COSMOS-Web overview paper is out!
November 18, 2022
COSMOS-Web is a 255 hour treasury program to be conducted by the James Webb Space Telescope (JWST) in its first cycle of observations. It is a contiguous 0.54 deg2 NIRCam imaging survey in four filters (F115W, F150W, F277W, and F444W) that will reach 5σ point source depths ranging ∼27.5–28.2 magnitudes. In parallel, we will obtain 0.19 deg2 of MIRI imaging in one filter (F770W). COSMOS-Web will build on the rich heritage of multiwavelength observations and data products available in the COSMOS field. It is the largest program, both in terms of area coverage and prime observing time, awarded during Cycle 1, covering an area 2.7 times larger with NIRCam and 3.5 times larger with MIRI than all other Cycle 1 programs' deep field observations combined. The design of COSMOS-Web is driven by three primary science goals:
- to discover thousands of galaxies in the Epoch of Reionization (6 < z < 11) and use their spatial distribution to map large scale structure during the Universe’s first billion years,
- to identify hundreds of rare quiescent galaxies at z > 4 (within 2 Gyr of the Big Bang) and place constraints on the formation of the Universe’s most massive galaxies, and
- to directly measure the evolution of the stellar mass to halo mass relation using weak gravitational lensing out to z ∼ 2.5 and measure its variance with galaxies’ star formation histories and morphologies.
We anticipate that COSMOS-Web’s legacy value will reach far beyond these core science goals and extend to many subfields of extra- galactic astronomy and beyond. This includes measuring galaxy morphologies, using spatially resolved SEDs to measure galaxy properties, placing constraints on the dust attenuation law, identifying and characterizing galaxy protoclusters, finding strong gravitational lenses, identifying direct collapse black hole candidates, studying the co-evolution of supermassive black holes and their host galaxies, searching for z > 10 pair instability supernovae, and identifying ultracool sub-dwarf stars in the Milky Way’s halo.
We hope that COSMOS-Web will be an important resource for the broader community and enable many scientific discoveries for years to come. The first observations are planned for this December, so stay tuned for future discoveries and publications!
For more details on observations and science, see our COSMOS-Web overview paper: Casey & Kartaltepe et al. (2022)
Caption: A map of the COSMOS-Web tiling pattern embedded within the Hubble ACS F814W mosaic of the full COSMOS field. The mosaic consists of 152 visits where NIRCam serves as the primary instrument (long wavelength detector coverage shown in blue) with MIRI in parallel (shown in orange).
A Deep View into the Epoch of Reionization
The Epoch of Reionization (EoR) marks the time after the Big Bang during which the strong radiation of galaxies ionizes the previously neutral Hydrogen in our Universe. About 1 Billion years after the Big Bang (redshift z = 6) the Universe is fully ionized. COSMOS-Web will find more than 4000 galaxies in the EoR. The contiguous area of the survey enables for the first time the study of how galaxies cluster (the so-called “cosmic web” or large scale structure of the Universe) to test models of the process of reionization and galaxy formation. In addition, the exquisite photometry from JWST will enable the first detailed physical and structural study of massive galaxies in the very early Universe.
Caption: COSMOS-Web enables the measurement of the large scale structure (also known as the “cosmic web”) of our Universe at very early cosmic times (here redshift of z = 7, corresponding to only 700 million years after the Big Bang). The image shows a cutout of a large scale structure generated by a computer simulation (left) and how COSMOS-Web will be able to reconstruct it (right).
Growth of the Universe’s Massive Galaxies
Today’s most massive galaxies are elliptical in shape and have very little current star formation, indicating that they built up their stellar masses at earlier time periods and then ceased forming new stars. We call these galaxies passive or quiescent since they are no longer forming new stars. Quiescent galaxies have been discovered at very high redshifts, within 2 billion years of the Big Bang. The existence of significant numbers of these so early in the Universe’s history poses a challenge to theoretical models because these galaxies must have formed a lot of stars very quickly after the Big Bang and then stopped forming new stars. With its wide-area coverage, COSMOS-Web will be able to detect these quiescent galaxies out to high redshifts, if they exist, and measure just how rare they are, which will place strong constraints on the possible formation mechanism of these massive galaxies.
Caption: A comparison of the number density of quiescent galaxies predicted by various cosmological simulations (solid lines) to the observed number densities from different studies (points). The dashed lines correspond to the number density of one object in the volume of COSMOS-Web (black), medium-volume JWST surveys such as CEERS and PRIMER (dark gray), and deep volume surveys such as JADES-Deep and NGDEEP (light gray). Only COSMOS-Web has the volume necessary to place strong constraints on the number densities of quiescent galaxies at z > 4 if they are indeed as rare as expected.
Linking Dark Matter to the Visible
Galaxies are formed early as gas is pulled into “knots” of dark matter by gravity. These so-called Dark Matter halos may impact the evolution of galaxies significantly during their lifetimes. For example, the halos can attract new gas needed for star formation, but can also lead to quenching of star formation. Simulations and observations in the late Universe suggest a relation between the stellar mass of galaxies and the Dark Matter mass of their halos. Differences in this relation and its evolution with redshift put strong constraints on galaxy formation models. COSMOS-Web will provide robust measurements of galaxy halos more than 11 billion years back in time (a redshift of z = 2.5), which will allow the statistical correlation between galaxy properties (e.g., their rate of star formation or structure) and their halo masses. These measurements will put strong constraints on cosmological simulations and models, and push our understanding of galaxy formation at early times to new levels.
Caption: Galaxies exhibit a relatively strong correlation between their stellar mass and the mass of their Dark Matter halo. COSMOS-Web will measure this relation out to z = 2.5, 11 billion years back in time (left). The correlation of the stellar mass to halo mass relation to galaxy properties (such as color gradients) teach us about the formation of galaxies and how they eventually stop forming stars.
