Astrophysics from Space Lecture 8: Dusty starburst galaxies Prof. Dr. M. Baes (UGent) Prof. Dr. C. Waelkens (KUL) Academic year

The interstellar medium The space between the stars in a galaxy is not empty cold molecular gas: mm line radiation (CO) cold atomic gas: 21 cm line radiation (HI) warm ionized gas: optical line radiation (Balmer lines) hot plasma: X-ray observations interstellar dust

Interstellar dust Dynamically not important (typically M gas /M dust ≈ 100) Still very important extinction of starlight FIR/submm emission interstellar chemistry and physics (e.g. H 2 formation, gas cooling and heating…) building blocks for all organic material

Extinction by interstellar dust Extremely efficient in absorbing and scattering UV/optical radiation continuum extinction most efficient for UV/blue radiation

Modelling dust extinction Modelling dust extinction is very complicated (particularly scattering) Computer models indicate spiral galaxies are moderately opaque.

Modelling dust extinction

Thermal emission by interstellar dust Energy balance: dust grains emit the energy they absorb If we know the intensity of the radiation field and the optical properties of the dust, we can calculate the dust temperature. Realistic values in the ISM yield temperatures of K. The corresponding emission peaks in the FIR/submm… Interstellar dust effectively converts optical/UV starlight to FIR/submm emission

Thermal emission by interstellar dust NGC891 in submm radiation (left) and optical light (right). Submm image from M51 in the FIR (Herschel)

Thermal emission by interstellar dust For most galaxies: no spatial FIR information available. Analysis of the spectral energy distribution. Major ISO result: typical spiral galaxies emit about 30% of their bolometric luminosity in the FIR ! Confirmed by Spitzer (and by Herschel…) ISO results

Thermal emission by interstellar dust For most galaxies: no spatial FIR information available. Analysis of the spectral energy distribution. Major ISO result: typical spiral galaxies emit about 30% of their bolometric luminosity in the FIR ! Confirmed by Spitzer (and by Herschel…) Spitzer results

Star formation in galaxies Traditional SF tracers: UV radiation Hα line radiation Subject to dust extinction...

Obscured star formation Dust energy balance in star forming regions: hotter dust. Emission at shorter wavelengths ( µm) Major role for Spitzer 24 µm

Obscured star formation: Antennae HST image (optical/UV) reveals hundreds of SF regions But 50% of the MIR emission comes from obscured region between nuclei: obscured star formation !

LIRGs and ULIRGs One of the major discoveries of IRAS: population of galaxies with extreme luminosities in the FIR LIRGs (L IR > L sun ) ULIRGs (L IR > L sun ) Most (U)LIRGs emit >90% of their entire bolometric luminosity in the FIR. Dust in (U)LIRGs reprocesses up to 99% of their stellar radiation to FIR emission

LIRGs and ULIRGs Two scenarios for the origin of ULIRGs AGN (same space density and luminosities as quasars) star formation (HST imaging reveals them as mergers)

LIRGs and ULIRGs: MIR spectroscopy Dominant feature in MIR spectra of galaxies: PAH emission lines. Observable using ISO and Spitzer spectroscopy.

LIRGs and ULIRGs: MIR spectroscopy Importance differences in PAH strength star forming galaxies show strong PAH features AGN show no PAH features (AGN destroy PAHs)