Sky & Telescope - April 2021 - 22

Trio in Leo

W DELICATE SPIRAL By far the most detailed galaxy of the Leo Triplet,
M66 exudes dynamism from its close encounter with NGC 3628. I used
408× with the 28-inch scope on the night of June 3, 2016, at the site of
the Oregon Star Party to make this sketch.

the [northern] arm seems a likely suspect... [it wasn't] Lots of
galaxy detail at 408×. 21.82 SQM. "

Sometimes my sketches do most of the talking in my
notebooks, and I only write down what the drawing doesn't
show. My notes imply that my priority that night was seeing the supernova, but I stopped looking for it after about
ﬁve minutes or so and concentrated on sketching the galaxy.
In retrospect, I'm not surprised I couldn't spot the 16thmagnitude supernova since it turned out to be in a relatively
bright part of the galaxy, but so what - this turned out to be
far-and-away the ﬁnest view of M66 I've had so far.

NGC 3628
I also have a soft spot for edge-on galaxies and tidal tails. At
the northern tip of the Triplet, NGC 3628 ﬁts the bill on both
counts. But it also has the lowest surface brightness of the
three galaxies, making it the most difﬁcult to see well. Under
a light-polluted suburban sky NGC 3628 may even be invisible, but it blossoms wonderfully under a truly dark sky.
The ﬁrst thing that's noticeable is the galaxy's elongated
east-west shape. Next you might see the fairly wide, diffuse
dark lane running through its center. The northern slice of
NGC 3628 above the dark lane is much brighter than the
slice below, and on some nights the southern portion may be
difﬁcult to detect at all. The dust lane often fades into the sky
22

APRIL 2 021 * SK Y & TELESCOPE

background under such conditions.
However, on the best nights the apparent length and width
of the galaxy is noticeably increased by a faint envelope of
fuzziness - an effect of tidal disruption by M66. You'll need
dark and transparent skies to see this feature.
Interestingly, NGC 3628 may also be a barred spiral galaxy. This is based on the subtle X pattern of its core, which
is evident in many images but to my knowledge hasn't been
seen visually. Although the X pattern isn't absolute proof this
is a barred spiral, it's an intriguing clue.
The portion of the tidal tail depicted in my sketch is only
the brightest part of a much longer stream. As you can see
in the deep-exposure photo at right, it extends to the east at
least three times the length of NGC 3628. Toward the end
is a knot that's recently been identiﬁed as the tidal dwarf
galaxy Leo-TDG. The formation of a dwarf galaxy within a
tidal tail is a wonderfully exotic phenomenon. Leo-TDG most
likely formed when NGC 3628 and M66 had their closest
encounter approximately 800 million years ago, when they
came within only 82,000 light-years of each other. That must
have been quite a sight! In addition, radio observations have
shown that NGC 3628 resides in a vast pool of neutral hydrogen that also reaches toward M66, another clue supporting
the tidal interaction of these two galaxies.
Although much too faint for me to see, there's an intriguing string of objects just south of the core of NGC 3628 that
Halton Arp described in a 2002 paper. Two of these objects
are quasars, which he claimed were ejected from the nucleus
of NGC 3628 due to their curious alignment with the galaxy's core and various associated outﬂows. The redshifts of
these quasars are quite different from each other, but Arp
maintained that redshifts aren't always an indication of distance when applied to quasars. Unfortunately, his insistence
on this may have contributed to toppling him from the ranks
of top-level astronomers.
Back in the brightness range I can actually see, my best
sketch of NGC 3628 was made from a dark site with my
28-inch scope:
" An excellent view, especially considering that it's 2/3 of the way
to the western horizon. The dark lane is prominent, and long,

HOWA RD BA NICH

X GALAXIES HAVE TAILS, TOO Note how NGC 3628 has a thick
envelope of galactic fuzz surrounding it, almost like it's wobbling along
its major axis and shaking stars out of its spiral arms. Look closely to see
the X-shaped core, an indication that NGC 3628 may be a barred spiral.
The long tidal tail stretches about three times the length of NGC 3628 to
the east (left) in this image, with the tidal dwarf galaxy (Leo-TDG), marked
with the gold lines just north of the background galaxy IC 2787. I know
of only one visual observation of this object - by Jimi Lowrey with his
48-inch scope from West Texas. Also, the inset box shows the objects
Halton Arp claimed were ejected from the core of NGC 3628.

Sky & Telescope - April 2021

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