Astrophysics from Space

Lecture 8: Dusty starburst galaxies

Prof. Dr. M. Baes (UGent)

Prof. Dr. C. Waelkens (KUL)

Academic year 2014-2015

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 Mgas/Mdust ≈ 100)

Still very important• extinction of starlight• FIR/submm emission• interstellar chemistry

and physics (e.g. H2 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 15-30 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 SCUBA@JCMT.

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 (20-100 µm)

Major role for Spitzer 24 µm

Obscured star formation: Antennae

HST image (optical/UV) reveals hundreds of SF regionsBut 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 (LIR > 1011 Lsun)• ULIRGs (LIR > 1012 Lsun)

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)