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Quasar brightness lower bound, real or my error?



 
 
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  #1  
Old December 26th 16, 06:31 PM posted to sci.astro.research
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Default Quasar brightness lower bound, real or my error?

For this plot of quasar Mag vs z

https://inspirehep.net/record/120804...dist1_dr10.png


There is a lower bound for quasar Mag that looks artificial. At
z=0.4 the lower limit is about M = -23. I tried to check if this
is due to a lower limit for sdss scope via using CalcTool for
apparent to absolute magnitude calculator
http://www.calctool.org/CALC/phys/as...star_magnitude

apparent Mag limit for sdss is 22 from their site, which yields
absolute mag limit at z=0.4 ~4e9 LYr from Ned Wrights calculator.

Entering those values, 22 apparent and 4e9 Lyr distance yields a
limit of -18.44 Mag.

This is 5 Mag brighter than the lower cut off in the plot,

Meaning?????

Quasars have a sharp lower brightness bound that gets brighter with
increasing z????

What am I doing wrong?

rt

[[Mod. note -- Could it be that you're seeing Malmquist bias?
(https://en.wikipedia.org/wiki/Malmquist_bias) Without doing the
numbers I can't see whether this is what you're seeing here, but
it would result in just such an observational-selection "sharp
lower brightness bound that gets brighter with increasing z".
-- jt]]
  #2  
Old December 27th 16, 03:24 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Default Quasar brightness lower bound, real or my error?

In article ,
writes:

For this plot of quasar Mag vs z

https://inspirehep.net/record/120804...dist1_dr10.png

There is a lower bound for quasar Mag that looks artificial. At
z=0.4 the lower limit is about M = -23.


Quasars are often DEFINED as AGN brighter than -23. That probably
explains your puzzle.
  #3  
Old December 28th 16, 04:45 AM posted to sci.astro.research
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Default Quasar brightness lower bound, real or my error?

On Monday, December 26, 2016 at 12:31:03 PM UTC-5, wrot=
e:
For this plot of quasar Mag vs z=20
=20
https://inspirehep.net/record/120804...dist1_dr10.png

=20
{snip}

What is the source of this figure? My guess would be that the text
of the document in which it appears will answer your question; it
could be a selection effect, due to the definition of quasar the
authors used, or something else (or some combo). I find it's a good
idea to read the text of the document in which such figures (plots)
appear, and when asking questions, quote the source.
  #4  
Old December 28th 16, 04:45 AM posted to sci.astro.research
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Default Quasar brightness lower bound, real or my error?

Thanks Jonathan,
Yes, it does look like Malmquist bias and has the right sense.

But it's off by 5 magnitudes....too dim. ie, the scope should be
able to see far dimmer objects. So there shouldn't be a cut off,
at least not on that graph at that brightness value....based on my
calculations

Sooooo, I'm guessing my calculations are wrong but can't see why.
rt


[[Mod. note -- Could it be that you're seeing Malmquist bias?
(https://en.wikipedia.org/wiki/Malmquist_bias) Without doing the
numbers I can't see whether this is what you're seeing here, but
it would result in just such an observational-selection "sharp
lower brightness bound that gets brighter with increasing z".
-- jt]]

  #5  
Old December 28th 16, 04:37 PM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 273
Default Quasar brightness lower bound, real or my error?

In article ,
writes:

On Monday, December 26, 2016 at 12:31:03 PM UTC-5, wrot=
e:
For this plot of quasar Mag vs z=20
=20
https://inspirehep.net/record/120804...dist1_dr10.png

=20
{snip}

What is the source of this figure? My guess would be that the text
of the document in which it appears will answer your question; it
could be a selection effect, due to the definition of quasar the
authors used, or something else (or some combo). I find it's a good
idea to read the text of the document in which such figures (plots)
appear, and when asking questions, quote the source.


From the paper: "this quasar sample is flux limited to i = 19.1".
  #6  
Old January 3rd 17, 07:04 AM posted to sci.astro.research
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Default Quasar brightness lower bound, real or my error?

On Wednesday, December 28, 2016 at 10:37:20 AM UTC-5, Phillip Helbig
(undress to reply) wrote:

{snip}

From the paper: "this quasar sample is flux limited to i = 19.1".


Thanks PH. I found a copy of the paper, Shen+ (2013), as an arXiv
preprint: https://arxiv.org/abs/1212.4526

rt: Does knowing that the sample is flux limited ("to i = 19.1") answer
the question you asked?

  #7  
Old January 23rd 17, 11:45 PM posted to sci.astro.research
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Posts: 35
Default Quasar brightness lower bound, real or my error?

On Monday, January 2, 2017 at 10:04:26 PM UTC-8, wrote:
On Wednesday, December 28, 2016 at 10:37:20 AM UTC-5, Phillip Helbig
(undress to reply) wrote:

{snip}

From the paper: "this quasar sample is flux limited to i = 19.1".


Thanks PH. I found a copy of the paper, Shen+ (2013), as an arXiv
preprint: https://arxiv.org/abs/1212.4526

rt: Does knowing that the sample is flux limited ("to i = 19.1") answer
the question you asked?


Yes, I think that is what I was missing from the start. Makes perfect
sense now, thanks.

rt

 




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