Sky & Telescope - April 2023 QR - 57

Going Deep by Ivan Maly
The Hickson Catalog's Deep End
These compact galaxy groups make for pleasing, but challenging, views in the eyepiece.
W
hen Canadian astronomer Paul
Hickson published his survey of
compact galaxy groups in 1982, he was
driven by his interest in their dynamical
peculiarities and their role in galaxy
evolution. Before the century was out,
however, he found himself a soughtafter
guest and lecturer among amateur
astronomers who gathered with their
telescopes under dark skies to observe
his groups visually. Amateurs' interest
in Hickson's catalog was emblematic of
the new direction the hobby was taking
as scientific information and largeaperture
telescopes were becoming more
accessible.
Visual observers have since pursued
other galaxy groups, such as those in
catalogs that American astronomer
James Rose or Armenian astronomer
Romelia Shakhbazian assembled. Yet
the 100 Hickson groups remain the
most popular. The more frequently
observed Hicksons comprise, or at least
are anchored by, galaxies from the New
General Catalogue of Nebulae and Clusters
of Stars, which are comparatively
bright. We have previously highlighted
some of these brighter groups (see,
e.g., S&T: Nov. 2017, p. 34). The typical
Hickson group, however, contains no
NGC galaxies at all.
As I'm completing my visual survey
of the Hickson catalog, the faintest
members of quite a few groups still
elude me, even though I have access to
very dark mountain skies and a 20-inch
scope. To decide if you are up for jumping
into this catalog's deep end, try
the following selection of targets well
placed in the April sky and see if they
whet your appetite for further pursuit.
Dipping Our Toes
Transiting near the zenith in the early
evening is Hickson Compact Group
36 in Cancer. Look for HCG 36 some
6.3° east-southeast of Delta (δ) Cancri.
This group is anchored by the
relatively large and bright 14.1-magnitude,
1.5′-long galaxy IC 528. The
companions, however, are from the
Catalogue of Principal Galaxies. The
galaxies of HCG 36 are all spirals, and
each displays a different inclination to
our line of sight. After tracking down
this group with the 20-inch telescope
of the Buffalo Astronomical Association
observatory under rural skies, I
detected all four members at 450×.
Components of a Hickson group are
referred to by letter designations, which
usually run from A through D in order
of brightness. HCG 36's easternmost
component, HCG 36D, displayed its
elongated form, as did the dominant
central galaxy, HCG 36A (IC 528). The
latter, however, appeared to terminate
south of HCG 36C, indicating that I was
only seeing its inner core. In the case
of the southernmost component, HCG
36B, which is a nearly edge-on spiral,
I only noted the compact central core.
Subsequently, observing with my own
20-inch from a much darker location, I
GRACEFUL GROUPINGS You'll need a big telescope and dark skies to observe the galaxy groups outlined here. But if you succeed, you'll be well
rewarded by having stepped up to the challenge. We've used Hickson's original lowercase labeling for the components in the images presented here.
HCG 47
40′ × 15′
b
c
d
a
HD 90445
7.3
HCG 47
450×, 10′
sk yand tele scope .o r g * APRIL 2023 57
ALL IMAGES POSS-II / STSCI / CALTECH / PALOMAR
OBSERVATORY; ALL SKETCHES IVAN MALY

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