Additional and observatory science

Fielding an IR-specified spectrograph in space, THESEUS would provide a unique resource for understanding the evolution of large samples of obscured galaxies and AGN. With a rapid slewing capability, and substantial mission duration, the mission provides a very flexible opportunity to take efficient images and spectra of large samples of galaxies with minute-to-many-hour-long cumulative integrations. A continuous spectral coverage with no blockages due to atmospheric opacity ensures that identical species~R lines can be tracked in extensive samples. The capability to cover the redshift range from 0.07<z<1.74 for H-alpha and 0.44<z<2.29 for H-beta enables Balmer decrement measurements of the extensive evolution of the AGN and galaxy luminosity functions at redshift ~0.5-1.5, a spectral region that simply cannot be covered from the ground. These key diagnostic rest-optical emission lines will be observed for galaxies in this substantial range of redshifts, reaching out towards the peak of AGN and galaxy formation activity, over a continuous redshift range where the bulge-blackhole mass relation is being built up and established, and the main sequence of star formation is well-studied. With excellent image quality, THESEUS~Rs R~500 grism can also provide spatially-resolved spectral information to highlight AGN emission, and identify galaxy asymmetries.

The imaging sensitivity of THESEUS is about 6 magnitudes lower than for JWST in the same exposure time; nevertheless, its availability ensures that many important statistical samples of active and evolved galaxies, selected from a wide range of sources can be compiled and diagnosed in detail at these interesting redshifts. Samples can be drawn from the very large WISE- and Herschel-selected infrared samples of galaxies, from EUCLID~Rs 24-mag large-area near-infrared galaxy survey, augmented by near-infrared selection in surveys from UKIDSS (whose deepest field reaches approximately 1 mag deeper than EUCLID~Rs wide-area survey in the H band) and VISTA, and in the optical from LSST and SDSS.

Spectra for rare and unusual galaxies and AGNs selected from wide-field imaging surveys can be obtained using the wide-field of THESEUS grism, thus building an extensive reference sample for studying the environments of the selected galaxies and AGNs, identifying large-scale structures and allowing overdensities to be measured. The parallel acquisition of spectral and imaging data over substantial areas would build up a clear picture of the environments, including serendipitiously-selected spectra, all taking advantage of the spectral resolution delivered for THESEUS’s primary science of investigating GRB afterglows.