Sky and Telescope - May 2017 - 31

HERON G A L A X Y: DOUG M AT THE WS & E. J. JONES / A DA M BLOCK / NOAO / AUR A / NSF; SK E TCH: DA LE HOLT; MICE G A L A X Y: N ASA / HOLL A ND FORD (JHU) / ACS SCIENCE TE A M / ESA

BRIGHT EYE
Even under difficult
skies, with the right
scope you might
be able to pull out
the luminous cores
of these interacting
galaxies, particularly
the bright "eye" of
the Heron, NGC
5394. This sketch
was drawn using a
20-inch (505-mm)
reflecting scope and
a deep-sky video
camera.

The Heron Galaxy

NGC 5394

NGC 5394
NGC 5395

NGC 5395. The opposing arm ("head" and "bill") was spied
as a tenuous smoke trail to the northwest.
A near-infrared study published in the Astronomical Journal
in October 2005 concluded, "This tightly wound structure [in
NGC 5395], resembling a broken ring, supports the scenario
of a strong interaction in the system, probably a collision
with NGC 5394 crossing through the disk of NGC 5395." A
spectroscopic investigation published in the Monthly Notices of
the Royal Astronomical Society in 2015 by astronomers working with Calar Alto Legacy Integral Field Area (CALIFA) survey
data reported key evidence of the earlier collision, with a ﬂurry
of star-forming activity a few hundred million years ago, as
well as detailing an ongoing nuclear starburst in NGC 5394.

NGC 5395

BRIGHT-EYED BIRD The interacting pair NGC 5394 and NGC
5395 lie about 165 million light-years from us in the direction of
Canes Venatici. This image was captured through the 20-inch f/8
Ritchey-Chrétien telescope at Kitt Peak Observatory
during the Overnight Telescope Observing Program.
N

The Mice

The Mice
Vorontsov-Velyaminov dubbed the two interacting members
of NGC 4676 that form Arp 242 (VV 224) the "Playing
Mice" based on their two oval bodies and long tails. This duo
lies in northern Coma Berenices, just 1.5° south-southeast of
the "Hockey Stick" galaxy NGC 4656, a well-known, highly
distorted edge-on. But NGC 4676 lies far in the background
at a distance of 300 million light-years in the outskirts of the
rich Coma Galaxy Cluster (Abell 1656).
Astronomers consider the Mice the archetypal prograde
merger of equal-mass disc galaxies - colliding with the same
sense of rotation and orbital motion. In a seminal 1972 paper
"Galactic Bridges and Tails," Alar Toomre (MIT) and Juri
Toomre (University of Colorado) used computer simulations
to trace the evolution at various time intervals and successfully reproduced the tidal features of the Mice. Joshua Barnes
(University of Hawai'i) used a more sophisticated dynamical
simulation in 2004 and concluded the collision occurred 170
million years ago with a ﬁnal head-on encounter and coalescence destined in 540 million years.
NGC 4676A is a small, high surface brightness oval.
Through the 48-inch, it extended 24″ × 16″. Its singular visual
feature is a thin tidal tail shooting 80″ due north and dimming
out just east of a 17.3-magnitude star. The surface brightness
of this spike is the highest for any long tidal tail and is a good
target for 12-inch or larger scopes under pristine skies.

NGC 4676A

NGC 4676B

PLAYMATES Interacting pair NGC 4676, commonly known as
"The Mice" because of the long tidal tails streaming from the
galaxies, lies 300 million light-years away. This image, taken with
the Hubble Space Telescope's Advanced Camera for
Surveys, shows the charming yet catastrophic merger.
N

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* MAY 2017

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Sky and Telescope - May 2017

Table of Contents for the Digital Edition of Sky and Telescope - May 2017