Sky & Telescope - April 2023 QR - 58

APRIL 2023 OBSERVING
Going Deep
DIGGING DEEP Use these images
as finders to guide you to each
HCG target. Letter designations
are from Hickson's 1993 catalog.
Dimensions are indicated in each.
A SEXTET OF GALAXY
GROUPS The author sketched the
targets outlined here at the eyepiece
of his 20-inch telescope. (Magnifications
and fields of view are
indicated.)
HCG 36
12′ × 12′
HD 78574
8.9
d
a
b
c
c b
d
a
HCG 39
17′ × 17′
HD 81980
6.3
could also see a 16th-magnitude galaxy
(SDSSCGB 51371.3) off the western edge
of IC 528. While Hickson didn't include
it in HCG 36, its redshift indicates that
it is indeed a member of the same group.
The galaxy-rich constellations that
HCG 36
450×, 5′
transit around midnight in April are
a treasure trove of intriguing Hickson
targets. To start with, HCG 39 in
Hydra may well be the most difficult
group in the catalog to resolve into
individual galaxies. HCG 39 lies a bit
more than 2½° west of 3.9-magnitude
Iota (ι) Hydrae and 8′ southeast
of 6.3-magnitude HD 81980. A magnification
of 570× wasn't enough for
me to resolve components A, B, and
C into anything other than a nearly
1′-long streak resembling a flat, edgeon
galaxy. Component D, 20″ off the
chain's northwestern end, was cleanly
separated with the same magnification.
Curiously, I noted that the galaxy
next to the star that lies 2′ southeast
of the group was not visible, although
photographically it appears at least as
prominent as HCG 39D. On the night
of my observation, I recorded poor seeing,
which undoubtedly didn't help the
task of separating the group's galaxies.
Unsteady seeing frequently accompanies
the kind of excellent transparency that
we had that night - a trade-off that
will often limit your ability to observe
faint Hicksons, unless you have access
to professional-quality astronomical
locations.
Next, head over to Ursa Major,
where you'll find HCG 41 around 4.2°
northwest of 3.5-magnitude Lambda
58 APRIL 2023 * SKY & TELESCOPE
(λ) Ursae Majoris, or Tania Borealis,
the Bear's hind toe. A 6.3-magnitude
star (HD 86166) stands guard some 11′
north-northeast. It's a beautiful little
collection of three edge-on spirals and
one lenticular. The central clustering
of three galaxies (A to C) is followed by
the smallest group member, D, to the
east. Photographs show an additional
edge-on galaxy just southwest of the
group. Under excellent conditions at my
remote observing site and with 360×, I
couldn't detect the non-member galaxy,
while HCG 41D looked like a soft
image of a threshold star. I did, however,
observe components A through
C, with their individual extents and
orientations. There are few sights in
the universe that, to my eye, match the
dynamism of a galaxy group seemingly
flying in all directions. HCG 41 presents
an excellent example of this appearance
- even though it's tiny and faint. From
a distance of roughly 400 million lightyears,
only HCG 41A is brighter than
magnitude 15 and exceeds 1′ in size.
In this instance, the impression of the
objects flying apart has something to do
with reality: The galaxies have widely
different redshifts and, as Hickson
noted in his 1993 opus, must be physically
unrelated.
HCG 39
570×, 5′
Diving Deeper
So, can one actually see any gravitationally
induced structural features
in a Hickson group? The answer is a
cautious " yes, " so long as your expectations
are tempered. Take, for example,
HCG 47 in Leo, which you'll find a bit
more than 4½° east-northeast of Regulus
and 1.6° west-southwest of 5.5-magnitude
46 Leonis. It consists mostly of
small, featureless Morphological Catalogue
of Galaxies members. Component
A, however, is 13th-magnitude UGC
5644, a nearly face-on spiral with two
arms that are visible in photographs. I
see the western arm as much brighter
and extending toward HCG 47B, which
suggests gravitational interaction to me.
At 450×, I was able to see a clear trace of
this arm even under the rural skies of
the Buffalo club's observatory. In fact,
I noted that this arm segment seemed
more prominent than the galaxy's core,
resulting in a very asymmetrical appearance.
As expected, component B is the
second-brightest member of this group.
Another close galaxy pair, comprising
members C and D, is immediately to
the north. HCG 47D appears larger, but
HCG 47C sports the brighter nucleus.
HCG 49 is our northernmost target
- look for it some 5½° north-northwest
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