Our Latest Results
Our research group has been one of the leaders in observational efforts
to explore galaxies in the first 2 billion years of the universe. The
first 2 billion years of the universe is exciting because it is during
this time that we expect galaxies to have built up from a very small
number of stars to the large galaxies we see today. It is also during
this time period that our universe was reionized and we expect galaxies
to have played the key role in this process.
We have a number on-going research projects aimed at finding galaxies
over this entire interval of time: from the very earliest times where
it is a challenge just to find a small number of sources to later
times where the goal is to collect large statistical samples, to
study these samples in detail, and to understand how the properties of
the galaxy population are changing. Our principal technique for
finding galaxies at early times has been the dropout technique, and
a detailed explanation can be found
here.
Here is a brief summary of our latest research results on early galaxy
formation -- ordered in terms of the amount of cosmic time from the Big
Bang:
400 million years (Redshift 10): Over the past few years, we have been
conducting a fairly comprehensive search for galaxies at these early times
using several months of NICMOS data from the Hubble Space Telescope. Three
years ago, we thought we had found a few galaxies which might be plausible
redshift 10 galaxy candidates because they appeared to dropout at
wavelengths bluer than 1300 nm.
However, all of these candidate redshift 10 candidates were later detected at
optical wavelengths in some very deep images that were subsequently acquired,
we now know that none of the sources are such high-redshift galaxies.
At present, we know of no sources in HST or ground-based data which we can
reasonably assert are z~10 candidates using the dropout selection technique.
However, just because we have not found these galaxies
does not mean they do not exist. Current thinking suggests that these
sources are simply too faint to be seen with current instrumentation on
large telescopes, and we will need for
more powerful telescopes to find them in large numbers.
700 million years (Redshift 7 and 8): Over the past few years, we have
been using a substantial fraction of the deep near-infrared data
available from the Hubble Space Telescope and large ground-based
telescopes to search for galaxies which appear to have emitted their
light at redshift z~7-8.
These data are available over 23 square arcminutes, or ~2% of
the area of the full moon in the case of the HST data and 250 square
arcminutes in the case of ground-based data (or ~25% of the area of
the full moon).
We identify candidate z~7-8 galaxies by looking for galaxies which
dropout at wavelengths of ~1000 nm and bluer. A very careful search
through the HST NICMOS data yield 9 reasonably robust z~7-8 galaxy
candidates. Some ~10 other z~7-8 candidates are found in searches
with wide-area surveys with near-IR cameras on large ground-based
telescopes like
Subaru or the
Very Large Telescope.
We have used our searches for z~7-8 galaxies to estimate the volume
density of these sources as a function of luminosity -- a quantity
known as the luminosity function. The luminosity function is of significant
interest to astronomers and allows us to learn something about how the
population of galaxies change as a function of cosmic time.
Comparisons of the luminosity function of galaxies at z~7-8 (700
million years after the Big Bang) with those at z~6 (900 million years
after the Big Bang) tell us that luminous galaxies at z~7-8 were much
less prevalent than they were at later cosmic times. Since we expect
the more luminous galaxies to build up gradually from less luminous
galaxies, we might have expected to find this deficit at bright
galaxies at early times. Yet, despite this qualitative agreement with
the models, the value in our measurements is that they provide
quantitative constraints on how rapidly this build up takes place.
900 million years (Redshift 6): Over the past few years, we have
taken advantage of all the deep optical data over the deepest HST fields
to compile a sample of
over 600 galaxies at redshift 6.
This sample allowed to study the properties of galaxies at these earliest times. Comparisons of this
sample with galaxies later on the history of the universe show that
galaxies at early times were much smaller, bluer, and lower luminosity
(on average) than galaxies which existed later on in the universe.
Each of these findings is consistent with the idea that galaxies build
up hierarchically from much smaller pieces. Our sample is still the
largest compilation of galaxies at these early times.
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