# Question about total absolute magnitudes of galaxies - negative or not?

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?

• To answer your comment below. Absolute magnitudes would conventionally be written as $M_I$. Whereas, just $I$ means apparent magnitude. – Rob Jeffries Jun 16 '15 at 22:51
• @ Rob Jeffries Thank you, this makes sense now! – Guest Jun 17 '15 at 14:54

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.

• 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 Jun 16 '15 at 21:20
• 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 Jun 17 '15 at 1:41
• 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 Jun 17 '15 at 21:47