Infrared and Submillimeter Astronomy

1
Infrared and Sub-millimeter Astronomy

• Introduction Overview
• Chris ODea

Acknowledgements Steve Beckwith, Don Figer,
Bernie Rauscher, Jeff Valenti
2
Outline

• Historical Overview
• IR Detectors
• Backgrounds
• The atmosphere
• Astronomical
• Radiative Processes
• IR Sub-mm Science
• NGST

3
What constitutes infrared ?

• Traditionally 1 mm 1 mm
• 1 mm is long wave cutoff of silicon CCDs and
  photographic emulsions
• Initial mm-wave observations with bolometers
• Now 1 mm 300 mm
• CCDs still limited InSb/HgCdTe to 0.6 mm
• High frequency heterodyne receivers to lt350 mm
• Bolometers still dominate broad band to 1.3 mm
• Note 1000 mm 1 mm 300 GHz

4
History

• Herschels detection of IR from Sun in 1800
• Johnsons IR photometry of stars (PbS) mid 60s
• Neugebauer Leighton 2mm Sky Survey (PbS), late
  60s
• Development of bolometer (Low) late 60s
• Development of InSb (mainly military) early 70s
• IRAS 1983
• Panoramic arrays (InSb, HgCdTe, SiAs IBCs)
  mid-80s
• NICMOS, 2MASS, IRTF, UKIRT, KAO, common-user
  instruments, Gemini

5
Discovery of Infrared Light in 1800

• Herschel used a prism to separate sunlight into
  colors.
• He used a thermometer to determine the
  temperature in each color. (Two were placed off
  to the side as controls).
• The highest temperature was found beyond red
  light (where no light was seen).

Artists illustration from SIRTF web page.
6
Historical motivation

• Exploration discovery
• Neugebauer, Leighton, Low, Fazio, Townes
• Technological opportunities
• Bolometer (Low), PbS (Neugebauer), balloons
  (Fazio), IR lasers interferometry (Townes)
• A few, key problems
• Bolometric luminosities (Herschel, Johnson)
• The Galactic Center (Becklin)
• Star formation many but especially Strom(s),
  Cohen, Rieke(s)

7
IR Bolometer and Array Detectors

• Photon Detection in PN Junctions- Review
  semiconductors- The PN Junction- Charge
  collection in PN junctions

8
Valence Conduction Bands in Semiconductors

• When atoms (a) come together to form a crystal,
  the outer energy levels overlap and blend to
  create bands (b).
• The outermost filled band is called the valence
  band (c).
• Above the valence band, one finds a forbidden
  energy gap -the band gap, and (at higher
  energies) conduction bands populated by thermally
  excited electrons.
• In metals, the valence and conduction bands
  overlap resulting in conduction. In insulators,
  the band gap is wider resulting in very poor
  conduction.

9
Periodic Table
Semiconductors occupy column IV of the Periodic
Table (and have 4 valence electrons per atom)
10
P N Type Semiconductors

• In a semiconductor, some electrons are promoted
  from the valence band into conduction by thermal
  excitation at room temperature.
• These promoted electrons leave behind positively
  charged holes.
• Both electrons in the conduction band, and holes
  in the valence band, contribute to conduction.

11
P N type Semiconductors Continued

• One can dope the semiconductor by adding
  impurities to the crystal. Adding an impurity
  with more valence electrons than the crystal will
  donate negative charges to the conduction band,
  thereby creating an n-type semiconductor.
• If the impurity has fewer valence electrons than
  the crystal, it will donate holes to the valence
  band giving rise to a p-type semiconductor.
• When p-type material is butted against n-type
  material, the result is a PN junction. In CCDs
  and most IR arrays that are in use today,
  photo-excited charge is collected in PN
  junctions.

12
PN Junctions

• In a PN junction, positively charged holes
  diffuse into the n-type material. Likewise,
  negatively charged electrons diffuse in the the
  p-type material.
• This process is halted by the resulting
  field.
• The effected volume is known as a depletion
  region.
• The charge distribution in the depletion region
  is electrically equivalent to a 2-plate capacitor.

13
Photon detection in PN junctions

• A photon can interact with the semiconductor to
  create an electron-hole pair.
• The electron will be drawn to the most positively
  charged zone in the PN junction, located in the
  depletion region in the n-type material.
• Likewise, the positively charged hole will seek
  the most negatively charged region.
• Each photon thus removes one unit of charge from
  the capacitor. This is how photons are detected
  in both CCDs and most IR arrays.

14
IR Arrays are Hybrid Sensors

• A photosensitive array of PN junctions is bump
  bonded to a silicone readout multiplexer (MUX).
• This is done because silicon technology is much
  more advanced than any other semiconductor
  electronics technology. A modern MUX has about as
  many transistors as the most advanced Pentium
  CPU.
• The bump bonds are made of indium, a very soft
  metal used for welding dissimilar materials.

15
Schematic View of an IR Array

• Note that each pixel has only one electrode.
• Charge collection occurs in the depletion region
  near a PN Junction.
• Charge is sensed in situ (it does not move as in
  a CCD).

16
Backgrounds

• The Atmosphere
• Astronomical Backgrounds

17
Atmospheric effects

• Absorption
• reduced source flux
• difficult calibrations
• Emission
• increased background noise
• reduced integration times
• Turbulence
• increased object size (seeing)
• All effects vary with wavelength, time, altitude,
  line-of-sight

18
Atmospheric absorption versus airmass

• The amount of absorbed radiation depends upon the
  number of absorbers along the line of sight

AM1
AM2
Atmosphere
19
Atmospheric absorption versus l

• Sharp cutoffs
• defined primarily by H2O
• shape wavebands
• Higher transmission between lines with higher
  resolution
• Can introduce large calibration errors for low
  resolution observations (MNRAS, 1994, 266, 497)

Altitude 4200m Airmass 1.0 H2O column
1.2mm Resolving power 3000
"These data, produced using the program IRTRANS4,
were obtained from the UKIRT worldwide web
pages.
http//www.jach.hawaii.edu/JACpublic/UKIRT/astrono
my/calib/atmos-index.html
20
Atmospheric absorption versus l - high res
Array defects
CO2 absorption lines
R l/Dl 23,000
Keck II 10-m Figer et al. 2000, ApJ, accepted
21
Atmospheric absorption versus altitude

• Particle number densities (n) for most absorbers
  fall off rapidly with increasing altitude.
• x0,H20 2 km, x0,CO2 7 km, x0,O3 15-30 km
• So, 95 of atmospheric water vapor is below the
  altitude of Mauna Kea.

22
Atmospheric Transparency on Mauna Kea
CSO web page.
23
Atmospheric Transmission (0.9-2.6 mm)
24
Atmospheric absorption versus altitude
25
Telluric OH and Thermal Emission
Mauna Kea NIRSPEC R2000
Sky Thermal Background
H
K
J
26
OH Airglow time variability
27
Atmospheric emission Blackbody
Total power onto a detector P h AW Dn
esky Bn(Tsky)
h transmission of all optics x Q.E. esky
emissivity of sky A telescope area W
solid angle subtended by focal plane
aperture Dn bandwidth Bn(Tsky) Planck
function
At 10 mm, typically h 0.2, e 0.1, AW
3x10-10 m2 Sr Dn 1.5 x 1013 Hz (10 mm filter),
T 270 K P 10-9 W or 4
x 1010 g s-1
28
Atmospheric Turbulence

• A diffraction-limited point spread function (PSF)
  has a full-width at half-maximum (FWHM) of
• In reality, atmospheric turbulence smears the
  image
• At Mauna Kea, r00.2 m at 0.5 mm.
• Isoplanatic patch is area on sky over which
  phase is relatively constant.

29
Atmospheric Turbulence
1.4O seeing
0.5O seeing
no seeing!
Lick 3-m Figer 1995PhD Thesis
Keck I 10-m Serabyn, Shupe, FigerNature 1998,
394, 448
HST/NICMOS 2.4-m Figer et al. 1999ApJ. 525, 750
30
Background - sources

• Atmosphere
• thermal
• molecular
• Telescope
• thermal
• scattering
• Zodiacal light
• Astronomical sources

31
Background - sources Atmosphere

• Thermal
• OH
• The average OH line intensity is approximately
  25,000 g s-1 m-2 asec-2 mm-1.
• The continuum between lines is about 50 times
  lower than this value (in the H band).

32
Background - sources Telescope - scattering

• Mirrors
• Baffle edges and walls
• Secondary support

33
Background - sources Astronomical

• Astronomical objects can be objects of interest
  or noise contributors, depending on the project.
• Sunlight, moonlight
• Light scattered by solar system dust (zodiacal)
• Light emitted (thermal) by solar system dust
  (zodiacal)
• Stars (especially in a crowded field)
• Light emitted by interstellar dust (cirrus)

34
Background - sources Astronomical
35
Radiation Processes
36
Absorption in Insulators resonance features
Lattice resonances
log(e)
0
Vibrational modes 1 30 mm
-2
-4
kn s(n) / mp n2
-6
-2
-1
0
1
2
log (n)
long wavelengths
37
Radiative heating isolated particle
Particle radius, a (spherical rapidly
spinning) Temperature, T
Distance, r
Emitted radiative power 4pa2 sT 4
Luminosity, L
Using en for small particles T r -2/5
cf L. Spitzer, Jr., Physical Processes in the
Interstellar Medium, ch. 9.1
38
Thermal emission
spectral radiance, brightness, specific
intensity In e cos q Bn(T) W m-2 Hz-1
sr-1
q
e º emissivity (dimensionless)
Planck (blackbody) function
Peak in nBn
Bn(T)
Flux density from surface
Total flux
F s T4 W m-2
Fn p Bn(T) W m-2 Hz-1
s 5.67 x 10-8 W m-2 K-4
39
Planck Function

• Assumptions
• Uniform temperature source
• Source is opaque
• Mathematical description

Emitting Area
40
Computed Blackbody Spectra
Rayleigh-Jeans Tail
Wien Law
41
Blackbody Curves
42
Wien Displacement Law

• Blackbody peak wavelength inversely proportional
  to temperature
• Find peak wavelength by solving

where
where
Wien Law
43
Relative dust extinction
44
IR Sub-mm Science
45
Current interest in infrared

• High redshift objects
• lobs l0 (1z) 5000 Å gt1 mm for z gt 1
• Classical problems require infrared data
• Obscuration by dust
• Al l-1.9 A2.2mm 0.1 AV
  J. S. Mathis 1990, ARAA, 28,
  37.
• Now important for
• Galactic nuclei, esp. AGN (unified model)
• Starburst galaxies
• Young stars

46
Current interest in infrared

• Very low mass objects extrasolar planets
• Tplanet 50 to 500 K lpeak 5 50 mm
• TBD 900 2000 K lpeak 1 5 mm

Extrasolar planets, brown dwarfs, and
circumstellar disks are optically faint but
infrared bright.
47
Structure of a protostar
after Stahler, Shu, and Taam 1980, Ap.J., 241,
637.
48
Young infrared star W33 A
after Soifer et al. 1979, Ap.J.Lett., 232, L53.
49
NICMOS
50
Mass Loss from Evolved Stars - 1

• Broad Scientific Goals Key Objectives
• Measure outflow characteristics for evolved stars
• Temperature, density, velocity, and composition
• Radial dependence for resolved sources
• Understand molecular and dust chemistry in
  outflows
• Nonequilibrium gas chemistry
• Dust formation mechanisms and rates
• Understand dynamical mechanisms driving outflows
• Radiative acceleration beyond a few stellar radii
• Adams MacCormack (1935), Spitzer (1938)
• Predictive model of mass loss from evolved stars
• Function of stellar age and initial stellar mass
• Feedback on interstellar structure and
  composition
• Test stellar evolution models for evolved stars
• Nuclear reaction pathways
• Internal mixing mechanisms

51
Mass Loss from Evolved Stars - 2

• Key Measurements
• Molecular lines at infrared and millimeter
  wavelengths
• Over 50 species detected in IRC10216
• Line ratios constrain temperature and density
• Line shifts and widths constrain velocity fields
• Isotopic abundance ratios constrain stellar
  models
• Infrared dust features
• A few dust families (silicates, graphites, ices,
  etc.)
• Band strengths constrain dust chemistry
• Angular resolution (10 mas)
• Resolves radial dependence of outflow
  characteristics
• Directly image clumps and general asymmetry
• Measure proper motion of clumps in nearest
  sources
• Spectral energy distribution constrains
  unresolved sources

52
Mass Loss from Evolved Stars - 3

• Samples
• Resolved outflow sources
• Cursory literature search
• Supergiants (I, II) and Miras
• Stellar angular diameter gt5 mas
• Outflows larger than photosphere
• Also proto-planetary nebulae
• Evolved stars in clusters
• Typical distance is 2 kpc
• Main sequence gives progenitor mass
• Interpret using detailed studies of resolved
  sources

53
Mass Loss from Evolved Stars - 6
Tsuji, Ohnaka, Hinkle, Ridgeway (1994, AA,
289, 469)
54
Mass Loss from Evolved Stars - 7
IRAS 09425-6040
AFGL 4106
Molster et al. (1999, AA, 350, 163)
Molster et al. (2001, AA, 366, 923)
55
Mass Loss from Evolved Stars - 8
Cernicharo, Guelin, Kahane (2000, AAS, 142,
181)
56
Planetary spectra
4
2
H2SO4
CO2
Venus
Jupiter
2
0
O3
H2O
2
Earth
0
0
Saturn
Mars
0
0
10
20
30
10
20
30
Relative, linear scales
Wavelength (mm)
57
Disks infrared emission
Beckwith Sargent 1996, Nature, 383, 139-144.
58
Circumstellar Dust
ASWG Marcia Rieke
Vega Disk Detection l Flux Contrast
(?m) (?Jy) Star/Disk 11?m 2.4
1.5x107 22?m 400 2x104 33?m 1300
3x103 Reflected emitted light detected with a
simple coronograph.
per Airy disk
NGST resolution at 24?m 5 AU at Vega, gt 10
pixels across the inner hole
59
Waelkens et al. 1996, AA, 315, L245.
Comet Hale-Bopp
6 Oct 1996
Fn(Jy)
Foresterite is a "primordial" constituent of
Solar dust
HD 100546
200
Fn(Jy)
Foresterite Mg2SiO4
100
PAH
0
10
40
20
30
Wavelength (mm)
60
HD 100546 - SWS and LWS all components
250
Short wavelength part - SWS
15
HD 100546
PAH
H2O - ice (50-80 µm)
Stellar photosphere
PAH
Hot continuum
Cold continuum
Total
10
PAH
200
H2O - ice
FLUX (Jy)
5
Br d
Pf d
Br a
Pf g
150
H2O - ice (43.8 µm)
0
2
6
8
4
OI (63.2 µm)
Crystalline pyroxene (40 µm)
Wavelength (µm)
PAH (11.3 µm)
FLUX (Jy)
100
OII (157.7 µm)
PAH (8.6 µm)
Hot cold continuum
PAH (7.8 µm)
PAH (3.3-3.4-3.5 µm)
50
PAH (6.2 µm)
Total
0
Crystalline forsterite
Amorphous olivine
-50
FeO
10
100
Wavelength (µm)
Malfait et al. 1998, AA, 332, L25
61
Radio to IR Spectrum of Luminous IR Galaxies
K-correction increases flux density for high-z
objects.
Carilli Yun 2000, ApJ, 530, 618
62
Mid-IR Observations of NGC1068
Imaging the starburst component.
(a) Mid-IR continuum. (b) PAH emission. (c) SCUBA
450 um on PAH. (d) CO on PAH.
Le Floch et al 2001, AA, 367, 487
63
Mid-IR Observations of NGC1068. II

(Top) Decomposition of Mid-IR spectrum into AGN
and starburst. (Bottom) ratio of unresolved flux
to extended (40) and total emission
Le Floch et al 2001, AA, 367, 487
64
Broad Band SED of 3C273

A large fraction of the bolometric luminosity is
re-emitted in the IR-submm band.
Average spectrum of 3C273. Dashed line is
extended jet. Dotted line is contribution from
host galaxy.
Turler etal 1999, AAS, 134, 89
65
Seeing through the dust in Cen A
66
COBE/DIRBE Image of the Sky
60 ?m blue 100 ?m green 240 ?m red.
Hauser etal.
67
COBE/DIRBE Image of the Sky
Zodiacal light removed. 60 ?m blue 100 ?m
green 240 ?m red.
Extragalactic Background (Galaxy removed). 240
?m image.

Hauser etal.
68
Cosmic UV to mm Extragalactic Background
Cosmic background can be produced by warm
M82-like star forming galaxies.
Genzel Cesarsky 2000, ARAA, 38, 761
69
NGST and the Future
70
Background - sources NGST
e-/s/pixel
71
The Future NGST
72
Near-infrared observing facilities
73
Sensitivity of Future IR Facilities
5s Flux Limits in 104 seconds
74
The End