Celestia Forums

Hi,

I have been flying around with the beautiful FT1 release, and noted something odd about M51 and NGC 5195. The view from Earth is undoubtely impressive, compared with the photographs. However, on travel to M51, I observed that NGC 5195 is closer to our system than her bigger companion. I was under the impression that the converse was true. I had this idea based on the fact that, in the photographs, one can see dark dust lanes belonging to one of M 51's arms, obscuring the light from NGC 5195. From which I concluded that the latter lies behind. I checked at SEDS and found that:

"It is thought that NGC 5195 has passed M51 roughly along our line of sight and is now behind its large neighbor."

Does anybody know more about this? Fridger, perhaps? Have you also observed this (since in the readme file of the release you mention M51)?

(The SEDS text is from http://www.seds.org/messier/xtra/ngc/n5195.html, whre there is also a picture like the one I described. The distance to both is given as 37000 kly.)

Regards,

Guille

abramson wrote:Hi,

I have been flying around with the beautiful FT1 release, and noted something odd about M51 and NGC 5195. The view from Earth is undoubtely impressive, compared with the photographs. However, on travel to M51, I observed that NGC 5195 is closer to our system than her bigger companion. I was under the impression that the converse was true. I had this idea based on the fact that, in the photographs, one can see dark dust lanes belonging to one of M 51's arms, obscuring the light from NGC 5195. From which I concluded that the latter lies behind. I checked at SEDS and found that:

"It is thought that NGC 5195 has passed M51 roughly along our line of sight and is now behind its large neighbor."

Does anybody know more about this? Fridger, perhaps? Have you also observed this (since in the readme file of the release you mention M51)?

(The SEDS text is from http://www.seds.org/messier/xtra/ngc/n5195.html, whre there is also a picture like the one I described. The distance to both is given as 37000 kly.)

Regards,

Guille

I believe that Fridger's methodology for generating his deep sky catalog derives the galaxy distances algorithmically based on apparent size, brightness, Hubble class, etc. Such an automatic approach, while important for data source visibility, cannot easily take into account special case factors such as dust lane superposition. Selden has noted that it would be useful to allow custom definitions to override the generated distance in special cases such as this, and it should be possible to do so eventually.

- Hank

Hi...


http://www.ewellobservatory.com/gallery ... 1/m51.html



bye
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abramson wrote:Hi,

I have been flying around with the beautiful FT1 release, and noted something odd about M51 and NGC 5195. The view from Earth is undoubtely impressive, compared with the photographs. However, on travel to M51, I observed that NGC 5195 is closer to our system than her bigger companion. I was under the impression that the converse was true. I had this idea based on the fact that, in the photographs, one can see dark dust lanes belonging to one of M 51's arms, obscuring the light from NGC 5195. From which I concluded that the latter lies behind. I checked at SEDS and found that:

"It is thought that NGC 5195 has passed M51 roughly along our line of sight and is now behind its large neighbor."

Does anybody know more about this? Fridger, perhaps? Have you also observed this (since in the readme file of the release you mention M51)?

(The SEDS text is from http://www.seds.org/messier/xtra/ngc/n5195.html, whre there is also a picture like the one I described. The distance to both is given as 37000 kly.)

Regards,

Guille

Guille,

I am not sure whether you saw my respective recent discussion on this issue in the FT1 release thread.

In any case the situation is quite complex. Certainly that value of 3.7Mly that you see in many places (copied without source reference) iis totally outdated (1975!). The distance of NGC 5194 (M51) has recently been determined by 2 precision methods and of NGC 5195 by a /different/ very accurate method. These data you find in my deepsky.dsc file. If you care, I can quote the original papers and discuss the merits of the methods used. Another problem is that the Hubble constant enters to set the basic scale and is only /very/ recently known very accurately from WMAP.

What's the specific problematics?:

1) From hires photos, is seems quite suggestive that NGC 5194 (M51) is /closer/ than NGC 5195 due to this dust tail from M51 being apparently in the foreground with NGC 5195 behind. But perspective effects could possibly play deceptive games, such that the main body of M51 might be further away than that dust tail in front of NGC 5195!

Also simulations of interacting galaxies come up with NGC 5195 orbiting M51 and today being pretty far behind M 51 as seen from Earth.

2) From the existing /precision/ distance methods

-- recession velocity (Hubble's law) in the CMB frame
-- Planetary Nebulae as "standard candles"

for NGC 5194 (M51) , the distance agrees very well and is


++++++++++++++++++
From the /independent/ distance determination via the /very accurate/ SBF Mehtod, NGC 5195 comes out to be 250-350 kly closer than NGC 5194 in some squeeze with phtographic evidence (1)!
++++++++++++++++++

Unfortunately, a SBF (=surface brightness fluctuations) determination of the M51 distance does NOT exist. In any case, all these distance data would at least have a 5-10% uncertainty attached, which may NEVER be forgotten!

I should however point out that many analyses of that system just ASSUME the distances being in the way the want it

As Hank emphasized above, my basic approach to data for Celestia is NOT to induce any personal prejudice, but rather only use values as published in scientific publications. Since the only independent "precision" measurements for NGC 5194 and NGC 5195 give a somewhat larger distance for M51 than for NGC 5195, you find these best distance determinations in my deepsky.dsc catalog.


With releasing a catalog with more than 10000 entries, any hand editing is neither a good idea from the scientific point of view, nor practicable.

Cheers,
Fridger

Thanks for the comments.

t00fri wrote:I am not sure whether you saw my respective recent discussion on this issue in the FT1 release thread.

I did not. I will look it up.

t00fri wrote:As Hank emphasized above, my basic approach to data for Celestia is NOT to induce any personal prejudice, but rather only use values as published in scientific publications. Since the only independent
...
With releasing a catalog with more than 10000 entries, any hand editing is neither a good idea from the scientific point of view, nor practicable.

I completely agree. We should stick to this policy.

Thanks again.

Guillermo