Sky & Telescope - July 2020 - 59

idly star-forming super spirals are all
smaller than the group median.
The high rate of star formation in
galaxies this massive is certainly curious
- normally they'd be populated with
old and red stars. Ogle and his team
studied various factors that contribute
to the star-formation rate, such as cold
gas accretion rates through the halo,
accretion shocks, as well as the density
of the intergalactic medium. They speculate that super spirals could be a remnant population of massive disk galaxies
that somehow still maintain active star
formation, i.e., the cold gas supply that
eventually becomes stars isn't cut off
for some reason. One scenario proposes
that when this gas supply is eventually quenched, super spirals evolve into
lenticulars or ellipticals. Identifying fossil giant disks devoid of star formation
around lenticular or elliptical galaxies
would support this theory.
Based on their sample, the team
estimates that there are about 60 super
spirals per cube of 3.3 billion light-years
on a side, which is only 6% of the space
density of elliptical galaxies of the same
luminosity. This makes super spirals
very rare. In fact, they're so rare that
even the most powerful cosmological
simulations don't have the capacity to
predict how many super spirals there
should be nor how they form.

FINDER IM AG ES: POSS-II / STSCI / CA LTECH / PA LO M A R OBSERVATORY (4); SOURCE
10 & 48 CLOSE-UPS: SLOA N DIGITA L SK Y SURV E Y; SOURCE 17 & 37 CLOSE-UPS:
PATRICK OG LE E T A L. / THE ASTR OPHYSICAL JOUR NAL 2016

Observing Super Spirals
What can amateurs expect to view in
the eyepiece? Fortunately, by deﬁ nition,
at 8 to 14 times the luminosity of the
Milky Way super spirals are relatively
bright. This makes them 2.5 to 3 magnitudes brighter than what a galaxy
similar to ours would be if placed at
the same distances.
In addition, AGNs give a boost to visibility - the halos of these galaxies are
much fainter than their cores. In some
cases, amateurs may see only the quasar
or Seyfert nucleus (as I did with one of
the objects I observed).
Just one of the 53 galaxies in Ogle's
sample was previously cataloged in
a familiar source: Number 50 in
the list is CGCG 122-067. The rest
either have Sloan Digital Sky Survey

This means that the largest spirals in the sample exceed
our Milky Way by up to 14 times in luminosity and four to
five times in size.
(SDSS) or Extended 2 Micron All-Sky
Survey (2MASX) designations - not
exactly tip-of-the-tongue monikers.
The sample's magnitudes range from
15.9 to 16.9, potentially making all of
them accessible to a 20-inch reﬂector.
Visual inspection of the photomontage of SDSS images on page 57 shows
that every galaxy has a moderately
bright nucleus and about two-thirds
have discernible spiral structure. For
amateur-size instruments the National
Geographic Society - Palomar Observatory Sky Survey (POSS) is a much better
gauge of visibility. Several of the larger
galaxies, such as numbers 16, 22, and
50, should show spiral structure in
large-aperture reﬂectors.
In moderately poor observing conditions at the 2018 Texas Star Party I
turned my 32-inch f/4 reﬂector on four
super spirals. Number 10 in Ogle's
list is a quasar, which I saw merely as
a 17th-magnitude stellar object, with
no sign of the surrounding galaxy. It
reportedly has the highest star-formation rate of the group, but this may be
overestimated from the infrared heating
effect of its active nucleus.
Number 17 is the brightest galaxy
in a cluster identiﬁed and cataloged by
Ogle and collaborators (OGC 0586).
This super spiral is also classiﬁed as
a Seyfert galaxy, in which the active
nucleus is partially obscured due to
viewing angle. In the eyepiece of the
32-inch, some traces of the galaxy
extensions were visible to the northwest and southeast. I'm sure I'd spot
some additional cluster members
in better conditions with prolonged
inspection, but I didn't this time.
Number 37 in Ogle's list is also
a brightest galaxy in a cluster. This
super spiral sports a binary nucleus,
one central and one offset, indicating a likely merger between a larger
and a smaller galaxy. It also displays
a markedly disturbed morphology

with tidal arms and tails splayed to
the northwest and southeast, but the
POSS-II image only hints at them. At
the Texas Star Party, I saw only the
central portion of the 16th-magnitude
galaxy, inclined at a position angle
of about 130°. I also spotted several
other members of the host cluster,
GMBCG J240.41924 +27.30444. In
fact, the two ellipticals around 50″ east
and east-southeast of the super spiral
actually appeared brighter.
The last source I observed in the hour
or so I spent on super spirals at the star
party was the nondescript Number 48
in the list. At magnitude 15.9, it was
the brightest of the four I viewed. It was
also the third smallest of Ogle's sample
at slightly less than 200,000 light-years
across. The SDSS image clearly shows
the surrounding galaxy, but in the
POSS-II image that's all but washed out.
There was no hint of outer spiral arms
in the eyepiece of the 32-inch, and it
would likely take perfect conditions to
tease them out at the galaxy's 1.8 billion light-year distance.
I plan to spend more time on this
project. I hope to dissect Number 50, the
closest superluminous spiral galaxy at
1.2 billion light-years, with the apparent largest size of about 35″ × 30″, and
Number 11, the most distant at 3.5
billion light-years. All but a few of these
goliaths are in the Northern Hemisphere
sky between 8 and 17 hours of right
ascension. Next time you have access to
a large reﬂector try adding a new type of
super galaxy to your collection.
¢ DAVE TOSTESON chases deep-sky
objects in between keeping up on all the
latest medical developments.
FURTHER READING: To read the original
paper go to https://is.gd/ogle_apj (Ogle,
P. M. et al., "Superluminous Spiral Galaxies," The Astrophysical Journal, Vol. 817,
p. 109, 2016).
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

https://www.is.gd/ogle_apj http://www.skyandtelescope.org

Sky & Telescope - July 2020

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