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Subj: These are the most peculiar galaxies you can observe
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These are the most peculiar galaxies you can observe
Crashing, colliding, and tangoing, these peculiar galaxies reveal the grand
evolution of the cosmos.
Forty years ago, I penned my first of many articles for Astronomy. It ran in
the February 1981 issue as "Observing Peculiar Galaxies."
The term peculiar galaxies refers to a class of galaxies whose distorted shapes
resemble neither classical spiral galaxies nor amorphous elliptical galaxies -
and at the time, they were still mysterious. Just 16 years earlier, Halton Arp
had published his Atlas of Peculiar Galaxies, a product of his documentation of
objects while examining the Palomar Sky Survey. It contained a sampling of some
the most unusual galaxies whose warped structures defied explanation.
After the passage of four decades, our understanding of what makes such
galaxies peculiar has improved - thanks to the Hubble Space Telescope, giant
new telescopes with adaptive optics, and more powerful computers that can model
the gravitational dances of interacting galaxies.
Eyepiece improvements and larger telescopes have enabled the humble amateur
astronomer to see details that were impossible to make out decades ago. Digital
imaging technology allows amateurs to take professional-quality images that
were unattainable with film. But at the same time, urban areas have seen
tremendous growth, making it necessary to travel farther to get under truly
dark skies. After all these changes, it's only fair to revisit that topic with
a new eye.
What makes a galaxy peculiar?
With the great diversity among "normal" galaxies, one may wonder if every
galaxy has its own peculiarities. To an extent, there may be some truth to that
statement. Edwin Hubble created the basis of the classification system that is
used today nearly 100 years ago.
The "tuning fork diagram" contains spirals - normal and barred - in the two
prongs, with ellipticals galaxies in the handle. Irregular galaxies are
outside. Over time, the structure has been tweaked with two additional
classifications. Lenticulars are spirals without active star formation that
share characteristics with ellipticals. And we now use the term transitional
barred spirals to refer to the so-called "SAB-type" in G‚rard de Vaucouleurs'
galaxy classification system that he developed in the late 1950s.
Galaxies tend to occur in clusters, just like stars. But unlike stars, the
sheer mass of galaxies encourages them to interact and even collide. A galaxy's
stars, dust, and hydrogen gas are all influenced by an interaction. Even a
passage separated by a hundred thousand light-years provides gravitational tugs
potent enough to distort galactic structure. This all happens in extreme slow
motion: The passage of galaxies relative to one another takes hundreds of
millions of years or longer.
As you might expect, the No. 1 cause of peculiarities is collision. An
estimated 5 percent of galaxies in the visible universe are interacting. And if
we had infrared vision, that number would rise to 25 percent; galaxies that
emit mostly infrared light seem to interact at a higher rate. Other causes
include bursts of star formation (related to collisions or interactions with
intergalactic hydrogen clouds) and exploding cores (due to massive black
holes). Some have anomalously large clouds of dust or glowing ionized hydrogen
gas (HII regions). A small percentage are still mysterious. Given time and
technology, these will become understood.
What can you see?
The good news for amateur observers is that plenty of peculiar galaxies are
bright enough to be seen in 4- to 10-inch telescopes. Several can even be
picked up in binoculars.
Observing detail, however, is another matter. If you have not dabbled in
peculiar galaxies before, it is better to simply find these targets and not
worry if you don't see a peculiar feature.
To bring out the most detail possible, nothing beats a large aperture and dark,
unpolluted skies. Another strategy to eke out shreds of detail is to avert your
vision slightly to one side of the object, relying on your retina's more
light-sensitive rod cells. Perhaps they will reveal a bit of asymmetry, a faint
companion, a bright patch or arm, or a shadowy dark nebula.
Some observers study a photo or two before looking at their target. Bright
stars in the field may help you locate the most conspicuous aspect of the
galaxy's peculiarity. Keep in mind that photos may be reversed from what you
see in the eyepiece.
I enjoy every aspect of observing peculiars, from checking off another "faint
fuzzy" to seeing detail. If you don't go in expecting to see many details, you
won't be disappointed - or, from a more optimistic perspective, you may be
pleasantly surprised!
Taking up the challenge
My 1981 article featured 12 of my favorite peculiar galaxies. Since then, I've
observed many other peculiar galaxies with larger optics. Here are 12 of my new
favorites:
NGC 1228, NGC 1229, NGC 1230, and IC 1892 make up a chain of galaxies in
Eridanus. These four are also known by their designation in Arp's atlas, Arp
332. They are a faint quartet, in the 13th- to 14th-magnitude range, a
challenge for a 10-inch scope. Why do galaxies line up? Perhaps it's random
chance, or maybe they evolved together in their own "local group." NGC 1229 is
a Seyfert-type galaxy - one with an active, energetic nucleus due to its
central, supermassive black hole gorging on material.
NGC 2207 and IC 2163 are a pair of spirals in Canis Major. Many observers are
aware of this constellation's numerous star clusters like M41. But the Big Dog
also contains a number of galaxies, including this pair in the early stages of
colliding. The smaller (and more distant) of the two, IC 2163, looks
comma-shaped, with arms stretched by tidal forces.
M82 (NGC 3034) in Ursa Major is a starburst galaxy, so called because it forms
stars at a much greater rate than most galaxies. Called the Cigar Galaxy
because it is long and narrow with rounded ends, it's easy to see with small
scopes. This Cuban is shrouded in smoky dark nebulae that can likewise be seen
in small telescopes. With larger optics, the detail in the dusty shreds is fun
to observe. How much detail can you see?
NGC 3226 and 3227 are nowhere close to being the brightest galaxies in Leo. But
I like them because they are a "four-fer": The bright double star Gamma (?)
Leonis lies less than a degree west of the pair (which makes them easy to
find). NGC 3226 is an elliptical galaxy. NGC 3227 is a spiral. And NGC 3227 has
an active nucleus. In a telescope, they appear as two oval glows with compact
nuclei.
NGC 4027 in Corvus is a hook-shaped barred spiral galaxy. Its form is
reminiscent of the more famous Antennae Galaxies of NGC 4038/9 (featured in the
opening and closing scenes of the 1999 sci-fi comedy Galaxy Quest). The
asymmetry of what I call the "Fishhook Galaxy" is due to a collision in its
distant past that triggered stars to form, brightening one side. It is readily
visible in moderate optics. After observing the large number of galaxies from
edge-on to face-on, seeing one that is curved is a pleasant surprise.
NGC 4485 and NGC 4490 are an interacting pair in Canes Venatici that is bright
enough to be seen in small optics, and close enough to show detail with more
aperture. NGC 4485 was originally thought to be irregular or a weak spiral like
the Large Magellanic Cloud. Its asymmetry is due to a grazing collision with
the larger galaxy NGC 4490. That clash caused star formation to occur in a
trail connecting the two galaxies, now separated by only 24,000 light-years.
That's only slightly closer than the Sun's distance to the center of our galaxy.
NGC 4656 and NGC 4657, located in Canes Venatici, is a near edge-on spiral that
is classified as SB(s)m, like the Large Magellanic Cloud. NGC 4657 forms the
clump of HII regions and star clouds. A thin ray with a short curve at one end
gives it the moniker "Hockey Stick Galaxy." It bears magnification well. Look
for it near NGC 4631 - a larger, brighter edge-on spiral - as this region of
the sky is a fun area to explore with a wide-field scope.
M60 (NGC 4649) and NGC 4647, in Virgo, are an unusual duo because they have
different classifications, masses, and overall luminosities. M60 is a large
elliptical, appearing nearly round (between E1 and E2 in Hubble's scheme) and
visible in small instruments. NGC 4647 is a face-on intermediate spiral (SABc)
with a low surface brightness. This is an example of a pair just beginning to
collide.
M64 (NGC 4826) is peculiar in having a large, asymmetric dark nebula that
blocks light from one half of its inner spiral disk. This feature was first
noted by William Herschel in the 1780s, leading him to coin the name "Black Eye
Nebula" for this galaxy. Located in Coma Berenices, M64 is a favorite for small
telescope users because its "black eye" is easily observed hugging the central
hub. Larger scopes will show the ragged nature of this dust cloud. Lesser known
is the fact that although M64 is a type 2 Seyfert galaxy, its bright central
emissions do not appear to originate from an active core, but rather from the
region just around it. This may be because the core is obscured along our line
of sight.
NGC 5426 and NGC 5427 in Virgo are a pair of near-twin Sc spirals in the early
stages of colliding. Also cataloged as Arp 271, they are nearly perpendicular
to one another. It would be interesting to see how they appear in a few hundred
million years. Visually, this is a faint pair and is best viewed with 8-inch
and larger optics. Knowing what you are seeing makes this a fun target.
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These are the most peculiar galaxies you can observe
FROM THE September 2021 ISSUE
These are the most peculiar galaxies you can observe
Crashing, colliding, and tangoing, these peculiar galaxies reveal the grand
evolution of the cosmos.
By Alan Goldstein | Published: Wednesday, September 15, 2021
RELATED TOPICS: PECULIAR GALAXIES | OBSERVING | AMATEUR ASTRONOMY
ASYPG0921_01RE
M82 is a nearby galaxy forming stars 10 times faster than the Milky Way. This
prodigious rate of production is due to gravitational interactions with its
neighboring galaxy, M81.
Patrick Winkler
Forty years ago, I penned my first of many articles for Astronomy. It ran in
the February 1981 issue as "Observing Peculiar Galaxies."
The term peculiar galaxies refers to a class of galaxies whose distorted shapes
resemble neither classical spiral galaxies nor amorphous elliptical galaxies -
and at the time, they were still mysterious. Just 16 years earlier, Halton Arp
had published his Atlas of Peculiar Galaxies, a product of his documentation of
objects while examining the Palomar Sky Survey. It contained a sampling of some
the most unusual galaxies whose warped structures defied explanation.
After the passage of four decades, our understanding of what makes such
galaxies peculiar has improved - thanks to the Hubble Space Telescope, giant
new telescopes with adaptive optics, and more powerful computers that can model
the gravitational dances of interacting galaxies.
Eyepiece improvements and larger telescopes have enabled the humble amateur
astronomer to see details that were impossible to make out decades ago. Digital
imaging technology allows amateurs to take professional-quality images that
were unattainable with film. But at the same time, urban areas have seen
tremendous growth, making it necessary to travel farther to get under truly
dark skies. After all these changes, it's only fair to revisit that topic with
a new eye.
ASYPG0921_04
The gravitational tango of NGC 4485 and NGC 4490 twirling around each other has
created a stream of stars that connects the two.
Dan Crowson
What makes a galaxy peculiar?
With the great diversity among "normal" galaxies, one may wonder if every
galaxy has its own peculiarities. To an extent, there may be some truth to that
statement. Edwin Hubble created the basis of the classification system that is
used today nearly 100 years ago.
The "tuning fork diagram" contains spirals - normal and barred - in the two
prongs, with ellipticals galaxies in the handle. Irregular galaxies are
outside. Over time, the structure has been tweaked with two additional
classifications. Lenticulars are spirals without active star formation that
share characteristics with ellipticals. And we now use the term transitional
barred spirals to refer to the so-called "SAB-type" in G‚rard de Vaucouleurs'
galaxy classification system that he developed in the late 1950s.
Galaxies tend to occur in clusters, just like stars. But unlike stars, the
sheer mass of galaxies encourages them to interact and even collide. A galaxy's
stars, dust, and hydrogen gas are all influenced by an interaction. Even a
passage separated by a hundred thousand light-years provides gravitational tugs
potent enough to distort galactic structure. This all happens in extreme slow
motion: The passage of galaxies relative to one another takes hundreds of
millions of years or longer.
As you might expect, the No. 1 cause of peculiarities is collision. An
estimated 5 percent of galaxies in the visible universe are interacting. And if
we had infrared vision, that number would rise to 25 percent; galaxies that
emit mostly infrared light seem to interact at a higher rate. Other causes
include bursts of star formation (related to collisions or interactions with
intergalactic hydrogen clouds) and exploding cores (due to massive black
holes). Some have anomalously large clouds of dust or glowing ionized hydrogen
gas (HII regions). A small percentage are still mysterious. Given time and
technology, these will become understood.
ASYPG0921_05
NGC 4027's distinctive shape is likely due to an interaction with an unseen
companion galaxy. It's located in the Crow, less than half a degree south of
the famous Antennae Galaxies.
ESO
What can you see?
The good news for amateur observers is that plenty of peculiar galaxies are
bright enough to be seen in 4- to 10-inch telescopes. Several can even be
picked up in binoculars.
Observing detail, however, is another matter. If you have not dabbled in
peculiar galaxies before, it is better to simply find these targets and not
worry if you don't see a peculiar feature.
To bring out the most detail possible, nothing beats a large aperture and dark,
unpolluted skies. Another strategy to eke out shreds of detail is to avert your
vision slightly to one side of the object, relying on your retina's more
light-sensitive rod cells. Perhaps they will reveal a bit of asymmetry, a faint
companion, a bright patch or arm, or a shadowy dark nebula.
Some observers study a photo or two before looking at their target. Bright
stars in the field may help you locate the most conspicuous aspect of the
galaxy's peculiarity. Keep in mind that photos may be reversed from what you
see in the eyepiece.
I enjoy every aspect of observing peculiars, from checking off another "faint
fuzzy" to seeing detail. If you don't go in expecting to see many details, you
won't be disappointed - or, from a more optimistic perspective, you may be
pleasantly surprised!
ScreenShot20210910at1.10.57PM
Astronomy: Roen Kelly after Antonio Ciccolella/M. De Leo, CC BY 3.0
Taking up the challenge
My 1981 article featured 12 of my favorite peculiar galaxies. Since then, I've
observed many other peculiar galaxies with larger optics. Here are 12 of my new
favorites:
NGC 1228, NGC 1229, NGC 1230, and IC 1892 make up a chain of galaxies in
Eridanus. These four are also known by their designation in Arp's atlas, Arp
332. They are a faint quartet, in the 13th- to 14th-magnitude range, a
challenge for a 10-inch scope. Why do galaxies line up? Perhaps it's random
chance, or maybe they evolved together in their own "local group." NGC 1229 is
a Seyfert-type galaxy - one with an active, energetic nucleus due to its
central, supermassive black hole gorging on material.
NGC 2207 and IC 2163 are a pair of spirals in Canis Major. Many observers are
aware of this constellation's numerous star clusters like M41. But the Big Dog
also contains a number of galaxies, including this pair in the early stages of
colliding. The smaller (and more distant) of the two, IC 2163, looks
comma-shaped, with arms stretched by tidal forces.
M82 (NGC 3034) in Ursa Major is a starburst galaxy, so called because it forms
stars at a much greater rate than most galaxies. Called the Cigar Galaxy
because it is long and narrow with rounded ends, it's easy to see with small
scopes. This Cuban is shrouded in smoky dark nebulae that can likewise be seen
in small telescopes. With larger optics, the detail in the dusty shreds is fun
to observe. How much detail can you see?
ASYPG0921_06
NGC 4656 and NGC 4657 combine to form the Hockey Stick Galaxy (middle). The
edge-on spiral NGC 4631 is located half a degree to the northwest.
Terry Hancock and Tom Masterson
NGC 3226 and 3227 are nowhere close to being the brightest galaxies in Leo. But
I like them because they are a "four-fer": The bright double star Gamma (?)
Leonis lies less than a degree west of the pair (which makes them easy to
find). NGC 3226 is an elliptical galaxy. NGC 3227 is a spiral. And NGC 3227 has
an active nucleus. In a telescope, they appear as two oval glows with compact
nuclei.
NGC 4027 in Corvus is a hook-shaped barred spiral galaxy. Its form is
reminiscent of the more famous Antennae Galaxies of NGC 4038/9 (featured in the
opening and closing scenes of the 1999 sci-fi comedy Galaxy Quest). The
asymmetry of what I call the "Fishhook Galaxy" is due to a collision in its
distant past that triggered stars to form, brightening one side. It is readily
visible in moderate optics. After observing the large number of galaxies from
edge-on to face-on, seeing one that is curved is a pleasant surprise.
NGC 4485 and NGC 4490 are an interacting pair in Canes Venatici that is bright
enough to be seen in small optics, and close enough to show detail with more
aperture. NGC 4485 was originally thought to be irregular or a weak spiral like
the Large Magellanic Cloud. Its asymmetry is due to a grazing collision with
the larger galaxy NGC 4490. That clash caused star formation to occur in a
trail connecting the two galaxies, now separated by only 24,000 light-years.
That's only slightly closer than the Sun's distance to the center of our galaxy.
NGC 4656 and NGC 4657, located in Canes Venatici, is a near edge-on spiral that
is classified as SB(s)m, like the Large Magellanic Cloud. NGC 4657 forms the
clump of HII regions and star clouds. A thin ray with a short curve at one end
gives it the moniker "Hockey Stick Galaxy." It bears magnification well. Look
for it near NGC 4631 - a larger, brighter edge-on spiral - as this region of
the sky is a fun area to explore with a wide-field scope.
ASYPG0921_07
M60 is a large elliptical with a supermassive black hole as massive as 4.5
billion Suns; it's flanked by NGC 4647, a smaller, younger spiral. Astronomers
think this odd couple is only in the early stages of its relationship - no new
star formation can be seen, but observations from the Hubble Space Telescope
suggest tidal interactions have just begun.
Dan Crowson
M60 (NGC 4649) and NGC 4647, in Virgo, are an unusual duo because they have
different classifications, masses, and overall luminosities. M60 is a large
elliptical, appearing nearly round (between E1 and E2 in Hubble's scheme) and
visible in small instruments. NGC 4647 is a face-on intermediate spiral (SABc)
with a low surface brightness. This is an example of a pair just beginning to
collide.
M64 (NGC 4826) is peculiar in having a large, asymmetric dark nebula that
blocks light from one half of its inner spiral disk. This feature was first
noted by William Herschel in the 1780s, leading him to coin the name "Black Eye
Nebula" for this galaxy. Located in Coma Berenices, M64 is a favorite for small
telescope users because its "black eye" is easily observed hugging the central
hub. Larger scopes will show the ragged nature of this dust cloud. Lesser known
is the fact that although M64 is a type 2 Seyfert galaxy, its bright central
emissions do not appear to originate from an active core, but rather from the
region just around it. This may be because the core is obscured along our line
of sight.
NGC 5426 and NGC 5427 in Virgo are a pair of near-twin Sc spirals in the early
stages of colliding. Also cataloged as Arp 271, they are nearly perpendicular
to one another. It would be interesting to see how they appear in a few hundred
million years. Visually, this is a faint pair and is best viewed with 8-inch
and larger optics. Knowing what you are seeing makes this a fun target.
ASYPG0921_08
NGC 5426 and NGC 5427 are practically twins: In addition to being nearly the
same size, their internal structures are also very similar.
Dan Crowson
NGC 6621 and NGC 6622 in Draco are another interacting pair that is fascinating
to observe with large telescopes. Smaller scopes can distinguish the larger NGC
6621, a distorted spiral with a long arm arcing over the galaxy's body. NGC
6622 is a compact barred spiral that has an unobscured nuclear region, while
the disk of the galaxy is mostly surrounded by dust and stars from NGC 6621.
NGC 6946 is not on most lists of peculiar galaxies. This near face-on spiral on
the Cepheus-Cygnus border has been nicknamed the Fireworks Galaxy because of
its prodigious number of supernovae. While the Milky Way produces an average of
one supernova per century, NGC 6946 produces 10 times that rate despite having
only one-third as many stars. This Sc-type spiral is also thought to be a
starburst galaxy. NGC 6946 is dimmed by interstellar dust from the Milky Way,
but is still bright enough for small scopes in dark skies. In large scopes, it
shows numerous HII regions and distinct spiral arms.
If you want to stretch the limits of your observing or imaging skills, peculiar
galaxies are ideal targets. They are links to the amazing evolution of galaxies
- starry halos with the heart of a black hole. Their incredible diversity of
form is what makes them worth a look.
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