JWST/MIRI
The Infrared Sky: Background
Considerations for JWST
Dean C. Hines & Christine Chen
MIRI Instrument Team
DCH-1
Space Telescope Science Institute
4/9/2015
JWST/MIRI
The Visible (Optical) Sky
DCH-2
Space Telescope Science Institute
4/9/2015
JWST/MIRI
The Visible vs the Infrared Sky
DCH-3
Space Telescope Science Institute
4/9/2015
JWST/MIRI
The Visible vs the Infrared Sky
DCH-4
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Infrared Background Sources

Three primary sources of diffuse light in the astronomical sky
– Extragalactic – light from unresolved objects (galaxies and QSOs)
– Galactic – mostly star-light reprocessed by material in the Interstellar
Medium (ISM)
– Solar System – sun light reprocessed by material within the solar
system

These astronomical components are highly wavelength
dependent
– Visible and near-Infrared (NIR) – dominated by scattered/reflected light
but with some thermal NIR emission from very hot dust
– Longer wavelengths – dominated by thermal emission from warm dust
and by broad solid-state and giant molecular emission features

Emission from the telescope and stray light also contribute to
the background
– Not a large issue for HST (except for NICMOS l > 1.8µm)
– For JWST, this is an issue for most wavelengths, and becomes a
dominant source for MIRI
DCH-5
Space Telescope Science Institute
4/9/2015
JWST/MIRI
HST vs JWST
Because JWST operates much colder than HST (~39-46K vs ~290K), diffuse
galactic and zodiacal emission dominate the background for l ≤ 16µm
JWST
HST
HST Thermal
Emission
Diffuse Galactic + Zodi
NICMOS Instrument Handbook
Glasse et al. (2010)
DCH-6
Space Telescope Science Institute
4/9/2015
JWST/MIRI
COBE/DIRBE
Blue = 1.25 µm
Green = 2.2 µm
Red = 3.5 µm
Zodi + Galaxy + Exgal
All of the images use 4
decade logarithmic color
scales.
Galaxy + Exgal
The lowest level is 3.95 kJy/sr
at 3.5 microns, 6.67 kJy/sr at
2.2 microns, and 7.08 kJy/sr
at 1.25 microns for the upper
and middle images, but 0.395
kJy/sr for the lower image.
DCH-7
Exgal
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Thermal Emission from Zodi Dust
Blue = 12 µm; Green = 60 µm; Red = 100 µm
IRAS
DCH-8
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Thermal Emission from Zodi Dust
Blue = 12 µm; Green = 60 µm; Red = 100 µm
IRAS
DCH-9
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Thermal Emission from Zodi Dust
Blue = 65 µm; Green = 90 µm; Red = 140 µm
Akari
DCH-10
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Zodi Dust Bands (IRAS)
DCH-11
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Zodi Bands & Spectrum
Optical
DCH-12
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Taurus Spitzer 24µm
Residual Zodiacal Dust
Band Emission after
subtraction of a smooth
zodiacal background
model based on
COBE/DIRBE data.
Zodiacal
Dust Bands
DCH-13
Space Telescope Science Institute
7.7˚
4/9/2015
JWST/MIRI
Galactic ISM (COBE)
Blue = 60 µm; Green = 100 µm; Red = 240 µm
DCH-14
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Galactic ISM (COBE)
Green = 100 µm; Red = 240 µm
DCH-15
Space Telescope Science Institute
4/9/2015
JWST/MIRI
PAH Emission from the ISM
PAH Emission Features — 3.29, 6.2, 7.7, 8.7, 11.3, and 12.7 µm
Wavelength (µm)
DCH-16
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Emission from the Galaxy
PAH Emission Features — 3.29, 6.2, 7.7, 8.7, 11.3, and 12.7 µm
DCH-17
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Zodi + Galactic
DCH-18
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Zodi + Galactic
DCH-19
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Conclusions

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DCH-20
The Infrared Sky is much different in structure and brightness
compared the visible sky
At visible wavelengths, the background is dominated by light
scattered from dust in the solar system (HST also contends
with geocoronal emission and earthshine)
The infrared background is dominated by emission from dust
in the solar system and the galaxy, plus broad-band emission
features from PAHs associated with diffuse galactic dust
These astronomical background sources will dominate the
JWST background for l ≤ 16µm, and will still be important for
longer wavelengths
The background will vary in time as our view of the zodiacal
emission changes during the year
STScI is working with the Spitzer Science Center (SSC) to
develop a background model appropriate for JWST
Space Telescope Science Institute
4/9/2015
JWST/MIRI
Fin
DCH-21
Space Telescope Science Institute
4/9/2015
JWST/MIRI
WISE
DCH-22
Space Telescope Science Institute
4/9/2015
JWST/MIRI
DCH-23

All of the images use 4 decade logarithmic color scales. The
lowest level is 3.95 kJy/sr at 3.5 microns, 6.67 kJy/sr at 2.2
microns, and 7.08 kJy/sr at 1.25 microns for the upper and
middle images, but 0.395 kJy/sr for the lower image.

The lowest level is 223.2 kJy/sr at 240 microns, 555.3 kJy/sr at
100 microns, and 508.2 kJy/sr at 60 microns for the upper and
middle images, but 30.7 kJy/sr for the lower image.
Space Telescope Science Institute
4/9/2015