Effects of Dust on the Observed SEDs of Galaxies

Andrew Schurer

INAF/SISSA & MAGPOP

Overview

• Interstellar Dust in Astrophysics:

– Why is the study of dust important?

– How can dust reprocessing be effectively modelled, e.g. GRASIL

• My Role within this field

– My current work,

– Future research possibilities.

The Importance of Dust in Astrophysics

• Trumpler (1930) found that distant stars dimmed by something in addition to inverse squared law; dust.

• Recent IR observations (IRAS) have shown how important this effect is. In the local universe 30% of starlight is dust reprocessed.

• Dust absorbs and scatters photons, mostly at UV wavelengths, emitting it in the IR

Dust Models• Seek to reproduce the SED of an astrophysical object including the

presence of dust.

• Unrealistic geometry -2D slab model

• Incomplete radiative transfer

– no emission

– Empirical fitting laws used.

• No link to galaxy evolution

– estimate of adsorbed stellar radiation

– many only interested in interpretation of a single object

• GRASIL (Silva 1998) - complete radiative transfer computation of

self-consistent galactic models including dust with realistic geometry. – Calculates SED from radio to far UV

– including PAH, molecular and nebular lines

GRASIL - geometry• Galaxies are composed of either a spheroid a disk or a combination of both.

• Two main Dust Components - Molecular clouds (where stars are born) - Interstellar medium

• Also Dust envelope around AGB stars, represents shell of expelled material.

Age Dependent Extinction

Chemical Evolution• Chemical Evolution - Requires history of SF, initial mass function,

metallically and residual gas function.– Semi Analytic Models e.g. Durham model - Galform– Monolithic chemical evolution code -che evo

• CHE EVO – SFR based on a Schmidt law with the possibility truncating the SF or

of adding a burst of star formation.

Observational Data Sample

Goldmine data set - Boselli (2003) (MAGPOP collaborator)

• Data available for wide range of wavelengths from radio to UV, including IR data from ISO CAM/PHOT and IRAS

• Optically selected Virgo sample (100 galaxies)

– Galaxies later than SO

– Whole range of morphologies and galaxy densities.

• Virgo serendipitous sample (18 galaxies)

– Representative of a mid-IR selected sample of nearby galaxies

• An additional 6 Virgo galaxies observered by ISOCAM as part of other projects

• A1367 and Coma clusters samples (contursi 2001) (18 galaxies)

– High star forming late time galaxies with peculiar morphologies (not a complete sample)

• Additional 3 galaxies in the Coma cluster

GRASIL Parameters• Che-Evo

– Star formation efficiency

– Infall Timescale

• Geometry of Galaxy

– For Bulge: one scale length

– For Disk: two scale lengths

– For a combination: disk to bulge ratio

• Other

– Radius and mass of MCs (one independent parameter)

– Fraction of residual gas in MCs

– Escape Timescale

Conclusions

• Shown GRASIL is capable of recreating observations of late type galaxies of different type.

• Several problems still. Particularly in UV, radio and when describing galaxies of earlier type,

• Future Work– Improve fits, by increasing size of library - more chemical evolutions.– Through an investigation into the parameters, typical values can be

found.– Analyse nebula emission lines and check if best fit models agree with

other observational evidence e.g. physical size.– Detect and correct any further problems found.

Future Work• Improved model of PAH bands

– First version of GRASIL based on pre-ISO data, (following Xu & De Zotti 1989)

– Following release of ISO data PAH treatment updated (Vega 2005 following Li & Draine 2001). ISO able to measure short wavelengths only in bright objects e.g. visual reflection nebula.

– PAH suppressed in the MCs to match observations.– IRAC aboard the Spitzer space telescope has increased sensitivity and

resolution, allowing for better treatment of the PAH lines (Draine 2006).– More bands and slight changes in existing bands, better treatment of

NIR continuum.– GRASIL should be updated to incorporate this.

• Evolution of Dust Grain Population

– Complex problem, need to explain how dust grains are produces and destroyed (Dwek 98, Morgan & Edmunds 2003).

– The main ways in which dust grains can be produced are in the stellar outflow of stars such as AGBs and supernova and also by being built up in the ISM.

– Processes which lead to their destruction include shock waves due to supernova and collisions with other grains.

– Not clear which of the processes dominate.

– GRASIL does not include any details of the evolution of the grain population. E.g. Birth and destruction of grains.

– By attempting to include these processes within the GRASIL model it should be possible to make it more physically consistent and possibly more accurate at high redshift.

• Modelling embedded clusters.– Young Massive Clusters - possibly evolutionary fore-runners of

Globular Clusters (Tagle 2003).– Two types blue YMCs, for whom the blue photosphere of the young

stars is directly visible and embedded YMCs.– Observations of embedded YMCs in a starburst environment require

an angular resolution in the MIR which has only become available recently.

– Using new data it is possible to try to model embedded YMCs in starburst environments, using a dust model such as GRASIL.

– Can improve our understanding of starburst phenomena, the formation of globular clusters as well as about star formation itself.

• Coupling to SAMs in Munich (MAGPOP node)– It is essential that any galaxy evolution model contains a correct

treatment of dust in order to compare models to observations.– Work with Munich to develop a GRASIL ‘black box’ which can be

‘attached’ to their SAMs.

Summary• Interstellar dust is a very important component of galaxies and a

rigorous treatment must be adopted for a wide range of studies including the modelling of SEDs and determination of the star formation rate.

• GRASIL is a state of the art model which incorporates the effects of dust reprocessing from the X-Ray to radio wavelengths within a

realistic galactic geometry. • It can be used to model a wide range of different galaxies with careful

treatment of the parameters of the model.

• Needs an updated treatment of the PAH lines based on the new SPITZER observations.

• It should include a more physical treatment of the evolution of the dust component.

• It can be used to study the evolution of Young Massive Clusters.

• It could be combined with the Munich SAMs.