fied 850 K greybody, augmented
with a mid-infrared, polycyclic
aromatic hydrocarbon emission template spectrum, as suggested by da Cunha et al. (2008).
The luminosity of the excess SED
component correlates with the
star-formation rate of the galaxy,
so the excess shows some promise as an extinction-free star formation tracer.
Figure 3.
The case for hot
(~ 1000 K) dust or
PAH continuum
emission in GDDS
galaxies, based on
multi-wavelength
rest-frame
photometry for
88 GDDS galaxies.
See Mentuch et
al. (2009), from
which this figure is
taken, for details.
Observations from
the Gemini Deep
Deep Survey nearly
always show a
disagreement
between purestellar models and
observations at
2-5 μm. As shown
in the next figure,
adding an 850 K
greybody+PAH
line emission fit
data well. Is this
related to star
formation, and if
so, how?
10
och is either a mix of different types of galaxies,
or possibly a unique class of objects.
Extragalactic Circumstellar Disks?
Shifting gears completely, another relatively
recent result from the GDDS, and presented
in a paper led by Erin Mentuch in 2009, focuses on the blue star-forming galaxies in the
GDDS. Of course, we originally set out to target a totally different (quiescent) population
of galaxies: the so-called red and dead galaxies. But since these systems are fairly rare, and,
since one has to fill up gaps in Gemini MultiObject Spectrograph masks with something,
we also targeted bluer objects when redder
ones were unavailable. As it turned out, the
survey did some of its most interesting work
on these “runt” galaxies.
Detailed modeling of the Spitzer colors of
these objects (Figure 3) shows clear evidence
for a near-infrared excess at around 3 microns, which, at the redshifts of these galaxies, is seen in the Infrared Array Camera (IRAC)
[5.8]-micron and [8.0]-micron bands. In a nice
surprise, Mentuch et al. modeled this excess
as an additional Spectral Energy Distribution (SED) component consisting of a modi-
GeminiFocus
HST imaging data hint that the
excess correlates with star-formation activity and morphology. But the main interest of the
excess lies in the interpretation
of its origin. The five best candidates for the excess emission
are: (1) active galactic nuclei (AGN); (2) the
high-redshift counterpart to the interstellar
cirrus emission seen in our own galaxy, (3)
reflection nebulae; (4) post-asymptotic giant
branch (AGB) stars/planetary nebulae; and (5)
proto-stellar/proto-planetary disks in massive
star-forming regions.
Mentuch et al. (2009) come down firmly in
favor of the last candidate, in effect attributing the excess light to the collective emission
from the thousands of flared circumstellar
disks around massive stars in galaxies at high
redshifts