There are a few reasons I'd like to inform TAC of a group effort to locate
NGC 6138.
1) We are among the first to know the answer to this 130 year old puzzle.
2) The story provides some insight into what the NGC/IC Project Team does.
3) I learned some new information that might be helpful to others.
Bob Czerwinski and I recently have been hunting down objects in the galaxy
cluster Abell 2197. On TAC, Bob had mentioned his difficulty finding NGC
6138. "The Sky" says it doesn't exist, as well as gives a location for it 2'
from where the sources below say it is. Uranometria and the NASA/IPAC
Extragalactic Database (NED) place NGC 6138 at RA 16h 24m 54.1s, DEC +41d
03m 03s. The Digitized Sky Survey also shows a galaxy at that location. Sure
enough, I was able to see it. For me, that would have been the end of the
story.
However, thinking that NGC 6138 was one of those NGC objects incorrectly
identified, Bob consulted the NGC/IC Project page. Bob posted on TAC that
the NGC/IC Project identified NGC 6138 as NGC 6263. This puzzled me, since I
had seen a galaxy at the "correct" location. In addition, NGC 6263 is nearly
15 degrees away!
So, I consulted the ultimate source - Steve Gottlieb. Steve is an amazing
resource who seems to have endless knowledge about amateur astronomy. He
impressed me by taking the time to immediately respond with the following
detailed information.
"Stephan's discovery was "announced" in his list II, which was published in
Monthly Notices of the Royal Society (November 1871). He gave a position
for 1870 as 16h 18m 28.76s +41d 14' 18.5" which precesses to 16h 22m 50.9s
+40d 56' 04" (2000). This position was micrometrically computed using an
offset star which was identified in the Monthly Notices paper as "2,821 Arg.
Z +41d". The actual micrometric offset is not given in the paper but the
position of the star is listed at 16h 17m 7.32s +41d 10m 39.7s (1870) or 16h
21m 30.1s +40d 52' 11" (2000) with a magnitude of 9.2. If you subtract the
RA and Dec positions for the offset star and NGC 6138 we can infer Javelle's
offsets as 1 minute 21 seconds in RA and a bit under 4' in declination. Fair
enough. But if you look up the position of 16h 21m 30.1s +40d 52' 11"
(2000) on sky charting software, there is no 9th magnitude star there! So,
clearly Javelle gave the wrong position for his offset star but I don't know
where the star he mentioned is located -- and even then it's possible he
misidentified the offset star. Corwin claims the modern designation for
this star is BD +41 deg 2821. If you can track down the 2000 coordinates
for this star, we could go from there."
Well, I did as Steve suggested and found BD +41 2821 in the Bonner
Durchmusterung catalog.
BD +41 2821
RA 17 16 38.5, DEC +41 11.6 (1855), mag 9.2
Stephan reported his discovery was near 2821 Arg. Z +41d.
RA 16 17 7.31, DEC +41 10 39.7 (1870)
It appears that the 16 and 17 for RA degrees and minutes were transposed in
Stephan's report. Fortunately, he named the star so it was possible to look
up the correct coordinates for it.
The 1855 coordinates for BD +41 2821 translate to:
RA 17 21 17.7, DEC +41 03.0 (2000).
There is a star very near these coordinates at RA 17 21 18.5, DEC +41 02
13.9. "The Sky" says it is a mag 10. The Guide Star Catalogue says it is a
9.88. It looks like a match to Stephan's reference star.
That puts NGC 6363 (not NGC 6263) at exactly the right offset. Stephan
apparently made a mistake and transposed the numbers for RA degrees and
minutes for his reference star. He actually discovered NGC 6363 and
calculated its position using the transposed numbers. That is why no object
appears at his coordinates. So, the galaxy cited as NGC 6138 in Uranometria
and NED is mislabeled. It should have one of its other designations, such as
MCG+7-34-20.
Apparently someone has figured this out before, but incorrectly wrote down
NGC 6263 instead of NGC 6363. Steve confirmed that this is true. The
correction will eventually make its way into the appropriate professional
and amateur databases.
This was a nice exercise since I learned quite a lot from it. Here are some
of the lessons learned.
1) TAC is a great tool for sharing observations. Bob's posts of his
observations set the ball rolling.
2) Steve is a wonderful resource. If he hadn't already done the background
research, hadn't the info at his fingertips, or hadn't been so responsive,
the ball would have stopped. He also directed me where to look.
3) The information readily available through the Internet continues to amaze
me. Steve probably gave me more credit than I deserved. I know very little
about star designations and had no idea what BD +41 deg 2821 meant. But a
search of "stellar databases" quickly gave me a review of various systems
for naming stars. I learned that BD stands for "Bonner Durchmusterung."
4) From my research on galaxy clusters, I was aware of various catalogues
and a variety of search methods for finding galaxies. I was very surprised
to learn that the Bonner Durchmusterung catalog
(http://vizier.u-strasbg.fr/viz-bin/Cat?I/122) of stars, first assembled in
1859-62, is available and searchable on-line. The updates in years 1903,
1951, and 1968 are also included. It was straightforward to search the
database using VizieR (http://vizier.u-strasbg.fr/viz-bin/VizieR-3). VizieR
can also be used to translate the older stated coordinates into modern Epoch
2000 coordinates.
As amateurs, we use the published databases extensively to select and/or
identify objects. Many errors still exist among objects readily observable
by us. We shouldn't hesitate to question the accuracy of what we read when
we discover discrepancies between our observations and the published data.
Doing so may allow us to make a small, yet satisfying contribution to
astronomy.
Albert