Sky & Telescope - July 2020 - 9

GALAXIES

Knife Edge Galaxy Throws Astronomers for a Loop
IN 2006, AMATEUR ASTRONOMER

T WO LOOPS: D. M A RTÍNE Z-DELG A DO E T A L. / THE ASTR OPHYSICAL
JOUR NAL 20 0 8; SING LE LOOP: VA N DOK K U M E T A L. /
ASTR OPHYSICAL JOUR NAL LE T TERS 2019; MERCURY TERR AIN: N ASA
/ JHU -A PL / CA R NEGIE INSTIT U TION OF WASHINGTON

R. Jay GaBany obtained an iconic image
of NGC 5907. Using his observatory, he
captured an eye-catching double loop
of diffuse starlight wrapping around
the edge-on galaxy, also known as the
Knife Edge Galaxy. The image appeared
in the December 10, 2008, Astrophysical
Journal as part of a team effort led by
David Martínez-Delgado (Astronomical Calculation Institute, Germany). In
addition to a known elliptical feature
- most likely a tidally disrupted dwarf
galaxy - GaBany's image clearly shows
a second loop.
Then the Dragonfly telephoto array
(S&T: May 2019, p. 64), which excels
at detecting low-surface-brightness
features, failed to see the second loop.
Pieter van Dokkum (Yale University)
and collaborators published these
results in the October 1, 2019, Astrophysical Journal Letters.
But Ignacio Trujillo (Institute of
Astrophysics of the Canary Islands,
Spain), who contributed to the 2008
paper, counters that the Dragonfly team
uses a nonstandard method to quantify
the depth of their images. "They [do]
not reach particularly deep low-surfacebrightness levels," he claims, much to
the annoyance of van Dokkum.

Now, Oliver Müller (University of
Strasbourg, France) and colleagues have
observed the galaxy with the 1.4-meter
Milankovic´ telescope in Serbia. And
they, too, failed to see signs of the second loop, as they write in the December
2019 Astronomy & Astrophysics.
Martínez-Delgado argues that at
least some of the Milankovic´ images
were taken when the galaxy was at
relatively low elevation, hampering the
detection of extremely diffuse features.
"Their data cannot shed light on this
controversy and are still consistent
with shallow images taken by tens of
amateurs with very modest equipment
in the last decade," he says.
Unfortunately, the raw data from
the 2006 image were lost in a harddrive crash and cannot be reprocessed
independently. But Martínez-Delgado
says more follow-up observations by his
team are in progress. "I'll give a suitable
answer [to the criticism] in a refereed
publication soon," he says. Adds Trujillo:
"I am confident that the mystery will be
finally solved later this year."
■ GOVERT SCHILLING
u Top: R. Jay GaBany captured a double loop
around NGC 5907. Bottom: Later images,
including this one from the Dragonfly telephoto
array, fail to show the second loop.

This suggests that volatile-rich materials
might be stored deep within large areas
of the planet's crust.
"It's fascinating work and I think it
will spur quite a lot of research," says
David Blewett (Johns Hopkins University Applied Physics Laboratory).
Confirmation of whether volatilerich materials still exist underground
- and what they are - could come from
the BepiColombo spacecraft, due to
arrive at the innermost planet in 2025.
■ JAVIER BARBUZANO
u NASA's Messenger spacecraft captured this
close view of chaotic terrain. The area in the image is 248 km across.
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 0

9


http://www.skyandtelescope.org

Sky & Telescope - July 2020

Table of Contents for the Digital Edition of Sky & Telescope - July 2020

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