- 4275 - GALAXIES - how to measure distances?- Why was it tricky to know the distances to galaxies JWST was seeing? One of the chief objectives of the James Webb Space Telescope (JWST) is to study the formation and evolution of the earliest galaxies in the Universe, which emerged more than 13 billion years ago.
---------------- 4275 - GALAXIES - how to measure distances?
- Scientists must
identify galaxies from different cosmological epochs to explore how their
properties have changed over time. This, in turn, requires precise dating
techniques so astronomers are able to determine when in the history of the
Universe an observed galaxy existed. The key is to measure the object’s
redshift, which indicates how long its light has been traveling through space.
-
- As distant light
travels through space it experiences that space is expanding. This stretches the lights wavelength towards
the red end of the spectrum. A stars
light might be in the ultraviolet wavelengths but by the time we see it
billions of years later it is stretched to the infrared.
-
- The purpose of the
“Cosmic Evolution Early Release Science Survey” (CEERS), is to analyze Webb
data to learn more about galactic evolution. These galaxies are known as
“high-redshift,” meaning that their light emissions are redshifted all the way
into the infrared spectrum.
-
- Galaxies that
existed 13 billion years ago can only be observed in the near-infrared
spectrum, which is now possible thanks to Webb’s Near-Infrared Camera (NIRCam).
Even so, obtaining accurate redshift measurements from such distant galaxies is
a very tricky, and requires advanced techniques.
-
- When observing
distant galaxies, astronomers will analyze their light using a spectrometer, a
device that breaks light down into its respective wavelengths, to measure for
redshift. The redshift determines how
long light has been traveling through space to reach us.
-
- Consistent with
Einstein’s Special Theory of Relativity, we know that the speed of light is
constant light’s regardless of the motion of the observer or source. However,
since the space between the source and observer is constantly expanding, the
light’s wavelength is elongated, causing it to shift towards the red end of the
spectrum (the “redshift”).
- By obtaining
accurate measurements of a galaxy’s redshift, astronomers can place it in the
context of cosmic history. If a galaxy has a redshirt value of 6 (z = 6), we
can conclude that its light has been traveling through space for roughly 12.7
billion years.
-
- This means that
the galaxy they are observing appears as it would have almost 13 billion years
ago. Obtaining these values is not easy, but astronomers have come to rely on
various techniques to make it easier.
-
- There are two
techniques involve measuring emission lines and using a “break” in a galactic
spectrum, abrupt changes in the light intensity at specific wavelengths. A
common break astronomers rely on is the spectral break of neutral hydrogen (
the Lyman Break), which corresponds to the amount of energy it can absorb
before it becomes ionized.
-
- Similarly, there’s
the “Balmer break”, where electrons become completely ionized directly from the
second energy level of a hydrogen atom. Galaxies that show both of these breaks
are called double-break galaxies.
-
- Since astronomers
know the wavelength of these breaks, they can target galaxies at certain
distances by looking for breaks with the right redshift. Some of the first
images Webb took were of double-break high redshift values (z = 7), meaning
they existed when the Universe was less than one billion years old (13 billion
years ago).
-
- However, these
galaxies were much brighter and larger than expected, which did not sit well
with prevailing cosmological models. This illustrates a flaw in the
double-break method, which is great for finding galaxies but can introduce
biases into the data.
-
- Astronomers can
take images using multiple filters to collect the object’s light in several
different colors. When we measure a galaxy’s “photometry”, or how bright it is
in an image, we’re measuring the brightness of the object averaged across the
full range of wavelengths transmitted by the filter. There is a lot of detailed information
hidden within each single measurement for every 0.3–1.0 microns in wavelength
coverage.
-
- The shape of a
galaxy’s spectrum is affected by several properties, including the numbers of
stars forming within it, how dusty it is, and how much its light has been
redshifted. The measured brightness of the galaxy is then compared in each
filter to what galaxy models predict, spanning a range of properties at a range
of redshifts.
-
- Based on how well
the model fits the data, astronomers can determine the galaxy is at a certain
“moment in history.” This analytical process, where a “best fit” redshift is
found, is known as the photometric redshift.
-
- In July 2022, teams
with the CEERS Survey used NIRCam images to identify two previously undetected
galaxies with photometric redshifts greater than z = 11, corresponding to more
than 13.4 billion years ago.
-
- While we can
estimate a best-fit redshift based on modeling the photometry, the resulting
probability distribution is often broad. Additionally, galaxies at different
redshifts can have similar colors in broadband filters, making it difficult to
distinguish their redshifts based only on photometry.
-
- Red, dusty
galaxies at redshifts less than 5 (or when the universe was 1.1 billion years
old or older) and cool stars in our own galaxy can sometimes mimic the same
colors of a high-redshift galaxy. We therefore consider all galaxies that are
selected based on their photometric redshifts to be high-redshift candidates
until we can obtain a more precise redshift.
-
- Astronomers will
attempt to constrain a galaxy’s redshift by obtaining a spectrum, using Webb‘s
Near-Infrared Spectrograph (NIRSpec).
They can improve their calculations of redshift probability distribution
by measuring the perceived brightness (photometry) of a galaxy in ever finer
wavelengths. As they go from broadband filters to narrower filters to a
spectrum, the distribution narrows until a very precise redshift measurement is
obtained, known as a spectroscopic redshift.
-
- In February 2023,
the CEERS team used the NIRSpec to obtain precise spectroscopic redshifts on
two previously identified high-redshift candidates. These were Maisie’s Galaxy
and CEERS-93316, which were initially thought to have redshifts of z = 16 and
16.4, which made them the farthest galaxies ever observed.
-
- But when another
team of astronomers analyzed the redshift measurements obtained by Webb, they
determined that Maisie’s Galaxy and CEERS-93316 had redshift values of z = 11.4
and 4.9, meaning that they existed roughly 390 million years and 1.2 billion
years after the Big Bang.
-
- Clearly, observing
objects at cosmological distances and constraining their properties is no easy
task. But with improved instruments and the sophisticated methods they allow,
our measurements of the cosmos are rapidly improving. This, in turn, will allow
astronomers to resolving the greatest cosmological mysteries of our time.
-
- These include the
existence of Dark Matter and Dark Energy and the discrepancy between distance
measurements to determine the rate of cosmic expansion, the Hubble Tension.
-
- Slowly, but
surely, we are getting closer to understanding how everything in our Universe
fits together! We have a long way to
go!
-
-
December 18, 2023
GALAXIES - how to measure distances? 4275
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