Astrophysics from Space Lecture 7: Infrared astronomy Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year

The atmosphere in the IR The atmosphere is almost completely opaque to IR radiation. Culprit: H 2 0 Observations from space seem unavoidable for FIR astronomy

Stratospheric observations Water vapor is a low-altitute phenomenon. Stratospheric or ground- based telescopes: cheaper diffraction limit !

Stratospheric observations KAO ( ) SOFIA (2010-)

Balloon observations Boomerang (microwave) BLAST (FIR/submm)

Observations from the ground Observations from the ground are possible in a few windows. Even though the transparency is modest and variable, this has advantages large collection area higher resolution cheaper longer lifetime Now even attempts to open THz window (200 µm)

Infrared/submm/mm detectors Photon detectors individual photons release charge carriers based on the photo-electric effect incoherent: phase information is lost Thermal detectors (bolometers) photon energy goes into heat measure the heat change via e.g. resistance incoherent Coherent detectors direct measurement of the electric field of the wave coherent: preserve phase information

Photon detectors for IR astronomy Normal semiconductors have limited cutoff wavelengths Si – 1.11 µm Ge – 1.85 µm HgCdTe – 12.4 µm Solutions doping (add different atoms to a pure crystal to create intermediate levels) Si:Sb – 28.8 µm Ge:Ga – 115 µm mechanical stress

Bolometers Principle: measure the change in heat input and convert it to current or voltage Two main parts: absorber and thermometer Main advantage: quantum efficiency is about 100% Main disadvantage: output per incident power does not depend on wavelength

Bolometers

Coherent detectors Measure the direct interaction of the electric field of the wave e.g. dipole antenna Limited by quantum noise (only doable at long wavelengths) Coherent detectors in (sub)mm astronomy: heterodyne receivers

Infrared Astronomical Satellite 1974: NASA AO for missions on Explorer spacecraft. Competition won by IRAS Joint project of NASA, NIVR (Netherlands) and SERC (UK) Operational in 1983 Liquid helium depleted after 10 months (still orbits the Earths but insensitive to IR emission)

Infrared Astronomical Satellite Primary mirror of 60 cm diameter Filters centered at 12, 25, 60 and 100 µm Main mission goal: diffraction-limited all-sky survey in the four bands. Extra instruments: LRS spectrograph (8 – 22 µm) CPC (bad)

IRAS legacy Main legacy: All-sky images and the associated catalogues objects ! Many interesting discoveries dusty starburst galaxies circumstellar disks asteroids Even today, IRAS is a huge resource for astronomers ! (many objects still unidentified)

IRAS all-sky survey (12, 60 and 100 µm)

Cosmic Background Explorer 1974: Three CMB experiments lost competition to IRAS. NASA invited astronomers from these programs to think about joint project 1979: COBE mission designed Nov 1989: Launch (delayed due to cost overruns on IRAS and the Challenger disaster) Sep 1990: He supply finished Dec 1993: instruments turned off

Cosmic Background Explorer Goal: measure the diffuse infrared and microwave background radiation Technology borrowed heavily on IRAS (IR detectors, dewar…) DIRBE: all-sky maps ( µm) DMR: CMB maps (3.3 – 9.5 mm) FIRAS: measure CMB spectrum precisely (100 µm – 10 mm)

COBE legacy COBE did fulfill all of its objectives (and even more…) More details in lecture on the Cosmic Microwave Background

COBE legacy 2006 Nobel Prize in Physics awarded to John C. Mather (FIRAS) and George F. Smoot (DRM) Nobel Prize committee: “The COBE project can be regarded as the starting point for cosmology as a precision science”

Infrared Space Observatory European follow-up mission for IRAS (infrared detector technology developed very fast) 1979: first ideas (before IRAS launch!) 1983: selection of ISO project Nov 1995: launch Feb 1996: start of the operations Apr 1998: depletion of liquid He

Infrared Space Observatory ISO orbit: highly elliptical geocentric orbit (24h period) Perigee inside Van Allen radiation belt: 7h shutdown/day All data transmitted in real time to ground stations

Infrared Space Observatory Telescope diameter 60 cm Main differences: elliptic orbit wavelength coverage to 240 µm spectroscopy: SWS and LWS increased spatial resolution sensitivity: 1000 times better ! no survey instrument ISO design and technology: borrowed heavily from IRAS experience

ISO legacy successful observations successful observing proposals 500+ principle investigators Full data set available to the community in the ISO archive

WMAP Success of COBE asked for a successor. 1995: MAP proposed 1996: mission selected (medium-sized mission in the Explorer program) 2001: MAP launched 2003: renamed WMAP 2010: mission ceased (original lifetime was 27 months)

WMAP Relatively simple mission: completely focused on detailed measurement of CMB and its anisotropies Novelties compared to COBE Lissajous orbit around Sun-Earth L 2 point 2 large dishes of 1.4 x 1.6 m 2 each 5 microwave bands between 3.2 and 13 mm 45x more sensitive 33x angular resolution Small and focused mission: price only 150 M$

WMAP legacy

Spitzer Space Telescope NASA’s follow-up mission for IRAS. Originally seen as a major facility for Spacelab (heavily relying on the Space Shuttle program) (seeing the success of IRAS): NASA goes for a space observatory Late 1980s and 1990: hard times for SIRTF Challenger explosion drastic budget cuts serious de-scope of the mission Finally launched in Aug 2003 – end of cold phase in Apr 2009

Spitzer Space Telescope

Main novelties compared to IRAS and ISO Earth-trailing heliocentric orbit warm-launch cryogenic architecture (only the instruments are cooled to cryogenic temperatures) spectacular increase in detector technology increased spatial resolution with 85 cm primary mirror

Spitzer legacy Similar to ISO: general observations (no survey instrument) Huge archive with loads of data still to be investigated Similar to HST: legacy programs Part of the Great Observatories: Good synergy with HST Attention for outreach

Herschel Space Observatory State-of-the-art FIR observatory Fourth cornerstone mission of ESA’s Cosmic Vision Contribution from NASA and CSA Proposed in 1982 as FIRST Launched 14 May 2009, together with Planck 2000 l of liquid helium exhausted in April 2013

Herschel Space Observatory Design choice: large, passively cooled, 3.5 m mirror (largest mirror ever launched in space) sensitivity spatial resolution Orbit around L 2 Wavelength coverage to submm wavelengths (unexplored territory)

Herschel legacy Mode of operations: 1/3 guaranteed time, 2/3 open time First results were spectacular (also the later ones…)

Planck ESA’s CMB mission, formerly COBRAS/SAMBA Main goal: improvement of COBE and WMAP results resolution: 3x better sensitivity: 10x times better larger wavelength coverage (350 µm – 10 mm) polarization Launched together with Herschel in May 2009, mission officially ended in October 2013.