Elliptical Galaxies
as Absolute Calibration Sources
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We analyze IRAC observations of
large elliptical galaxies with the ultimate goal to derive the extended
source calibration for resolved bjects. The analysis includes
a detailed comparison with a set of SED templates and spectra that
represent the NIR/MIR light distribution of elliptical galaxies.
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Method
Extended source calibration depends on the source size and spectral
shape. The goal is to create aperture (flux) corrections based on
these parameters. To begin the process, we elect to start with the
simplest cases, moving on to more challenging cases as our knowledge
of these calibration issues increases.
The simplest extended sources to model and analyze are
early-type elliptical galaxies, whose dust-free starlight is
well understood in the near and mid-infrared window.
Elliptical Galaxies
Early-type elliptical galaxies have very homogeneus properties,
resulting in smooth, featureless spectral energy distributions
that are dominated by the evolved luminous population.
Ellipticals and spheroids have low star formation, minimal dust content, and
relatively high surface brightness. Single burst population synthesis
models do an adequate job of describing the mid-infrared properties of
elliptical galaxies.
Photometry & Measurements
The mid-infrared IRAC imaging comes from GTO, SINGS and Spitzer-IST programs.
The IRAC images are convolved with the 2MASS PSF so that direct comparison
can be made at small radii (reliable photometry should be possible for
r > 3 arcsec).
2MASS mosaic imaging is obtained from the Large Galaxy Atlas (Jarrett et al 2003)
and IRAC mosaic imaging from the SSC archive and from special calibration
observations. Foreground (Milky Way) stars are removed from the images.
Photometry is carried out using a nested set of circular (or elliptical, where noted:
the shape of the photometry aperture is not critical, as either
circular or elliptical apertures may be used)
apertures centered on
the brightest part of the nucleus (note: since ellptical galaxies are dominated
by starlight from the evolved population, the near-infrared and mid-infrared
positions will be identical).
The background light offset is removed. Local background is estimated using
an annulus that encircles the target, located just beyond the last aperture used
to measure the flux of the target.
The measurements are flux calibrated using
the zero-point mag in header of the 2MASS images and using the standard (point source) flux
calibration for IRAC.
Spectral Energy Distribution
We construct the elliptical galaxy SED from
near-infrared measurements (2MASS) and mid-infrared measurements
(IRAC).
Integrated flux measurements are in units of flux density.
The model SEDs are then fit to the galaxy SEDs using the
K-band (2.2 µm) measurement. In some cases we have
corrected the photometry for cosmic reddening (the k-correction).
Tenets:
- elliptical galaxies are dust-free
- the near-infrared measurements
of 2MASS are free of their own flux biases
- the near-infrared (2.2 µm)
is tracing the "old" stellar population and is unaffected by ISM dust
That is, we
assume
the near-infrared 2MASS light represents ground truth.
Any deviations that are seen for the IRAC measurements compared to
the E-galaxy model SED represent discrepancies in the IRAC flux calibration.
Models
We will be comparing IRAC photometric measurements
with the expected light distribution of an old galaxy (t = 11 & 13 Gyrs)
derived from the Bruzual & Charlot (2003) and GRASIL population synthesis, respectivelhy.
Additional templates are shown below, including
versions with AGB dusty shell populations (e.g. the Piovan models A&A 408, 559).
Finally, two galaxies in the sample (N1404 and N4552) are from SINGS, which therefore includes high quality
IRS SL spectra (from 5um and longward). In order to match the SED templates, we've normalized the
SINGS spectra (which have units of MJy/sr) by the *expected* K-band SB, as deduced from "old" galaxy SED
comparing K-band to the 8um window (see table below).
Elliptical Galaxy Model Templates after
normalization in the K-band (2.2 µm);
three spectra are included: K2III, M0III, M2.5III;
and equivalent IRAC photometric points shown with magenta triangles
(derived from the templates and M-giant spectra).
Also shown is the IRS-SL spectrum for the SINGS galaxy
NGC1404. Here are the same templates overlayed with
ISOPHOT-S results for a set of ellipticals (some of which
have IRAS dust emission); data comes from Nanyao Lu (2006).
Finally,
here is a set of templates that
include a dusty late-type galaxy to demonstrate
PAH emission bands in the MIR.
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Relative Spectral Response of an Elliptical Galaxy
with 2MASS and Spitzer filters
| band | wavelength | f/f_K | |
| J | 1.25 | 1.020 | |
| H | 1.65 | 1.239 | |
| K | 2.18 | 1.000 | |
| 1 | 3.6 | 0.473 | |
| 2 | 4.5 | 0.302 | |
| 3 | 5.8 | 0.204 | |
| 4 | 8.0 | 0.125 | |
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The following set of templates show the
effect of adding a (relatively) young population
of evolved (AGB) stars with dusty shells.
The younger the population, the bigger the effect
(i.e., MIR emission in the long IRAC bands).
Finally, here is the 10.2 Gyr AGB with the SINGS
IRS-SL spectrum for NGC1404 & NGc4552 overlayed. I've normalized the SINGS spectrum assuming
a K-band to 8um flux ratio consistent with ellipticals (see table to the left).
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**The assumption that elliptical galaxies are completely dust-free
is not true for all galaxies in the sample, but it does seem likely
that most of the sample is dust free (based on the SINGS spectroscopy
and MIPS imaging.) However, if it turns out that AGB dust is present
in the bulk of the sample, then the results will be severely compromised
(as the AGB models seen above are abundantly clear) and the
extended source aperture corrections wil be systematically too
large.**
Galaxy Photometry Results
The following table summarizes the flux calibration results for 13 galaxies
analyzed in detail. The results include
detailed SEDs and IRS SL spectra (when available) for each galaxy.
* denotes sources with IRS spectral measurements.
Extended Source Aperture Correction Results
IRAC-1 (3.5 µm) extended source
correction factors for the galaxy sample. The magenta line denotes
"swire-1".
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IRAC-2 (4.5 µm) extended source
correction factors for the galaxy sample. The magenta line denotes
"swire-1".
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IRAC-3 (5.8 µm) extended source
correction factors for the galaxy sample.
N4552 is discrepant due to the presence of dust (PAH emission).
The magenta line denotes
"swire-1".
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IRAC-4 (8.0 µm) extended source
correction factors for the galaxy sample.
N4552 is discrepant due to the presence of dust (PAH emission).
The magenta line denotes
"swire-1".
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IRAC-1 (3.5 µm) extended source
correction factors combined; points with error bars represent
the median and RMS in the sample.
The doted line denotes N4552 (dusty
elliptical).
The magenta line denotes "swire-1".
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IRAC-2 (4.5 µm) extended source
correction factors combined; points with error bars represent
the median and RMS in the sample.
The dotted line denotes N4552 (dusty
elliptical).
The magenta line denotes "swire-1".
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IRAC-3 (5.8 µm) extended source
correction factors combined; points with error bars represent
the median and RMS in the sample. The dotted line denotes N4552 (dusty
elliptical).
The magenta line denotes "swire-1".
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IRAC-4 (8.0 µm) extended source
correction factors combined; points with error bars represent
the median and RMS in the sample. The dotted line denotes N4552 (dusty
elliptical).
The magenta line denotes "swire-1".
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Further Analysis & Links
Last updated by T. Jarrett on April 12, 2007
