In this Section, we discuss the results of our mid-IR photometric survey.
Because we required a
detection for each source at
and
, the effective survey area for each
galaxy is the overlap of the IRAC
and
image mosaics.
Table1 lists the effective
survey area, gas-phase (H
) star formation rates adopted from Khan et al. (2013) and
the number of point-sources cataloged in each galaxy followed by the
number of matches at
and
pixel, duplicates
between the
and
source-lists, and the number of
counterparts identified
at the three longer wavelength images for each galaxy.
Tables
list the coordinates (J
; RA and Dec) of the
point-sources followed by their Vega calibrated apparent magnitudes (
), the associated
uncertainties (
), and (for the
bands) the differences
between the PSF and aperture photometry magnitudes (
).
For the
,
and
bands,
implies that the associated photometric measurement is a
flux upper limit, and
(as well as
) indicates that no reliable photometric measurement could be
obtained for that location. For the IRAC bands,
implies
that one or both of the associated photometric
measurements did not yield a
flux measurement.
Figures 4 and 5 present the
vs.
,
vs.
, and
vs.
color magnitude diagrams (CMDs) for
each galaxy. For comparison, we include the mid-IR CMDs for all
sources in a
deg
region (see Khan et al.2013, for details)
of the NOAO Bootes Field produced
from the Spitzer Deep Wide Field Survey
(SDWFS, Ashby et al.2009) data. Our catalogs
simply inventory all the sources present on the image mosaics
and do not attempt to
distinguish between sources actually associated with the
galaxies and unrelated contaminants. The contamination
is significant for galaxies like NGC247, which is highly inclined
and covers a smaller fraction of the Spitzer images,
than for the larger and more face-on galaxies (see
Figure1). Indeed, the CMDs of NGC247
(Figure5, second column) clearly show two
distinct population of sources, with the break appearing
near
. The CMDs of the two closest galaxies
(NGC6822 and M33) show distinguishable sequence of
bright and red AGB stars (near
), but for the more
distant galaxies this feature is less prominent (see Khan et al.2010
for relevant discussion). The CMDs of NGC7793
has the fewest identified point-ources both due to its lower mass
and because of relatively poorer quality (more systematic artifacts) of its image
mosaics.
Figure 6
show the apparent magnitude histograms of all sources in the catalog,
with the shaded regions showing the sources in M33.
It is apparent that the M33 catalog
is mag shallower than the other catalogs in all bands.
Overall, our source-lists become incomplete at
,
,
and
. Figure7
show the mid-IR color histograms of all sources in the catalog,
with the shaded regions showing the sources with
uncertainty in color
.
It demonstrates the absence of any surprises in the color distributions and
that the mid-color distributions are not due to large uncertainties.
All normal stars have the same mid-IR color in the first two IRAC bands,
because of the Rayleigh-Jeans
tails of their spectra, and we see this from the sharp peak of
color distribution at
.
The longer wavelength detections are increasingly dominated by dusty stars,
with color distribution peaks at
,
,
and
. Figures 6 and 7
also show the apparent magnitude and color histograms of the SDWFS point
sources (dotted lines). They show that our catalogs are
mag deeper than the
SDWFS catalog and the mid-IR color distribution of
sources in a random extragalactic field (the NOAO Bootes Field) significantly differs from that
of fields containing nearby galaxies. In particular, at longer wavelengths,
the galaxies contain more red sources (dusty stars) while the
random extragalactic field sources are generally bluer.
Although we report the photometry in the catalogs,
these measurements have limited utility due to the lower spatial
resolution of this band.
The aperture used for this band commonly includes
objects other than the intended target and is contaminated
by emission from cold interstellar dust. Nevertheless,
as we showed in Figure5 of Khan et al. (2013), the spectral energy
distributions of the normal stars show the expected negative slope for the Rayleigh-Jeans tail
of their SEDs between
and
.
The
photometry can be very useful in specific cases, such as
for studying evolved massive stars (see Khan et al.2015, for details),
despite the resolution limitation.
Where Thompson et al. (2009) identified sources
in M33, and Khan et al. (2010) identified
sources
in NGC300 and
sources in M81, now we catalog
,
and
sources in these galaxies
respectively. This is due to a number of factors. First, both of the
earlier studies only cataloged the central regions of these
galaxies, while here we analyze the full mosaics.
Second, we use a larger matching radius
of
pixel (rather than
pixel) to define point-sources and
a higher fraction of our cataloged sources
can be co-incidental matches between the
and
.
Third, the catalogs presented here
are deeper due to improved photometry and search methods based on
lessons learned from Khan et al. (2013); Khan et al. (2011); Khan et al. (2015). For example, Thompson et al. (2009)
noted that their M
point-source-list becomes incomplete at
,
while we reach
mag deeper for this galaxy and
mag
deeper for the rest. We have compared our M33, NGC300 and M81
catalogs with the catalogs published by Thompson et al. (2009) and
Khan et al. (2010), and
found no notable discrepency or significant photometric mismatch.
This catalog is a resource as an archive for studying mid-IR transients
and for planning observations with the James Webb Space Telescope.
Our survey is being expanded to
galaxies with x higher integrated star formation rate
than for these seven galaxies.
While we have shown that surveys for stellar populations are feasible
using archival Spitzer data, JWST will be a far more powerful probe
of stars in the mid-IR. The nearly order-of-magnitude higher resolution
(Gardner et al.2006) of JWST compared to Spitzer
can be used either to greatly reduce the problem of confusion
in these galaxies or to greatly expand the survey volume.
This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration (NASA). We extend our gratitude to the SINGS Legacy Survey and the LVL Survey for making their data publicly available. RK is supported through a JWST Fellowship hosted by the Goddard Space Flight Center and awarded as part of the NASA Postdoctoral Program operated by the Oak Ridge Associated Universities on behalf of NASA.