Pascal Oesch


I am currently a YCAA Fellow at Yale University working on very deep Hubble and Spitzer Space Telescope data to study galaxy evolution across cosmic time. In particular, my research is focused on understanding the build-up and assembly of the first generations of galaxies.

Additional research interests include

  1. Finding the First Passive Galaxies

  2. Rest-Frame Optical Emission Line Properties at z>3

  3. Morphological Evolution of Galaxies

  4. Photometric Redshifts


Email: pascal.oesch[at]

Phone: 203-432-1265

Office: JWG 469

Postal Address:

Yale Center for Astronomy and Astrophysics

P.O. Box 208120

New Haven, CT 06520-8120


Publications: Link (ADS)


Research Interests

Legacy UV Imaging of the CANDELS-DEEP Fields

I am the PI of a cycle 22 legacy UV imaging survey of the two CANDELS-Deep fields in GOODS-North and -South. Our program will build on existing WFC3/UVIS data to obtain deep images at 250-350 nm (in F275W and F336W). These data will provide the first complete census of low-luminosity star-forming galaxies at z~0.5-2 and enable a wealth of research by the community. This includes measuring the physical properties of sub-L* galaxies, and characterizing resolved stellar populations to decipher the build-up of the Hubble sequence from sub-galactic clumps.
To ensure these legacy data are fully exploited, we will release high-level science data products to the community as soon as they are available, which will be announced on our survey webpage.

Recent results on z~7-10 galaxies

Due to an increase in efficiency by a factor about 40, the installation of the WFC3/IR camera on the Hubble Space Telescope revolutionized our knowledge about galaxies in the reionization epoch. From the combination of our extremely deep HUDF09 survey with additional wide-area imaging it became possible to identify large samples of galaxies at z~7-8, and we were even able to find one first galaxy at z~10, only 500 Myr after the Big Bang.

We recently created a new multi-color image of the Hubble Ultra Deep Field (called XDF), including all the available data in the optical and NIR that were taken over this field with HST over the last decade. This new image provides the deepest view of the universe ever and is shown below. The full release can be found on The actual multi-wavelength science data are also available on or from MAST.


Some recent research highlights are shown below. For more information see also and our page:

Thanks to large surveys with HST's WFC3/IR camera over the last few years, we have been able to push the observational frontier of galaxies out to z~10, when the universe was only ~500 Myr old. Even though small, the first samples of z>8 galaxies allowed for first constraints on the cosmic star-formation rate density (SFRD) in the heart of the cosmic reionization epoch. This is shown in the figure above, where the current estimates of the cosmic star-formation rate density are plotted for Lyman Break galaxies from z~3 to z~11 (limited at a fixed minimum star-formation rate of 0.7 M/yr). While star-formation appears to decline steadily from z~3 to z~8 (shown by the gray shaded area), we seem to be seeing a more dramatic decrease at z>8 (black solid line). However, due to the small sample sizes, the current uncertainties at the highest redshifts are still very large. Using new HST data that is being taken now as part of the Frontier Field Initiative, we will be able to make significant progress in the near future. All the information on the above plot and how it is derived can be found in Oesch et al. 2013b and Oesch et al. 2013c.

Images in different filters of four bright galaxy candidates at z~9-10 that we recently identified in the WFC3/IR data of the CANDELS survey. At these early times, the neutral hydrogen in the inter-galactic medium absorbs all the light at wavelengths shorter than ~1.3 microns. Therefore, we can only significantly see these galaxies in HST's reddest band (F160W). Amazingly, however, these galaxies are also detected in extremely deep Spitzer/IRAC imaging. This is the first ever robust individual detection of rest-frame optical light at ~500 Myr from the Big Bang, allowing us to constrain the stellar masses of these galaxies. The high luminosity of these sources is somewhat surprising. These galaxies are about an order of magnitude brighter than previous z~10 galaxy candidates, and no other field revealed such bright sources. These galaxies therefore indicate that star-formation in the very early universe was likely highly stochastic, resulting in large field-to-field variations. The individual detections of these galaxies with IRAC motivated our approved Exploration Science program to push to even deeper limits with Spitzer/IRAC over the GOODS fields in order to individually detect a large fraction of known galaxy candidates in the reionization epoch in order to study the stellar mass build-up at early times.

WFC3/IR Grism Observations of GN-z10-1

In cycle 22, we were awarded 12 orbits of WFC3/IR grism observations to obtain a continuum spectroscopic redshift of the brightest GOODS-N source shown above. Stay tuned.

A link to all my first-author publications is: Link (ADS)