Infrared Astronomy. Generally ~ 1μm (10,000 Å) few hundred μm

Transcription

1 Infrared Astronomy Generally ~ 1μm (10,000 Å) few hundred μm Atmospheric transmission: grey regions are observable from the ground. Two regimes for IR astronomy Ground- based near/mid- IR astronomy through atmospheric windows. Space- based near/mid/far- IR astronomy

2 Infrared Astronomy: MoJvaJon Cool stars: λ max = 0.29 cm/t(k), for a 3000K star gives λ max = 1μm Low mass main sequence stars, red giants, red supergiants For galaxies, most of stellar mass is in low mass stars Brown dwarfs: even cooler

3 Infrared Astronomy: MoJvaJon Penetrate Dust in near- IR AV = 3.2 E(B- V), but AK=0.28 E(B- V) Mapping galaxy structure: Milky Way Studying Star formajon: Eagle Nebula

4 Infrared Astronomy: MoJvaJon Dust emission at mid- and far- IR Con:nuum: λ max = 0.29 cm/t(k), for 30K dust gives λ max ~ 100μm Line emission: dust has large, complex molecules called Polycyclic AromaJc Hydrocarbons (PAHs) which radiate line emission in the mid- IR. Galliano 2004

5 Infrared Astronomy: MoJvaJon Dust emission at mid- and far- IR Mapping galaxy ISM: Milky Way Mapping galaxy ISM: M81

6 Infrared Astronomy: MoJvaJon Dust emission at mid- and far- IR

7 Infrared Astronomy: MoJvaJon Hidden star forma6on in galaxies Herchel 70/100/160μm HST opjcal

8 Infrared Astronomy: MoJvaJon Low- energy lines Molecular rotajonal/vibrajonal transijons Atomic fine structure lines: e.g., OI (63μm) and CII (158μm). Dust/PAHs

9 Infrared Astronomy: MoJvaJon Studies at higher redshi> λ obs = (1+z)λ em So at z=1, opjcal imaging probes intrinsic UV emission

10 Infrared Astronomy: MoJvaJon Studies at higher redshi> λ obs = (1+z)λ em and opjcal features have shiked into the IR

11 Infrared Astronomy: Techniques/Issues Spa6al Resolu6on DiffracJon limit: θ min = 1.22λ/D. So for Spitzer, D=0.85m: 5μm è 1.5 resolujon 70μm è 20 resolujon (JWST will have a 6.5m primary) Ground- based Backgrounds High atmospheric opacity, largely due to water vapor. (get high and dry!) Very high sky background: OH lines in the near- IR thermal background at longer wavelengths (T=300K è λ max = 10μm) Poisson noise in background >> source intensity Atmospheric emission is highly variable on Jmescales < 1s Good news: atmospheric turbulence drops at longer wavelength, so 10m class telescopes are diffracjon limited at a few μm.

12 Sky Backgrounds opjcal

13 Infrared Astronomy: Techniques/Issues Spa6al Resolu6on DiffracJon limit: θ min = 1.22λ/D. So for Spitzer, D=0.85m: 5μm è 1.5 resolujon 70μm è 20 resolujon (JWST will have a 6.5m primary) Ground- based Backgrounds High atmospheric opacity, largely due to water vapor. (get high and dry!) Very high sky background: OH lines in the near- IR thermal background at longer wavelengths (T=300K è λ max = 10μm) Poisson noise in background >> source intensity Atmospheric emission is highly variable on Jmescales < 1s Good news: atmospheric turbulence drops at longer wavelength, so 10m class telescopes are diffracjon limited at a few μm.

14 Infrared Detectors (Rieke 2007 ARAA) CCDs Like opjcal CCDs, semiconductors which detect individual photons. Different materials: Material Silicon HgCdTe ( mercad ) InSb Si:As Ge/Ga Wavelength < 1 μm μm μm 5 28 μm μm Beyond a few microns, detectors need to be cooled to liquid Helium temperatures (4K) Detector consists of an photon- sensijve layer coupled to a Si- FET array which measures the photoelectrons. Each pixel has its own FET, so pixels are not electronically coupled. Array can be read out at high speed in non- destrucjve mode.

15 Infrared Detectors (Rieke 2007 ARAA) Bolometers Absorb and thermalize photons, energy increase (heat!) is detected. Must be cooled to mk temperatures. Example: SCUBA- 2 on JCMT (15m) Array of four 32x40 detectors 450μm and 850μm (sub- mm) Detectors operate at 80mK FWHM: FOV: ~ 45 arcmin 2 Scan the detector across the sky to reconstruct a map.

16 450μm

17 Infrared Observing : Ground Based Jansky (Jy) = W/m 2 /Hz = erg/s/cm 2 /Hz Example: 10μm Thermal flux from telescope: 144 Jy/arcsec 2 è 1.4x10 7 e - /sec/pix on detector! In a 10s exposure, photon noise will be sqrt(1.4x10 8 )=12,000 e - per pixel Arcturus: 600 Jy, so for a 0.7 aperture, only 2/3 of the flux comes from the star. Sources must be bright, and background subtracjon is crijcal! Exposures: Chopping and Nodding A typical 10s exposure actually consists of hundreds of ~ 20 msec frames. Chopping: rapidly switching between source and background (~15 offset) on ~ few Hz frequencies. Done with a chopping secondary mirror. By subtracjng the background, most of the sky can be corrected out. Nodding: Chopping gives residual background since light path through telescope is changing. Nodding moves the telescope every few seconds to offset these differences. In spectroscopic mode (or near- IR), background is lower and exposures can be longer; oken can get away without chopping ( stare and nod ).

18 Major Infrared Satellite Missions Why space? All wavelengths accessible Backgrounds! Telescope cool, no atmospheric background. Go to Earth- trailing orbits to avoid thermal background from Earth. IRAS (1983): 0.57m primary, all- sky IR survey at 12, 25, 60, and 100μm. ResoluJon: 12μm, Spitzer (2003- ): 0.85m primary, pointed. Imaging + spectroscopic capabilijes from 3-180μm. Instruments: IRAC, IRS, MIPS. Ran out of liquid Helium in 2009, only near- IR imaging now. Herschel ( ): 3.5m primary, pointed. Imaging and spectroscopic capabilijes from μm. WISE ( ): 0.4m primary, all- sky survey at 3.4, 4.6, 12, and 22μm. Much more sensijve than IRAS. JWST ( ?): 6.5m primary, pointed μm, imaging + spectroscopic capabilijes.