Sky & Telescope - October 2023 - 25

SYMONS ET AL. / ASTROPHYSICAL JOURNAL 2023
verse's Extra Light
Out Yonder
This was the state of affairs when LORRI entered the picture.
New Horizons carried this imager to Pluto in 2015
(S&T: Nov. 2015, p. 18) and to 486958 Arrokoth in 2019
(S&T: Apr. 2019, p. 8) in order to image these fascinating
worlds in the outer solar system. But Tod Lauer (NSF's
NOIRLab), Marc Postman (Space Telescope Science Institute),
and others realized the camera could be put to a different
purpose. By traveling to the outer solar
system, the spacecraft and the camera it
carries have passed far beyond the zodiacal
dust. That's akin to removing the bedside
lamp from the hypothetical room, rather
than trying to shield it from view.
Lauer, Postman, and fellow members
of the New Horizons science team led two
studies, one in 2021 and another in 2022,
that used a selection of LORRI images to
look for the EBL. And they found it. The
detection is significant to more than 10
sigma - in other words, there's almost
zero chance this measurement is a fluke.
The EBL the team measured is surprisingly
bright, between 14.9 and 17.8 nanowatts
per square meter per steradian. (The units
indicate the power coming from each unit
uMASKED The first step in looking for background
light is to remove the light from stars and
galaxies. The image at bottom shows the same
LORRI image as at top, but with a mask applied
to remove light from stars and galaxies. An optical
ghost at the center, caused by a bright star just
outside the field of view, is also masked out.
THE SPACE BETWEEN Some astronomers find that, in the space
between stars and galaxies, there's a subtle glow (not visible here) that
can't be explained by more distant - and thus unresolved - stars and
galaxies. What is the source of this background light?
area on the sky and landing on each unit area of the detector;
hereafter nW/m2/sr.) The result is about twice as bright
as expected from faint, unresolved stars and galaxies.
Even without the zodiacal light in play, there's a lot that
goes into this accounting to cancel out all
the light from stars and galaxies. First, the
team removed the objects actually detected
in the images. Then they removed the dim
halo of light around stars and galaxies just
outside LORRI's field of view. The team also
subtracted out the light coming from dust
scattered along the galactic plane as well as
light from optical or camera defects.
The remainder ought to come from the
light of faint, unresolved sources, mostly
galaxies. But according to galaxy counts
made using Hubble observations, faint galaxies
should only contribute 8 nW/m2/sr,
explaining roughly half of the EBL that
LORRI detects.
Postman says his initial reaction wasn't
elation at discovery but rather, " What did
we miss? " Right away, the researchers went
through every possible source of light other
than stars and galaxies. After all, Postman
adds, " We are using the LORRI camera on
New Horizons in a way for which it was
never intended. " And the team was thorough.
The researchers checked for potensk
yand tele scope .o r g * OCTOBER 2023 25
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Sky & Telescope - October 2023

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