Stellar mass And GAlaxy CEnsus in the first 2 billion years of the Universe – SAGACE

Our project is based on the ongoing program SPLASH of 1700h with the IRAC camera onboard of the Spitzer telescope, the only instrument that probes the rest frame optical fluxes needed to measure the galaxy stellar masses at z>3. We cover a large field of 2 deg2 to get tens of rare and massive galaxies at z ~ 7-8, a mass-selected sample of more than a hundred thousand of galaxies at z>3 and exquisite data to secure their distances (Hyper Suprime-Cam in optical and UltraVISTA in near-infrared). This work relies on our ability to create a robust catalogue of a million galaxies, including their oberved flux in >30 bands, then to measure their distances and their stellar mass purely based on the multi-color data. Our main challenge is to measure the distances and stellar masses for sources with a really faint emission in optical, specially for the z>6 not detected in optical. The goal of this project is to measure the global star formation history and the evolution of the galaxy growth rate at 3<z<8. We adopt a complementary and independent view based on the stellar mass census. Almost all studies in this domain are based on the light emitted by massive, short-lived stars while on the contrary, we based our estimate on the old stars light. This new measurement is crucial since it constraints the physical processes introduced to convert this baryonic gas into stellar populations. By comparing our results with the expected properties from the hydrodynamical simulations HORIZON-AGN, we bring some insight on our knowledge of these physical processes. Finally, we will organise a spectroscopic follow-up of the massive sources at z>6 with X-Shooter, KMOS, HST and we will propose high redshift targets for JWST in 2017.

The first 30 months of this ANR were dedicated to the creation of our multi-color and redshift catalogues. One important milestone was the release of the final catalogues to the world. The COSMOS2015 catalogue contains precise PSF-matched photometry, photometric redshifts and stellar masses for more than half a million of sources (excluding masked area) on the COSMOS field. Including all the already existing optical bands, new YJHKs images from the UltraVISTA-DR2 survey, Y -band from Hyper Suprime-Cam and infrared data from SPLASH Spitzer legacy program, this near-infrared selected catalog is highly optimised for the study of galaxy evolution and environments in the early Universe. We release this catalogue world-wide (ftp://ftp.iap.fr/pub/from_users/hjmcc/COSMOS2015/) and published an associated paper (Laigle, McCracken, Ilbert et al. 2016, ApJS, 2016, 224, 24).

We also published a paper «Spitzer Bright, UltraVISTA Faint Sources in COSMOS: The Contribution to the Overall Population of Massive Galaxies at z = 3-7« Caputi, Ilbert, Laigle C., et al., 2015, ApJ, 810, 73 to study the abundance of massive galaxies in the early Universe (press release www.eso.org/public/news/eso1545). We also measure the evolution of the stellar mass function out to z~6 («The COSMOS2015 galaxy stellar mass function: 13 billion years of stellar mass assembly in 10 snapshots«, Davidzon, Ilbert, Laigle et al. 2017).

As expected, our catalogue contains several galaxy candidates in the early Universe. We identified 9 galaxies at z>7 which could be the most massive galaxies known at such distance. In order to confirm their distances, we plan a spectroscopic follow-up at VLT and with HST.

Using galaxy stellar mass functions measured by Davidzon et al. 2017 between 3<z<6, we are able to derive the cosmic Star Formation History and the evolution of the specific Star Formation Rate, which is the main scientific goal of our project. We find interesting preliminary results with a steep increase of the sSFR with redshift. Such increase is expected if the baryonic mass falling into the galaxy follows the dark matter accretion rate into the halo.<br />
Finally, we started a project to apply exactly the same observational methods to the predictions of an hydrodynamical simulation. The goal is to perform a comparison free from the systematic bias introduced by the observers. With such comparison in hand, we could really conclude on the physical processes at play in the mass function evolution.

1. Caputi, Ilbert, Laigle C., et al., 2015, ApJ, 810, 73
2. Laigle, McCracken, Ilbert et al. 2016, ApJS, 2016, 224, 24
3. Davidzon, Ilbert, Laigle et al. 2017, A&A, in press, arXiv:1701.02734

One of the primary frontiers in extragalactic astronomy is observing the build-up of the galaxies in the first 1–2 billion years of the universe. The first galaxies could appear 200 million years after the Big-Bang. These primordial systems will grow and they could end up into huge galaxies composed of several hundred billions of stars, such as our own Milky Way. We propose to measure at which rate the galaxies assembled their stellar populations in their infancy, i.e. by catching massive galaxies when the Universe was only 500 million years old. Then, we will follow their evolution over the following two billion years.

Numerous physical mechanisms play a crucial role in shaping the galaxy mass assembly, but their relative importance is unknown at z>3. For instance, some simulations show that the star formation could be boosted in the early Universe by dark matter filaments penetrating directly the galaxy, i.e. a process called “cold accretion”. But the observational evidences of such mechanism are tiny. Major mergers could contribute in building the massive galaxies, but we do not know how often such events could occur at z>3. We also do not know when the supernovae and Active Galaxy Nuclei activities started to have a significant impact in regulating/quenching the star formation.

We need observables to constrain the relative importance of these physical processes and bring together a robust scenario of galaxy formation. We will produce statistical analysis extracted from the largest samples of massive galaxies at the current frontier in extra-galactic astronomy.

We propose to make measurements of unparalleled precision of the buildup of stellar mass and the star-formation rate at 3<z<6 and produce the largest census of the massive galaxies at 6<z<8. This project is nowadays possible thanks to the ongoing program SPLASH of 1700h with the IRAC camera onboard of the Spitzer telescope, the only instrument that probes the rest frame optical fluxes needed to measure the galaxy stellar masses at z>3. We will cover a large field of 2 deg2 to get tens of rare and massive galaxies at z ~ 7-8, a mass-selected sample of more than a hundred thousand of galaxies at z>3 and exquisite data to secure their distances (Hyper Suprime-Cam in optical and UltraVISTA in near-infrared). By comparing our results with semi-analytical and phenomenological models, we will quantify the relative importance of the different physical processes powering galaxy formation. Our work will provide a solid legacy for the forthcoming deep extra-galactic international surveys in which French laboratories are deeply involved, and a treasure of targets for the JWST and ELT to proceed later with detailed observations.