The Brightest of Reionizing Galaxies Survey is a large Hubble Space Telescope Program focused on finding rare and bright galaxy candidates at redshift z~8-10 that is when the Universe was about 500 to 650 million years old. We use Hubble's Wide Field Camera 3, which is highly efficient in the near-infrared bands (Y, J, H) that are needed to identify these very distant galaxies. The near-infrared filters are complemented by deep optical imaging.


BoRG is a pure-parallel survey. This means that the data are acquired while Hubble is pointing for primary spectroscopic observations (typically distant QSOs or extrasolar planets).

BoRG was originally designed in Cycle 17 (GO 11700) to search for z~8 sources and continued with observations in Cycle 19 (GO 12572) and Cycle 20 (GO 12905), augmented by assimilation of other parallel imaging in the Hubble archive (see Bradley et al. 2012 and Schmidt et al. 2014 for the list of pointings and associated programs). Currently, we are boldly moving to Hubble's detection frontier at z~9-10, and the BoRG Cycle22 observations (GO 13767) are optimized for identifying J-band dropouts that is galaxies observed when the Universe was about 500 Myr old.

We are releasing all science images through the Mikulski Archive for Space Telescopes.

Magnification bias in the BoRG luminosity function

BoRG graduate student Charlotte Mason (UCSB) has developed a Bayesian framework to account for the magnification bias from gravitational lensing in luminosity functions, and used the BoRG z~8 LF as a test case. Magnification bias can distort the bright end of high redshift LFs so it is important to understand how it affects our results.

The work has been accepted for publication in the Astrophysical Journal.


We find that magnification bias is currently not significant for the luminosity range of galaxies in BoRG: the effect is only significant for the very brightest galaxies, with M_{UV}<-22 . However, we also show that the effect will dominate future wide-area surveys such as Euclid and WFIRST, which will detect the rarest very bright galaxies, so it will be crucial to account for magnification bias in these surveys.

BoRG Cycle 22 is approved for 32 days of observing time!

A new BoRG survey has been approved for Cycle 22! We boldly proposed to extend the search for the brightest galaxies that Hubble can detect, focusing on redshift z~10 (just 500 Myr after the Big Bang), and we obtained what might well be the largest allocation of Hubble observing time awarded in this cycle: 480 parallel orbits (more than 750 hours).

Pending a successful Phase II proposal, and scheduling, Hubble will spend more than one month observing for our BoRG program!

While we wait for JWST, this new dataset has the potential of being "transformative" (borrowing the word from the reviewers' comments), and the BoRG team are looking forward to planning, and then analyzing the upcoming observations.

For further information see the associated KICC press release.


Latest BoRG luminosity function at z~8

BoRG postdoc Kasper Borello Schmidt published the latest determination of the z~8 BoRG luminosity function in the Astrophysical Journal.

The luminosity function measured by combining BoRG data with deeper data from the Hubble Ultradeep Field and ERS observations is shown here:


The luminosity function measure shows a very strong evolution in the few hundred Myr separating z~8 from z~6, and like our previous determination in 2012, it finds a steep faint end slope ( \alpha ~ -1.87 \pm 0.26).

We also used the BoRG z~8 luminosity function to perform inference on the epoch of reionization, and in particular on the neutral fraction of hydrogen when the universe was only ~600 Myr old. The BoRG data prefer a 'late reionization' scenario, where a significant fraction of the IGM is neutral at z~8, i.e. the reionization was still underway at that time. This is in very good agreement with the results of our recent MOSFIRE follow-up campaign.

In addition to obtaining the largest area determination of the z~8 luminosity function published to date in a refereed journal, Kasper developed a new rigorous Bayesian approach to fit the luminosity function without resorting to binning of the data, commonly employed by previous studies.

BoRG Cool Stars

BoRG CoI Benne Holwerda (Leiden University) took advantage of the random pointings nature of BoRG to investigate the distribution of cool stars (primarily M dwarfs) in the Milky Way. The work has been accepted for publication in the Astrophysical Journal and is available on arXiv.

The distribution of BoRG lines of sight in galactic coordinates is shown here:


Each colored point corresponds to a different WFC3 BoRG pointing, with size proportional to the surface density of dwarfs. Two intriguing conclusions of the study are:

  1. There is a North-South asymmetry, with North fields showing a ~2 \sigma evidence of overdensity, despite the fact that the sun is above the galactic plane. The work confirms and strengthens the asymmetry previously noted on just two lines of  sight of GOODS north vs. south.
  2. We clearly detect the Sagittarius Stream as an overdensity of dwarf stars (star point in the figure).

The work by Benne demonstrates the legacy value of BoRG imaging beyond its key contributions on galaxy formation during the epoch of reionization, and complements studies of the Milky Way structure by tracing stars that are much fainter than those that Gaia will observe (BoRG identifies stars to about m_{AB} ~24 in the H band).

BoRG@Keck: Witnessing reionization in progress

BoRG CoI Tommaso Treu led a spectroscopic follow-up of 13 BoRG z~8 galaxies to search for Ly- \alpha emission. The results have been published in the Astrophysical Journal.

The lack of Ly- \alpha detection to a faint flux (median sensitivity of 25 \mathring{A} at 5 \sigma ) sets very interesting constraints on the evolution of the Ly- \alpha equivalent width distribution, which is most naturally interpreted as evidence of an increase in the neutral gas content at z>6: