<blockquote>
  <p>The morphological properties of galaxies between 21 mag &lt; I &lt; 25 mag
  in the Hubble Deep Field are investigated using a quantitative
  classification system based on mea- surements of the central
  concentration and asymmetry of galaxian light.</p>
</blockquote>

<p>Those are apparent magnitudes, not absolute.</p>

<p>The convention is to use $M_X$ for absolute magnitude and just $X$ for apparent mag (or m$_X$), where X is the bandpass symbol (U, B, V, R, I, J, L, H, K, etc).</p>

<p>And, the $V_T$ and $M_{V_T}$ (note the correct subscripting, which the authors failed to do) means the V band radial profile is extrapolated to infinite radius to give a total for the whole galaxy.  </p>

CC BY-SA 3.0
2015-06-16T21:20:48.157Z
It doesn't seem like they state this and I'm not very familiar with this field yet so I'm sorry if this obvious, but how would one be able to tell when authors are talking about an apparent or absolute magnitude? Is it because those values in the second paper are positive and that means they cannot be absolute since these are galaxies? Thank you for your reply!
Guest
CC BY-SA 3.0
2015-06-17T01:41:42.190Z
Would a meteorologist write 128°F when talking about Antarctica? Nobody would ever omit the negative sign if it's negative magnitude. 25th visible light absolute magnitude is just over 1% of a light so dim that I've never seen the standard diagram of star types' scale go that dim.
user6784
CC BY-SA 3.0
2015-06-17T21:47:41.870Z
In the second quote, the fact that the authors used "I" (capital-i) indicates that they are talking about an apparent magnitude in the "I" bandpass. An absolute magnitude in "I" would be designated by M_I.
Peter Erwin
CC BY-SA 3.0
2015-06-16T20:49:46.317Z
2015-06-26T17:32:38.713Z
2015-06-26T17:32:38.713Z
eshaya
<p>I have seen this in a couple of places, but when the authors in the paper - <a href="http://chandra.as.utexas.edu/~kormendy/kfcb-accepted.pdf" rel="nofollow">http://chandra.as.utexas.edu/~kormendy/kfcb-accepted.pdf</a> -
wrote:</p>

<blockquote>
  <p>"All 10 of our ellipticals with total absolute magnitudes M_VT ≤ −21.66 have cuspy cores – “missing light” – at small radii. Cores are well known and naturally scoured by binary black holes formed in dissipationless (“dry”) mergers. All 17 ellipticals with −21.54 ≤ M_VT ≤ −15.53 do not have cores."</p>
</blockquote>

<p>Since absolute magnitude is the magnitude of that object at 10 pc away from Earth, then this is why the galaxies have such negative (meaning, very bright) values for total absolute magnitude, correct?</p>

<p>However, in this paper - <a href="http://arxiv.org/pdf/astro-ph/9602044v1.pdf" rel="nofollow">http://arxiv.org/pdf/astro-ph/9602044v1.pdf</a> - the author writes:</p>

<blockquote>
  <p>"The morphological properties of galaxies between 21 mag &lt; I &lt; 25 mag in the Hubble Deep Field are investigated using a quantitative classification system based on mea- surements of the central concentration and asymmetry of galaxian light."</p>
</blockquote>

<p>The galaxies here are between 21 to 25 absolute magnitudes (?) in the infrared (I?) band, but why are these magnitudes not negative? </p>

CC BY-SA 3.0
2015-06-16T22:51:20.353Z
To answer your comment below. Absolute magnitudes would conventionally be written as $M_I$. Whereas, just $I$ means apparent magnitude.
ProfRob
CC BY-SA 3.0
2015-06-17T14:54:24.057Z
@ Rob Jeffries Thank you, this makes sense now!
Guest
<galaxy><magnitude>
Question about total absolute magnitudes of galaxies - negative or not?
Guest
# Question about total absolute magnitudes of galaxies - negative or not?

(Asked by: username_0 on 2015-06-16)

I have seen this in a couple of places, but when the authors in the paper - <http://chandra.as.utexas.edu/~kormendy/kfcb-accepted.pdf> -
wrote:

> "All 10 of our ellipticals with total absolute magnitudes M\_VT ≤ −21.66 have cuspy cores – “missing light” – at small radii. Cores are well known and naturally scoured by binary black holes formed in dissipationless (“dry”) mergers. All 17 ellipticals with −21.54 ≤ M\_VT ≤ −15.53 do not have cores."

Since absolute magnitude is the magnitude of that object at 10 pc away from Earth, then this is why the galaxies have such negative (meaning, very bright) values for total absolute magnitude, correct?

However, in this paper - <http://arxiv.org/pdf/astro-ph/9602044v1.pdf> - the author writes:

> "The morphological properties of galaxies between 21 mag < I < 25 mag in the Hubble Deep Field are investigated using a quantitative classification system based on mea- surements of the central concentration and asymmetry of galaxian light."

The galaxies here are between 21 to 25 absolute magnitudes (?) in the infrared (I?) band, but why are these magnitudes not negative?

**Tags:** galaxy, magnitude

> **Comments:**
> **username_1**: To answer your comment below. Absolute magnitudes would conventionally be written as $M_I$. Whereas, just $I$ means apparent magnitude.  
> **username_0**: @ Rob Jeffries Thank you, this makes sense now!  

---

## 1 answers

### Answer by username_2 on 2015-06-16. Score: 2 | Accepted answer

> The morphological properties of galaxies between 21 mag < I < 25 mag
> in the Hubble Deep Field are investigated using a quantitative
> classification system based on mea- surements of the central
> concentration and asymmetry of galaxian light.

Those are apparent magnitudes, not absolute.

The convention is to use $M\_X$ for absolute magnitude and just $X$ for apparent mag (or m$\_X$), where X is the bandpass symbol (U, B, V, R, I, J, L, H, K, etc).

And, the $V\_T$ and $M\_{V\_T}$ (note the correct subscripting, which the authors failed to do) means the V band radial profile is extrapolated to infinite radius to give a total for the whole galaxy.

> **Comments:**
> **username_0**: It doesn't seem like they state this and I'm not very familiar with this field yet so I'm sorry if this obvious, but how would one be able to tell when authors are talking about an apparent or absolute magnitude? Is it because those values in the second paper are positive and that means they cannot be absolute since these are galaxies? Thank you for your reply!  
> **username_3**: Would a meteorologist write 128°F when talking about Antarctica? Nobody would ever omit the negative sign if it's negative magnitude. 25th visible light absolute magnitude is just over 1% of a light so dim that I've never seen the standard diagram of star types' scale go that dim.  
> **username_4**: In the second quote, the fact that the authors used "I" (capital-i) indicates that they are talking about an apparent magnitude in the "I" bandpass. An absolute magnitude in "I" would be designated by M_I.  

---

