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A.2 Photometric Quantities
The nomenclature of photometric quantitities in use in astronomical literature
is far from standard and sometimes ambiguous. Here we therefore give a brief
summary of the definitions and units of measure of these quantities as they
are used in this study.
- The Luminosity of a source is the energy radiated by the whole
surface of the source per unit time, that is
|
(A.1) |
- The Brightness of a source is the energy radiated by the whole
surface of the source per unit time per unit area (of the receiver), that is
|
(A.2) |
- Most galaxies, unlike most stars, are resolved objects, so that in
addition to measuring their total energy flux, we can in principle
measure the energy flux per unit solid angle of the source coming
from different regions.
The Surface Brightness of a region of a diffuse source is
the energy radiated by the region per unit time, per unit area
(of the receiver) and per unit solid angle (of the source), that is
|
(A.3) |
Astronomers, however, generally express brightness and surface brightness
in logarithmic units, i.e. in magnitudes (mag) and magnitudes per square
second of arc (mag/arcsec), respectively. To define a magnitude
scale, one has to arbitrarily choose a reference brightness , and the
corresponding reference surface brightness
of per square
second of arc. The brightness of a source expressed in magnitudes is then
|
(A.4) |
while the surface brightness of a region of a diffuse source in magnitudes
per square second of arc is
|
(A.5) |
is called the zero-point of the adopted magnitude scale
since for (and thus for
).
Note that these definitions equally apply to bolometric measurements
and to measurements in a given photometric band.
Note also that the sky background is often expressed in different units,
such as those described by [Leinert et al. 1998].
Next: B. The Historical Development
Up: A. Units of Measure,
Previous: A.1 Angular Quantities
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Mattia Vaccari
2000-12-05