In this Section, we detail how we obtained the photometric measurements at various wavelengths and combined them to construct the point-source catalogs. Although the procedures followed here are derived from the techniques developed by Khan et al. (2010) and Khan et al. (2013), there are some key differences, as we now carry out an inventory of all point-sources rather than targeting a particular sub-class with desired photometric properties.
Specifically, Khan et al. (2010) searched
for very red mid-IR (
) sources near or at the detection limit
of the first two IRAC bands, and therefore included in their primary
source-list objects that were detected in the
but
not the
image, as well as objects that could only be detected in
the
difference image but not in either of the individual images.
Khan et al. (2013) focused on
the most luminous mid-IR (
) sources with a
spectral energy distribution (SED) that is either flat across the four IRAC
bands or rising towards the longer wavelengths, and therefore included in
the primary source-list all objects
that had
in any of the first three
(
) IRAC bands.
In this work, we implement strict detection criteria by
selecting all sources detected at
in both the
and
images within a certain
matching radius as point-sources. Next,
we search for
detections of these point-sources
in the
and
images within the same matching
radius. If no counterpart is found, we attempt
to measure the flux at the location of the
point-source through PSF fitting,
and failing that, through aperture photometry. For the MIPS
images, we only use aperture photometry
due to the much lower resolution and larger PSF size compared to the IRAC images.
Finally, for all objects that do not have a
detection
at
,
and
, we estimate the
flux upper
limits. The fluxes and upper limits are transformed to Vega-calibrated magnitudes using
the flux
zero-points
and aperture corrections provided in the Spitzer Data Analysis
Cookbook
.
Given this broad outline, we now describe the specific technical details of how we
performed the measurements at the various stages of constructing the catalogs.
We used the DAOPHOT/ALLSTAR PSF-fitting and photometry package (Stetson1992)
to construct the PSFs, to identify the sources, and to measure their
flux at all
IRAC bands. The different roll-angles of the various Spitzer observations
made it necessary to construct
the PSFs for each galaxy in each band independently.
Next, we empirically determined the optimal radius to match the
and
source-lists. Figure2 shows the distribution of
distances to the nearest
source for each
source in
M33. In this case, over
have a match within
pixel. The density of
nearest matches falls rapidly between
pixel (
additional
matches), while the number of duplicates increases
(
duplicate matches), and then the distribution essentially flattens.
Similar distributions are observed for the other six galaxies
(see Table1). We therefore adopted an empirically
motivated matching radii of 1pixel in order to maximize
the number of matches for a minimal number of duplicates.
We used the IRAF ApPhot/Phot tool for performing aperture photometry
at the point-source locations for all IRAC bands and the MIPS
band.
For the four IRAC bands, we use an extraction aperture of
, a local background
annulus of
, and aperture corrections of
,
,
,
and
respectively.
For the MIPS
band, we use an extraction aperture of
, a local background
annulus of
, and an aperture correction of
.
We estimate the local background using a
outlier rejection procedure in
order to exclude sources located in the local sky annulus, and correct for the
excluded pixels assuming a Gaussian background distribution. Using a
background annulus immediately next to the signal aperture minimizes the effects of
background variations in the crowded fields of the galaxies. We also determine
the
flux upper limit for each aperture location using the
local background estimate.
Ideally, flux measurements of an isolated point-source through either aperture or
PSF photometry would produce the same
results after appropriate aperture corrections (in the first case)
and small zero-point offsets (in the second case) to account for flux
underestimation due to PSF fitting up to finite radii rather than to infinity.
We derive this small (usually mag) offset for each image from the mean
difference between the magnitudes of relatively isolated, unsaturated bright sources measured through aperture and PSF
photometry. For fainter sources, especially in limited spatial
resolution images of crowded fields, the aperture and PSF photometry measurements can vary significantly.
Figure3 shows the differences between apparent magnitudes
determined through aperture and PSF photometry as a function of PSF magnitude
for the
sources in NGC6822 and M81.
For the less
crowded case of NGC6822, the two measurements generally agree for the very
brightest sources in all four IRAC bands, with the scatter increasing
for the fainter sources. The same is true for the
and
images of M81, but at
, the scatter
is much worse, although a sequence of bright sources with good agreement
between the two sets of measurements can still be seen. However, for
, such a sequence cannot be clearly identified,
and we found this to be true for
the other galaxies as well. Mismatches between the two magnitudes are a
good indicator of when crowding is significantly effecting the magnitude estimates,
and in the catalogs we list the difference
between the PSF and aperture photometry magnitudes for each source.
Because of these crowding problems,
we do not attempt to fine-tune the PSF photometry
measurements by applying the small
linear offset, although
we do this for the other three IRAC bands. For all IRAC bands, we
universally prefer PSF photometry over aperture photometry,
because PSF fitting is more successful at extracting relatively fainter
sources in crowded fields. Aperture photometry can
significantly over-subtract the sky and underestimate the source flux,
or overestimate the source flux by failing to remove
contamination from nearby brighter sources. Nevertheless, aperture photometry proves very useful
for validating the PSF photometry measurements at the bright end in all IRAC bands,
for estimating
and
fluxes
where PSF fitting fails and
flux
where the lower resolution makes PSF photometry infeasible,
and for determining flux upper limits.
To summarize, we implement strict detection criteria by requiring a
detection of all cataloged sources at
and
.
We then complement those measurements
at the
,
and
bands through a combination of PSF and
aperture photometry, preferring PSF fitting over aperture photometry at
and
, and exclusively using aperture photometry at
. For all objects that do not have a
detection
at these three longer wavelengths, we estimate
flux upper
limits.