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Statistics of galaxies brighter than given magnitude

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#1 Franta

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Posted 04 August 2019 - 05:38 PM

I made some plots for my own observing projects, and since they provided me with more information than I initially expected, I give here a link in order they are interesting for someone else: https://franta-d.raj...iven_magnitude/

(try this link https://www.rajce.net/f1390563728 if the previous link does not work). Please find a brief description of the plots in the link. I apologise to observers from the southern hemisphere, because the maps aim mainly at the northern sky.

 

The maps showed me several interesting features: when we have a telescope which sees one 1 mag further, it shows 3x more galaxies. This trend continues from 10 mag up to 14.5 mag, and then the increment is smaller - but this is likely because of the fact that the catalogue is incomplete for fainter objects. Another feature is that Virgo cluster, which is a conspicuous overdensity for mag < 13 galaxies, it becomes only one of many galactic clusterings when we exceed mag > 14.5. I am also surprised how many galaxy clusters are there already for mag ~ 15. Although I have seen many overdensities in galaxies in Uranometria, I expected that they are mainly beyond reach of my telescope (with perhaps the exceptioon of the brightest galaxy, but seeing only one galaxy does not seem to be that attractive). We can also easily see the substructure of the Virgo cluster (mainly around M 61, M 98 and M 60) on the zoom-in plots (files Vir_et_Com).  It is also clear how the Milky Way obstructs our view outside.

 

Since my humble 250mm skywatcher dobsonian reaches to 15-15.5 mag on galaxies under excellent skyes, I am tempted to try more of these objects.



#2 TOMDEY

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Posted 04 August 2019 - 06:12 PM

To pretty good accuracy, it should be a simple computation, assuming that the distribution of galaxies in the universe is homogeneous and a few other reasonable assumptions. I'm leaving for the deep woods park, to hike five miles before it gets dark. Will compute that when I get back... if no one else picks up the gauntlet.

 

It should follow that relationship until one reaches SO far out that the younger universe kicks in... which for us, with our dinky scopes... would be interesting... but rather moot.

 

Later - Tom



#3 Starman1

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Posted 04 August 2019 - 06:28 PM

So if you make a list using total integrated magnitudes, the list would have a particular sequence.

If you reorganized the list by surface brightness, which may be more indicative of the visibility of the galaxies,

the list would have an entirely different sequencing.

Other than the brightest 50 galaxies (your personal ranking matters more here than measurements), I'd look at the SB ranking

for numbers, since that will have more relevance for observing. 



#4 VariableBob

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Posted 04 August 2019 - 07:51 PM

Many of those dimmer magnitude galaxies are pretty small and would almost appear to be stellar like in the eyepiece.  This is one time that one would go to higher magnification.  Really need dark, clear skies.  One factor that one needs to take in consideration is that the distribution of galaxies are not uniform.  Other than that get out and observe.

 

Clear skies

 

Bob



#5 Redbetter

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Posted 04 August 2019 - 07:52 PM

The ~3x factor per magnitude is one I had derived as well from various datasets.  It holds up pretty well somewhere into the 16's, perhaps to 17+, but it will begin to taper off at some point.

 

The issue that arises is that the 3x factor is largely the result of extending the horizon as well as filling in fainter galaxies within the prior "outer boundary".  But by about 2 billion light years the redshift reduces the visual luminosity to an increasing degree.  By 3 billion the effective visual luminosity is becoming quite poor and by perhaps 4 billion it is largely non-existent.  The result is that while one can use larger aperture to extend the magnitude range, the visual universe has some bounds.  Quasars and other shorter wavelength sources can still be observed despite the shift, but the galaxies themselves are lost to ever redder wavelengths.   

 

If it was not for redshift, then doubling the aperture would allow galaxies with the same intrinsic brightness/absolute magnitude to be seen that were ~1.5 magnitude dimmer, at roughly twice the distance, and half the apparent diameter, but the same surface brightness.  However, there would be some loss even in this ideal case because the eye will have more trouble detecting a target of half the apparent diameter with the same surface brightness...and seeing also limits how much magnification can be applied.

 

When gaining 1 magnitude of reach, the new horizon is about 1.58x as distant, ignoring redshift.  Targets of the same intrinsic brightness will be ~0.63x as wide at the same magnification.

 

When redshift is added the very distant galaxies suffer in multiple ways in addition the traditional factors already noted above:  their intrinsic visual magnitude plummets because the emissions in the very short wavelengths are much less and continue to taper off (these are the wavelengths that are redshifted into the visual range, while the former visual range is shifted to deep red/infrared.)  Likewise their surface brightness and apparent visual dimensions are reduced. 

 

The 2 billion light year plus galaxies look very orange in the digital sky survey images...and our scotopic (rod) vision is poor at detecting these things.  I have targeted a few and seen them with the 20", but they are just barely detected wisps mostly.  Looking up g magnitudes where possible reveals why:  they tend to be in the 17 to ~18 g mag range as the upper bound, and this is the wavelength range that the eye can detect for threshold extended objects at night. 

 

It would be an interesting project to play around with some models of the populations with distance, some minimum apparent size (magnification dependent) factor in the redshift, and determine what the theoretical factors might be as aperture is increased.  I haven't explored this.


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#6 havasman

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Posted 04 August 2019 - 08:34 PM

The maps showed me several interesting features: when we have a telescope which sees one 1 mag further, it shows 3x more galaxies. 

 

Since my humble 250mm skywatcher dobsonian reaches to 15-15.5 mag on galaxies under excellent skyes, I am tempted to try more of these objects.

One of the dangers of such analytics is that they only consider magnitude when declaring the observability of a galaxy but when you go out and look other factors often dominate the determination - surface brightness, extinction, redshift, etc.

 

And of course site conditions dominate.




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