Sky & Telescope - June 2021 - 11

GALAXIES

Amateur Astronomers
Reveal Galactic Echoes

DUILIA DE MELLO / DIM TE A M

IN A PROMISING proof of concept, a
professional astronomer has teamed
up with ﬁve amateurs to capture the
echoes of long-ago mergers ringing
nearby galaxies. The team is now looking to expand their number of galaxies
- and observers.
Duilia de Mello (Catholic University
of America) is leading the Deep Images
of Mergers (DIM) project to capture
the faint shells of stars on the outskirts
of a class of galaxies thought to have
had a merger in their past. The shells
are likely the remains of a less massive
galaxy disrupted during the collision.
To test this idea, de Mello, who is
from Brazil herself, engaged ﬁve Brazilian amateurs - Marcelo Wagner Silva
Domingues, Cristóvão Jacques Lage
de Faria, João Antônio Mattei, Eduardo de Jesus Oliveira, and Sérgio José
Gonçalves da Silva - to observe such
galaxies, including Centaurus A and
Arp 230.
Centaurus A was the proof of concept: The Victor M. Blanco 4-meter tele-

S The DIM team's image of Centaurus A (left) reveals shells of stars after processing (right).

ago merger. With 10 hours of integration, the team revealed four shells that
Hubble had also detected - and possibly
something more.
" We have hints of another outer
shell not seen in the Hubble image, but
it needs to be conﬁrmed with deeper
images, " de Mello says.
De Mello presented the preliminary
results at the January meeting of the
American Astronomical Society. If
you're interested in contributing observations to the team, learn more and
sign up at https://is.gd/DIMproam.

scope at the Cerro Tololo Inter-American Observatory in Chile had made
deep observations of the dusty edge-on
galaxy 13 million light-years away.
With more than 40 hours of combined
integration time, the team was able to
reveal the stellar shells the professional
telescope had seen previously.
The team then turned to Arp 230,
a galaxy about ﬁve times farther away.
Serendipitously, this galaxy turned out
to have Hubble observations, but the
team selected it for the large amounts
of neutral hydrogen gas along its edges,
probably knocked loose during a long-

MONICA YOUNG

IN BRIEF

First Black Hole Upsized

Farfarout Orbit Confirmed

Star Cluster Has Blackhole-filled Center

Discovered in 1964, Cygnus X-1 is the
first-known black hole orbiting a supergiant
companion. Now, new observations reported
in the February 18th Science suggest it's
even more massive than originally thought:
21 solar masses rather than 15. While previous radio parallax measurements had shown
Cygnus X-1 to be 6,000 light-years away,
James Miller-Jones (International Centre for
Radio Astronomy Research-Curtin University,
Australia) and colleagues used the Very Long
Baseline Array to fine-tune parallax measurements conducted over a seven-year baseline,
putting Cygnus X-1 at more than 7,000 lightyears away. So the black hole must be more
massive to power the same amount of radiation. The larger mass suggests that the star
- which astronomers already thought directly
collapsed into a black hole, sans supernova
- lost even less bulk than thought via stellar
winds toward the end of its life.
JULIE FREYDLIN

Astronomers have confirmed the most distant object in the solar system: a 400-km
rock nicknamed " Farfarout. " The team -
Chad Trujillo (Northern Arizona University),
Scott Sheppard (Carnegie Institution for
Science), and David Tholen (University
of Hawai'i) - found the object in January 2018, following their discovery of the
previous record-holder, dubbed " Farout. "
With additional observations they have
confirmed Farfarout's orbit and thus its
distance; it's now provisionally designated
as 2018 AG37. And it really is far, far out
at 132 astronomical units (a.u.) from the
Sun, nearly four times farther away than
Pluto. The orbit is elongated, though, so
while Farfarout travels out to 175 a.u., it
also comes in as close as 27 a.u., inside
Neptune's orbit. Its trajectory suggests a
past gravitational tussle with Neptune; the
two may interact again in the future.
MONICA YOUNG

The globular cluster NGC 6397 packs
more than 100,000 stars into a ball a few
dozen light-years across. Its crowded
center is a prime location for an intermediate-mass black hole (IMBH) with at least
100 times the Sun's mass. Eduardo Vitral
and Gary Mamon (both at the Institute of
Astrophysics of Paris) sought to detect an
IMBH hiding in the cluster's core by analyzing the stars' motions. But there wasn't
just one black hole in the middle - there
were several. The " dark mass " extends
to a few percent of the size of the cluster,
Vitral explains. While some of that might
be white dwarfs and neutron stars, the
astronomers calculate that most is stellarmass black holes. The results appear in
February's Astronomy & Astrophysics.
MONICA YOUNG

sk yandtelescope.org * JUNE 2 021

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Sky & Telescope - June 2021

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