Galactic Cirrus and Zodiacal Light

60K 50AU Dyson shell

\Large B(\lambda) = \LARGE { { 2 h c } \over { \lambda^3 \left( e^{ h c / \lambda k T } - 1 \right) } } = { { 4e19 ~ Jy / sr ~ {\mu}m^3 } \over { \lambda^3 \left( e^{ 240 {\mu} m / \lambda } - 1 \right) } }

This is for an infinite resolution imager - at large distances, the power is the image angular area divided by the pixel angular area. This scales the intensity way down for distant Dyson shells. If the ratio of shell intensity to background is F , the pixel size in arcseconds is px , the shell radius in AU is r , and the distance in parsecs is R , then the angular size A is:

A ~ = ~ \pi ( r / R )^2

F ~ = ~ px^2 / A = ( px ~ R / r ) ^2 / \pi

R ~ = ~ ( 50" / px ) \sqrt{ \pi F }

Detectability

px is 90" (90 arcseconds) for IRAS, 12" for WISE, and .11" for JWST. It is 2.3" for Spitzer MIPS (Multiband Imaging Photometer for Spitzer) 24 μm (128x128 pixels) and 9.4" for Spitzer 70 μm (16x32 pixels).

R ~=~ 0.96 \sqrt{ F } for IRAS, 7.2 \sqrt{ F } for WISE, and 790 \sqrt{ F } for JWST. Of those three, only IRAS has sensors for wavelengths longer than 25 μm. R ~=~ 38 \sqrt{ F } for 24 μm Spitzer and 9.2 \sqrt{ F } for 70 μm Spitzer.

Disk

Assume 3 pixels wide for identification of a disk. IRAS can "see" a 270" disk ( 100/270 = 0.37 pc away - nothing expected), WISE can see a disk (100/12 = 8.3 pc away ), etc:

Pixel

Image

IRAS

90"

0.37

WISE

12"

2.8

Spitzer 24 μm

2.3"

14.5

JWST 25 μm

0.11"

300

attachment:cobeslide12.jpg from http://lambda.gsfc.nasa.gov/product/cobe/cobe_image_table.cfm

The graphs at the NASA site are not scaled, but they resemble logarithmic-scaled graphs in Kelsall, 1998, Figure 2. Units MJy/sr.

Color

Blue

Blue

Blue

Blue

Cyan

Cyan

Gren

Gren

Yelw

Yelw

Orng

Orng

Red

Red

Gray

Gray

Whit

4.9 μm

0.10

0.14

0.19

0.26

0.35

0.49

0.67

0.92

1.26

1.72

2.37

3.25

4.46

6.12

8.39

11.5

15.8

12 μm

1.58

2.02

2.58

3.29

4.21

5.37

6.86

8.77

11.2

14.3

18.3

23.3

29.8

38.1

48.7

62.2

79.4

25 μm

3.98

4.80

5.79

6.98

8.41

10.1

12.2

14.7

17.8

21.4

25.8

31.2

37.6

45.3

54.6

65.9

79.4

60 μm

1.00

1.31

1.73

2.27

2.99

3.92

5.16

6.78

8.91

11.7

15.4

20.2

26.6

35.0

46.0

60.4

79.4

The zodiac is highly inclined compared to the galaxy, but these plots show it intersecting near galactic center. This is fortunate if true - we are unlikely to distinguish a Dyson shell where the background is very dense (too confusing) or very thin (radially outwards, fewer candidates) - this puts two problem areas in the same place.

attachment:Ellisfig58b.jpg

Band(um)

12μm

25μm

60μm

70 μm

100μm

frequency

25 THz

12 THz

5 THz

4.3 THz

3 THz

Z nW/m2-sr

2800

1800

190

130

42

Z Jy-sr

1.12e7

1.5e7

3.8e6

3.0e6

1.4e6

C nW/m2-sr

4.8

2.3

3.2

4.5

13

C Jy-sr

1.9e4

1.9e4

6.4e4

1.1e5

4.3e5

B Jy-sr

4.8e7

1.7e11

3.5e12

3.9e12

4.0e12

B/(C+Z)

4.3

1.1e4

9.1e5

1.3e6

2.2e6

we may ignore zodiacal measurements

F=B/C

2.5e3

8.9e6

5.5e7

3.5e7

9.3e6

assume measurements in galactic plane

R IRAS pc

48

2900

7100

--

2900

R WISE pc

360

21000

--

--

--

R Spitzer pc

--

110000

--

54000

--

R JWST pc

40000

2400000

--

--

--

Question: In the very short wavelengths, does the presence of a shell block luminosity in a detectable way - a "round hole" in a uniform nebula?