Sky & Telescope - May 2020 - 59

N
M82
Hoag 1

p CHALLENGING TRIPLET Hoag 1 appears
as a faint dot southeast of M82 in this 7′ × 10′
blue POSS-II image. The arrows indicate the
positions of Hoag 2 and Hoag 3.

nitude of 19.5 and a redshift of 2.05.
I viewed this object's ten-and-a-halfbillion-year-old light on the night of
May 13, 2017, with my 32-inch reﬂector
at 650× in seeing of 4/10 and transparency of 6-7/10. It was stellar, of course,
and difﬁcult in those conditions. At the
eyepiece there was no hint of the two
fainter, nearby Hoag quasars. I plan to
try for those two on a better night.

galaxy rings: polar, collisional, accretion,
and those arising from secular evolution.
Polar-ring galaxies are likely formed
when a smaller galaxy is gravitationally
captured by a larger lenticular or elliptical galaxy, and waves of star formation ensue. I've observed NGC 4650A,
NGC 660, and NGC 2685, which are
examples from this category. On a
recent trip to Chile I was thrilled to also
spot NGC 5128 (Centaurus A) with
my unaided eye. It's arguably the most
famous galaxy in this group.
Galaxies sporting collisional rings
are perhaps best exempliﬁed by the
Cartwheel Galaxy (ESO 350-40) in
Sculptor. In this case, an interloper has
collided head-on with another galaxy,
giving rise to nearly symmetric density
waves of gravitational disruption that
trigger star formation. I have viewed
the 15th-magnitude Cartwheel with my

HOAG 1: POSS-II / STSCI / CA LTECH / PA LO M A R OBSERVATORY;
NGC 4650A: THE HUBBLE HERITAG E TE A M / N ASA / ESA

Several Kinds of Rings
After the National Geographic Society - Palomar Observatory Sky Survey
(POSS) was completed in 1958, astronomers trawled this jewel of a resource
for previously unknown objects. The
Russian astronomer Boris VorontsovVel'yaminov searched systematically for
interacting and unusual galaxies. His
Atlas and Catalog of Interacting Galaxies,
published in 1959 and later expanded
in the 1970s to 852 items, listed many
ring-type galaxies, including a category
of "detached-ring" objects similar to
Hoag's ﬁnd. Three of Halton Arp's
peculiar galaxies (numbers 146, 147,
and 148) have ring structures. During
the past seven decades reports of new
ring galaxy discoveries have trickled
in. Proliferation can foster confusion
until organizing principles of formation and evolution are established. The
ﬁeld appears to be without consensus,
but agreement is simmering in some
areas. In a paper published in 2017,
Burçin Mutlu Pakdil of the University of
Minnesota and her collaborators nicely
summarize the four main categories of

32-inch reﬂector at the Okie-Tex Star
Party, but I couldn't distinguish the
"spokes." On the POSS plates and in the
eyepiece, the Cartwheel appears reverse
to Hoag's Object: The ring is brighter
than the core.
IC 2006 in Eridanus is a good
example of a galaxy with a ring that
formed through accretion processes.
In these cases, an early-type galaxy
accreted external matter - possibly
through interactions at a distance with
other galaxies - that went into structuring the ring. IC 2006 is ephemeral
at best on the red POSS plates, and on
the blue plates it's large (4′ in diameter),
circular, and ghostly faint. I have yet to
observe this galaxy.
The most common ring galaxies
are likely forged via secular evolution
through slow, steady interaction with
their environment. This class is largely
composed of barred spirals, although
non-barred spirals have also been
observed to sport rings. These galaxies
morph over time via internal processes,
which include star formation induced by
spiral density waves and bulge growth
through the funneling of gas into the
core in barred spirals (which could also
contribute to starburst activity).
So, into which of these ring-forming scenarios does Hoag's Object ﬁt?
Although consensus hasn't been reached,
observational clues largely point to a
major accretion event some 2 to 3 billion
years ago onto the core of the central
elliptical galaxy, likely triggered by interaction with another galaxy.

Epilogue
The proliﬁc science and sci-ﬁ writer
Isaac Asimov is credited with the phrase
"That's odd!" in reference to a moment
of discovery. The legacy of Arthur
Hoag's ﬁnds will live on if we pursue
our personal and collective dreams of
exploring outliers in the sky. Beauty -
even when it's "odd"- is in the eyepiece
of the beholder.
p RING AROUND A GALAXY A prime
example of a polar-ring galaxy, NGC 4650A
lies around 130 million light-years away in Centaurus. As the category name implies, the ring
rotates around the poles of the central galaxy.

¢ DAVE TOSTESON enjoys searching
the literature and news outlets for unusual and yet-to-be-explained objects
to observe.
s k y a n d t e l e s c o p e . o r g * M AY 2 0 2 0

http://www.skyandtelescope.org

Sky & Telescope - May 2020

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