Sky and Telescope - November 2017 - 38

Hickson Compact Groups

HCG 16

Eridanus

NGC 835

NGC 833

NGC 838

NGC 839
N

HCG 31

NGC 1741
PGC 16573

PGC 16570

HCG 21 lies about 1° north of Tau (τ) Eridani and is comprised of ﬁve well-separated members. They make easy targets
for large-aperture scopes, and the three more obvious members will show in an 8-inch in dark enough skies. NGC 1099,
the brightest in the group, is a spiral that's worth examining
closely for any hint of spiral structure extending away from a
luminous core.
Only a little dimmer is NGC 1100, which lies about 5′
east-northeast of NGC 1099. Its lucent core anchors an elongated halo. NGC 1098, 7′ northwest of NGC 1099, looks like
a very typical elliptical galaxy: round with a visible core. The
last two members of the group, NGC 1092 and NGC 1091,
form a pair to the north. Look for two similar patches of fuzz
that brighten toward their centers.
More challenging is HCG 31, which lies about 1° 35′
northwest of Beta (β) Eridani. I spotted two objects rather
easily with my 18-inch scope. Most vivid is the combined
light of NGC 1741 and PGC 16573, which are unresolved.
This mottled, oval patch is irregular in appearance. About a
minute to the southwest, PGC 16570 (some sources identify
this as NGC 1741B) appears as a streak angled northeastsouthwest. A circular patch 2½′ to the southeast is IC 399.
The SIMBAD Astronomical Database (simbad.u-strasbg.fr)
lists this as the "g" component of HCG 31, but few other
sources include it in the group. No other members are seen.

HCG 7

NGC 196

t p DOUBLE ENTRY Top: With his 14-inch scope and deep-sky video
set-up, Dale Holt was able to detect structure in what appears to be the
central object of HCG 16, the face-on spiral galaxy NGC 835. Middle:
Because compact galaxy groups frequently contain closely interacting
galaxies, many of the entries in the Hickson catalog can also be found
in Halton Arp's Atlas of Peculiar Galaxies (1966). Shown here is the red
plate used to identify what Arp considered to be the "irregular clump"
galaxies of Arp 259, which also make up the core of HCG 31.

NGC 197

NGC 192

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

Cetus hosts two groups on our sampler tour, both quartets of
relatively easily seen galaxies. HCG 16 is 4½° degrees east of
Zeta (ζ) Ceti. Its four members arc around a 9th-magnitude
star. NGC 835 is a mottled oval with a bright core and a hint
of spiral structure in its fainter halo. NGC 833, nearly in
contact to the west of NGC 835, is more elongated and angled
east-west with a noticeable core. NGC 838 lies about 4′ east
of the pair and appears round with more obvious center.
The faintest member of the group is the spiral galaxy
NGC 839, southeast of NGC 838. It's a bit elongated, angled

t SLOW AND STEADY This "close-up" image of HCG 7, showing two
spiral galaxies (NGC 192 and NGC 197) and a lenticular galaxy (NGC
196), comes to us courtesy of the Hubble Space Telescope. HCG 7, like
many other entries in the Hickson catalog, appears to be remarkably
calm for a group of interacting galaxies. Astronomers expect to see high
rates of star formation in merging galaxies, but the production of HCG 7
members seems to have been relatively steady over time. It's possible
that the galaxies have (and are still) producing stars with only endemic
gas not affected by the gravitational forces of galactic mergers, but
explanatory models for such a situation are still being worked out.

HCG 16: DA LE HOLT; HCG 31: ATL AS OF PECUL AR G AL A XIES: H A LTON A RP / N ASA / IPAC E X TR AG A L ACTIC
DATA BASE (NED) / CA LTECH / CA R NEGIE OBSERVATORIES; HGC 7: ESA / N ASA / HUBBLE SPACE TELESCOPE

Cetus

http://simbad.u-strasbg.fr

Sky and Telescope - November 2017

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