ASTRO: NGC 3646 and 3649

NGC 3646 is a very peculiar spiral galaxy in Leo. It's inner spiral
structure is that of a normal flocculent spiral but the outer structure
is chaotic at best. One note at NED reads: "...The spiral pattern is
very odd. One spiral arm emerges from the SE end of the inner disk and
turns counterclockwise. Another emerges from the NW side of the inner
disk and turns clockwise. These two arms appear to merge on the SW side
of the galaxy, joining in a point. The arm structure on the west side is
very well defined, and HSB, with many bright knots. On the east side,
the arms are diffuse and patchy, with fewer knots. There is an LSB outer
disk on the west side. The referee suggests, and we agree, that this may
be an example of a galaxy with visible leading and trailing arms." I'm
not sure I see the bi-directional arms but two certainly meet at a point
on the southwest side.

Another note has a somewhat different view: "The morphology of NGC 3646
is peculiar, as if the result of an encounter. The outer spiral pattern
cannot be traced even as to the sense of direction, but the inner spiral
pattern is regular. NGC 3646 forms a physical pair with NGC 3649 (SBa;
not in the RSA) at an angular separation of 8'. The redshifts from the
catalog of Karachentsev (1987) are v_o(3646) = 4227 km/s and v_o(3649) =
4322 km/s. At the mean redshift distance of 85 Mpc (H = 50), the
projected linear separation is 200 kpc. However, there is no indication
of morphological distortion in the spiral pattern of NGC 3649. The
peculiar outer ring structure in NGC 3646 here cannot be attributed to
interaction with NGC 3649.

"NGC 3649 is much smaller in angular diameter than NGC 3646. The
diameters are 80" for NGC 3649 and 260" for the outer ring of NGC 3646.
Note that the inner part of NGC 3646 resembles a normal Sbc spiral of
the same size as NGC 3649. The linear diameter of this inner part is 33
kpc - a normal value for most RSA spirals. However, the outer ring of
NGC 3646 has a linear diameter of 107 kpc, which is abnormally large.

"The fact that the inner image of NGC 3646 has a normal morphology and
is of normal size suggests that the ring is a result of a dynamical
process such as the dropping of one galaxy through another, as
postulated by Theys and Spiegel (1976, 1977) and by Lynds and Toomre
(1976) in other ring galaxies. The abnormal velocity field and an early
comment on the large linear diameter of the ring are given by Burbidge,
Burbidge, and Prendergast (1961)."

HSB=High Surface Brightness, LSB=Low Surface Brightness, RSA=Revises
Shapley-Ames Catalog of Bright Galaxies. A parsec is 3.26 light-years
so a diameter of 107 kpc translates to almost 350 thousand light-years.

I think the idea it is the result of some sort on interaction is quite
likely correct. I wonder if a merger is involved rather than a
collision as I find no bullet in the area. Also there are some odd
plumes. One to the southwest is just visible toward the orange star.
Others come off the northeast side and are more easily seen. Maybe
someone who can put dozens of hours into this one can find traces of
looping plumes that would pretty well cinch the merger idea or not find
them thus giving more weight to a fast interaction being the cause.

NGC 3649, while called a companion has a somewhat different redshift and
appears rather normal. As the notes say, it is not interacting with NGC
3646 nor does it appear to have done so in the past. Redshift puts it
at 240 million light-years while NGC 3646 has a redshift distance of 210
million light-years. Tully-Fisher distance estimates place it 174 to 203
million light-years away with the more modern measurements favoring the
shorter distance. The pair are located in Leo. NED classes NGC 3646 as
SAr)bc pec (ring). The NGC Project uses the simpler classification of
Sc I. Irregular or peculiar it certainly is different yet one Arp
didn't include. It would have fit his miscellaneous category, I'd
think. NGC 3649 is classed SB(s)a by NED and S0 by the NGC project. I
can't rationalize S0 for this obvious spiral galaxy. It has two sets of
arms much like NGC 3646. One set tight to the core and outer arms that
form a ring structure about it giving it a Saturn-like appearance.

The galaxy cluster at the top of my image, left of center has oddly
differing redshift data for the cluster and the BCG anchoring it. Both
are labeled as photographic which isn't as accurate as spectroscopic
measurements. This may account for the difference. The cluster is WHL
J112213.1+201752. NED shows it as containing 14 galaxies but gives no
diameter. Toward the eastern side near the middle is another cluster
with a different redshift (again both photographic) for the anchor
galaxy. Though in this case the difference is small and not seen in the
two significant digits I use for annotated images. That cluster is WHL
J112242.1+200748 and has only 4 members in an undefined area.

While NED lists thousands of galaxies in my image except for these two
cluster anchoring galaxies as well as the two NGC galaxies, none have
redshift data. Hardly made the annotated image worth it. But I started
it before discovering the lack of data. I'm including it anyway.

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick
--
Prefix is correct. Domain is arvig dot net

Great image Rick.
NGC 3646 is one of my favourites because it looks like a human eye.

Stefan

"Rick Johnson" schrieb im Newsbeitrag
. com...
NGC 3646 is a very peculiar spiral galaxy in Leo. It's inner spiral
structure is that of a normal flocculent spiral but the outer structure
is chaotic at best. One note at NED reads: "...The spiral pattern is
very odd. One spiral arm emerges from the SE end of the inner disk and
turns counterclockwise. Another emerges from the NW side of the inner
disk and turns clockwise. These two arms appear to merge on the SW side
of the galaxy, joining in a point. The arm structure on the west side is
very well defined, and HSB, with many bright knots. On the east side,
the arms are diffuse and patchy, with fewer knots. There is an LSB outer
disk on the west side. The referee suggests, and we agree, that this may
be an example of a galaxy with visible leading and trailing arms." I'm
not sure I see the bi-directional arms but two certainly meet at a point
on the southwest side.

Another note has a somewhat different view: "The morphology of NGC 3646
is peculiar, as if the result of an encounter. The outer spiral pattern
cannot be traced even as to the sense of direction, but the inner spiral
pattern is regular. NGC 3646 forms a physical pair with NGC 3649 (SBa;
not in the RSA) at an angular separation of 8'. The redshifts from the
catalog of Karachentsev (1987) are v_o(3646) = 4227 km/s and v_o(3649) =
4322 km/s. At the mean redshift distance of 85 Mpc (H = 50), the
projected linear separation is 200 kpc. However, there is no indication
of morphological distortion in the spiral pattern of NGC 3649. The
peculiar outer ring structure in NGC 3646 here cannot be attributed to
interaction with NGC 3649.

"NGC 3649 is much smaller in angular diameter than NGC 3646. The
diameters are 80" for NGC 3649 and 260" for the outer ring of NGC 3646.
Note that the inner part of NGC 3646 resembles a normal Sbc spiral of
the same size as NGC 3649. The linear diameter of this inner part is 33
kpc - a normal value for most RSA spirals. However, the outer ring of
NGC 3646 has a linear diameter of 107 kpc, which is abnormally large.

"The fact that the inner image of NGC 3646 has a normal morphology and
is of normal size suggests that the ring is a result of a dynamical
process such as the dropping of one galaxy through another, as
postulated by Theys and Spiegel (1976, 1977) and by Lynds and Toomre
(1976) in other ring galaxies. The abnormal velocity field and an early
comment on the large linear diameter of the ring are given by Burbidge,
Burbidge, and Prendergast (1961)."

HSB=High Surface Brightness, LSB=Low Surface Brightness, RSA=Revises
Shapley-Ames Catalog of Bright Galaxies. A parsec is 3.26 light-years
so a diameter of 107 kpc translates to almost 350 thousand light-years.

I think the idea it is the result of some sort on interaction is quite
likely correct. I wonder if a merger is involved rather than a
collision as I find no bullet in the area. Also there are some odd
plumes. One to the southwest is just visible toward the orange star.
Others come off the northeast side and are more easily seen. Maybe
someone who can put dozens of hours into this one can find traces of
looping plumes that would pretty well cinch the merger idea or not find
them thus giving more weight to a fast interaction being the cause.

NGC 3649, while called a companion has a somewhat different redshift and
appears rather normal. As the notes say, it is not interacting with NGC
3646 nor does it appear to have done so in the past. Redshift puts it
at 240 million light-years while NGC 3646 has a redshift distance of 210
million light-years. Tully-Fisher distance estimates place it 174 to 203
million light-years away with the more modern measurements favoring the
shorter distance. The pair are located in Leo. NED classes NGC 3646 as
SAr)bc pec (ring). The NGC Project uses the simpler classification of
Sc I. Irregular or peculiar it certainly is different yet one Arp
didn't include. It would have fit his miscellaneous category, I'd
think. NGC 3649 is classed SB(s)a by NED and S0 by the NGC project. I
can't rationalize S0 for this obvious spiral galaxy. It has two sets of
arms much like NGC 3646. One set tight to the core and outer arms that
form a ring structure about it giving it a Saturn-like appearance.

The galaxy cluster at the top of my image, left of center has oddly
differing redshift data for the cluster and the BCG anchoring it. Both
are labeled as photographic which isn't as accurate as spectroscopic
measurements. This may account for the difference. The cluster is WHL
J112213.1+201752. NED shows it as containing 14 galaxies but gives no
diameter. Toward the eastern side near the middle is another cluster
with a different redshift (again both photographic) for the anchor
galaxy. Though in this case the difference is small and not seen in the
two significant digits I use for annotated images. That cluster is WHL
J112242.1+200748 and has only 4 members in an undefined area.

While NED lists thousands of galaxies in my image except for these two
cluster anchoring galaxies as well as the two NGC galaxies, none have
redshift data. Hardly made the annotated image worth it. But I started
it before discovering the lack of data. I'm including it anyway.

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick
--
Prefix is correct. Domain is arvig dot net

Hadn't thought of it that way but now that you mention it....

I'd like to know how it got that way and so huge. Likely a common
element involving mergers. Still that orderly core as big as our galaxy
surrounded by that huge, chaotic ring-like structure seems difficult to
explain.

Rick

On 5/7/2012 2:52 PM, Stefan Lilge wrote:
Great image Rick.
NGC 3646 is one of my favourites because it looks like a human eye.

Stefan

"Rick schrieb im Newsbeitrag
. com...
NGC 3646 is a very peculiar spiral galaxy in Leo. It's inner spiral
structure is that of a normal flocculent spiral but the outer structure
is chaotic at best. One note at NED reads: "...The spiral pattern is
very odd. One spiral arm emerges from the SE end of the inner disk and
turns counterclockwise. Another emerges from the NW side of the inner
disk and turns clockwise. These two arms appear to merge on the SW side
of the galaxy, joining in a point. The arm structure on the west side is
very well defined, and HSB, with many bright knots. On the east side,
the arms are diffuse and patchy, with fewer knots. There is an LSB outer
disk on the west side. The referee suggests, and we agree, that this may
be an example of a galaxy with visible leading and trailing arms." I'm
not sure I see the bi-directional arms but two certainly meet at a point
on the southwest side.

Another note has a somewhat different view: "The morphology of NGC 3646
is peculiar, as if the result of an encounter. The outer spiral pattern
cannot be traced even as to the sense of direction, but the inner spiral
pattern is regular. NGC 3646 forms a physical pair with NGC 3649 (SBa;
not in the RSA) at an angular separation of 8'. The redshifts from the
catalog of Karachentsev (1987) are v_o(3646) = 4227 km/s and v_o(3649) =
4322 km/s. At the mean redshift distance of 85 Mpc (H = 50), the
projected linear separation is 200 kpc. However, there is no indication
of morphological distortion in the spiral pattern of NGC 3649. The
peculiar outer ring structure in NGC 3646 here cannot be attributed to
interaction with NGC 3649.

"NGC 3649 is much smaller in angular diameter than NGC 3646. The
diameters are 80" for NGC 3649 and 260" for the outer ring of NGC 3646.
Note that the inner part of NGC 3646 resembles a normal Sbc spiral of
the same size as NGC 3649. The linear diameter of this inner part is 33
kpc - a normal value for most RSA spirals. However, the outer ring of
NGC 3646 has a linear diameter of 107 kpc, which is abnormally large.

"The fact that the inner image of NGC 3646 has a normal morphology and
is of normal size suggests that the ring is a result of a dynamical
process such as the dropping of one galaxy through another, as
postulated by Theys and Spiegel (1976, 1977) and by Lynds and Toomre
(1976) in other ring galaxies. The abnormal velocity field and an early
comment on the large linear diameter of the ring are given by Burbidge,
Burbidge, and Prendergast (1961)."

HSB=High Surface Brightness, LSB=Low Surface Brightness, RSA=Revises
Shapley-Ames Catalog of Bright Galaxies. A parsec is 3.26 light-years
so a diameter of 107 kpc translates to almost 350 thousand light-years.

I think the idea it is the result of some sort on interaction is quite
likely correct. I wonder if a merger is involved rather than a
collision as I find no bullet in the area. Also there are some odd
plumes. One to the southwest is just visible toward the orange star.
Others come off the northeast side and are more easily seen. Maybe
someone who can put dozens of hours into this one can find traces of
looping plumes that would pretty well cinch the merger idea or not find
them thus giving more weight to a fast interaction being the cause.

NGC 3649, while called a companion has a somewhat different redshift and
appears rather normal. As the notes say, it is not interacting with NGC
3646 nor does it appear to have done so in the past. Redshift puts it
at 240 million light-years while NGC 3646 has a redshift distance of 210
million light-years. Tully-Fisher distance estimates place it 174 to 203
million light-years away with the more modern measurements favoring the
shorter distance. The pair are located in Leo. NED classes NGC 3646 as
SAr)bc pec (ring). The NGC Project uses the simpler classification of
Sc I. Irregular or peculiar it certainly is different yet one Arp
didn't include. It would have fit his miscellaneous category, I'd
think. NGC 3649 is classed SB(s)a by NED and S0 by the NGC project. I
can't rationalize S0 for this obvious spiral galaxy. It has two sets of
arms much like NGC 3646. One set tight to the core and outer arms that
form a ring structure about it giving it a Saturn-like appearance.

The galaxy cluster at the top of my image, left of center has oddly
differing redshift data for the cluster and the BCG anchoring it. Both
are labeled as photographic which isn't as accurate as spectroscopic
measurements. This may account for the difference. The cluster is WHL
J112213.1+201752. NED shows it as containing 14 galaxies but gives no
diameter. Toward the eastern side near the middle is another cluster
with a different redshift (again both photographic) for the anchor
galaxy. Though in this case the difference is small and not seen in the
two significant digits I use for annotated images. That cluster is WHL
J112242.1+200748 and has only 4 members in an undefined area.

While NED lists thousands of galaxies in my image except for these two
cluster anchoring galaxies as well as the two NGC galaxies, none have
redshift data. Hardly made the annotated image worth it. But I started
it before discovering the lack of data. I'm including it anyway.

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick
--
Prefix is correct. Domain is arvig dot net





--
Prefix is correct. Domain is arvig dot net