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(IAAC) Re: (SNES) How Bright are the Galaxies - Really?
Sorry about the length on this one, Doug et al. I always try to be thorough!
But I do hope and believe, that it is worth the time it will take to read. :)
Very experienced deep-sky observer Doug Stewart posted the following to the
"Southern New England Stargazers" email list recently...
(Interested folks can find the full text of Doug's initial post at:
http://snegazers.brainiac.com/snegazers/archive/0112/msg00006.html
)
>[T]he popular star charting program, SkyMap Pro, is now available. Version 8
>features a number of advances and refinements... One of the most interesting
>changes in version 8 is the inclusion of the revised New General Catalog
>(NGC) database, published earlier this year...
>
>I find the numbers for M81 to be of particular interest (6.9 vs. 7.8), since
>so many reports have surfaced of observers seeing this galaxy naked eye. I was
>skeptical about these claims when M81 was listed at mag. 6.9. I am even more
>so with the new figure of 7.8 - a value which jibes much better with my
>observations over the years, using everything from binoculars to large
>telescopes. Overall I find the new higher magnitudes to be much more realistic.
>M33, as an example, makes much more sense at magnitude 6.2 than at 5.7 - for
>only under the very darkest FDO skies have I ever seen it with the naked eye.
>I'd be interested in hearing comments from other observers.
The question of how "published magnitudes" relate to visibility of "extended
objects" (like galaxies) is a very interesting one - and one that I have never
seen adequately covered in any publication or Website: even the authoritative
(and conflicting) discussions on the topic in Luginbuhl&Skiff, Kepple&Sanner,
and Roger Clark's old book _Visual Astronomy of the Deep Sky_, don't seem to
have "covered all bases" in this confusing (i.e., poorly understood?) topic.
========
Anyway, I was intrigued enough by the conflict between the magnitudes Doug
quoted, and the fact of these naked-eye observing reports (among them mine),
to actually do a little on-line literature research. One of the best sources
for finding out the "real deal" on astrophysical quantities associated with
galaxies and galaxy groupings, is an online source usually only used by the
professionals, but which IS available to us amateurs, known as "NED"! This
is short for the "NASA/IPAC Extragalactic Database", found at this URL:
http://nedwww.ipac.caltech.edu/
Doing a search for M81's "photometric data points" (that is, actual magnitude
measurements at various wavelengths throughout the EM spectrum), I found:
http://nedwww.ipac.caltech.edu/cgi-bin/nph-datasearch?search_type=Photo_id&objid=58480&objname=MESSIER%20081
This page contains some interesting numbers for M81. For example, the fainter
magnitude which Doug mentions from the revised NGC, 7.8, *is* quoted here...
as a Blue ("B_T") magnitude, rather than a visual one, however!
The only magnitude references I could find on "Visual" band (the only band in
use by professionals that even approximates what the eye sees), were these:
http://nedwww.ipac.caltech.edu/cgi-bin/nph-datasearch?search_type=Photo_id&objid=58480&objname=MESSIER%20081#No11
http://nedwww.ipac.caltech.edu/cgi-bin/nph-datasearch?search_type=Photo_id&objid=58480&objname=MESSIER%20081#No12
And interestingly enough, these appear to be the sources for what Doug calls
the "old" total magnitude measurements, of 6.9 (or even 6.6) rather than 7.9.
Now INTERESTINGLY, one often finds in the literature that professionals will
refer to a "B" (blue band) magnitude as "visual"! I'd imagine that for those
folks trying to compile databases for amateur use, this must lead to all kinds
of nasty confusion - and maybe that is exactly the explanation in this case?
========
By the way, "RC3.9" is a reference to the "THIRD REFERENCE CATALOGUE OF BRIGHT
GALAXIES, VERSION 3.9", by de Vaucouleurs, de Vaucouleurs, Buta et al., 1991.
And that brings up one CAUTION in all this: Is it possible that NED has simply
not been updated with more recent total V magnitude measures for M81...? In an
attempt to verify this, I did an Abstract Search (http:// adswww.harvard.edu),
and did find at least one possibly apposite reference, "SBF Survey of Galaxy
Distances. IV. SBF Magnitudes, Colors, and Distances", 2001ApJ...546..681T.
I don't have access to this publication of course (ApJ costs a few thou/year!),
but this reference - and all the other recent ones I noticed - appear to relate
to magnitude estimates in OTHER BANDS such as 'I', 'V', etc... In other words,
all the recent research I could find only relates to how bright the galaxy M81
would appear in infrared, ultraviolet, x-ray or other non-amateur telescopes!
Here is another survey of "CO" (radio) band emissions, called "BIMA Survey Of
Nearby Galaxies" or "BIMA SONG", that SEEMS to rely on some new "B" magnitude
references in an indirect way: 2001Ap&SS.276.1131H. But again, I don't have
access to the full journal article: http://kapis1.wkap.nl/oasis.htm/337009
========
Ultimately, I suspect the V magnitude value of "about 6.9" may have to stand,
at least until some reference that can be followed up is found for the higher
(fainter) values which are given out in that newest version of SkyMap Pro.
That said, I thought I would open this question up to the larger audience of
experienced deep-sky observers AND amateur researchers on IAAC. (I'd love to
post this to 'amastro' also, but find that to be sadly untenable for me...)
So out of respect for Doug's long experience, I have to ask these experienced
amateurs: what have I missed in my research?? And what *is* the "experienced
amateurs'" consensus on M81, and in particular its naked-eye visibility?
Now even assuming a V=6.9, does that mean I think M81 should be *EASY* to see
with the unaided eye?? *NO WAY!* But on that subject, please read on... :)
========
Finally, one other note: this is in response to a telling comment in Doug's
later post to SNES, about Comet C/2000 WM1 (LINEAR):
>The new figure for tonight is magnitude 5.6 (vs. an earlier prediction of
>4.7). So it will apparently not reach [***]naked eye magnitude[***] while
>still visible from our northern latitude (only about another 10 days).
In fact, I observed this comet with the unaided eye, as did several other
experienced naked-eye (meteor) observers, back on 18 and 19 November! At
that time, the predicted magnitude was somewhere down low in the 7s, but
using open clusters M37 and M52 for naked-eye comparison, I estimated the
comet magnitude around 6.5. Now *FOR SURE*, my experience with observing
comets is *VERY* limited - so it's possible and even likely that my magni-
tude estimate was off by a half magnitude or more either way... But there
can be no doubt, for us on those nights, that it WAS a "naked-eye comet".
Always keep in mind that whether an object is only rarely seen from one
site, with its own individual light and particulate pollution problems,
may or may not have any bearing on whether the same object is observable
elsewhere, and in particular by a different observer entirely...
Clear (and really, really DARK) skies everyone,
Lew Gramer
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