In Figures 2 and 3, we show the mid-IR luminosity ,
SED slope
, and fraction
of
Car (``
''), the Carina nebula excluding
Car (``
''),
and the entire complex including
Car (``
''). It is apparent from these figures that
even if the Carina nebula was not resolved: (1) we would select analogs of
Car and
unresolved dusty stellar complexes hosting such analogs, (2) while it is close, we would not
select a stellar complex that is identical to the Carina nebula excluding
Car,
and (3) there are no sources with
,
, and
comparable
to
Car in M81. Indeed, this last point is true for each galaxy we studied.
There are, however, far more compact star clusters among the candidates such as M33-5, M33-8
and M81-10 (see Section3) where HST images are required to recognize their spatial extent.
Even in these cases it is unlikely we would lose a candidate. First, it
would require a ``conspiracy'' of a sort, namely that the SED of the hotter circumstellar dust
around the star (with characteristic K and
)
seamlessly merges with the colder SED of the interstellar dust (with characteristic
K and
) in the cluster. Typically we find that this leads to
SEDs with ``bumps'' which we do not observe.
Possibly more constraining is the requirement that for a compact
cluster to hide an Car analog it must still contain large
amounts of interstellar gas and dust several million years after the
cluster formed to allow for the time that even the most massive stars
require to evolve away from the main sequence. However, a cluster sufficiently
luminous to hide an
Car analog must host many luminous stars
with strong UV radiation fields and winds, which will likely clear the
cluster of gas and dust needed to produce strong
mid-IR emission. For example, 30Dor,
which harbors stars possibly as massive as
and
is about
years old (e.g., Crowther et al.2010), is
a weak source of
emission (see, e.g., Figure1 of Zhang & Stanek2012).