of the total mass of the ISM it is an important agent in star formation. Studying dust properties in galaxies is thus untimely linked to one of the most fundamental problems in present-day astrophysics, i.e. understanding how stars form in galaxies.

CONCERTO can probe the long wavelength emissivity of dust and see how it changes as a function of different galactic environments i.e. disc, spiral arm, nucleus. Combining CONCERTO, NIKA2 and Herschel data would provide additional input into comprehensive SED models of galaxies (e.g. Galliano et al. 2011) and would enable to investigate changes in the emissivity index. Of particular interest is the possibility of excess long wavelength emission over and above that predicted by the Herschel data. This excess is not clearly understood. It may indicate a significant cold dust component (T<15 K) and/or physically different dust.

Understanding how dust properties evolve as a function of metal enrichment can provide important constraints for galaxy evolution studies. Dwarf galaxies in the local Universe are ideal targets for such a study as many of them have low metallicity and high star-formation activity. As such, they present star formation properties and ISM conditions that are the closest analogues to those thought to be present in the primordial environments of the early Universe. Accurate maps of the dust mass surface density obtained using CONCERTO can be combined with the HI and H2 (CO) maps to give the spatial variation of the gasto-dust ratio. The gas-to-dust ratio is a crucial input into chemical evolution models that predict the metallicity as a function of the gas fraction. When combined with the gas phase metallicity the fraction of metals in both gas and dust can be measured. Using samples ranging from metal-poor to metal-rich environments, CONCERTO would allow to derive the gas-to-dust mass ratios as a function of metallicities.

High-redshift galaxies:

Active star formation in dusty, optically obscured galaxies manifests itself in far-infrared to millimeter wavelength observations. The key feature that makes (sub-)millimeter wave observations of distant galaxies fascinating is the ability to sample the spectral energy distribution (SED) of a target galaxy at wavelengths for which the SED is an increasing function of frequency. This ensures that distant galaxies are observed at a rest-frame wavelength closer to the peak of their SED. Consequently, there is a strong, negative K correction, which leads to high-redshift galaxies being relatively easy to detect at (sub-)millimeter wavelengths as compared with their low-redshift counterparts. Observing extragalactic fields deeply with CONCERTO could:
1. Give a complementary view of SFR at high redshift: At present, mostly UV selected galaxy populations have been used to study the cosmic SFR history of the Universe at z > 3 (Madau & Dickinson, 2014). However, the UV selection prevents the detection of the massive dusty star-forming population, and hence is strongly biased. A complete census on the SFR history of galaxies is therefore missing and large-area millimeter surveys of the z>3 Universe will have to be performed. This could be one of the major goals of CONCERTO deep field observations.
2. Determine the star formation / dark-matter halo mass connection: Thanks to the FOV and its mapping speed, CONCERTO would be particularly powerful to probe the clustering of galaxies. It could then provide particularly strong constraints on models for the evolution of large-scale structures, giving the link between star formation and dark-mater halos mass.
3. Provide us the evolutionary history of the galaxy populations that dominate the CIB.