To explicate: blue galaxies emit more blue light (fiducial band 0.450--0.495 μm) than red galaxies---and red galaxies emit more red light (fiducial range 0.625--0.740 μm) than blue galaxies.
green valley galaxies are just between blue galaxies and red galaxies in emission.
All of these types just look they emit white light??? in "absolute" true color---which is just what psychophysical response of the unshielded human eye would be seeing them while the unshielded human eye was off in outer space.
For further explication of galaxy colors, see file galaxy_colors.html.
UNDER RECONSTRUCTION BELOW
Thus, the short explication of
blue galaxies
is that their maximum
of emission is blueward
of that of
red galaxies---whose
maximum
of emission is redward
of that of
blue galaxies.
But both
blue galaxies
and red galaxies
look more or less like they
emit white light???
in "absolute" true color---which
is just what psychophysical response
of the unshielded human eye would be
seeing them while the unshielded human eye
was off in outer space.
But that "absolute" true color
is just a special case of our ordinary
psychophysical response.
It is NOT especially meaningful.
The makers of astronomical images often try to make the
coloration
of galaxies truish,
but meaningful.
To further explicate
blue galaxies:
Galaxies with
star formation do have strong
blue light (fiducial band 0.450--0.495 μm)
emission near
star formation regions because of
hot young blue
stars
(i.e., OB stars) that
are born in star formation regions
and the largest and brightest of which
(i.e., those of ∼> 8
M_☉)
die within ∼< 30 Myr as
supernovae very near to
the star formation regions in which
they were born.
In images of galaxies
with enhanced
blue light (fiducial band 0.450--0.495 μm)
over "absolute" true color,
galaxies with
star formation regions
do look blue near said
star formation regions.
Such galaxies are
are blue galaxies
in the sense described above.
Such blue-light-enhanced images usually
show beautiful
complex coloration
with
blue light (fiducial range 0.450--0.495 μm),
red light (fiducial range 0.625--0.740 μm),
and brown light (mixed wavelength bands).
Spiral galaxies
and irregular galaxies
are usually
blue galaxies
and are often shown
blue-light-enhanced images.
See, for example,
the spiral galaxies
in Image 2
below.
To further explicate
red galaxies:
Red galaxies
do NOT look red
(note red light (fiducial range 0.625--0.740 μm))
in "absolute"
true color nor
in most images.
In both, they usually look
white-yellow.
However, their overall spectra do usally rise
to the red end of the
visible band (fiducial range 0.4--0.7 μm)
and peak in the
infrared band (fiducial range 0.7 μm -- 0.1 cm)???.
However, our psychophysical response is
white-yellow.
So they are redder
than blue galaxies
which do NOT have such a rise or as much of a rise.
It is maybe true that galaxies
at high
cosmological redshift z
look red
in true color
if NOT deredshifted.
But no one seems to spit out this
factoid.
There are, however,
relatively rare
green pea galaxies
which do look green
in enhanced truish color if
their actual emission
green light (fiducial range 0.495--0.570 μm is enhanced
enough
(see Wikipedia: Pea galaxies:
Description;
SDSS photometry passband filters;
Robert Lee, 2023,
"Rare 'green pea' galaxy may be the most 'chemically primitive' galaxy ever discovered").
Local file: local link: galaxy_colors.html.
Galaxies shine by primarily by starlight, and
so their overall spectra are sort of
an average
stellar spectra
which means their spectra
are very roughly an average
of blackbody spectra
with a maximum
of emission at some wavelength.
To inverse what was said of
blue galaxies,
the short explication of
red galaxies
is that their maximum
of emission is redward
of that of
blue galaxies---whose
maximum
of emission is blueward
of that of
red galaxies.
File: Galaxies file:
galaxy_colors.html.