ngc 6689/90 One ordinary yet strange galaxy

NGC 6689/90 is a near edge on spiral galaxy in Draco. It is classified as SBcd with HII emission. It's distance isn't all that well determined. Redshift puts it only 18 million light-years away but readings that close are highly unreliable. Tully-Fisher estimates vary from 40 to 54 million light-years with a median value of 45 million which is also about the mean value. Adopting that distance it is about 57,000 light-years in diameter. It went on my to-do list due to its odd dust lane and the lack of much real hint of spiral structure. The dust lane seen in the side slightly toward us makes an odd almost V shaped curve to the south then heads north cutting across what little spiral structure the galaxy has and vanishes off the top end rather than following the expected spiral pattern.

Notes at NED indicate it has a strange rotation curve. "Both sides of the rotation curve behave differently: the redshifted side has a solid body rotation curve, whereas the blueshifted side exhibits a plateau around 35 km s^-1^ up to 40 arcsec, then it increases again without ever reaching the redshifted side. There is no companion in the vicinity that could explain such an asymmetry in the rotation curve." Also radio observations show the HI disk is strongly warped. So while no companion is seen it is likely there is one or a recently "digested" one. The digestion idea might explain the odd dust lane as well.

The galaxy has no hint of a bright core and I see little hint of a true central bulge. At first glance the galaxy appears rather "normal" but when you look more closely it is quite odd indeed. I wish we could see it more face on.

The galaxy has the odd honor of being discovered 4 times by only 2 observers. Seems Heinrich d'Arrest found it first on August 22, 1863 then again sometime later. I can't find that date. While he didn't catch it was the same object the positions match. This results in it being listed as NGC 6689 by Dryer. Then on August 16, 1884 Lewis Swift made his first discovery of it. Later (again I can't find a date) he recorded it a second time with a position about 1 minute from that of his first discovery and the positions d'Arrest found. Dryer caught that the first position was the same as d'Arrest but failed to realize the second position was in error and thus it went on the NGC as NGC 6690. I think it safe to say this second position isn't related to the unseen possible companion. You can read about this mix-up at the entries for either NGC entry at the NGC Project.

Since no other galaxy in the field had any redshift data at NED I didn't prepare an annotated image. This was taken on one of the rare good nights here this year. Even the fires out west kept their smoke out of my skies this night. I used to have skies like this regularly but the last two years they've been exceedingly rare I'm sorry to report.

I've also attached an HST image of the core region of this galaxy. Too me its resolution pretty well supports the 45 million light-year distance give or take 20%.

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

Rick

Rick,

another one I have not imaged, but at least it already was on my list :-)

Stefan


"WA0CKY" schrieb im Newsbeitrag ...


NGC 6689/90 is a near edge on spiral galaxy in Draco. It is classified
as SBcd with HII emission. It's distance isn't all that well
determined. Redshift puts it only 18 million light-years away but
readings that close are highly unreliable. Tully-Fisher estimates vary
from 40 to 54 million light-years with a median value of 45 million
which is also about the mean value. Adopting that distance it is about
57,000 light-years in diameter. It went on my to-do list due to its odd
dust lane and the lack of much real hint of spiral structure. The dust
lane seen in the side slightly toward us makes an odd almost V shaped
curve to the south then heads north cutting across what little spiral
structure the galaxy has and vanishes off the top end rather than
following the expected spiral pattern.

Notes at NED indicate it has a strange rotation curve. "Both sides of
the rotation curve behave differently: the redshifted side has a solid
body rotation curve, whereas the blueshifted side exhibits a plateau
around 35 km s^-1^ up to 40 arcsec, then it increases again without ever
reaching the redshifted side. There is no companion in the vicinity that
could explain such an asymmetry in the rotation curve." Also radio
observations show the HI disk is strongly warped. So while no companion
is seen it is likely there is one or a recently "digested" one. The
digestion idea might explain the odd dust lane as well.

The galaxy has no hint of a bright core and I see little hint of a true
central bulge. At first glance the galaxy appears rather "normal" but
when you look more closely it is quite odd indeed. I wish we could see
it more face on.

The galaxy has the odd honor of being discovered 4 times by only 2
observers. Seems Heinrich d'Arrest found it first on August 22, 1863
then again sometime later. I can't find that date. While he didn't
catch it was the same object the positions match. This results in it
being listed as NGC 6689 by Dryer. Then on August 16, 1884 Lewis Swift
made his first discovery of it. Later (again I can't find a date) he
recorded it a second time with a position about 1 minute from that of
his first discovery and the positions d'Arrest found. Dryer caught that
the first position was the same as d'Arrest but failed to realize the
second position was in error and thus it went on the NGC as NGC 6690. I
think it safe to say this second position isn't related to the unseen
possible companion. You can read about this mix-up at the entries for
either NGC entry at the NGC Project.

Since no other galaxy in the field had any redshift data at NED I didn't
prepare an annotated image. This was taken on one of the rare good
nights here this year. Even the fires out west kept their smoke out of
my skies this night. I used to have skies like this regularly but the
last two years they've been exceedingly rare I'm sorry to report.

I've also attached an HST image of the core region of this galaxy. Too
me its resolution pretty well supports the 45 million light-year
distance give or take 20%.

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

Rick


--
WA0CKY