The Spitzer instruments

The three science instruments for imaging and spectroscopy are the Infrared Array Cam- era (IRAC), the Multiband Imaging Photometer for Spitzer (MIPS) and the InfraRed Spectrograph (IRS). A schematic view of the Spitzer focal plane in Figure 3.1 (taken from Chapter 2 of the Spitzer Observer’s Manual - Version 7.1 issued by the Spitzer Science Center December 8 2006; hereafter referred to as the “SOM”) shows how the science instru- ment apertures are projected onto the sky. A summary of the instrument characteristics is given in Table 3.1.

The instruments, all three of which were used for the data presented in this thesis, are described in the following sections.

Table 3.1:Summary of Spitzer instrumentation, adapted from Table 2.2 of the SOM.

Wavelength Mode (IRAC, MIPS)/ Array Type Resolving Field of Pixel Sensitivitya[µJy]

[µm] Module (IRS) Power View Size (5 σ in 500 s

[00

] incl. confusion)

IRAC: InfraRed Array Camera 3.6 Ch1 - Imaging InSb 4.7 5.0_21×5.0

21 1.221 1.6 (3.4)b

4.5 Ch2 - Imaging InSb 4.4 5.0

18×5.0

18 1.213 3.1 (4.3) 5.8 Ch3 - Imaging Si:As (IBC) 4.0 5.0_21×5.0

21 1.222 20.8 (21) 8.0 Ch4 - Imaging Si:As (IBC) 2.8 5.0

21×5.0

21 1.220 26.9 (27)

MIPS: Multiband Imaging Photometer for Spitzer 24 Ch1 - Imaging Si:As (IBC) 5 5.0_4×5.0

4 2.55 110c

70 Ch2 - WideFOV Ge:Ga 4 5.0

2×2.0

6 9.98 7.2 mJyd

Ch2 - NarrowFOV/ Ge:Ga 4 2.0_7×1.0

4 5.20 14.4 mJy Super Res

55 – 95e _{Ch2 - SED Ge:Ga 15-25 0.}0_32×3.0

8 10.1 82/201/447 mJy (at 60/75/90 µm) 160 Ch3 - Imaging Ge:Ga(stressed) 5 0.053×5.03 16×18 29 (40) mJyf

IRS: Infrared Spectrograph 5.2 – 14.5 Short-Low Si:As (IBC) 60 – 127 3.00

7×57.00

0 1.8 250g

13.5 – 18.7 Peak-up Blue ( Si:As (IBC)h _{∼3 1.}0_0×1.0

2 1.8 116 18.5 – 26.0 Peak-up Red 80 9.9 – 19.6 Short-High Si:As (IBC) ∼600 4.00_7×11.00

3 2.3 1.2 ×10−18_{W m}−2

14.0 – 38.0 Long-Low Si:Sb (IBC) 57 – 126 10.00

6×168.00

0 5.1 1500 18.7 – 37.2 Long-High Si:Sb (IBC) ∼600 11.00

1×22.00

3 4.5 2 ×10−18_{W m}−2 a_{Sensitivities are for point sources, and are only representative. See Chapters 6–8 of the SOM for}

more detail.

b_{IRAC sensitivity is given for intermediate background. The first number in each case is without}

confusion, and the second number (in parentheses) includes confusion. See also IRAC Chapter 6 of the SOM.

c_{MIPS sensitivity is given for low background.}

d_{70 µm can be confusion limited; see the MIPS Chapter 8 of the SOM for more details.}

e_{Because of a bad readout at one end of the slit, the spectral coverage for 4 columns of the array is}

reduced to about 65–90 µm.

f_{160 µm is often confusion limited; the first number is without confusion, and the second number}

(in parentheses) includes confusion.

g_{IRS sensitivity is given for low background at high ecliptic latitude. Note that for IRS, sensitivity}

is a strong function of wavelength.

Figure 3.1:Schematic diagram of the Spitzer focal plane showing the nominal field-of-view locations of the science instruments projected onto the sky. Taken from Chapter 2 of the SOM).

The Infrared Array Camera - IRAC

The Infrared Array Camera (IRAC) is a four-channel camera that provides simultaneous 5.0

2 × 5.0

2 broadband images at 3.6, 4.5, 5.8, and 8 µm (Fazio et al. 2004). Two adjacent fields of view are imaged in pairs (3.6 and 5.8 µm; 4.5 and 8.0 µm) using dichroic beamsplitters. The diffraction limited angular resolution of Spitzer in the four IRAC channels is 1.00

07 at 3.6 µm, 1.00_{33 at 4.5 µm, 1.}00_{72 at 5.8 µm and 2.}00

37 at 8.0 µm. All four detector arrays in the camera have dimensions of 256 pixels × 256 pixels, with a pixel size of ∼100.

2 × 100.

2. The two short wavelength channels use indium antimonide (InSb) detector arrays and the two longer wavelength channels use arsenic-doped silicon (Si:As) impurity band conduction (IBC) detectors.

Figure 3.2 shows the IRAC cryogenic assembly. The Focal Plane Assemblies include the detector arrays and associated components. The IRAC optical layout is shown in

Figure 3.2: The IRAC cryogenic assembly model, with the top cover removed to show the inner components. Taken from IRAC Chapter 6 of the SOM.

Figures 3.3 and 3.4. Channels 1 and 3 view the same telescope field (within a few pixels), and Channels 2 and 4 view a different field simultaneously (see Figure 3.1 of the Spitzer field of view).

The Multiband Imaging Photometer for Spitzer - MIPS

The Multiband Imaging Photometer for Spitzer (MIPS) provides simultaneous broadband imaging at wavelengths of 24, 70, and 160 µm, and low-resolution spectroscopy in the 55 and 95 µm wavelength region (Rieke et al. 2004).

The instrument contains 3 separate detector arrays each of which resolves the telescope Airy disk with pixel sizes ≤ λ/2D. All three arrays view the sky simultaneously, with multiband imaging at a given point provided via telescope motions. The 24 µm camera provides a 5.0_{4 × 5.}0_{4 field of view. The 70 µm camera was designed to have a ∼5.}0_{0 × 5.}0

0 field of view, but a cabling problem compromising the outputs of half the array results in a field of view of 5.0_{2 × 2.}0_{6. The 70 µm array also has a narrow field of view/higher}

magnification mode, and addionally can be used in a spectroscopic mode. The 160 µm array projects to the equivalent of a 0.0_{5 × 5.}0_{3 field of view. The diffraction limited angular}

resolution of Spitzer in the three MIPS channels is 7.00_{11 at 24 µm, 20.}00_{72 at 70 µm and 47.}00

37 at 160 µm. Corresponding pixel sizes on the detectors are 2.00_{55 at 24 µm, 9.}00

98 at 70 µm for the 5.0_{2 × 2.}0

6 field of view, and 1600× 1800

Figure 3.3: The IRAC optical layout, top view. The layout is similar for both pairs of channels; the light enters the doublet and the long wavelength passes through the beamsplitter to the Si:As detector (Channels 3 and 4), with the short wavelength light reflected to the InSb detector (Channels 1 and 2). Taken from IRAC Chapter 6 of the SOM.

Figure 3.4: The IRAC optical layout, side view. The Si:As detectors are shown at the far right of the figure, the InSb arrays are behind the beamsplitters. Taken from IRAC Chapter 6 of the SOM.

Figure 3.5: Schematic diagram of the optical layout within the MIPS cold assembly: detectors (Focal Plane Arrays), and optical elements and paths. Two mirror facets are attached to the Cryogenic Scan Mirror Mechanism (CSMM): one mirror feeds the 70 µm optical train (normal field-of-view, narrow field-of-view and spectrometer/SED), while the second mirror feeds both the 24 µm and 160 µm optical trains. Spitzer’s central axis and the telescope focal plane are to the right in this view. Taken from MIPS Chapter 8 of the SOM.

At 24 µm, Si:As IBC detectors are used, whereas for the two longer-wavelength chan- nels, gallium-doped germanium (Ge:Ga) photoconductors are the only detectors that can operate at the temperature available in the Spitzer cryostat.

The MIPS cryogenic scan mirror mechanism (CSMM) is intrinsic to all observational operations, enabling image motion compensation during scanned imaging and one di- mensional dithering for all 3 arrays, as well as selection of band and observing mode.

Figure 3.5 provides a schematic illustration of the physical layout of the major optical elements within the MIPS cold assembly: The 24 µm Si:As, 70 µm Ge:Ga, and 160 µm stressed Ge:Ga Focal Plane Arrays (FPAs), movable scan mirror, and fixed mirrors and grating. Two mirrors in the telescope focal plane deflect light into the instrument where it is reflected back by two mirrors to form pupils at the two facets of the CSMM. The CSMM deflects the light into the desired optical train: light is simultaneously sent into

Figure 3.6:Schematic representation of the IRS slits and peak-up apertures. Note that the IRS slits are not parallel in the Spitzer focal plane - Figure 3.1 shows the relative positions and angles of the slits/apertures. Taken from IRS Chapter 7 of the SOM.

the 3 widefield optical trains, or into the 70 µm narrow field-of-view train, or into the 55 µm SED optical train.

MIPS had four operating modes: Photometry, Scan Mapping, Spectral Energy Dis- tribution (SED), and Total Power Measurement. See Section 3.2.2 for a summary of the Spitzer observing modes. The majority of the MIPS 24 µm observations presented in this thesis were obtained in photometry mode, although the SINGS Legacy observations (described in Section 3.3.1) with MIPS were obtained in Scan Mapping mode.

The Infrared Spectrograph - IRS

The Infrared Spectrograph (IRS) contains the primary spectroscopic functions for Spitzer, with four modules capable of low and moderate resolution spectroscopy in the 5.2 to 38.0 µm wavelength range (Houck et al. 2004). There are also two small imaging sub- arrays (“peak-up arrays”) which allow for accurate positioning of sources in any of the IRS slits, and can provide science-quality images for photometric or structural information – known as IRS Peak-Up Imaging (PUI). Both a red (18.5–26.0 µm; λe f f = 22.3 µm) and a

blue (13.3–18.7 µm;λe f f = 15.8 µm) filter are available, the latter of which was designed to

fill the wavelength gap between IRAC and MIPS (for ease of reference, the blue PUI filter is referred to as the 16 µm filter throughout this thesis). Parallel red and blue peak-up images are obtained simultaneously, with a field-of-view seen by each filter of ∼5500× 8100,

separated by a 3300-wide vignetted zone (see Figure 3.6). The images are 30 × 45 pixels in size and have a native plate scale of ∼1.00

8 per pixel.

For the data presented here, the IRS has been mainly used for its imaging capabilities, i.e., PUI mode, with the blue peak-up array at 16 µm during Spitzer Cycles 3 and 4. The diffraction limited angular resolution of Spitzer is 4.00

74 at 16 µm.

Table 3.1 provides details of the instrument characteristics for the spectroscopic and imaging modes of the IRS. Figure 3.6 provides a schematic representation of the various IRS modules.