Sky & Telescope - October 2021 - 57

Going Deep by Ivan Maly
The Odd World
of Peculiar Galaxies
Galaxies with singular features present intriguing
views in the eyepiece.
" E
very galaxy is peculiar. " So wrote
Halton Arp in the introduction
to his pioneering work, Atlas of Peculiar
Galaxies (1966), specifying, " when
looked at closely enough. " At the time,
Arp was working at the largest telescope
in the world - the 200-inch at Palomar.
Arp's mentor, Edwin Hubble, had
famously classified galaxies into various
subsets, depending on their elongation
and how tightly their arms (if present)
wound around the core. An ever-increasing
body of data, however, suggested that
galaxies not conforming to this classification
system abounded in the universe.
Moreover, deep photographs showed
unexpected - and odd - details in
familiar and seemingly orderly galaxies.
Arp obtained hours-long photographic
exposures of these distant
" island universes " (as 18th-century
philosopher Immanuel Kant described
galaxies) that revealed subtle structural
features in many of them. He used
these details to expand upon his mentor's
original classification scheme. Arp
subsequently compiled a list of galaxies
with unusual morphologies, in what
eventually became the Atlas of Peculiar
Galaxies. The astrophysical interpretation
of these peculiarities has changed
- in many cases quite dramatically -
since the time of that seminal publication,
but nevertheless galaxies from
Arp's catalog continue to attract the
attention of professional astronomers.
Seeing any level of detail in a galaxy
visually using amateur telescopes can be
a challenge. However, any observer who
has mastered the art might soon begin
to wonder if so-called grand-design spirals
are the only ones out there. This is
where Arp's catalog can come to the rescue.
With the advent of more
affordable, large-aperture
telescopes, amateurs
began to observe
these often-faint
Arp galaxies systematically.
As with
other challenging
deep-sky objects,
dark skies can be at
least as important, if
not more so, than telescope
aperture when striving
to see all 338 objects in the
Atlas. High magnification is essential for
discerning the details of the peculiarities
of these galaxies, as are finder charts for
guiding you to their locations.
Let's take a look at a sample of
targets from the Atlas - hopefully this
selection will whet your appetite for
these space oddities.
The Peculiar Set
Arp No.
Object
164
121
121
100
100
133
308
308
230
126
NGC 455
MCG-1-3-51
MCG-1-3-52
IC 18
IC 19
NGC 541
NGC 545
NGC 547
IC 51
UGC 1449
Arp 164
(7′, 360×)
ARP 164 Also known as NGC 455,
this Arp specimen in Pisces is a prime
example of a peculiar galaxy. Like many
objects in Arp's Atlas, NGC 455's messy
look is likely due to a close encounter with
another galaxy.
SKETCHES AT THE EYEPIECE The
drawings (all north up) represent the view as
seen through the author's 20-inch scope.
Surface
Brightness Mag(v)
13.4
13.8
12.8
14.0
13.8
13.2
13.5
12.7
13.2
12.5
12.7
13.7
14.1
14.6
14.1
12.1
12.2
12.2
12.8
13.3
Size
1.9′ × 1.2′
2.1′ × 0.6′
1.1′ × 0.6′
1.0′ × 0.7′
1.8′ × 1.7′
2.4′ × 1.6′
1.3′ × 1.3′
1.5′ × 1.1′
1.1′ × 0.5′
RA
0.6′ × 0.6′ 00h 59.4m
00h 28.6m
00h 28.7m
01h 25.7m
01h 26.0m
01h 26.0m
00h 46.4m
01h 58.1m
Dec.
01h 16.0m +05° 11′
00h 59.4m
-04° 48′
-04° 48′
-11° 35′
-11° 38′
-01° 23′
-01° 20′
-01° 21′
-13° 26′
+03° 05′
Visually, an object's size is often smaller than the cataloged value and varies according to the aperture
and magnification of the viewing instrument. Right ascension and declination are for equinox 2000.0.
sk yand tele scope .o r g * OCTOBER 2021 57
SKETCH: IVAN MALY; ARP 164: JUDY SCHMIDT / CC BY 2.0

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Sky & Telescope - October 2021

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